Abstract: We demonstrate a 40 Gb/s selfsynchronizing, alloptical packet
clock recovery circuit designed for efficient packetmode traffic. The circuit
locks instantaneously and enables subnanosecond packet spacing due to the
low clock persistence time. A lowQ FabryPerot filter is used as a passive
resonator tuned to the linerate that generates a retimed clockresembling
signal. As a reshaping element, an optical powerlimiting gate is
incorporated to perform bitwise pulse equalization. Using two preamble
bits, the clock is captured instantly and persists for the duration of the data
packet increased by 16 bits. The performance of the circuit suggests its
suitability for future alloptical packetswitched networks with reduced
transmission overhead and fine network granularity.
Abstract: In this paper, we survey the current stateoftheart in middleware and systems for Wireless Sensor Networks (WSN). We provide a discussion on the definition of WSN middleware, design issues associated with it, and the taxonomies commonly used to categorize it. We also present a categorization of a number of such middleware platforms, using middleware functionalities and challenges which we think will play a crucial role in developing software for WSN in the near future. Finally, we provide a short discussion on WSN middleware trends.
Abstract: Based on our experience in designing, building and maintaining an information system for supporting a large scale electronic lottery, we present in this paper a unified approach to the design and implementation of electronic lotteries with the focus on pragmatic trust establishment. This approach follows closely the methodologies commonly employed in the development of general information systems. However, central to the proposed approach is the decomposition of a security critical system into layers containing basic trust components so as to facilitate the management of trust, first along the layers, and then as we move from layer to layer. We believe that such a structured approach, based on layers and trust components, can help designers of security critical applications produce demonstrably robust and verifiable systems that people will not hesitate to use.
Abstract: Implementation of a commercial application to a
grid infrastructure introduces new challenges in managing the
qualityofservice (QoS) requirements, most stem from the fact
that negotiation on QoS between the user and the service provider
should strictly be satisfied. An interesting commercial application
with a wide impact on a variety of fields, which can benefit from
the computational grid technologies, is three–dimensional (3D)
rendering. In order to implement, however, 3D rendering to a
grid infrastructure, we should develop appropriate scheduling
and resource allocation mechanisms so that the negotiated (QoS)
requirements are met. Efficient scheduling schemes require
modeling and prediction of rendering workload. In this paper
workload prediction is addressed based on a combined fuzzy
classification and neural network model. Initially, appropriate
descriptors are extracted to represent the synthetic world. The
descriptors are obtained by parsing RIB formatted files, which
provides a general structure for describing computergenerated
images. Fuzzy classification is used for organizing rendering
descriptor so that a reliable representation is accomplished which
increases the prediction accuracy. Neural network performs
workload prediction by modeling the nonlinear inputoutput
relationship between rendering descriptors and the respective
computational complexity. To increase prediction accuracy, a
constructive algorithm is adopted in this paper to train the neural
network so that network weights and size are simultaneously
estimated. Then, a grid scheduler scheme is proposed to estimate
the queuing order that the tasks should be executed and the
most appopriate processor assignment so that the demanded
QoS are satisfied as much as possible. A fair scheduling policy is
considered as the most appropriate. Experimental results on a real
grid infrastructure are presented to illustrate the efficiency of the
proposed workload prediction — scheduling algorithm compared
to other approaches presented in the literature.
Abstract: We consider the Railway Traveling Salesman Problem. We
show that this problem can be reduced to a variant of the generalized
traveling salesman problem, defined on an undirected graph G = (V,E)
with the nodes partitioned into clusters, which consists in finding a mini
mum cost cycle spanning a subset of nodes with the property that exactly
two nodes are chosen from each cluster. We describe an exact exponen
tial time algorithm for the problem, as well we present two mixed integer
programming models of the problem. Based on one of this models pro
posed, we present an efficient solution procedure based on a cutting plane
algorithm. Extensive computational results for instances taken from the
railroad company of the Netherlands Nederlandse Spoorwegen and involv
ing graphs with up to 2182 nodes and 38650 edges are reported.
Abstract: Many of the network security protocols employed today utilize symmetric block ciphers (DES, AES and CAST etc). The majority of the symmetric block ciphers implement the crucial substitution operation using look up tables, called substitution boxes. These structures should be highly nonlinear and have bit dispersal, i.e. avalanche, properties in order to render the cipher with resistant to cryptanalysis attempts, such as linear and differential cryptanalysis. Highly secure substitution boxes can be constructed using particular Boolean functions as components that have certain mathematical properties which enhance the robustness of the whole cryptoalgorithm. However, enforcing these properties on SBoxes is a highly computationally intensive task. In this paper, we present a distributed algorithm and its implementation on a computing cluster that accelerates the construction of secure substitution boxes with good security properties. It is fully parametric since it can employ any class of Boolean functions with algorithmically definable properties and can construct SBoxes of arbitrary sizes. We demonstrate the efficiency of the distributed algorithm implementation compared to its sequential counterpart, in a number of experiments.
Abstract: We here present the Forward Planning Situated Protocol (FPSP), for scalable, energy efficient and fault tolerant data propagation in situated wireless sensor networks. To deal with the increased complexity of such deeply networked sensor systems, instead of emphasizing on a particular aspect of the services provided, i.e. either for lowenergy periodic, or lowlatency eventdriven, or highsuccess querybased sensing, FPSP uses two novel mechanisms that allow the network operator to adjust the performance of the protocol in terms of energy, latency and success rate on a pertask basis. We emphasize on distributedness, direct or indirect interactions among relatively simple agents, flexibility and robustness.
The protocol operates by employing a series of plan & forward phases through which devices selforganize into forwarding groups that propagate data over discovered paths. FPSP performs a limited number of long range, high power data transmissions to collect information regarding the neighboring devices. The acquired information, allows to plan a (parameterizable long by {\"e}) sequence of short range, low power transmissions between nearby particles, based on certain optimization criteria. All particles that decide to respond (based on local criteria) to these long range transmissions enter the forwarding phase during which information is propagated via the acquired plan. Clearly, the duration of the forwarding phases is characterized by the parameter {\"e}, the transmission medium and the processing speed of the devices. In fact the parameter {\"e} provides a mechanism to adjust the protocol performance in terms of the latencyenergy tradeoff. By reducing {\"e} the latency is reduced at the cost of spending extra energy, while by increasing {\"e}, the energy dissipation is reduced but the latency is increased.
To control the success rateenergy tradeoff, particles react locally on environment and context changes by using a set of rules that are based on response thresholds that relate individuallevel plasticity with networklevel resiliency, motivated by the natureinspired method for dividing labor, a metaphor of social insect behavior for solving problems [1]. Each particle has an individual response threshold {\`E} that is related to the "local" density (as observed by the particle, [2]); particles engage in propagation of events when the level of the taskassociated stimuli exceeds their thresholds. Let s be the intensity of a stimulus associated with a particular sensing task, set by the human authorities. We adopt the response function T_{{\`e}}(s) = s^{n}over s^{n} + {\`e}^{n}, the probability of performing the task as a function of s, where n > 1 determines the steepness of the threshold. Thus, when {\`e} is small (i.e. the network is sparse) then the response probability increases; when s increases (i.e. for critical sensing tasks) the response probability increases as well.
This rolebased approach where a selective number of devices do the high cost planning and the rest of the network operates in a low cost state leads to systems that have increased energy efficiency and high faulttolerance since these long range planning phases allow to bypass obstacles (where no sensors are available) or faulty sensors (that have been disabled due to power failure or other natural events).
Abstract: eVoting is considered to be one of the most challenging domains of modern eGovernment and one of the main vehicles for increasing eParticipation among citizens. One of the main obstacles for its wide adoptionis the reluctance of citizens to participate in electronic voting procedures. This reluctance can be partially attributed to the low penetration of technology among citizens. However, the main reason behind this reluctance is the lack of trust which stems from the belief of citizens that systems implementing an eVoting process will violate their privacy. The departure point of this approach is that the emergence of such a belief can be considerably facilitated by designing and building systems in a way that evidence about the system’s properties is produced during the design process. In this way, the designers can demonstrate the respect in privacy using this evidence that can be understood and checked by the specialist and the informed layman. These tools and models should provide sufficient evidence that the target system handles privacy concerns and requirements that can remove enough of the fears towards eVoting. This paper presents the efforts of the authors‘ organization, the Computer Technology Institute and Press “Diophantus” (CTI), towards the design and implementation of an eVoting system, called PNYKA, with demonstrable security properties. This system was based on a trustcentered engineering approach for building general security critical systems. The authors‘ approach is pragmatic rather than theoretical in that it sidesteps the controversy that besets the nature of trust in information systems and starts with a working definition of trust as people’s positive attitude towards a system that transparently and demonstrably performs its operations, respecting their privacy. The authors also discuss the social side of eVoting, i.e. how one can help boost its acceptance by large social groups targeting the whole population of the country. The authors view eVoting as an innovation that must be diffused to a population and then employ a theoretical model that studies diffusion of innovation in social network, delineating structural properties of the network that help diffuse the innovation fast. Furthermore, the authors explain how CTI’s current situation empowers CTI to realize its vision to implement a privacy preserving, discussion and public consultation forum in Greece. This forum will link, together, all Greek educational institutes in order to provide a privacy preserving discussion and opinion gathering tool useful in decision making within the Greek educational system.
Abstract: Future Grid Networks should be able to provide Quality
of Service (QoS) guarantees to their users. In this work
we propose a framework for Grid Networks that provides
deterministic delay guarantees to its Guaranteed Service
(GS) users and best effort service to its Best Effort (BE)
users. The proposed framework is theoretically and experimentally
analyzed. We also define four types of computational
resources based on the type of users (GS, BE) these
resources serve and the priority they give them. We implement
the proposed QoS framework for Grids and verify that
it not only satisfies the delay guarantees given to GS users,
but also improves performance in terms of deadlines missed
and resource use. In our simulations, data from a real Grid
Network are used, validating in this way the appropriateness
and usefulness of the proposed framework.
Abstract: Paul Spirakis is an eminent, talented, and influential researcher that contributed significantly to computer science. This article is a modest attempt of a biographical sketch of Paul, which we drafted with extreme love and honor.
Abstract: In this paper we present a CPU scavenging
architecture suitable for desktop resources,
and we study its appropriateness in exploiting the
PC Laboratory resources of the Greek School
Network and their integration to the existing
HellasGrid national infrastructure. School laboratories
form an extensive network equipped with
computational systems and fast Internet connections.
As this infrastructure is utilized at most 8 h per day and 5 days per week, it could be
made available during its remaining idle time for
computational purposes through the use of Grid
technology. The structure and organization of the
school laboratories and backbone network enables
the CPU scavenging service, as an independent
and additional service, which will not violate
the operational rules and policies of the school
network, while it will add additional resources
to the current HellasGrid infrastructure with low
adaptation cost.
Abstract: This work addresses networked embedded systems enabling the seam
less interconnection of smart building automations to the Internet and
their abstractions as web services. In our approach, such abstractions are
used to primarily create a exible, holistic and scalable system and allow
external endusers to compose and run their own smart/green building
automation application services on top of this system.
Towards this direction, in this paper we present a smart building test
bed consisting of several sensor motes and spanning across seven rooms.
Our testbed's design and implementation simultaneously addresses sev
eral corresponding system layers; from hardware interfaces, embedded
IPv6 networking and energy balancing routing algorithms to a RESTful
architecture and over the web development of sophisticated, smart, green
scenarios. In fact, we showcase how IPv6 embedded networking combined
with RESTful architectures make the creation of building automation ap
plications as easy as creating any other Internet Web Service.
Abstract: With the proliferation of wireless sensor net
works and mobile technologies in general, it is possible to
provide improved medical services and also to reduce costs
as well as to manage the shortage of specialized personnel.
Monitoring a person’s health condition using sensors pro
vides a lot of benefits but also exposes personal sensitive
information to a number of privacy threats. By recording
userrelated data, it is often feasible for a malicious or
negligent data provider to expose these data to an unau
thorized user. One solution is to protect the patient’s pri
vacy by making difficult a linkage between specific
measurements with a patient’s identity. In this paper we
present a privacypreserving architecture which builds
upon the concept of
k
anonymity; we present a clustering
based anonymity scheme for effective network manage
ment and data aggregation, which also protects user’s
privacy by making an entity indistinguishable from other
k
similar entities. The presented algorithm is resource
aware, as it minimizes energy consumption with respect to
other more costly, cryptographybased approaches. The
system is evaluated from an energyconsuming and net
work performance perspective, under different simulation
scenarios.
Abstract: In mobile adhoc networks (MANETs), the mobility of the nodes is a complicating factor that significantly affects the effectiveness and performance of the routing protocols. Our work builds upon recent results on the effect of node mobility on the performance of available routing strategies (i.e.~path based, using support) and proposes a protocol framework that exploits the usually different mobility rates of the nodes by adapting the routing strategy during execution. We introduce a metric for the relative mobility of the nodes, according to which the nodes are classified into mobility classes. These mobility classes determine, for any pair of an origin and destination, the routing technique that best corresponds to their mobility properties. Moreover, special care is taken for nodes remaining almost stationary or moving with high (relative) speeds. Our key design goal is to limit the necessary implementation changes required to incorporate existing routing protocols in to our framework. We provide extensive evaluation of the proposed framework, using a wellknown simulator (NS2). Our first findings demonstrate that the proposed framework improves, in certain cases, the performance of the existing routing protocols.
Abstract: In adhoc mobile networks (MANET), the mobility of the nodes is a complicating factor that significantly affects the effectiveness and performance of the routing protocols. Our work builds upon the recent results on the effect of node mobility on the performance of available routing strategies (i.e.~path based, using support) and proposes a protocol framework that exploits the usually different mobility rates of the nodes by adopting the routing strategy during execution. We introduce a metric for the relative mobility of the nodes, according to which the nodes are classified into mobility classes. These mobility classes determine, for any pair of origin and destination, the routing technique that best corresponds to their mobility properties. Moreover, special care is taken for nodes remaining almost stationary or moving with high (relative) speeds. Our key design goal is to limit the necessery implementation changes required to incorporate existing routing protocols in our framework. We provide extensive evaluation of the proposed framework, using a wellknown simulator (NS2). Our first findings demonstrate that the proposed framework improves, in certain cases, the performance of the existing routing protocols.
Abstract: We propose a simple obstacle model to be used while simulating wireless sensor networks. To the best of our knowledge, this is the first time such an integrated and systematic obstacle model for these networks has been proposed. We define several types of obstacles that can be found inside the deployment area of a wireless sensor network and provide a categorization of these obstacles based on their nature (physical and communication obstacles, i.e. obstacles that are formed out of node distribution patterns or have physical presence, respectively), their shape and their change of nature over time. We make an eXtension to a custommade sensor network simulator (simDust) and conduct a number of simulations in order to study the effect of obstacles on the performance of some representative (in terms of their logic) data propagation protocols for wireless sensor networks. Our findings confirm that obstacle presence has a significant impact on protocol performance, and also that different obstacle shapes and sizes may affect each protocol in different ways. This provides an insight into how a routing protocol will perform in the presence of obstacles and highlights possible protocol shortcomings. Moreover, our results show that the effect of obstacles is not directly related to the density of a sensor network, and cannot be emulated only by changing the network density.
Abstract: We design and implement a multicost impairment aware routing and wavelength assignment algorithm for online traffic. In transparent optical networks the quality of a transmission degrades due to physical layer impairments. To serve a connection, the proposed algorithm finds a path and a free wavelength (a lightpath) that has acceptable signal quality performance by estimating a quality of transmission measure, called the Q factor. We take into account channel utilization in the network, which changes as new connections are established or released, in order to calculate the noise variances that correspond to physical impairments on the links. These, along with the time invariant eye impairment penalties of all candidate network paths, form the inputs to the algorithm. The multicost algorithm finds a set of so called nondominated Q paths from the given source to the given destination. Various objective functions are then evaluated in order to choose the optimal lightpath to serve the connection. The proposed algorithm combines the strength of multicost optimization with low execution time, making it appropriate for serving online connections.
Abstract: We propose and evaluate a new burst assembly algorithm based on the average delay of the packets comprising a burst. This
method ﬁxes the average delay of the packets belonging to an assembled burst to a desired value TAVE that may be different for
each forwarding equivalence class (FEC). We show that the proposed method signiﬁcantly improves the delay jitter experienced
by the packets during the burst assembly process, when compared to that of timerbased and burst lengthbased assembly policies.
Minimizing packet delay jitter is important in a number of applications, such as realaudio and streamingvideo applications. We
also ﬁnd that the improvement in the packet delay jitter yields a corresponding signiﬁcant improvement in the performance of TCP,
whose operation depends critically on the ability to obtain accurate estimates of the roundtrip times (RTT).
c
2007 Elsevier B.V. All rights reserved.
Abstract: We present a new dynamic graph structure specifically suited
for largescale transportation networks that provides simultaneously three
unique features: compactness, agility and dynamicity. We demonstrate
its practicality and superiority by conducting an experimental study for
shortest route planning in largescale European and US road networks
with a few dozen millions of nodes and edges. Our approach is the first
one that concerns the dynamic maintenance of a largescale graph with
ordered elements using a contiguous memory part, and which allows an
arbitrary online reordering of its elements.
Abstract: In this work we present the architecture and implementation of WebDust, a software platform for managing multiple, heterogeneous (both in terms of software and hardware), geographically disparate sensor networks. We describe in detail the main concepts behind its design, and basic aspects of its implementation, including the services provided to endusers and developers. WebDust uses a peertopeer substrate, based on JXTA, in order to unify multiple sensor networks installed in various geographic areas. We aim at providing a software framework that will permit developers to deal with the new and critical aspects that networks of sensors and tiny devices bring into global computing, and to provide a coherent set of high level services, design rules and technical recommendations, in order to be able to develop the envisioned applications of global sensor networks. Furthermore, we give an overview of a deployed distributed testbed, consisting of a total 56 nodes and describing in more detail two specific testbed sites and the integration of the related software and hardware technologies used for its operation with our platform. Finally, we describe the design and implementation of an interface option provided to endusers, based on the popular Google Earth application.
Abstract: Tourists become increasingly dependent on mobile city guides to locate tourist services and retrieve information about nearby points of interest (POIs) when visiting unknown destinations. Although several city guides support the provision of personalized tour recommendations to assist tourists visiting the most interesting attractions, existing tour planners only consider walking tours. Herein, we introduce eCOMPASS, a contextaware mobile application which also considers the option of using public transit for moving around. Far beyond than just providing navigational aid, eCOMPASS incorporates multimodality (i.e. time dependency) within its routing logic aiming at deriving nearoptimal sequencing of POIs along recommended tours so as to best utilize time available for sightseeing and minimize waiting time at transit stops. Further advancing the state of the art, eCOMPASS allows users to define arbitrary start/end locations(e.g. the current location of a mobile user) rather than choosing among a fixed set of locations. This paper describes the routing algorithm which comprises the core functionality of eCOMPASS
and discusses the implementation details of the mobile application using the metropolitan area of Berlin (Germany) as case study
Abstract: We study the problem of data propagation in sensor networks,
comprised of a large number of very small and lowcost nodes,
capable of sensing, communicating and computing. The distributed
cooperation of such nodes may lead to the accomplishment of large
sensing tasks, having useful applications in practice. We present
a new protocol for data propagation towards a control center
(``sink") that avoids flooding by probabilistically favoring
certain (``close to optimal") data transmissions.
This protocol is very simple to implement in sensor devices
and operates under total absence
of coordination between sensors. We consider a network model of randomly deployed sensors of sufficient density.
As shown by a geometry analysis,
the protocol is correct, since it always propagates data
to the sink, under ideal network conditions (no failures). Using
stochastic processes, we show that the protocol is very energy efficient. Also, when part of the network is inoperative, the
protocol manages to propagate data very close to the sink, thus in
this sense it is robust. We finally present and discuss
largescale experimental findings validating the analytical
results.
Abstract: We study the problem of data propagation in sensor networks,
comprised of a large number of very small and lowcost nodes,
capable of sensing, communicating and computing. The distributed
cooperation of such nodes may lead to the accomplishment of large
sensing tasks, having useful applications in practice. We present a new protocol for data propagation towards a control center ("sink") that avoids flooding by probabilistically favoring certain ("close to optimal") data transmissions. Motivated by certain applications and also as a starting point for a rigorous analysis, we study here latticeshaped sensor networks. We however show that this lattice shape emerges even in randomly deployed sensor networks of sufficient sensor density. Our work is inspired and builds upon the directed diffusion paradigm.
This protocol is very simple to implement in sensor devices, uses only local information and operates under total absence of coordination between sensors. We consider a network model of randomly deployed sensors of sufficient density. As shown by a geometry analysis, the protocol is correct, since it always propagates data to the sink, under ideal network conditions (no failures). Using stochastic processes, we show that the protocol is very energy efficient. Also, when part of the network is inoperative, the protocol manages to propagate data very close to the sink, thus in this sense it is robust. We finally present and discuss largescale experimental findings validating the analytical results.
Abstract: Raising awareness among young people on the
relevance of behaviour change for achieving energy savings is widely considered as a key approach towards longterm and costeffective energy efficiency policies. The GAIA Project aims to deliver a comprehensive solution for both increasing awareness on energy efficiency and achieving energy savings in school buildings. In this framework, we present a novel rule engine that, leveraging a resourcebased graph model encoding relevant application domain knowledge, accesses IoT data for producing energy savings recommendations. The engine supports configurability, extensibility and easeofuse requirements, to be easily applied and customized to different buildings. The paper introduces the main design and implementation details and presents a set of preliminary performance results.
Abstract: We describe the design and implementation
of a secure and robust architectural data management
model suitable for cultural environments. Usage and exploitation
of the World Wide Web is a critical requirement
for a series of administrative tasks such as collecting, managing
and distributing valuable cultural and artistic information.
This requirement introduces a great number of
Internet threats for cultural organizations that may cause
huge data and/or financial losses, harm their reputation
and public acceptance as well as people’s trust on them.
Our model addresses a list of fundamental operational
and security requirements. It utilizes a number of cryptographic
primitives and techniques that provide data safety
and secure user interaction on especially demanding online
collaboration environments. We provide a reference
implementation of our architectural model and discuss
the technical issues. It is designed as a standalone solution
but it can be flexibly adapted in broader management
infrastructures.
Abstract: Usage and exploitation of the Internet is a critical requirement for managing and distributing valuable digital assets. This requirement introduces a great number of threats for commercial (or not) organizations that may cause huge data and financial losses, harm their reputation as well as people's trust on them. In this paper we present the research challenges for secure digital asset management over the web by proposing a model that provides data safety and secure user interaction on especially demanding online collaboration environments.
Abstract: In this paper, a new service oriented networking
paradigm is presented, where network nodes (peers) are self
organized into individual service entities. The key idea relies on
the overlay approach, where there exists a virtual service plane,
fragmented into selforganized and selfmanaged entities called
islands of service transparency. The islands are formed in an
upstream, adhoc mode from the nonnetworking resources (i.e
VoD, grid server, etc) towards all ingress routers of the network,
using link state advertisements and multicost path selection
algorithms (i.e residual bandwidth, server capacity, storage, etc).
Organization and reorganization of nodes around nonnetwork
resources is transparent to endusers, and thus any request
within a specific service island is transparently routed to the
island’s resource for execution. A service proxy is commissioned
to resolve service addresses and service attributes to QoS metrics.
In this paper, we present the main notations and metrics of the
proposed architecture as well as node behavior and potential
GMPLS extensions for implementation issues
Abstract: We propose a MAC protocol for mobile ad hoc networks that
uses power control for the RTS/CTS and DATA frame
transmissions in order to improve energy and capacity
utilization efficiency. Unlike IEEE 802.11, in our scheme the
RTS frames are not sent using the maximum transmission
power to silence neighbouring nodes, and the CTS frames do
not silence all receiving nodes to the same degree. In contrast,
the transmission power of the RTS frames follows a slow
start principle, while the CTS frames, which are sent at
maximum transmission power, prevent the neighbouring
nodes from transmitting their DATA frames with power more
than a computed threshold, while allowing them to transmit at
power levels less than that threshold. This is done by
including in the RTS and the CTS frames additional
information, such as the power of the transmissions, and the
interference tolerance of the nodes. Moreover the DATA
frames are sent at the minimum required transmission power
increased by a small margin to ensure connectivity with the
intended receiver, so as to cause minimal interference to
neighbouring nodes and allow for future interference to be
added to the receiver of the DATA frames. The power to be
used by the transmitter is computed by the recipient of the
RTS frame and is included in the CTS frame. It is expected
that a network with such a power management scheme would
achieve a better throughput performance and more power
savings than a network without such a scheme.
Abstract: We propose a MAC protocol for mobile ad hoc networks that
uses power control for the RTS/CTS and DATA frame
transmissions in order to improve energy and capacity
utilization efficiency. Unlike IEEE 802.11, in our scheme the
RTS frames are not sent using the maximum transmission
power to silence neighbouring nodes, and the CTS frames do
not silence all receiving nodes to the same degree. In contrast,
the transmission power of the RTS frames follows a slow
start principle, while the CTS frames, which are sent at
maximum transmission power, prevent the neighbouring
nodes from transmitting their DATA frames with power more
than a computed threshold, while allowing them to transmit at
power levels less than that threshold. This is done by
including in the RTS and the CTS frames additional
information, such as the power of the transmissions, and the
interference tolerance of the nodes. Moreover the DATA
frames are sent at the minimum required transmission power
increased by a small margin to ensure connectivity with the
intended receiver, so as to cause minimal interference to
neighbouring nodes and allow for future interference to be
added to the receiver of the DATA frames. The power to be
used by the transmitter is computed by the recipient of the
RTS frame and is included in the CTS frame. It is expected
that a network with such a power management scheme would
achieve a better throughput performance and more power
savings than a network without such a scheme.
Abstract: As water supplies become scarce and polluted, there is an urgent
need to irrigate more efficiently in order to optimize water
use. In this paper, we present a WSN based, smart homeirrigation
system that consists of heterogeneous motes, special
sensors and actuators. The system is fully adaptive not
only to environmental conditions but also to the specific water
needs that different plants may have. This way, it manages
to perform efficient home irrigation, while it provides
an IPv6capable managing system.
Abstract: eVoting is a challenging approach for increasing eParticipation. However, lack of citizens¢ trust seems to be a main obstacle that hinders its successful realization. In this paper we propose a trustcentered engineering approach for building eVoting systems that people can trust, based on transparent design and implementation phases. The approach is based on three components: the decomposition of eVoting systems into “layers of trust” for reducing the complexity of managing trust issues in smaller manageable layers, the application of a risk analysis methodology able to identify and document security critical aspects of the eVoting system, and a cryptographically secure eVoting protocol. Our approach is pragmatic rather than theoretical in the sense that it sidesteps the controversy that besets the nature of trust in information systems and starts with a working definition of trust as people¢s positive attitude towards a system that performs its operations transparently.
Abstract: Designing wireless sensor networks is inherently complex; many aspects such as energy efficiency, limited resources, decentralized collaboration, fault tolerance have to be tackled. To be effective and to produce applicable results, fundamental research has to be tested, at least as a proofofconcept, in large scale environments, so as to assess the feasibility of the new concepts, verify their large scale effects (not only at technological level, but also as for their foreseeable implications on users, society and economy) and derive further requirements, orientations and inputs for the research. In this paper we focus on the problems of interconnecting existing testbed environments via the Internet and providing a virtual unifying laboratory that will support academia, research centers and industry in their research on networks and services. In such a facility important issues of trust, security, confidentiality and integrity of data may arise especially for commercial (or not) organizations. In this paper we investigate such issues and present the design of a secure and robust architectural model for interconnecting testbeds of wireless sensor networks.
Abstract: In this work, we present a concrete framework, based on web servicesoriented architecture, for integrating small programmable objects in the web of things.
Functionality and data gathered by the Small Programmable Objects (SPO) are exposed using Web Services. Based on this, by exploiting XML encoding, SPO can be comprehensible by any web application. The architecture proposed is focused in providing secure and efficient interoperability between SPO and the web.
Additionally, the proposed architecture provides management capabilities for deploying, maintaining and operating SPO applications across multiple networks.
We present the multilayer architecture of our system and its implementation, which uses a combination of Java Standard and Micro Editions. Finally, we present a case study presenting our implementation. In this application we use SunSPOTs, which are wireless network motes developed by Sun Microsystems.
Abstract: Motivated by emerging applications, we consider sensor networks where the sensors themselves (not just the sinks) are mobile. Furthermore, we focus on mobility scenarios characterized by heterogeneous, highly changing mobility roles in the network. To capture these high dynamics of diverse sensory motion we propose a novel network parameter,
the mobility level, which, although simple and local, quite accurately takes into account both the spatial and speed characteristics of motion. We then propose adaptive data dissemination protocols that use the mobility level estimation to optimize performance, by basically exploiting high mobility (redundant message ferrying) as a costeffective replacement of flooding, e.g. the sensors tend to dynamically propagate less data in the presence
of high mobility, while nodes of high mobility are favored for moving data around. These dissemination schemes are enhanced by a distancesensitive probabilistic message flooding inhibition mechanism that further reduces communication cost, especially for fast nodes of high mobility level, and as distance to data destination decreases. Our simulation findings
demonstrate significant performance gains of our protocols compared to nonadaptive protocols, i.e. adaptation increases the success rate and reduces latency (even by 15%) while at the same time significantly reducing energy dissipation (in most cases by even 40%). Also, our adaptive schemes achieve significantly higher message delivery ratio and
satisfactory energylatency tradeoffs when compared to flooding when sensor nodes have
limited message queues.
Abstract: We introduce a new modelling assumption for wireless sensor networks, that of node redeployment (addition of sensor devices during protocol evolution) and we extend the modelling assumption of heterogeneity (having sensor devices of various types). These two features further increase the highly dynamic nature of such networks and adaptation becomes a powerful technique for protocol design. Under these modelling assumptions, we design, implement and evaluate a new power conservation scheme for efficient data propagation. Our scheme is adaptive: it locally monitors the network conditions (density, energy) and accordingly adjusts the sleepawake schedules of the nodes towards improved operation choices. The scheme is simple, distributed and does not require exchange of control messages between nodes.
Implementing our protocol in software we combine it with two wellknown data propagation protocols and evaluate the achieved performance through a detailed simulation study using our extended version of the network simulator ns2. We focus on highly dynamic scenarios with respect to network density, traffic conditions and sensor node resources. We propose a new general and parameterized metric capturing the tradeoffs between delivery rate, energy efficiency and latency. The simulation findings demonstrate significant gains (such as more than doubling the success rate of the wellknown Directed Diffusion propagation protocol) and good tradeoffs achieved. Furthermore, the redeployment of additional sensors during network evolution and/or the heterogeneous deployment of sensors, drastically improve (when compared to ``equal total power" simultaneous deployment of identical sensors at the start) the protocol performance (i.e. the success rate increases up to four times} while reducing energy dissipation and, interestingly, keeping latency low).
Abstract: Clustering is a crucial network design approach to enable largescale wireless sensor networks (WSNs) deployments. A large variety of clustering approaches has been presented focusing on different performance metrics. Such protocols usually aim at minimizing communication overhead, evenly distributing roles among the participating nodes, as well as controlling the network topology. Simulations on such protocols are performed using theoretical models that are based on unrealistic assumptions like the unit disk graph communication model, ideal wireless communication channels and perfect energy consumption estimations. With these assumptions taken for granted, theoretical models claim various performance milestones that cannot be achieved in realistic conditions. In this paper, we design a new clustering protocol that adapts to the changes in the environment and the needs and goals of the user applications. We address the issues that hinder its performance due to the real environment conditions and provide a deployable protocol. The implementation, integration and experimentation of this new protocol and it's optimizations, were performed using the \textsf{WISEBED} framework. We apply our protocol in multiple indoors wireless sensor testbeds with multiple experimental scenarios to showcase scalability and tradeoffs between network properties and configurable protocol parameters. By analysis of the real world experimental output, we present results that depict a more realistic view of the clustering problem, regarding adapting to environmental conditions and the quality of topology control. Our study clearly demonstrates the applicability of our approach and the benefits it offers to both research \& development communities.
Abstract: Wireless Sensor Networks are by nature highly dynamic and communication between sensors is completely ad hoc, especially when mobile devices are part of the setup. Numerous protocols and applications proposed for such networks
operate on the assumption that knowledge of the neighborhood is a priori available to all nodes. As a result, WSN deployments need to use or implement from scratch a neighborhood discovery mechanism. In this work we present a new protocol based on adaptive periodic beacon exchanges. We totally avoid continuous beaconing by adjusting the rate of broadcasts using the concept of consistency over the understanding of neighborhood that nearby devices share. We propose, implement and evaluate our adaptive neighborhood discovery protocol over our experimental testbed and using large scale simulations. Our results indicate that the
new protocol operates more eciently than existing reference implementations while it provides valid information to applications that use it. Extensive performance evaluation indicates that it successfully reduces generated network traffic by 90% and increases network lifetime by 20% compared to existing mechanisms that rely on continuous beaconing.
Abstract: Motivated by emerging applications, we consider sensor networks where the sensors themselves
(not just the sinks) are mobile. Furthermore, we focus on mobility
scenarios characterized by heterogeneous, highly changing mobility
roles in the network.
To capture these high dynamics of diverse sensory motion
we propose a novel network parameter, the mobility level, which, although
simple and local, quite accurately takes into account both the
spatial and speed characteristics of motion. We then propose
adaptive data dissemination protocols that use the
mobility level estimation to optimize performance, by basically
exploiting high mobility (redundant message ferrying) as a costeffective
replacement of flooding, e.g., the sensors tend to dynamically propagate
less data in the presence of high mobility, while nodes of high mobility
are favored for moving data around.
These dissemination schemes are enhanced by a distancesensitive
probabilistic message flooding inhibition mechanism that
further reduces communication cost, especially for fast nodes
of high mobility level, and as distance to data destination
decreases. Our simulation findings demonstrate significant
performance gains of our protocols compared to nonadaptive
protocols, i.e., adaptation increases the success rate and reduces
latency (even by 15\%) while at the same time significantly
reducing energy dissipation (in most cases by even 40\%).
Also, our adaptive schemes achieve significantly
higher message delivery ratio and satisfactory energylatency
tradeoffs when compared to flooding when sensor nodes have limited message queues.
Abstract: Motivated by emerging applications, we consider sensor networks where the sensors themselves
(not just the sinks) are mobile. We focus on mobility
scenarios characterized by heterogeneous, highly changing mobility
roles in the network.
To capture these high dynamics
we propose a novel network parameter, the mobility level, which, although
simple and local, quite accurately takes into account both the
spatial and speed characteristics of motion. We then propose
adaptive data dissemination protocols that use the
mobility level estimation to improve performance. By basically
exploiting high mobility (redundant message ferrying) as a costeffective
replacement of flooding, e.g., the sensors tend to dynamically propagate
less data in the presence of high mobility, while nodes of high mobility
are favored for moving data around.
These dissemination schemes are enhanced by a distancesensitive
probabilistic message flooding inhibition mechanism that
further reduces communication cost, especially for fast nodes
of high mobility level, and as distance to data destination
decreases. Our simulation findings demonstrate significant
performance gains of our protocols compared to nonadaptive
protocols, i.e., adaptation increases the success rate and reduces
latency (even by 15\%) while at the same time significantly
reducing energy dissipation (in most cases by even 40\%).
Also, our adaptive schemes achieve significantly
higher message delivery ratio and satisfactory energylatency
tradeoffs when compared to flooding when sensor nodes have limited message queues.
Abstract: Data propagation in wireless sensor networks can be performed either by hopbyhop single transmissions or by multipath broadcast of data. Although several energyaware MAC layer protocols exist that operate very well in the case of single pointtopoint transmissions, none is especially designed and suitable for multiple broadcast transmissions. The key idea of our protocols is the passive monitoring of local network conditions and the adaptation of the protocol operation accordingly. The main contribution of our adaptive method is to proactively avoid collisions by implicitly and early enough sensing the need for collision avoidance. Using the above ideas, we design, implement and evaluate three different, new strategies for proactive adaptation. We show, through a detailed and extended simulation evaluation, that our parameterbased family of protocols for multipath data propagation significantly reduce the number of collisions and thus increase the rate of successful message delivery (to above 90%) by achieving satisfactory tradeoffs with the average propagation delay. At the same time, our protocols are shown to be very energy efficient, in terms of the average energy dissipation per delivered message.
Abstract: We consider sensor networks where the sensor nodes are attached on entities that move in a highly dynamic, heterogeneous manner. To capture this mobility diversity we introduce a new network parameter, the directionaware mobility
level, which measures how fast and close each mobile node is expected to get to the data destination (the sink). We then provide local, distributed data dissemination protocols
that adaptively exploit the node mobility to improve performance. In particular, "high" mobility is used as a low cost replacement for data dissemination (due to the ferrying of data), while in the case of "low" mobility either a) data propagation redundancy is increased (when highly mobile neighbors exist) or b) longdistance data transmissions are used (when the entire neighborhood is of low mobility) to accelerate data dissemination towards the sink. An extensive performance comparison to relevant methods from
the state of the art demonstrates signicant improvements i.e. latency is reduced by even 4 times while keeping energy dissipation and delivery success at very satisfactory levels.
Abstract: We investigate the problem of ecient wireless energy recharging in Wireless Rechargeable Sensor Networks (WRSNs). In
such networks a special mobile entity (called the Mobile Charger) traverses the network and wirelessly replenishes the energy
of sensor nodes. In contrast to most current approaches, we envision methods that are distributed, adaptive and use limited
network information. We propose three new, alternative protocols for ecient recharging, addressing key issues which we
identify, most notably (i) to what extent each sensor should be recharged (ii) what is the best split of the total energy between
the charger and the sensors and (iii) what are good trajectories the MC should follow. One of our protocols (
LRP
) performs
some distributed, limited sampling of the network status, while another one (
RTP
) reactively adapts to energy shortage alerts
judiciously spread in the network. As detailed simulations demonstrate, both protocols signicantly outperform known state
of the art methods, while their performance gets quite close to the performance of the global knowledge method (
GKP
) we
also provide, especially in heterogeneous network deployments.
Abstract: Smart cities are becoming a vibrant application domain for a number of science fields. As such, service providers and stakeholders are beginning to integrate cocreation aspects into current implementations to shape the future smart city solutions. In this context, holistic solutions are required to test such aspects in real cityscale IoT deployments, considering the complex city ecosystems. In this work, we discuss OrganiCity¢s implementation of an ExperimentationasaService framework, presenting a toolset that allows developing, deploying and evaluating smart city solutions in a onestop shop manner. This is the first time such an integrated toolset is offered in the context of a largescale IoT infrastructure, which spans across multiple European cities. We discuss the design and implementation of the toolset, presenting our view on what ExperimentationasaService should provide, and how it is implemented. We present initial feedback from 25 experimenter teams that have utilized this toolset in the OrganiCity project, along with a discussion on two detailed actual use cases to validate our approach. Learnings from all experiments are discussed as well as architectural considerations for platform scaling. Our feedback from experimenters indicates that ExperimentationasaService is a viable and useful approach.
Abstract: Wireless sensor networks are comprised of a vast number of
ultrasmall autonomous computing, communication and sensing devices,
with restricted energy and computing capabilities, that cooperate
to accomplish a large sensing task. Such networks can be very useful
in practice, e.g.~in the local monitoring of ambient conditions and
reporting them to a control center. In this paper we propose a
distributed group key establishment protocol that uses mobile agents
(software) and is particularly suitable for energy constrained,
dynamically evolving adhoc networks. Our approach totally avoids
the construction and the maintenance of a distributed structure that
reflects the topology of the network. Moreover, it tradesoff
complex message exchanges by performing some amount of additional
local computations in order to be applicable at dense and dynamic
sensor networks. The extra computations are simple for the devices
to implement and are evenly distributed across the participants of
the network leading to good energy balance. We evaluate the
performance of our protocol in a simulated environment and compare
our results with existing group key establishment protocols. The
security of the protocol is based on the DiffieHellman problem and
we used in our experiments its elliptic curve analog. Our findings
basically indicate the feasibility of implementing our protocol in
real sensor network devices and highlight the advantages and
disadvantages of each approach given the available technology and
the corresponding efficiency (energy, time) criteria.
Abstract: Optical network design problems fall in the broad
category of network optimization problems. We give a short
introduction on network optimization and general algorithmic
techniques that can be used to solve complex and difficult
network design problems. We apply these techniques to address
the static Routing and Wavelength Assignment problem that is
related to planning phase of a WDM optical network. We present
simulation result to evaluate the performance of the proposed
algorithmic solution.
Abstract: For a number of optimization problems on random graphs
and hypergraphs, e.g., kcolorings, there is a very big gap between the
largest average degree for which known polynomialtime algorithms can
find solutions, and the largest average degree for which solutions provably
exist. We study this phenomenon by examining how sets of solutions
evolve as edges are added.We prove in a precise mathematical sense that,
for each problem studied, the barrier faced by algorithms corresponds
to a phase transition in the problems solutionspace geometry. Roughly
speaking, at some problemspecific critical density, the set of solutions
shatters and goes from being a single giant ball to exponentially many,
wellseparated, tiny pieces. All known polynomialtime algorithms work
in the ball regime, but stop as soon as the shattering occurs. Besides
giving a geometric view of the solution space of random instances our
results provide novel constructions of oneway functions.
Abstract: As a result of recent significant technological advances, a new computing and communication environment, Mobile Ad Hoc Networks (MANET), is about to enter the mainstream. A multitude of critical aspects, including mobility, severe limitations and limited reliability, create a new set of crucial issues and tradeoffs that must be carefully taken into account in the design of robust and efficient algorithms for these environments. The communication among mobile hosts is one among the many issues that need to be resolved efficiently before MANET becomes a commodity.
In this paper, we propose to discuss the communication problem in MANET as well as present some characteristic techniques for the design, the analysis and the performance evaluation of distributed communication protocols for mobile ad hoc networks. More specifically, we propose to review two different design techniques. While the first type of protocols tries to create and maintain routing paths among the hosts, the second set of protocols uses a randomly moving subset of the hosts that acts as an intermediate pool for receiving and delivering messages. We discuss the main design choices for each approach, along with performance analysis of selected protocols.
Abstract: The Lov{\'a}sz Local Lemma (LLL) is a powerful tool that can be used to prove that an object
having none of a set of bad properties exists, using the probabilistic method. In many applications
of the LLL it is also desirable to explicitly construct the combinatorial object. Recently it was
shown that this is possible using a randomized algorithm in the full asymmetric LLL setting [R.
Moser and G. Tardos, 2010]. A strengthening of the LLL for the case of dense local neighborhoods
proved in [R. Bissacot et al., 2010] was recently also made constructive in [W. Pegden, 2011]. In
another recent work [B. Haupler, B. Saha, A. Srinivasan, 2010], it was proved that the algorithm
of Moser and Tardos is still efficient even when the number of events is exponential. Here we
prove that these last two contributions can be combined to yield a new version of the LLL.
Abstract: An ever growing emphasis is put nowadays in developing personalized journey planning and renewable mobility services in smart cities. These services combine means of scheduledbased public transport and electric vehicles or bikes, using crowdsourcing techniques for collecting realtime traffic information and for assessing the recommended routes. The goal is to develop an information system that will allow the fast, realtime computation of best routes.
The main challenges in developing such an information system are both technological and algorithmic. The technological challenge concerns the collection, storage, management, and updating of a huge volume of transport data that are usually timedependent, and the provision (through these data) of personalized renewable mobility services in smartphones. This challenge is typically confronted by creating a cloud infrastructure that on the one hand will support the storage, management, and updating of data, while on the other hand it will handle the necessary data feed to the smartphone applications for providing the users with the requested best routes.
The algorithmic challenge concerns the development of innovative algorithms for the efficient provision of journey planning services in smartphones, based on data they will receive from the cloud infrastructure. These services guarantee the computation of realistic and useful best routes, as well as the updating of the precomputed (route and timetable) information, in case of delays of scheduled public transport vehicles, so that the users can online update their routes to destination. The goal is to develop an algorithmic basis for supporting modern renewable mobility services (information systems), such as "mobility on demand'' (where the next leg of a journey is decided in realtime) and "doortodoor'' personalized mobility, in urban scheduledbased public transport environments. Scheduledbased public transport information systems should not only compute in realtime enduser queries requesting best routes, but also to update the timetable information in case of delays.
The core algorithmic issues of mobility and journey planning (regarding the computation of optimal routes under certain criteria) in scheduledbased public transport systems concern the efficient solution of the fundamental earlier arrival (EA) problem (compute a journey from station S to station T minimizing the overall traveling time required to complete the journey), the minimum number of
transfers (MNT) problem (compute a journey from station S to station T minimizing the number of times a passenger is required to change vehicle), and the efficient updating of timetable information system in case of vehicle delays. The EA and MNT problems have been extensively studied in the literature under two main approaches: the arraybased modeling (where the timetable is represented as an array) and the graphbased modeling (where the timetable is represented as a graph). Experimental results have shown so far that the arraybased approaches are faster in terms of query time than graphbased ones, as they are able to better exploit data locality and do not rely on priority queues. On the other hand, the arraybased approaches have not been theoretically or experimentally studied as far as the efficient updating of timetable information, in case of delays, is concerned.
In this thesis, new graphbased models are being developed that solve efficiently the aforementioned fundamental algorithmic mobility problems in urban scheduledbased public transport information systems, along with a mobile application (journey planner) running on Androidbased smartphones that includes a service for the evaluation of the recommended routes by the users. In particular:
(a) An extensive comparative evaluation was conducted on graphbased dynamic models that represent big data volumes regarding their suitability for representing timetable information. The study confirmed that the realistic timeexpanded model is the most suitable for representing timetable information.
(b) Two new graphbased models have been developed for representing timetable information (in a timetable information system), the reduced timeexpanded model and the dynamic timetable model (DTM), both of which are more spaceefficient with respect to the realistic timeexpanded model. For both of the new models, new efficient algorithms were developed for fast answering of EA and MNT queries, as well as for updating the timetable information representation in case of delays.
(c)An experimental evaluation was conducted with the new graphbased models and their associated query and update algorithms on a set of 14 realworld scheduledbased transportation systems, including the metropolitan areas of Berlin, Athens, London, Rome, and Madrid. The experimental results showed that the query algorithms of the reduced timeexpanded model are superior to those of the DTM model, while the reverse is true regarding the update algorithms. In addition, the experimental study showed that the query algorithms of the new graphbased models compete favorably with those of the best arraybased models.
(d) A mobile, cloudbased, journey planner (information system) was developed whose core algorithmic engine builds upon the new graphbased models. The mobile application is accompanied by a service that allows the users to assess the recommended journeys. The journey planner demonstrates the practicality of the new graphbased models and their associated query and update algorithms.
Abstract: In this work we study the important problem of colouring squares of planar graphs (SQPG). We design and implement two new algorithms that colour in a different way SQPG. We call these algorithms MDsatur and RC. We have also implemented and experimentally evaluated the performance of most of the known approximation colouring algorithms for SQPG [14, 6, 4, 10]. We compare the quality of the colourings achieved by these algorithms, with the colourings obtained by our algorithms and with the results obtained from two wellknown greedy colouring heuristics. The heuristics are mainly used for comparison reasons and unexpectedly give very good results. Our algorithm MDsatur outperforms the known algorithms as shown by the extensive experiments we have carried out.
The planar graph instances whose squares are used in our experiments are “nonextremal” graphs obtained by LEDA and hard colourable graph instances that we construct.
The most interesting conclusions of our experimental study are:
1) all colouring algorithms considered here have almost optimal performance on the squares of “nonextremal” planar graphs. 2) all known colouring algorithms especially designed for colouring SQPG, give significantly better results, even on hard to colour graphs, when the vertices of the input graph are randomly named. On the other hand, the performance of our algorithm, MDsatur, becomes worse in this case, however it still has the best performance compared to the others. MDsatur colours the tested graphs with 1.1 OPT colours in most of the cases, even on hard instances, where OPT denotes the number of colours in an optimal colouring. 3) we construct worst case instances for the algorithm of Fotakis el al. [6], which show that its theoretical analysis is tight.
Abstract: We introduce a new modelling assumption in wireless sensor networks, that of node redeployment (addition of sensor devices during the protocol evolution) and we extend the modelling assumption of heterogeneity (having sensor devices of various types). These two features further increase the highly dynamic nature of such networks and adaptation becomes a powerful technique for protocol design. Under this model, we design, implement and evaluate a power conservation scheme for efficient data propagation. Our protocol is adaptive: it locally monitors the network conditions (density, energy) and accordingly adjusts the sleepawake schedules of the nodes towards best operation choices. Our protocol operates does not require exchange of control messages between nodes to coordinate.Implementing our protocol we combine it with two wellknown data propagation protocols and evaluate the achieved performance through a detailed simulation study using our extended version of Ns2. We focus in highly dynamic scenarios with respect to network density, traffic conditions and sensor node resources. We propose a new general and parameterized metric capturing the tradeoff between delivery rate, energy efficiency and latency. The simulation findings demonstrate significant gains (such as more than doubling the success rate of the wellknown Directed Diffusion propagation paradigm) and good tradeoffs. Furthermore, redeployment of sensors during network evolution and/or heterogeneous deployment of sensors drastically improve (when compared to equal total "power" simultaneous deployment of identical sensors at the start) the protocol performance (the success rate increases up to four times while reducing energy dissipation and, interestingly, keeping latency low).
Abstract: In this paper, a novel configuration is proposed for
the implementation of an almost alloptical switch architecture
called the scheduling switch, which when combined with appropriate
waitforreservation or tellandgo connection and flow
control protocols provides lossless communication for traffic
that satisfies certain smoothness properties. An alloptical 2 2
exchange/bypass (E/B) switch based on the nonlinear operation
of a semiconductor optical amplifier (SOA) is considered as the
basic building block of the scheduling switch as opposed to active
SOAbased space switches that use injection current to switch
between ON and OFF states. The experimental demonstration of
the optically addressable 2 2 E/B, which is summarized for
10–Gb/s data packets as well as synchronous digital hierarchy
(SDH)/STM64 data frames, ensures the feasibility of the proposed
configuration at high speeds, with low switching energy and low
losses during the scheduling process. In addition, it provides
reduction of the number of required components for the construction
of the scheduling switch, which is calculated to be 50% in the
number of active elements and 33% in the fiber length.
Abstract: The use of Augmented Reality (AR) technologies is currently being investigated in numerous and diverse application domains. In this work, we discuss the ways in which we are integrating AR into educational inclass activities for the GAIA project, aiming to enhance existing tools that target behavioral changes towards energy efficiency in schools. We combine realtime IoT data from a sensing infrastructure inside a fleet of school buildings with AR software running on tablets and smartphones, as companions to a set of educational lab activities aimed at promoting energy awareness in a STEM context. We also utilize this software as a means to ease access to IoT data and simplify device maintenance. We report on the design and current status of our implementation, describing functionality in the context of our target applications, while also relaying our experiences from the use of such technologies in this application domain.
Abstract: The “small world” phenomenon, i.e., the fact that the
global social network is strongly connected in the sense
that every two persons are interrelated through a small
chain of friends, has attracted research attention and has
been strongly related to the results of the social
psychologist¢s Stanley Milgram experiments; properties
of social networks and relevant problems also emerge in
peertopeer systems and their study can shed light on
important modern network design properties.
In this paper, we have experimentally studied greedy
routing algorithms, i.e., algorithms that route information
using “longrange” connections that function as
shortcuts connecting “distant” network nodes. In
particular, we have implemented greedy routing
algorithms, and techniques from the recent literature in
networks of line and grid topology using parallelization
for increasing efficiency. To the best of our knowledge, no
similar attempt has been made so far
Abstract: A temporal graph is, informally speaking, a graph that changes with time. When time is discrete and only the relationships between the participating entities may change and not the entities themselves, a temporal graph may be viewed as a sequence G1,G2…,Gl of static graphs over the same (static) set of nodes V. Though static graphs have been extensively studied, for their temporal generalization we are still far from having a concrete set of structural and algorithmic principles. Recent research shows that many graph properties and problems become radically different and usually substantially more difficult when an extra time dimension in added to them. Moreover, there is already a rich and rapidly growing set of modern systems and applications that can be naturally modeled and studied via temporal graphs. This, further motivates the need for the development of a temporal extension of graph theory. We survey here recent results on temporal graphs and temporal graph problems that have appeared in the Computer Science community.
Abstract: We present the conceptual basis and the initial planning for an open source management architecture for wireless sensor networks (WSN). Although there is an abundance of open source tools serving the administrative needs of WSN deployments, there is a lack of tools or platforms for high level integrated WSN management. This is because of a variety of factors, including the lack of open source management tools, the immaturity of tools that offer manageability for WSNs, the limited high level management capabilities of sensor devices and architectures, and the lack of standardization. The current work is, to our knowledge, the first effort to conceptualize, formalize and design a remote, integrated management platform for the support of WSN research laboratories. The platform is based on the integration and extension of two innovative platforms: jWebDust, a WSN operation and management platform, and OpenRSM, an open source integrated remote systems and network management platform. The proposed system architecture can support several levels of integration (infrastructure management, functionality integration, firmware management), corresponding to different usecases and application settings.
Abstract: In this paper we propose a new methodology for determining
approximate Nash equilibria of noncooperative bimatrix games and,
based on that, we provide an efficient algorithm that computes 0.3393
approximate equilibria, the best approximation till now. The methodology
is based on the formulation of an appropriate function of pairs of
mixed strategies reflecting the maximum deviation of the players¢ payoffs
from the best payoff each player could achieve given the strategy
chosen by the other. We then seek to minimize such a function using
descent procedures. As it is unlikely to be able to find global minima
in polynomial time, given the recently proven intractability of the problem,
we concentrate on the computation of stationary points and prove
that they can be approximated arbitrarily close in polynomial time and
that they have the above mentioned approximation property. Our result
provides the best till now for polynomially computable approximate
Nash equilibria of bimatrix games. Furthermore, our methodology for
computing approximate Nash equilibria has not been used by others.
Abstract: We study the problem of greedy, single path data propaga
tion in wireless sensor networks, aiming mainly to minimize
the energy dissipation. In particular, we rst mathemat
ically analyze and experimentally evaluate the energy e
ciency and latency of three characteristic protocols, each one
selecting the next hop node with respect to a dierent cri
terion (minimum projection, minimum angle and minimum
distance to the destination). Our analytic and simulation
ndings suggest that any single criterion does not simulta
neously satisfy both energy eciency and low latency. To
wards parameterized energylatency tradeos we provide as
well hybrid combinations of the two criteria (direction and
proximity to the sink). Our hybrid protocols achieve sig
nicant perfomance gains and allow netuning of desired
performance. Also, they have nice energy balance proper
ties, and can prolong the network lifetime.
Abstract: We introduce a new model of adhoc mobile networks,
which we call hierarchical, that are comprised of
dense subnetworks of mobile users (corresponding to highly
populated geographical areas), interconnected across access
ports by sparse but frequently used connections.
To implement communication in such a case, a possible
solution would be to install a very fast (yet limited) backbone
interconnecting such highly populated mobile user areas, while
employing a hierarchy of (small) supports (one for each lower level
site). This fast backbone provides a limited number of access
ports within these dense areas of mobile users.
We combine here theoretical analysis (average case estimations based on
random walk properties) to claim and validate
results showing that such a hierarchical routing approach is,
for this class of adhoc mobile networks, significantly
more efficient than a simple extension of the
basic ``support'' idea presented in [WAE00,DISC01].
Abstract: Consider k particles, 1 red and k–1 white, chasing each other on the nodes of a graph G. If the red one catches one of the white, it ldquoinfectsrdquo it with its color. The newly red particles are now available to infect more white ones. When is it the case that all white will become red? It turns out that this simple question is an instance of information propagation between random walks and has important applications to mobile computing where a set of mobile hosts acts as an intermediary for the spread of information.
In this paper we model this problem by k concurrent random walks, one corresponding to the red particle and k–1 to the white ones. The infection time Tk of infecting all the white particles with red color is then a random variable that depends on k, the initial position of the particles, the number of nodes and edges of the graph, as well as on the structure of the graph.
We easily get that an upper bound on the expected value of Tk is the worst case (over all initial positions) expected meeting time m* of two random walks multiplied by THgr (log k). We demonstrate that this is, indeed, a tight bound; i.e. there is a graph G (a special case of the ldquolollipoprdquo graph), a range of values k
Abstract: We consider the Moran process, as generalized by Lieberman, Hauert and Nowak (Nature, 433:312316, 2005). A population resides on the vertices of a finite, connected, undirected graph and, at each time step, an individual is chosen at random with probability proportional to its assigned 'fitness' value. It reproduces, placing a copy of itself on a neighbouring vertex chosen uniformly at random, replacing the individual that was there. The initial population consists of a single mutant of fitness r>0 placed uniformly at random, with every other vertex occupied by an individual of fitness 1. The main quantities of interest are the probabilities that the descendants of the initial mutant come to occupy the whole graph (fixation) and that they die out (extinction); almost surely, these are the only possibilities. In general, exact computation of these quantities by standard Markov chain techniques requires solving a system of linear equations of size exponential in the order of the graph so is not feasible. We show that, with high probability, the number of steps needed to reach fixation or extinction is bounded by a polynomial in the number of vertices in the graph. This bound allows us to construct fully polynomial randomized approximation schemes (FPRAS) for the probability of fixation (when r≥1) and of extinction (for all r>0).
Abstract: We consider approval voting elections in which each voter votes for a (possibly empty) set of candidates and the outcome consists of a set of k candidates for some parameter k, e.g., committee elections. We are interested in the minimax approval voting rule in which the outcome represents a compromise among the voters, in the sense that the maximum distance between the preference of any voter and the outcome is as small as possible. This voting rule has two main drawbacks. First, computing an outcome that minimizes the maximum distance is computationally hard. Furthermore, any algorithm that always returns such an outcome provides
incentives to voters to misreport their true preferences.
In order to circumvent these drawbacks, we consider approximation algorithms, i.e., algorithms that produce an outcome that approximates the minimax distance for any given instance. Such algorithms can be considered as alternative voting rules. We present a polynomialtime 2approximation algorithm that uses a natural linear programming relaxation for the underlying optimization problem and deterministically
rounds the fractional solution in order to compute the outcome; this result improves upon the previously best known algorithm that has an approximation ratio of 3. We are furthermore interested in approximation algorithms that are resistant to manipulation by (coalitions of) voters, i.e., algorithms that do not motivate voters to misreport their true preferences in order to improve their distance from the outcome. We complement previous results in the literature with new upper and lower bounds on strategyproof and groupstrategyproof algorithms.
Abstract: The study of the path coloring problem is motivated by the allocation of optical bandwidth to communication requests in alloptical networks that utilize Wavelength Division Multiplexing (WDM). WDM technology establishes communication between pairs of network nodes by establishing transmitterreceiver paths and assigning wavelengths to each path so that no two paths going through the same fiber link use the same wavelength. Optical bandwidth is the number of distinct wavelengths. Since stateoftheart technology allows for a limited number of wavelengths, the engineering problem to be solved is to establish communication minimizing the total number of wavelengths used. This is known as the wavelength routing problem. In the case where the underlying network is a tree, it is equivalent to the path coloring problem.
We survey recent advances on the path coloring problem in both undirected and bidirected trees. We present hardness results and lower bounds for the general problem covering also the special case of sets of symmetric paths (corresponding to the important case of symmetric communication). We give an overview of the main ideas of deterministic greedy algorithms and point out their limitations. For bidirected trees, we present recent results about the use of randomization for path coloring and outline approximation algorithms that find path colorings by exploiting fractional path colorings. Also, we discuss upper and lower bounds on the performance of online algorithms.
Abstract: The Greek School Network (GSN) has developed and put into production a number of elearning services, including synchronous and asynchronous teleeducation, electronic class management, blogs, videoondemand, podcasts and multimedia libraries. These new services complement established and accepted elearning services, such as teleconferencing, user wikis, forums, email, electronic publishing, and emagazines. This report presents the most prominent digital elearning services offered by GSN, with emphasis on the asynchronous teleeducation service, which is presented in detail. Its implementation platform, the Moodle course management system, is compared against wellknown asynchronous open source teleeducation platforms such as COSE, Claroline, Fle3, ILIAS, Manhattan, KEWL, Comentor, eClass and Eledge. The evaluation of the asynchronous teleeducation platforms is based on detailed comparisons of their characteristics and of the methodology they adopt in order to deliver educational services. The comparison is based on evaluation criteria derived from the documented experiences of research institutes and educational bodies and also from the experience of GSN itself. The paper concludes with the presentation of an extension to Moodle for implementing communities of practice (CoPs) that facilitate the creation and delivery of electronic educational open content for teachers in a synergetic manner.
Abstract: We study here the effect of concurrent greedy moves of players in atomic congestion games
where n selfish agents (players) wish to select a resource each (out of m resources) so that her selfish delay there is not much. Such games usually admit a global potential that decreases by sequential and selfishly improving moves. However, concurrent moves may not always lead to global convergence. On the other hand, concurrent play is desirable because it might essentially improve the system convergence time to some balanced state. The problem of ?maintaining? global progress while allowing concurrent play is
exactly what is examined and answered here. We examine two orthogonal settings : (i) A game where the players decide their moves without global information, each acting ?freely? by sampling resources randomly and locally deciding to migrate (if the new resource is better) via a random experiment. Here, the resources can have quite arbitrary latency that is load dependent. (ii) An ?organised? setting where the players are prepartitioned into selfish groups (coalitions) and where each coalition does an improving coalitional move.
Here the concurrency is among the members of the coalition. In this second setting, the resources have latency functions that are only linearly dependent on the load, since this is the only case so far where a global potential exists. In both cases (i), (ii) we show that the system converges to an ?approximate? equilibrium very fast (in logarithmic rounds where the logarithm is taken on the maximum value of the global potential). This is interesting, since two quite orthogonal settings lead to the same result. Our work considers concurrent selfish play for arbitrary latencies for the first time. Also, this is the first time where fast coalitional convergence
to an approximate equilibrium is shown. All our results refer to atomic games (ie players are finite and distinct).
Abstract: A new model for intrusion and its propagation through various attack
schemes in networks is considered. The model is characterized by the number of
network nodes n, and two parameters f and g. Parameter f represents the probability
of failure of an attack to a node and is a gross measure of the level of security of
the attacked system and perhaps of the intruder¢s skills; g represents a limit on
the number of attacks that the intrusion software can ever try, due to the danger
of being discovered, when it issues them from a particular (broken) network node.
The success of the attack scheme is characterized by two factors: the number of
nodes captured (the spread factor) and the number of virtual links that a defense
mechanism has to trace from any node where the attack is active to the origin of
the intrusion (the traceability factor). The goal of an intruder is to maximize both
factors. In our model we present four different ways (attack schemes) by which an
intruder can organize his attacks. Using analytic and experimental methods, we first
show that for any 0 < f < 1, there exists a constant g for which any of our attack
schemes can achieve a {\`E}(n) spread and traceability factor with high probability,
given sufficient propagation time. We also show for three of our attack schemes
that the spread and the traceability factors are, with high probability, linearly related
during the whole duration of the attack propagation. This implies that it will not be
easy for a detection mechanism to trace the origin of the intrusion, since it will have
to trace a number of links proportional to the nodes captured.
Abstract: A new model for intrusion and its propagation through various attack
schemes in networks is considered. The model is characterized by the number of
network nodes n, and two parameters f and g. Parameter f represents the probability
of failure of an attack to a node and is a gross measure of the level of security of
the attacked system and perhaps of the intruder¢s skills; g represents a limit on
the number of attacks that the intrusion software can ever try, due to the danger
of being discovered, when it issues them from a particular (broken) network node.
The success of the attack scheme is characterized by two factors: the number of
nodes captured (the spread factor) and the number of virtual links that a defense
mechanism has to trace from any node where the attack is active to the origin of
the intrusion (the traceability factor). The goal of an intruder is to maximize both
factors. In our model we present four different ways (attack schemes) by which an
intruder can organize his attacks. Using analytic and experimental methods, we first
show that for any 0 < f < 1, there exists a constant g for which any of our attack
schemes can achieve a (n) spread and traceability factor with high probability,
given sufficient propagation time. We also show for three of our attack schemes
that the spread and the traceability factors are, with high probability, linearly related
during the whole duration of the attack propagation. This implies that it will not be
easy for a detection mechanism to trace the origin of the intrusion, since it will have
to trace a number of links proportional to the nodes captured.
Abstract: We design and implement an algorithm for solving the static RWA problem based on an LP relaxation formulation. This formulation is capable of providing integer optimal solutions despite the absence of integrality constraints for a large subset of RWA input instances. In static RWA there is no apriori knowledge of the channels usage and the interference among them cannot be avoided once the solution has been found. To take into consideration adjacent channel interference, we extend our formulation and model the interference by a set of analytical formulas as additional constraints on RWA.
Abstract: In this paper, we present BAD, an applicationlevel multi
cast infrastructure. BAD is designed to improve the perfor
mance of multicast dissemination trees, under both a static
and a dynamic environment, where the eective bandwidth
of the network links changes with time. Its main goal is
to improve the data rate that end users perceive during
a multicast operation. BAD can be used for the creation
and management of multicast groups. It can be deployed
over any DHT retaining its fundamental advantages of band
width improvement. BAD consists of a suit of algorithms
for node joins/leaves, bandwidth distribution to heteroge
neous nodes, tree rearrangement and reduction of overhead.
We have implemented BAD within the FreePastry system.
We report on the results of a detailed performance evalua
tion which testies for BAD's eciency and low overhead.
Specically, our experiments show that the improvement on
the minimum bandwidth ranges from 40% to 1400% and the
improvement on the average bandwidth ranges from 60% to
250%.
Abstract: We examine multiplayer pervasive games that rely on the
use of adhoc mobile sensor networks. The unique feature in
such games is that players interact with each other and their
surrounding environment by using movement and presence
as a means of performing gamerelated actions, utilizing sen
sor devices. We brie
y discuss the fundamental issues and
challenges related to these type of games and the scenar
ios associated with them. We have also developed a frame
work, called Fun in Numbers (FinN) that handles a number
of these issues, such as such as neighbors discovery, local
ization, synchronization and delaytolerant communication.
FinN is developed using Java and is based on a multilayer ar
chitecture, which provides developers with a set of templates
and services for building and operating new games.
Abstract: We study the fundamental naming and counting problems in networks that are anonymous, unknown, and possibly dynamic. Network dynamicity is modeled by the 1interval connectivity model [KLO10]. We first prove that on static networks with broadcast counting is impossible to solve without a leader and that naming is impossible to solve even with a leader and even if nodes know n. These impossibilities carry over to dynamic networks as well. With a leader we solve counting in linear time. Then we focus on dynamic networks with broadcast. We show that if nodes know an upper bound on the maximum degree that will ever appear then they can obtain an upper bound on n. Finally, we replace broadcast with onetoeach, in which a node may send a different message to each of its neighbors. This variation is then proved to be computationally equivalent to a fullknowledge model with unique names.
Abstract: Purpose – To examine broadband competition and broadband penetration in a set of countries that employ the same regulation framework. To define the policy and strategy required to promote broadband in weak markets that do not employ alternative infrastructures.
Design/methodology/approach – Study penetration and competition level statistics from 2002 to 2005 in a set of countries with different infrastructures deployed, services provided as well as in their socialeconomic structures but employing the same regulation framework. Measure the level of interplatform and intraplatform competition as well as the availability of bitstream access versus the incumbents' shares.
Findings – The paper concludes that a mature broadband market is the one that exhibits a high penetration ratio in combination with a high competition level. Bitstream access can counterbalance the inexistence of alternative broadband infrastructures, especially in weak markets. In particular the availability of numerous bitstream access types in combination with the proper price differentiation can fuel broadband adoption in relatively weak broadband markets.
Originality/value – The paper challenges the general rule that only platform (also known as facility) based competition guarantees longterm growth of the broadband market. Bitstream and resale access do not lag local loop unbundling and can be used in weak markets that do not employ alternative infrastructures to fuel competition in the relevant markets. Different policies and strategies must be followed, in that case, on behalf of the local NRA.
Abstract: In this work we present a platformagnostic framework for intergrating heterogeneous Smart Objects in the Web of Things. Our framework, consists of 4 different hardware platforms, Arduino, SunSPOT, TelosB, iSense. These hardware platforms are the most representative ones, as used by the relevant research community. A first contribution of our work is a careful description of the necessary steps to make such a heterogeneous network interoperate and the implementation of a network stack, in the form of a software library, named mkSense, which enables their intercommunication. Moreover, we describe the design and implementation of software library which can be used for building “intelligent software” for the Web of Things.
Abstract: Ever since the creation of the first human society, people have understood that the only way of sustaining and improving their societies is to rely on each other for exchanging services. This reliance have traditionally built on developing, among them, {\em trust}, a vague, intuitive to a large extend and hard to define concept that brought together people who worked towards the progress we all witness around us today. % Today's society is, however, becoming increasingly massive, collective, and complex and includes not only people, but huge numbers of machines as well. It is no overstatement to say that machines interconnected together into a complex communication fabric as well as communicating directly with people will very soon outnumber people by several orders of magnitude. Thus, trust, being already a difficult concept to define and measure when applied to a few people that form a cooperating group or a set of acquaintances, it is far more difficult to pinpoint when applied to large communities whose members may hardly know each other in person or to interconnected machines employed by these communities. In this paper we attempt to take a pragmatic position with regard to trust definition and measurement. We employ several formalisms, into each of which we define a reasonable notion of trust, and show that inherent weaknesses of these formalisms result in an inability to have a concrete and fully measurable trust concept. % We then argue that trust in the modern intertwined WWW society must, necessarily, incorporate to some degree nonformalizable elements, such as common sense and intuition. Although this may sound pessimistic, our view is that it is not, since by understanding these limitations of formalism with regard to trust, increases selfawareness and caution on people's part and avoid problems that result if one relies only on automation in order to deduce trust.
Abstract: In this work, we study the propagation of influence and computation in dynamic networks that are possibly disconnected at every instant. We focus on a synchronous message passing communication model with broadcast and bidirectional links. To allow for bounded endtoend communication we propose a set of minimal temporal connectivity conditions that bound from the above the time it takes for information to make progress in the network. We show that even in dynamic networks that are disconnected at every instant information may spread as fast as in networks that are connected at every instant. Further, we investigate termination criteria when the nodes know some upper bound on each of the temporal connectivity conditions. We exploit our termination criteria to provide efficient protocols (optimal in some cases) that solve the fundamental counting and alltoall token dissemination (or gossip) problems. Finally, we show that any protocol that is correct in instantaneous connectivity networks can be adapted to work in temporally connected networks.
Abstract: The Team Orienteering Problem with Time Windows (TOPTW)
deals with deriving a number of tours comprising a subset of candidate
nodes (each associated with a \prot" value and a visiting time window)
so as to maximize the overall \prot", while respecting a specied time
span. TOPTW has been used as a reference model for the Tourist Trip
Design Problem (TTDP) in order to derive nearoptimal multipleday
tours for tourists visiting a destination featuring several points of inter
est (POIs), taking into account a multitude of POI attributes. TOPTW
is an NPhard problem and the most ecient known heuristic is based on
Iterated Local Search (ILS). However, ILS treats each POI separately;
hence it tends to overlook highly protable areas of POIs situated far
from the current location, considering them too timeexpensive to visit.
We propose two clusterbased extensions to ILS addressing the afore
mentioned weakness by grouping POIs on disjoint clusters (based on
geographical criteria), thereby making visits to such POIs more attrac
tive. Our approaches improve on ILS with respect to solutions quality,
while executing at comparable time and reducing the frequency of overly
long transfers among POIs.
Abstract: This paper presents ongoing work on using data mining clustering to support the evaluation of software
systems' maintainability. As input for our analysis we employ software measurement data extracted from
Java source code. We propose a twosteps clustering process which facilitates the assessment of a system's
maintainability at rst, and subsequently an incluster analysis in order to study the evolution of each
cluster as the system's versions pass by. The process is evaluated on Apache Geronimo, a J2EE 1.4 open
source Application Server. The evaluation involves analyzing several versions of this software system in
order to assess its evolution and maintainability over time. The paper concludes with directions for future
work.
Abstract: One problem that frequently arises is the establishment of a
secure connection between two network nodes. There are many key
establishment protocols that are based on Trusted Third Parties or
public key cryptography which are in use today. However, in the
case of networks with frequently changing topology and size
composed of nodes of limited computation power, such as the adhoc
and sensor networks, such an approach is difficult to apply. One
way of attacking this problem for such networks is to have the two
nodes share some piece of information that will, subsequently,
enable them to transform this information into a shared
communication key.
%
Having each pair of network nodes share some piece of information
is, often, achieved through appropriate {\em key predistribution}
schemes. These schemes work by equipping each network node with a
set of candidate key values, some of which shared with other
network nodes possessing other keys sets. Later, when two nodes
meet, they can employ a suitable key establishment protocol in
order to locate shared values and used them for the creation of
the communication key.
%
In this paper we give a formal definition of collusion resistant
key predistribution schemes and then propose such a scheme based
on probabilistically created set systems. The resulting key sets
are shown to have a number of desirable properties that ensure the
confidentiality of communication sessions against collusion
attacks by other network nodes.
Abstract: An intersection graph of n vertices assumes that each vertex is equipped with a subset of a global label set. Two vertices share an edge
when their label sets intersect. Random Intersection Graphs (RIGs) (as defined in [18, 31]) consider label sets formed by the following experiment:
each vertex, independently and uniformly, examines all the labels (m in total) one by one. Each examination is independent and the vertex
succeeds to put the label in her set with probability p. Such graphs nicely capture interactions in networks due to sharing of resources among nodes.
We study here the problem of efficiently coloring (and of finding upper bounds to the chromatic number) of RIGs. We concentrate in a range
of parameters not examined in the literature, namely: (a) m = n{\'a} for less than 1 (in this range, RIGs differ substantially from the Erd¨os Renyi random graphs) and (b) the selection probability p is quite high
(e.g. at least ln2 n m in our algorithm) and disallows direct greedy colouring methods.
We manage to get the following results:
For the case mp ln n, for any constant < 1 − , we prove that np colours are enough to colour most of the vertices of the graph with high probability (whp). This means that even for quite dense
graphs, using the same number of colours as those needed to properly colour the clique induced by any label suffices to colour almost all of the vertices of the graph. Note also that this range of values of m, p
is quite wider than the one studied in [4].
� We propose and analyze an algorithm CliqueColour for finding a proper colouring of a random instance of Gn,m,p, for any mp >=ln2 n. The algorithm uses information of the label sets assigned to the
vertices of Gn,m,p and runs in O (n2mp2/ln n) time, which is polynomial in n and m. We also show by a reduction to the uniform random
intersection graphs model that the number of colours required by the algorithm are of the correct order of magnitude with the actual
chromatic number of Gn,m,p.
⋆ This work was partially supported by the ICT Programme of the European Union under contract number ICT2008215270 (FRONTS). Also supported by Research Training Group GK693 of the Paderborn Institute for Scientific Computation
(PaSCo).
� We finally compare the problem of finding a proper colouring for Gn,m,p to that of colouring hypergraphs so that no edge is monochromatic.We show how one can find in polynomial time a kcolouring of the vertices of Gn,m,p, for any integer k, such that no clique induced by only one label in Gn,m,p is monochromatic. Our techniques are novel and try to exploit as much as possible the hidden structure of random intersection graphs in this interesting range.
Abstract: We investigate random intersection graphs, a combinatorial model that quite accurately abstracts distributed networks with local interactions between nodes blindly sharing critical resources from a limited globally available domain. We study important combinatorial properties (independence and hamiltonicity) of such graphs. These properties relate crucially to algorithmic design for important problems (like secure communication and frequency assignment) in distributed networks characterized by dense, local interactions and resource limitations, such as sensor networks. In particular, we prove that, interestingly, a small constant number of random, resource selections suffices to make the graph hamiltonian and we provide tight evaluations of the independence number of these graphs.
Abstract: This chapter aims at presenting certain important aspects of the design of lightweight, eventdriven algorithmic solutions for data dissemination in wireless sensor networks that provide support for reliable, efficient and concurrencyintensive operation. We wish to emphasize that efficient solutions at several levels are needed, e.g.~higher level energy efficient routing protools and lower level power management schemes. Furthermore, it is important to combine such different level methods into integrated protocols and approaches. Such solutions must be simple, distributed and local. Two useful algorithmic design principles are randomization (to tradeoff efficiency and faulttolerance) and adaptation (to adjust to high network dynamics towards improved operation). In particular, we provide a) a brief description of the technical specifications of stateoftheart sensor devices b) a discussion of possible models used to abstract such networks, emphasizing heterogeneity, c) some representative power management schemes, and d) a presentation of some characteristic protocols for data propagation. Crucial efficiency properties of these schemes and protocols (and their combinations, in some cases) are investigated by both rigorous analysis and performance evaluations through large scale simulations.
Abstract: The problem of communication among mobile nodes is one of the most fundamental problems in ad hoc mobile networks and is at the core of many algorithms, such as for counting the number of nodes, electing a leader, data processing etc. For an exposition of several important problems in ad hoc mobile networks. The work of Chatzigiannakis, Nikoletseas and Spirakis focuses on wireless mobile networks that are subject to highly dynamic structural changes created by mobility, channel fluctuations and device failures. These changes affect topological connectivity, occur with high frequency and may not be predictable in advance. Therefore, the environment where the nodes move (in threedimensional space with possible obstacles) as well as the motion that the nodes perform are \textit{input} to any distributed algorithm.
Abstract: In this work, we overview some results concerning communication combinatorial properties in random intersection graphs and uniform random intersection graphs. These properties relate crucially to algorithmic design for important problems (like secure communication and frequency assignment) in distributed networks characterized by dense, local interactions and resource limitations, such as sensor networks. In particular, we present and discuss results concerning the existence of large independent sets of vertices whp in random instances of each of these models. As the main contribution of our paper, we introduce a new, general model, which we denote G(V, χ, f). In this model, V is a set of vertices and χ is a set of m vectors in ℝm. Furthermore, f is a probability distribution over the powerset 2χ of subsets of χ. Every vertex selects a random subset of vectors according to the probability f and two vertices are connected according to a general intersection rule depending on their assigned set of vectors. Apparently, this new general model seems to be able to simulate other known random graph models, by carefully describing its intersection rule.
Abstract: We study the problem of maintaining connectivity in a wireless
network where the network nodes are equipped with
directional antennas. Nodes correspond to points on the
plane and each uses a directional antenna modeled by a sector
with a given angle and radius. The connectivity problem
is to decide whether or not it is possible to orient the antennas
so that the directed graph induced by the node transmissions
is strongly connected. We present algorithms for
simple polynomialtimesolvable cases of the problem, show
that the problem is NPcomplete in the 2dimensional case
when the sector angle is small, and present algorithms that
approximate the minimum radius to achieve connectivity for
sectors with a given angle. We also discuss several extensions
to related problems. To the best of our knowledge, the
problem has not been studied before in the literature.
Abstract: We design and implement various algorithms for
solving the static RWA problem with the objective of minimizing
the maximum number of requested wavelengths based on LP
relaxation formulations. We present a link formulation, a path
formulation and a heuristic that breaks the problem in the two
constituent subproblems and solves them individually and
sequentially. The flow cost functions that are used in these
formulations result in providing integer optimal solutions despite
the absence of integrality constraints for a large subset of RWA
input instances, while also minimizing the total number of used
wavelengths. We present a random perturbation technique that is
shown to increase the number of instances for which we find
integer solutions, and we also present appropriate iterative fixing
and rounding methods to be used when the algorithms do not yield
integer solutions. We comment on the number of variables and
constraints these formulations require and perform extensive
simulations to compare their performance to that of a typical minmax
congestion formulation.
Abstract: Here we survey various computational models for Wireless Sensor Networks (WSNs). The population protocol model (PP) considers networks of tiny mobile finitestate artifacts that can sense the environment and communicate in pairs to perform a computation. The mediated population protocol model (MPP) enhances the previous model by allowing the communication links to have a constant size buffer, providing more computational power. The graph decision MPP model (GDM) is a special case of MPP that focuses on the MPP's ability to decide graph properties of the network. Another direction towards enhancing the PP is followed by the PALOMA model in which the artifacts are no longer finitestate automata but Turing Machines of logarithmic memory in the population size. A different approach to modeling WSNs is the static synchronous sensor field model (SSSF) which describes devices communicating through a fixed communication graph and interacting with their environment via input and output data streams. In this survey, we present the computational capabilities of each model and provide directions for further research.
Abstract: This paper addresses the problem of counting the size of a network where (i) processes have the same identifiers (anonymous nodes) and (ii) the et
work topology constantly changes (dynamic network). Changes are riven by a powerful adversary that can look at internal process states and add and remove edges in order to contrast the convergence of the algorithm to the correct count. The paper proposes two leaderbased counting algorithms. Such algorithms are based on a technique that mimics an energytransfer between network nodes. The first algorithm assumes that the adversary cannot generate either disconnected network graphs or network graphs where nodes have degree greater than D. In such algorithm, the leader can count the size of the network and detect the counting termination in a finite time (i.e., conscious counting algorithm). The second algorithm assumes that the adversary only keeps the network graph connected at any time and we prove that the leader can still converge to a correct count in a finite number of rounds, but it is not conscious when this convergence happens.
Abstract: In this article, we present a detailed performance
evaluation of a hybrid optical switching (HOS)
architecture called Overspill Routing in Optical Networks
(ORION). The ORION architecture combines
(optical) wavelength and (electronic) packet switching,
so as to obtain the individual advantages of both switching
paradigms. In particular, ORION exploits the possible insertions/extractions, to reduce the necessary
interfaces, do not deteriorate performance and thus the
use of traffic concentrators assure ORION’s economic
viability.
idle periods of established lightpaths to transmit
packets destined to the next common node, or even
directly to their common enddestination. Depending
on whether all lightpaths are allowed to simultaneously
carry and terminate overspill traffic or overspill is restricted
to a subset of wavelengths, the architecture
limits itself to constrained or unconstrained ORION. To
evaluate both cases, we developed an extensive network
simulator where the basic features of the ORION architectureweremodeled,
including suitable edge/core node
switches and loadvarying sources to simulate overloading
traffic conditions. Further, we have assessed various
aspects of the ORION architecture including two
basic routing/forwarding policies and various buffering
schemes. The complete network study shows that
ORION can absorb temporal traffic overloads, as intended,
provided sufficient buffering is present.We also
demonstrate that the restriction of simultaneous packet
Abstract: In this work, we expanded the Arduino's
capabilities by adding an 802.15.4 wireless module, in order to
expose its functionality as a Web of Things node. The second
contribution of our work is a careful description of the necessary
steps to make a heterogeneous network interoperate and the
implementation of a network stack for the 4 most representative
hardware platforms, as used by the relevant research community
(Arduino, SunSPOT, TelosB, iSense), in the form of a software
library, named mkSense, which enables their
intercommunication. Moreover, we describe the design and
implementation of a software library which can be used for
building “intelligent software” for the Web of Things.
Abstract: Everincreasing bandwidth demands and higher flexibility are the main challenges for the next generation optical core networks. A new trend in order to address these challenges is to consider the impairments of the lightpaths during the design of optical networks. In our work, we focus on translucent optical networks, where some lightpaths are routed transparently, whereas others go through a number of regenerators. We present a cost analysis of design strategies, which are based either on an exact Quality of Transmission (QoT) validation or on a relaxed one and attempt to reduce the amount of regenerators used. In the exact design strategy, regenerators are required if the QoT of a candidate lightpath is below a predefined threshold, assuming empty network conditions. In the relaxed strategy, this predefined threshold is lower, while it is assumed that the network is fully loaded. We evaluate technoeconomically the suggested design solutions and also show that adding more flexibility to the optical nodes has a large impact to the total infrastructure cost.
Abstract: Counting in general, and estimating the cardinality of (multi) sets in particular, is highly desirable for a large variety of applications, representing a foundational block for the efficient deployment and access of emerging internetscale information systems. Examples of such applications range from optimizing query access plans in internetscale databases, to evaluating the significance (rank/score) of various data items in information retrieval applications. The key constraints that any acceptable solution must satisfy are: (i) efficiency: the number of nodes that need be contacted for counting purposes must be small in order to enjoy small latency and bandwidth requirements; (ii) scalability, seemingly contradicting the efficiency goal: arbitrarily large numbers of nodes nay need to add elements to a (multi) set, which dictates the need for a highly distributed solution, avoiding serverbased scalability, bottleneck, and availability problems; (iii) access and storage load balancing: counting and related overhead chores should be distributed fairly to the nodes of the network; (iv) accuracy: tunable, robust (in the presence of dynamics and failures) and highly accurate cardinality estimation; (v) simplicity and ease of integration: special, solutionspecific indexing structures should be avoided. In this paper, first we contribute a highlydistributed, scalable, efficient, and accurate (multi) set cardinality estimator. Subsequently, we show how to use our solution to build and maintain histograms, which have been a basic building block for query optimization for centralized databases, facilitating their porting into the realm of internetscale data networks.
Abstract: Random walks in wireless sensor networks can serve as fully
local, very simple strategies for sink motion that reduce energy dissipa
tion a lot but increase the latency of data collection. To achieve satis
factory energylatency tradeoffs the sink walks can be made adaptive,
depending on network parameters such as density and/or history of past
visits in each network region; but this increases the memory require
ments. Towards better balances of memory/performance, we propose two
new random walks: the Random Walk with Inertia and the Exploreand
Go Random Walk; we also introduce a new metric (Proximity Varia
tion) that captures the different way each walk gets close to the network
nodes. We implement the new walks and experimentally compare them
to known ones. The simulation findings demonstrate that the new walk¢s
performance (cover time) gets close to the one of the (much stronger)
biased walk, while in some other respects (partial cover time, proximity
variation) they even outperform it. We note that the proposed walks
have been finetuned in the light of experimental findings.
Abstract: We present a new overlay, called the Deterministic Decentral
ized tree (D2tree). The D2tree compares favourably to other overlays for
the following reasons: (a) it provides matching and better complexities,
which are deterministic for the supported operations; (b) the manage
ment of nodes (peers) and elements are completely decoupled from each
other; and (c) an e±cient deterministic loadbalancing mechanism is pre
sented for the uniform distribution of elements into nodes, while at the
same time probabilistic optimal bounds are provided for the congestion
of operations at the nodes.
Abstract: DAP (Distributed Algorithms Platform) is a generic and homogeneous simulation environment aiming at the implementation, simulation, and testing of distributed algorithms for wired and wireless networks. In this work, we present its architecture, the most important design decisions, and discuss its distinct features and functionalities. DAP allows the algorithm designer to implement a distributed protocol by creating his own customized environment, and programming in a standard programming language in a style very similar to that of a realworld application. DAP provides a graphical user interface that allows the designer to monitor and control the execution of simulations, visualize algorithms, as well as gather statistics and other information for their experimental analysis and testing.
Abstract: The Greek School Network (GSN) is the nationwide network that connects all units of primary and secondary education in Greece. GSN offers a significant set of diverse services to more than 15.000 schools and administrative units, and more than 60.000 teachers, placing GSN second in infrastructure size nationwide. GSN has relied on the emerging power of open source software to build cuttingedge services capable of covering internal administrative and monitoring needs, end user demands, and, foremost, modern pedagogical requirements for tools and services. GSN provides a wide set of advanced services, varying from web mail to virtual classrooms and synchronous/asynchronous teleeducation. This paper presents an evaluation of GSN open source services based on the opinions of users who use GSN for educational purposes, and on usage and traffic measurement statistics. The paper reaches the conclusion that open source software provides a sound technological platform that meets the needs for cutting edge educational services deployment, and innovative, competitive software production for educational networks.
Abstract: In this work we introduce two practical and interesting models of adhoc mobile networks: (a) hierarchical adhoc networks, comprised of dense subnetworks of mobile users interconnected by a very fast yet limited backbone infrastructure, (b) highly changing adhoc networks, where the deployment area changes in a highly dynamic way and is unknown to the protocol. In such networks, we study the problem of basic communication, i.e., sending messages from a sender node to a receiver node. For highly changing networks, we investigate an efficient communication protocol exploiting the coordinated motion of a small part of an adhoc mobile network (the ldquorunners supportrdquo) to achieve fast communication. This protocol instead of using a fixed sized support for the whole duration of the protocol, employs a support of some initial (small) size which adapts (given some time which can be made fast enough) to the actual levels of traffic and the (unknown and possibly rapidly changing) network area, by changing its size in order to converge to an optimal size, thus satisfying certain Quality of Service criteria. Using random walks theory, we show that such an adaptive approach is, for this class of adhoc mobile networks, significantly more efficient than a simple nonadaptive implementation of the basic ldquorunners supportrdquo idea, introduced in [9,10]. For hierarchical adhoc networks, we establish communication by using a ldquorunnersrdquo support in each lower level of the hierarchy (i.e., in each dense subnetwork), while the fast backbone provides interconnections at the upper level (i.e., between the various subnetworks). We analyze the time efficiency of this hierarchical approach. This analysis indicates that the hierarchical implementation of the support approach significantly outperforms a simple implementation of it in hierarchical adhoc networks. Finally, we discuss a possible combination of the two approaches above (the hierarchical and the adaptive ones) that can be useful in adhoc networks that are both hierarchical and highly changing. Indeed, in such cases the hierarchical nature of these networks further supports the possibility of adaptation.
Abstract: In this work we investigate the problem of communication among mobile hosts, one of the most fundamental problems in adhoc mobile networks that is at the core of many algorithms. Our work investigates the extreme case of total absence of any fixed network backbone or centralized administration, instantly forming networks based only on mobile hosts with wireless communication capabilities, where topological connectivity is subject to frequent, unpredictable change.
For such dynamically changing networks we propose a set of protocols which exploit the coordinated (by the protocol) motion of a small part of the network in order to manage network operations. We show that such protocols can be designed to work correctly and efficiently for communication by avoiding message flooding. Our protocols manage to establish communication between any pair of mobile hosts in small, apriori guaranteed expected time bounds. Our results exploit and further develop some fundamental properties of random walks in finite graph.
Apart from studying the general case, we identify two practical and interesting cases of adhoc mobile networks: a) hierarchical adhoc networks, b) highly changing adhoc networks, for which we propose protocols that efficiently deal with the problem of basic communication.
We have conducted a set of extensive experiments, comprised of thousands of mobile hosts in order to validate the theoretical results and show that our protocols achieve very efficient communication under different scenaria.
Abstract: Evaluating target tracking protocols for wireless sensor networks that can localize multiple mobile devices, can be a very challenging task. Such protocols usually aim at minimizing communication overhead, data processing for the participating nodes, as well as delivering adequate tracking information of the mobile targets in a timely manner. Simulations on such protocols are performed using theoretical models that are based on unrealistic assumptions like the unit disk graph communication model, ideal network localization and perfect distance estimations. With these assumptions taken for granted, theoretical models claim various performance milestones that cannot be achieved in realistic conditions. In this paper we design a new localization protocol, where mobile assets can be tracked passively via software agents. We address the issues that hinder its performance due to the real environment conditions and provide a deployable protocol. The implementation, integration and experimentation of this new protocol and it's optimizations, were performed using the WISEBED framework. We apply our protocol in multiple indoors wireless sensor testbeds with multiple experimental scenarios to showcase scalability and tradeoffs between network properties and configurable protocol parameters. By analysis of the real world experimental output, we present results that depict a more realistic view of the target tracking problem, regarding power consumption and the quality of tracking information. Finally we also conduct some very focused simulations to assess the scalability of our protocol in very large networks and multiple mobile assets.
Abstract: We have designed and implemented a platform that enables monitoring and actuation in multiple buildings, that has been utilised in the context of a research project in Greece, focusing on public school buildings. The Green Mindset project has installed IoT devices in 12 Greek public schools to monitor energy consumption, along with indoor and outdoor environmental parameters. We present the architecture and actual deployment of our system, along with a first set of findings.
Abstract: In this paper, we present a Programmable Packet Processing Engine suitable for deep header processing in highspeed networking systems.
The engine, which has been – fabricated as part of a complete network processor, consists of a typical RISCCPU, whose register
Wle has been modiWed in order to support eYcient context switching, and two simple specialpurpose processing units. The engine can be
used in a number of network processing units (NPUs), as an alternative to the typical design practice of employing a large number of simple
general purpose processors, or in any other embedded system designed to process mainly network protocols. To assess the performance
of the engine, we have proWled typical networking applications and a series of experiments were carried out. Further, we have
compared the performance of our processing engine to that of two widely used NPUs and show that our proposed packetprocessing
engine can run speciWc applications up to three times faster. Moreover, the engine is simpler to be fabricated, less complex in terms of
hardware complexity, while it can still be very easily programmed.
Abstract: Pervasive games are a new type of digital games that combines game and physical reality within the gameplay. This novel game type raises unprecedented research and design challenges for developers and urges the exploration of new technologies and methods to create high quality game experiences and design novel and compelling forms of content for the players. This chapter follows a systematic approach to explore the landscape of pervasive gaming. First, the authors approach pervasive games from a theoretical point of view, defining the four axes of pervasive games design, introducing the concept of game world persistency, and describing aspects of spatially/temporally/socially expanded games. Then, they present ten pervasive game projects, classified in five genres based on their playing environment and features. Following that, the authors present a comparative view of those projects with respect to several design aspects: communication and localization, context and personal awareness aspects, information model, player equipment, and game space visualization. Last, the authors highlight current trends, design principles, and future directions for pervasive games development.
Abstract: In the recent years we have seen an increased popularity in game development using Smartphones, which has provided an increasingly ubiquitous platform for designing games. In this paper we wish to investigate the use of a modern Smartphone’s capabilities in game
development by implementing and evaluating a classic game on the iPhone platform. We identify the limitations and possibilities that this field offers to the different aspect of game design.
Abstract: Wireless Sensor Networks consist of a large number of small, autonomous devices, that are able to interact with their inveronment by sensing and collaborate to fulfill their tasks, as, usually, a single node is incapable of doing so; and they use wireless communication to enable this collaboration. Each device has limited computational and energy resources, thus a basic issue in the applicastions of wireless sensor networks is the low energy consumption and hence, the maximization of the network lifetime.
The collected data is disseminated to a static control point – data sink in the network, using node to node  multihop data propagation. However, sensor devices consume significant amounts of energy in addition to increased implementation complexity, since a routing protocol is executed. Also, a point of failure emerges in the area near the control center where nodes relay the data from nodes that are farther away. Recently, a new approach has been developed that shifts the burden from the sensor nodes to the sink. The main idea is that the sink has significant and easily replenishable energy reserves and can move inside the area the sensor network is deployed, in order to acquire the data collected by the sensor nodes at very low energy cost. However, the need to visit all the regions of the network may result in large delivery delays.
In this work we have developed protocols that control the movement of the sink in wireless sensor networks with nonuniform deployment of the sensor nodes, in order to succeed an efficient (with respect to both energy and latency) data collection. More specifically, a graph formation phase is executed by the sink during the initialization: the network area is partitioned in equal square regions, where the sink, pauses for a certain amount of time, during the network traversal, in order to collect data.
We propose two network traversal methods, a deterministic and a random one. When the sink moves in a random manner, the selection of the next area to visit is done in a biased random manner depending on the frequency of visits of its neighbor areas. Thus, less frequently visited areas are favored. Moreover, our method locally determines the stop time needed to serve each region with respect to some global network resources, such as the initial energy reserves of the nodes and the density of the region, stopping for a greater time interval at regions with higher density, and hence more traffic load. In this way, we achieve accelerated coverage of the network as well as fairness in the service time of each region.Besides randomized mobility, we also propose an optimized deterministic trajectory without visit overlaps, including direct (onehop) sensortosink data transmissions only.
We evaluate our methods via simulation, in diverse network settings and comparatively to related state of the art solutions. Our findings demonstrate significant latency and energy consumption improvements, compared to previous research.
Abstract: Wireless sensor networks are a recently introduced category of ad hoc computer networks, which are comprised by nodes of small size and limited computing and energy resources. Such nodes are able of measuring physical properties such as temperature, humidity, etc., wireless communication between each other and in some cases interaction with their surrounding environments (through the use of electromechanical parts).
As these networks have begun to be widely available (in terms of cost and commercial hardware availability), their field of application and philosophy of use is constantly evolving. We have numerous examples of their applications, ranging from monitoring the biodiversity of a specific outdoor area to structural health monitoring of bridges, and also networks ranging from few tens of nodes to even thousands of nodes.
In this PhD thesis we investigated the following basic research lines related to wireless sensor networks:
a) their simulation,
b) the development of data propagation protocols suited to such networks and their evaluation through simulation,
c) the modelling of ``hostile'' circumstances (obstacles) during their operation and evaluation of their impact through simulation,
d) the development of a sensor network management application.
Regarding simulation, we initially placed an emphasis to issues such as the effective simulation of networks of several thousands of nodes, and in that respect we developed a network simulator (simDust), which is extendable through the addition of new data propagation protocols and visualization capabilities. This simulator was used to evaluate the performance of a number of characteristic data propagation protocols for wireless sensor networks. Furthermore, we developed a new protocol (VRTP) and evaluated its performance against other similar protocols. Our studies show that the new protocol, that uses dynamic changes of the transmission range of the network nodes, performs better in certain cases than other related protocols, especially in networks containing obstacles and in the case of nonhomogeneous placement of nodes.
Moreover, we emphasized on the addition of ``realistic'' conditions to the simulation of such protocols, that have an adversarial effect on their operation. Our goal was to introduce a model for obstacles that adds little computational overhead to a simulator, and also study the effect of the inclusion of such a model on data propagation protocols that use geographic information (absolute or relative). Such protocols are relatively sensitive to dynamic topology changes and network conditions. Through our experiments, we show that the inclusion of obstacles during simulation can have a significant effect on these protocols.
Finally, regarding applications, we initially proposed an architecture (WebDust/ShareSense), for the management of such networks, that would provide basic capabilities of managing such networks and developing applications above it. Features that set it apart are the capability of managing multiple heterogeneous sensor networks, openess, the use of a peertopeer architecture for the interconnection of multiple sensor network. A large part of the proposed architecture was implemented, while the overall architecture was extended to also include additional visualization capabilities.
Abstract: Wireless sensor networks are comprised of a vast number of devices, situated in an area of interest that self organize in a structureless network, in order to monitor/record/measure an environmental variable or phenomenon and subsequently to disseminate the data to the control center.
Here we present research focused on the development, simulation and evaluation of energy efficient algorithms, our basic goal is to minimize the energy consumption. Despite technology advances, the problem of energy use optimization remains valid since current and emerging hardware solutions fail to solve it.
We aim to reduce communication cost, by introducing novel techniques that facilitate the development of new algorithms. We investigated techniques of distributed adaptation of the operations of a protocol by using information available locally on every node, thus through local choices we improve overall performance. We propose techniques for collecting and exploiting limited local knowledge of the network conditions. In an energy efficient manner, we collect additional information which is used to achieve improvements such as forming energy efficient, low latency and fault tolerant paths to route data. We investigate techniques for managing mobility in networks where movement is a characteristic of the control center as well as the sensors. We examine methods for traversing and covering the network field based on probabilistic movement that uses local criteria to favor certain areas.
The algorithms we develop based on these techniques operate a) at low level managing devices, b) on the routing layer and c) network wide, achieving macroscopic behavior through local interactions. The algorithms are applied in network cases that differ in density, node distribution, available energy and also in fundamentally different models, such as under faults, with incremental node deployment and mobile nodes. In all these settings our techniques achieve significant gains, thus distinguishing their value as tools of algorithmic design.
Abstract: A central problem in distributed computing and telecommunications
is the establishment of common knowledge between two computing
entities. An immediate use of such common knowledge is in the
initiation of a secure communication session between two entities
since the two entities may use this common knowledge in order to
produce a secret key for use with some symmetric cipher.
%
The dynamic establishment of shared information (e.g. secret key)
between two entities is particularly important in networks with no
predetermined structure such as wireless mobile adhoc networks.
In such networks, nodes establish and terminate communication
sessions dynamically with other nodes which may have never been
encountered before in order to somehow exchange information which
will enable them to subsequently communicate in a secure manner.
%
In this paper we give and theoretically analyze a protocol that
enables two entities initially possessing a string each to
securely eliminate inconsistent bit positions, obtaining strings
with a larger percentage of similarities. This can help the nodes
establish a shared set of bits and use it as a key with some
shared key encryption scheme.
Abstract: We here present Fun in Numbers (FinN), a framework for developing pervasive applications and interactive installations for entertainment and educational purposes. Using ad hoc mobile wireless sensor network nodes as the enabling devices, FinN allows for the quick prototyping of applications that utilize input from multiple physical sources (sensors and other means of interfacing), by offering a set of programming templates and services, such as topology discovery, localization and synchronization, that hide the underlying complexity. We present the target application domains of FinN, along with a set of multiplayer games and interactive installations. We describe the overall architecture of our platform
and discuss some key implementation issues of the application domains. Finally, we present the experience gained by deploying the applications developed with our platform.
Abstract: We describe our experiences from implementing and integrating a new job scheduling algorithm in the gLite Grid middleware and present experimental results that compare it to the existing gLite scheduling algorithms. It is the first time that gLite scheduling algorithms are put under test and compared with a new algorithm under the same conditions. We describe the problems that were encountered and solved, going from theory and simulations to practice and the actual implementation of our scheduling algorithm. In this work we also describe the steps one needs to follow in order to develop and test a new scheduling algorithm in gLite. We present the methodology followed and the testbed that was set up for the comparisons. Our research sheds light on some of the problems of the existing gLite scheduling algorithms and makes clear the need for the development of new.
Abstract: The domain of smart cities is currently burgeoning, with a lot of potential for scientific and socioeconomic innovation gradually being revealed. It is also becoming apparent that crossdiscipline research will be instrumental in designing and building smarter cities, where IoT technology is becoming omnipresent. SmartSantander is an FP7 project that built a massive
cityscale IoT testbed aiming to provide both a tool for the research community and a functional system for the local government to implement operational city
services. In this work, we present key smart cities projects, main application domains and representative smart city frameworks that reflect the latest advances in the smart cities domain and our own experience through SmartSantander. The project has deployed 51.910 IoT endpoints, offering a massive infrastructure to the community, as well as functional system services and a number of enduser applications. Based on these aspects, we identify and
discuss a number of key scientific and technological challenges. We also present an overview of the developed system components and applications, and
discuss the ways that current smart city challenges were handled in the project.
Abstract: Direct routing is the special case of bufferless routing where N packets, once injected into the network, must be routed along specific paths to their destinations without conflicts. We give a general treatment of three facets of direct routing: (i) Algorithms. We present a polynomial time greedy algorithm for arbitrary direct routing problems whch is worstcase optimal, i.e., there exist instances for which no direct routing algorithm is better than the greedy. We apply variants of this algorithm to commonly used network topologies. In particular, we obtain nearoptimal routing time using for the tree and ddimensional mesh, given arbitrary sources and destinations; for the butterfly and the hypercube, the same result holds for random destinations. (ii) Complexity. By a reduction from Vertex Coloring, we show that Direct Routing is inapproximable, unless P=NP. (iii) Lower Bounds for Buffering. We show the existence of routing problems which cannot be solved efficiently with direct routing. To solve these problems, any routing algorithm needs buffers. We give nontrivial lower bounds on such buffering requirements for general routing algorithms.
Abstract: We present the first approximate distance oracle for sparse directed networks with timedependent arctraveltimes determined by continuous, piecewise linear, positive functions possessing the FIFO property. Our approach precomputes (1 + {\aa}) −approximate distance summaries from selected landmark vertices to all other vertices in the network, and provides two sublineartime query algorithms that deliver constant and (1 + {\'o}) −approximate shortesttraveltimes, respectively, for arbitrary origindestination pairs in the network. Our oracle is based only on the sparsity of the network, along with two quite natural assumptions about traveltime functions which allow the smooth transition towards asymmetric and timedependent distance metrics.
Abstract: In this survey, we describe the state of the art for research on experimentallydriven research on networks of tiny artifacts. The main topics are existing and planned practical testbeds, software simulations, and hybrid approaches; in addition, we describe a number of current studies undertaken by the authors.
Abstract: Wireless sensor networks are comprised of a vast number of ultrasmall fully autonomous computing, communication and sensing devices, with very restricted energy and computing capabilities, which cooperate to accomplish a large sensing task. Such networks can be very useful in practice in applications that require finegrain monitoring of physical environment subjected to critical conditions (such as inaccessible terrains or disaster places). Very large numbers of sensor devices can be deployed in areas of interest and use selforganization and collaborative methods to form deeply networked environments. Features including the huge number of sensor devices involved, the severe power, computational and memory limitations, their dense deployment and frequent failures, pose new design and implementation aspects. The efficient and robust realization of such large, highlydynamic, complex, nonconventional environments is a challenging algorithmic and technological task. In this work we consider certain important aspects of the design, deployment and operation of distributed algorithms for data propagation in wireless sensor networks and discuss some characteristic protocols, along with an evaluation of their performance.
Abstract: This paper deals with systems of multiple mobile robots each of which observes the positions of the other robots and moves to a new position so that eventually the robots form a circle. In the model we study, the robots are anonymous and oblivious, in the sense that they cannot be distinguished by their appearance and do not have a common xy coordinate system, while they are unable to remember past actions.
We propose a new distributed algorithm for circle formation on the plane. We prove that our algorithm is correct and provide an upper bound for its performance. In addition, we conduct an extensive and detailed comparative simulation experimental study with the DK algorithm described in [7]. The results show that our algorithm is very simple and takes considerably less time to execute than algorithm DK.
Abstract: An ad hoc mobile network is a collection of mobile hosts, with wireless communication capabilities, forming a temporary network without the aid of any established fixed infrastructure. In such networks, topological connectivity is subject to frequent, unpredictable change. Our work focuses on networks with high rate of such changes to connectivity. For such dynamically changing networks we propose protocols which exploit the coordinated (by the protocol) motion of a small part of the network. We show that such protocols can be designed to work correctly and efficiently even in the case of arbitrary (but not malicious) movements of the hosts not affected by the protocol. We also propose a methodology for the analysis of the expected behavior of protocols for such networks, based on the assumption that mobile hosts (those whose motion is not guided by the protocol) conduct concurrent random walks in their motion space. In particular, our work examines the fundamental problem of communication and proposes distributed algorithms for it. We provide rigorous proofs of their correctness, and also give performance analyses by combinatorial tools. Finally, we have evaluated these protocols by experimental means.
Abstract: An ad hoc mobile network is a collection of mobile hosts, with wireless communication capabilities, forming a temporary network without the aid of any established fixed infrastructure. In such networks, topological connectivity is subject to frequent, unpredictable change. Our work focuses on networks with high rate of such changes to connectivity. For such dynamically changing networks we propose protocols which exploit the coordinated (by the protocol) motion of a small part of the network. We show that such protocols can be designed to work correctly and efficiently even in the case of arbitrary (but not malicious) movements of the hosts not affected by the protocol. We also propose a methodology for the analysis of the expected behavior of protocols for such networks, based on the assumption that mobile hosts (those whose motion is not guided by the protocol) conduct concurrent random walks in their motion space. In particular, our work examines the fundamental problem of communication and proposes distributed algorithms for it. We provide rigorous proofs of their correctness, and also give performance analyses by combinatorial tools. Finally, we have evaluated these protocols by experimental means.
Abstract: Counting items in a distributed system, and estimating the cardinality of multisets in particular,
is important for a large variety of applications and a fundamental building block for emerging Internetscale information systems. Examples of such applications range from optimizing query access plans in peertopeer data sharing, to computing the significance (rank/score) of data items in distributed information retrieval. The general formal problem addressed in this article is computing the networkwide distinct number of items with some property (e.g., distinct files with file name
containing “spiderman”) where each node in the network holds an arbitrary subset, possibly overlapping the subsets of other nodes. The key requirements that a viable approach must satisfy are:
(1) scalability towards very large network size, (2) efficiency regarding messaging overhead, (3) load
balance of storage and access, (4) accuracy of the cardinality estimation, and (5) simplicity and easy
integration in applications. This article contributes the DHS (Distributed Hash Sketches) method
for this problem setting: a distributed, scalable, efficient, and accurate multiset cardinality estimator.
DHSis based on hash sketches for probabilistic counting, but distributes the bits of each counter
across network nodes in a judicious manner based on principles of Distributed Hash Tables, paying
careful attention to fast access and aggregation as well as update costs. The article discusses various
design choices, exhibiting tunable tradeoffs between estimation accuracy, hopcount efficiency, and
load distribution fairness. We further contribute a fullfledged, publicly available, opensource implementation of all our methods, and a comprehensive experimental evaluation for various settings.
Abstract: In this paper we describe a new simulation platform for heterogeneous distributed systems comprised of small programmable objects (e.g., wireless sensor networks) and traditional networked processors. Simulating such systems is complicated because of the need to coordinate compilers and simulators, often with very different interfaces, options, and fidelities.
Our platform (which we call ADAPT) is a flexible and extensible environment that provides a highly scalable simulator with unique characteristics. While the platform provides advanced functionality such as realtime simulation monitoring, custom topologies and scenarios, mixing real and simulated nodes, etc., the effort required by the user and the impact to her code is minimal. We here present its architecture, the most important design decisions, and discuss its distinct features and functionalities. We integrate our simulator to the Sun SPOT platform to enable simulation of sensing applications that employ both lowend and highend devices programmed with different languages that are internetworked with heterogeneous technologies. We believe that ADAPT will make the development of applications that use small programmable objects more widely accessible and will enable researchers to conduct a joint research approach that combines both theory and practice.
Abstract: Topk query processing is a fundamental building block for efficient ranking in a large number of applications. Efficiency is a central issue, especially for distributed settings, when the data is spread across different nodes in a network. This paper introduces novel optimization methods for topk aggregation queries in such distributed environments. The optimizations can be applied to all algorithms that fall into the frameworks of the prior TPUT and KLEE methods. The optimizations address three degrees of freedom: 1) hierarchically grouping input lists into topk operator trees and optimizing the tree structure, 2) computing dataadaptive scan depths for different input sources, and 3) dataadaptive sampling of a small subset of input sources in scenarios with hundreds or thousands of queryrelevant network nodes. All optimizations are based on a statistical cost model that utilizes local synopses, e.g., in the form of histograms, efficiently computed convolutions, and estimators based on order statistics. The paper presents comprehensive experiments, with three different reallife datasets and using the ns2 network simulator for a packetlevel simulation of a large Internetstyle network.
Abstract: We address the issue of measuring distribution fairness in Internetscale networks. This problem has several interesting instances encountered in different applications, ranging from assessing the distribution of load between network nodes for load balancing purposes, to measuring node utilization for optimal resource exploitation, and to guiding autonomous decisions of nodes in networks built with marketbased economic principles. Although some metrics have been proposed, particularly for assessing load balancing algorithms, they fall short. We first study the appropriateness of various known and previously proposed statistical metrics for measuring distribution fairness. We put forward a number of required characteristics for appropriate metrics. We propose and comparatively study the appropriateness of the Gini coefficient (G) for this task. Our study reveals as most appropriate the metrics of G, the fairness index (FI), and the coefficient of variation (CV) in this order. Second, we develop six distributed sampling algorithms to estimate metrics online efficiently, accurately, and scalably. One of these algorithms (2PRWS) is based on two effective optimizations of a basic algorithm, and the other two (the sequential sampling algorithm, LBSHL, and the clustered sampling one, EBSS) are novel, developed especially to estimate G. Third, we show how these metrics, and especially G, can be readily utilized online by higherlevel algorithms, which can now know when to best intervene to correct unfair distributions (in particular, load imbalances). We conclude with a comprehensive experimentation which comparatively evaluates both the various proposed estimation algorithms and the three most appropriate metrics (G, CV, andFI). Specifically, the evaluation quantifies the efficiency (in terms of number of the messages and a latency indicator), precision, and accuracy achieved by the proposed algorithms when estimating the competing fairness metrics. The central conclusion is that the proposed metric, G, can be estimated with a small number of messages and latency, regardless of the skew of the underlying distribution.
Abstract: Using a set of geometric containers to speed up shortest path queries in a weighted graph has been proven a useful tool for dealing with large sparse graphs. Given a layout of a graph G=(V,E), we store, for each edge (u,v)set membership, variantE, the bounding box of all nodes tset membership, variantV for which a shortest utpath starts with (u,v). Shortest path queries can then be answered by DijkstraImage restricted to edges where the corresponding bounding box contains the target.
In this paper, we present new algorithms as well as an empirical study for the dynamic case of this problem, where edge weights are subject to change and the bounding boxes have to be updated. We evaluate the quality and the time for different update strategies that guarantee correct shortest paths in an interesting application to railway information systems, using realworld data from six European countries.
Abstract: Service Oriented Computing and its most famous implementation technology Web Services (WS) are becoming an important enabler of networked business models. Discovery mechanisms are a critical factor to the overall utility of Web Services. So far, discovery mechanisms based on the UDDI standard rely on many centralized and areaspecific directories, which poses information stress problems such as performance bottlenecks and fault tolerance. In this context, decentralized approaches based on Peer to Peer overlay networks have been proposed by many researchers as a solution. In this paper, we propose a new structured P2P overlay network infrastructure designed for Web Services Discovery. We present theoretical analysis backed up by experimental results, showing that the proposed solution outperforms popular decentralized infrastructures for web discovery, Chord (and some of its successors), BATON (and it¢s successor) and SkipGraphs.
Abstract: We present a new overlay, called the
Deterministic Decentralized tree
(
D
2

tree). The
D
2
tree compares favorably to other overlays for the following reasons: (a)
it provides matching and better complexities,which are deterministic for the supported
operations; (b) the management of nodes (peers) and elements are completely decoupled from each other; and (c) an efficient deterministic loadbalancing mechanism
is presented for the uniform distribution of elements into nodes, while at the same
time probabilistic optimal bounds are provided for the congestion of operations at the
nodes. The loadbalancing scheme of elements into nodes is deterministic and general
enough to be applied to other hierarchical treebased overlays. This loadbalancing
mechanism is based on an innovative lazy weightbalancing mechanism, which is
interesting in its own right.
Abstract: We present a new overlay, called the Deterministic Decentralized tree ( TeXtree). The TeXtree compares favorably to other overlays for the following reasons: (a) it provides matching and better complexities, which are deterministic for the supported operations; (b) the management of nodes (peers) and elements are completely decoupled from each other; and (c) an efficient deterministic loadbalancing mechanism is presented for the uniform distribution of elements into nodes, while at the same time probabilistic optimal bounds are provided for the congestion of operations at the nodes. The loadbalancing scheme of elements into nodes is deterministic and general enough to be applied to other hierarchical treebased overlays. This loadbalancing mechanism is based on an innovative lazy weightbalancing mechanism, which is interesting in its own right.
Abstract: This paper deals with early obstacles recognition in wireless sensor networks under various traffic
patterns. In the presence of obstacles, the efficiency of routing algorithms is increased by voluntarily avoiding some regions in the vicinity of obstacles, areas which we call deadends. In this paper, we first propose a fast convergent routing algorithm with proactive deadend detection together with a formal definition and description of deadends. Secondly, we present a generalization of this algorithm which improves performances in all to many and all to all traffic patterns. In a third part we prove that this algorithm produces paths that are optimal up to a
constant factor of 2ð+1. In a fourth part we consider the reactive version of the algorithm which is an extension of a previously known early obstacle detection algorithm. Finally we give experimental results to illustrate the efficiency of our algorithms in different scenarios.
Abstract: In this paper we present the efficient burst reservation protocol (EBRP) suitable
for bufferless optical burst switching (OBS) networks. The EBRP protocol is a
twoway reservation scheme that employs timed and inadvance reservation of
resources. In the EBRP protocol timed reservations are relaxed, introducing a
reservation time duration parameter that is negotiated during call setup phase.
This feature allows bursts to reserve resources beyond their actual size to
increase their successful forwarding probability and can be used to provide
qualityofservice (QoS) differentiation. The EBRP protocol is suitable for
OBS networks and can guarantee a low blocking probability for bursts that can
tolerate the roundtrip delay associated with the twoway reservation.We present
the main features of the proposed protocol and describe in detail the timing
considerations regarding the call setup phase and the actual reservation process.
Furthermore, we show evaluation results and compare the EBRP performance
against two other typical reservation schemes, a tellandwait and a tellandgo
(justenoughtime) like protocol. EBRP has been developed for the control plane
of the ISTLASAGNE project.
Abstract: Andrews et al. [Automatic method for hiding latency in high bandwidth networks, in: Proceedings of the ACM Symposium on Theory of Computing, 1996, pp. 257–265; Improved methods for hiding latency in high bandwidth networks, in: Proceedings of the Eighth Annual ACM Symposium on Parallel Algorithms and Architectures, 1996, pp. 52–61] introduced a number of techniques for automatically hiding latency when performing simulations of networks with unit delay links on networks with arbitrary unequal delay links. In their work, they assume that processors of the host network are identical in computational power to those of the guest network being simulated. They further assume that the links of the host are able to pipeline messages, i.e., they are able to deliver P packets in time O(P+d) where d is the delay on the link.
In this paper we examine the effect of eliminating one or both of these assumptions. In particular, we provide an efficient simulation of a linear array of homogeneous processors connected by unitdelay links on a linear array of heterogeneous processors connected by links with arbitrary delay. We show that the slowdown achieved by our simulation is optimal. We then consider the case of simulating cliques by cliques; i.e., a clique of heterogeneous processors with arbitrary delay links is used to simulate a clique of homogeneous processors with unit delay links. We reduce the slowdown from the obvious bound of the maximum delay link to the average of the link delays. In the case of the linear array we consider both links with and without pipelining. For the clique simulation the links are not assumed to support pipelining.
The main motivation of our results (as was the case with Andrews et al.) is to mitigate the degradation of performance when executing parallel programs designed for different architectures on a network of workstations (NOW). In such a setting it is unlikely that the links provided by the NOW will support pipelining and it is quite probable the processors will be heterogeneous. Combining our result on clique simulation with wellknown techniques for simulating shared memory PRAMs on distributed memory machines provides an effective automatic compilation of a PRAM algorithm on a NOW.
Abstract: We present three new coordination mechanisms for schedul
ing n sel¯sh jobs on m unrelated machines. A coordination
mechanism aims to mitigate the impact of sel¯shness of jobs
on the e±ciency of schedules by de¯ning a local schedul
ing policy on each machine. The scheduling policies induce
a game among the jobs and each job prefers to be sched
uled on a machine so that its completion time is minimum
given the assignments of the other jobs. We consider the
maximum completion time among all jobs as the measure
of the e±ciency of schedules. The approximation ratio of
a coordination mechanism quanti¯es the e±ciency of pure
Nash equilibria (price of anarchy) of the induced game. Our
mechanisms are deterministic, local, and preemptive in the
sense that the scheduling policy does not necessarily process
the jobs in an uninterrupted way and may introduce some
idle time. Our ¯rst coordination mechanism has approxima
tion ratio O(logm) and always guarantees that the induced
game has pure Nash equilibria to which the system con
verges in at most n rounds. This result improves a recent
bound of O(log2 m) due to Azar, Jain, and Mirrokni and,
similarly to their mechanism, our mechanism uses a global
ordering of the jobs according to their distinct IDs. Next
we study the intriguing scenario where jobs are anonymous,
i.e., they have no IDs. In this case, coordination mechanisms
can only distinguish between jobs that have diffeerent load
characteristics. Our second mechanism handles anonymous
jobs and has approximation ratio O
¡ logm
log logm
¢
although the
game induced is not a potential game and, hence, the exis
tence of pure Nash equilibria is not guaranteed by potential
function arguments. However, it provides evidence that the
known lower bounds for nonpreemptive coordination mech
anisms could be beaten using preemptive scheduling poli
cies. Our third coordination mechanism also handles anony
mous jobs and has a nice \costrevealing" potential func
tion. Besides in proving the existence of equilibria, we use
this potential function in order to upperbound the price of stability of the induced game by O(logm), the price of an
archy by O(log2 m), and the convergence time to O(log2 m)
approximate assignments by a polynomial number of best
response moves. Our third coordination mechanism is the
¯rst that handles anonymous jobs and simultaneously guar
antees that the induced game is a potential game and has
bounded price of anarchy.
Abstract: We examine a task scheduling and data migration problem for grid networks, which we refer to as the Data Consolidation (DC) problem. DC arises when a task concurrently requests multiple pieces of data, possibly scattered throughout the grid network, that have to be present at a selected site before the task¢s execution starts. In such a case, the scheduler and the data manager must select (i) the data replicas to be used, (ii) the site where these data will be gathered for the task to be executed, and (iii) the routing paths to be followed; this is assuming that the selected datasets are transferred concurrently to the execution site. The algorithms or policies for selecting the data replicas, the data consolidating site and the corresponding paths comprise a Data Consolidation scheme. We propose and experimentally evaluate several DC schemes of polynomial number of operations that attempt to estimate the cost of the concurrent data transfers, to avoid congestion that may appear due to these transfers and to provide fault tolerance. Our simulation results strengthen our belief that DC is an important problem that needs to be addressed in the design of data grids, and can lead, if performed efficiently, to significant benefits in terms of task delay, network load and other performance parameters.
Abstract: In many cryptographic applications it is
necessary to generate elliptic curves (ECs) whose order
possesses certain properties. The method that is usually
employed for the generation of such ECs is the
socalled Complex Multiplication method. This method
requires the use of the roots of certain class field polynomials
defined on a specific parameter called the discriminant.
The most commonly used polynomials are
the Hilbert and Weber ones. The former can be used
to generate directly the EC, but they are characterized
by high computational demands. The latter have usually
much lower computational requirements, but they
do not directly construct the desired EC. This can be
achieved if transformations of their roots to the roots of the corresponding (generated by the same discriminant)
Hilbert polynomials are provided. In this paper we present
a variant of the Complex Multiplicationmethod that
generates ECs of cryptographically strong order. Our
variant is based on the computation of Weber polynomials.
We present in a simple and unifying manner a
complete set of transformations of the roots of aWeber
polynomial to the roots of its corresponding Hilbert
polynomial for all values of the discriminant. In addition,
we prove a theoretical estimate of the precision required
for the computation ofWeber polynomials for all values
of the discriminant.We present an extensive experimental
assessment of the computational efficiency of the
Hilbert and Weber polynomials along with their precision
requirements for various discriminant values and
we compare them with the theoretical estimates.We further
investigate the time efficiency of the new Complex
Multiplication variant under different implementations
of a crucial step of the variant. Our results can serve as
useful guidelines to potential implementers of EC cryptosystems
involving generation of ECs of a desirable
order on resource limited hardware devices or in systems
operating under strict timing response constraints
Abstract: We call radiation at a point of a wireless network the total amount of electromagnetic quantity (energy or power density) the point is exposed to. The impact of radiation can be high and we believe it is worth studying and control; towards radiation aware wireless networking we take (for the first time in the study of this aspect) a distributed computing, algorithmic approach. We exemplify this line of research by focusing on sensor networks, studying the minimum radiation path problem of finding the lowest radiation trajectory of a person moving from a source to a destination point in the network region. For this problem, we sketch the main ideas behind a linear program that can provide a tight approximation of the optimal solution, and then we discuss three heuristics that can lead to low radiation paths. We also plan to investigate the impact of diverse node mobility to the heuristics' performance.
Abstract: Intuitively, Braess's paradox states that destroying a part
of a network may improve the common latency of selsh
ows at Nash
equilibrium. Such a paradox is a pervasive phenomenon in realworld
networks. Any administrator, who wants to improve equilibrium delays
in selsh networks, is facing some basic questions: (i) Is the network
paradoxridden? (ii) How can we delete some edges to optimize equilibrium
ow delays? (iii) How can we modify edge latencies to optimize
equilibrium
ow delays?
Unfortunately, such questions lead to NPhard problems in general. In
this work, we impose some natural restrictions on our networks, e.g.
we assume strictly increasing linear latencies. Our target is to formulate
ecient algorithms for the three questions above.We manage to provide:
{ A polynomialtime algorithm that decides if a network is paradoxridden,
when latencies are linear and strictly increasing.
{ A reduction of the problem of deciding if a network with arbitrary
linear latencies is paradoxridden to the problem of generating all
optimal basic feasible solutions of a Linear Program that describes
the optimal trac allocations to the edges with constant latency.
{ An algorithm for nding a subnetwork that is almost optimal wrt
equilibrium latency. Our algorithm is subexponential when the number
of paths is polynomial and each path is of polylogarithmic length.
{ A polynomialtime algorithm for the problem of nding the best
subnetwork, which outperforms any known approximation algorithm
for the case of strictly increasing linear latencies.
{ A polynomialtime method that turns the optimal
ow into a Nash
ow by deleting the edges not used by the optimal
ow, and performing
minimal modications to the latencies of the remaining ones.
Our results provide a deeper understanding of the computational complexity
of recognizing the Braess's paradox most severe manifestations,
and our techniques show novel ways of using the probabilistic method
and of exploiting convex separable quadratic programs.
Abstract: Intuitively, Braess’s paradox states that destroying a part of a network may improve the common latency of selfish flows at Nash equilibrium. Such a paradox is a pervasive phenomenon in realworld networks. Any administrator who wants to improve equilibrium delays in selfish networks, is facing some basic questions:
– Is the network paradoxridden?
– How can we delete some edges to optimize equilibrium flow delays?
– How can we modify edge latencies to optimize equilibrium flow delays?
Unfortunately, such questions lead to View the MathML sourceNPhard problems in general. In this work, we impose some natural restrictions on our networks, e.g. we assume strictly increasing linear latencies. Our target is to formulate efficient algorithms for the three questions above. We manage to provide:
– A polynomialtime algorithm that decides if a network is paradoxridden, when latencies are linear and strictly increasing.
– A reduction of the problem of deciding if a network with (arbitrary) linear latencies is paradoxridden to the problem of generating all optimal basic feasible solutions of a Linear Program that describes the optimal traffic allocations to the edges with constant latency.
– An algorithm for finding a subnetwork that is almost optimal wrt equilibrium latency. Our algorithm is subexponential when the number of paths is polynomial and each path is of polylogarithmic length.
– A polynomialtime algorithm for the problem of finding the best subnetwork which outperforms any known approximation for the case of strictly increasing linear latencies.
– A polynomialtime method that turns the optimal flow into a Nash flow by deleting the edges not used by the optimal flow, and performing minimal modifications on the latencies of the remaining ones.
Our results provide a deeper understanding of the computational complexity of recognizing the most severe manifestations of Braess’s paradox, and our techniques show novel ways of using the probabilistic method and of exploiting convex separable quadratic programs.
Abstract: We study the problem of localizing and tracking multiple moving targets in wireless sensor
networks, from a network design perspective i.e. towards estimating the least possible number
of sensors to be deployed, their positions and operation chatacteristics needed to perform the
tracking task. To avoid an expensive massive deployment, we try to take advantage of
possible coverage ovelaps over space and time, by introducing a novel combinatorial model
that captures such overlaps.
Under this model, we abstract the tracking network design problem by a combinatorial
problem of covering a universe of elements by at least three sets (to ensure that each point in
the network area is covered at any time by at least three sensors, and thus being localized). We
then design and analyze an efficient approximate method for sensor placement and operation,
that with high probability and in polynomial expected time achieves a (log n) approximation
ratio to the optimal solution. Our network design solution can be combined with alternative
collaborative processing methods, to suitably fit different tracking scenaria.
Abstract: We study the problem of localizing and tracking multiple moving targets in wireless sensor networks, from a network design perspective i.e. towards estimating the least possible number of sensors to be deployed, their positions and operation characteristics needed to perform the tracking task. To avoid an expensive massive deployment, we try to take advantage of possible coverage overlaps over space and time, by introducing a novel combinatorial model that captures such overlaps.
Under this model, we abstract the tracking network design problem by a combinatorial problem of covering a universe of elements by at least three sets (to ensure that each point in the network area is covered at any time by at least three sensors, and thus being localized). We then design and analyze an efficient approximate method for sensor placement and operation, that with high probability and in polynomial expected time achieves a {\`E}(logn) approximation ratio to the optimal solution. Our network design solution can be combined with alternative collaborative processing methods, to suitably fit different tracking scenarios.
Abstract: We investigate the problem of ecient wireless energy recharging in Wireless Rechargeable Sensor Networks (WRSNs). In
such networks special mobile entities (called the Mobile Chargers) traverse the network and wirelessly replenish the energy
of sensor nodes. In contrast to most current approaches, we envision methods that are distributed and use limited network
information. We propose four new protocols for ecient recharging, addressing key issues which we identify, most notably (i)
what are good coordination procedures for the Mobile Chargers and (ii) what are good trajectories for the Mobile Chargers.
Two of our protocols (
DC,DCLK
) perform distributed, limited network knowledge coordination and charging, while two others
(
CC,CCGK
) perform centralized, global network knowledge coordination and charging. As detailed simulations demonstrate,
one of our distributed protocols outperforms a known state of the art method, while its performance gets quite close to the
performance of the powerful centralized global knowledge method.
Abstract: Orthogonal Frequency Division Multiplexing (OFDM)
has recently been proposed as a modulation technique for optical networks, because of its good spectral efficiency, flexibility, and tolerance to impairments. We consider the planning problem of an OFDM optical network, where we are given a traffic matrix that includes the requested transmission rates of the connections to be served. Connections are provisioned for their requested rate by elastically allocating spectrum using a variable number of OFDM subcarriers and choosing an appropriate modulation level, taking into account the transmission distance. We introduce the Routing, Modulation Level and Spectrum Allocation (RMLSA) problem, as opposed to the typical Routing and Wavelength Assignment (RWA) problem of traditional WDM networks, prove that is also NPcomplete and present various algorithms to solve it. We start by presenting an optimal ILP RMLSA algorithm that minimizes the spectrum used to serve the traffic matrix, and also present a decomposition method that breaks RMLSA into its two
substituent subproblems, namely, (i) routing and modulation level, and (ii) spectrum allocation (RML+SA), and solves them sequentially. We also propose a heuristic algorithm that serves connections onebyone and use it to solve the planning problem by sequentially serving all the connections in the traffic matrix. In the sequential algorithm, we investigate two policies for defining the order in which connections are considered. We also use a simulated annealing metaheuristic to obtain even better orderings. We examine the performance of the proposed algorithms through simulation experiments and evaluate the spectrum utilization benefits that can be obtained by utilizing OFDM elastic bandwidth allocation, when compared to a traditional WDM network.
Abstract: We describe the design and implementation of secure and robust protocol and system for a national electronic lottery. Electronic lotteries at a national level are a viable cost effective alternative to mechanical ones when there is a business need to support many types of rdquogames of chancerdquo and to allow increased drawing frequency. Electronic lotteries are, in fact, extremely high risk financial application: If one discovers a way to predict or otherwise claim the winning numbers (even once) the result is huge financial damages. Moreover, the elottery process is complex, which increases the possibility of fraud or costly accidental failures. In addition, a national lottery must adhere to auditability and (regulatory) fairness requirements regarding its drawings. Our mechanism, which we believe is the first one of its kind to be described in the literature, builds upon a number of cryptographic primitives that ensure the unpredictability of the winning numbers, the prevention of their premature leakages and prevention of fraud. We also provide measures for auditability, fairness, and trustworthiness of the process. Besides cryptography, we incorporate security mechanisms that eliminate various risks along the entire process. Our system which was commissioned by a national organization, was implemented in the field and has been operational and active for a while, now.
Abstract: One of the most important applications of wireless sensor
networks is building monitoring and more specically, the
early detection of emergency events and the provision of
guidance for safe evacuation of the building. In this pa
per, we describe a demo application that, in the event of a
re inside a monitored building, uses the information from
the deployed sensor network in order to nd the shortest
safest path away from the emergency and provides naviga
tion guidance to the occupants (modelled by a mobile robot),
in order to safely evacuate the building. For this demo, we
developed our own adhoc robotsensor interconnection us
ing expansion connectors and programming in a lowlevel
language.
Abstract: Modern Wireless Sensor Networks offer an easy, lowcost
and reliable alternative to the backend for monitoring
and controlling large geographical areas like Buildings
and Industries. We present the design and implementation details of an open and efficient Prototype System as a solution for lowcost BMS that comprises of heterogeneous, smallfactor wireless devices. Placing that in the context of Internet of Things we come up with a solution that can cooperate with other systems installed on the same site to lower power consumption and costs as well as benefit humans that use its services in an transparent way. We evaluate and assess key aspects of the performance of our prototype. Our findings indicate specific approaches
to reduce the operation costs and allow the development of open applications.
Abstract: Urban ecosystems are becoming one of the most potentially attractive scenarios for innovating new services and technologies. In parallel, city managers, urban utilities and other stakeholders are fostering the intensive use of advanced technologies aiming at improving present city performance and its sustainability. The deployment of such technology entails the generation of massive amounts of information which in many cases might become useful for other services and applications. Hence, aiming at taking advantage of such massive amounts of information and deployed technology as well as breaking the potential digital barrier that can be raised, some easytouse tools have to be made available to the urban stakeholders. These tools integrated in a platform, operated directly or indirectly by the city, provides a singular opportunity for exploiting the concept of connected city whilst fostering innovation in all city dimensions and making the cocreation concept a reality and eventually impacting on government policies.
Abstract: This work is an attempt to present and describe the design and implementation of a system for the cooperative multiplayer control of gaming and entertainmentrelated software, based on the use of mobile devices with wireless networking capabilities. We are currently using wireless sensor networking devices as the enabling platform, and our prototype application is based on Google Earth�s integrated flight simulator.
Abstract: We study the problem of energybalanced data propagation
in wireless sensor networks. The energy balance property
guarantees that the average per sensor energy dissipation
is the same for all sensors in the network, during
the entire execution of the data propagation protocol. This
property is important since it prolongs the network¢s lifetime
by avoiding early energy depletion of sensors.
We propose a new algorithm that in each step decides
whether to propagate data onehop towards the final destination
(the sink), or to send data directly to the sink. This
randomized choice balances the (cheap) onehop transimssions
with the direct transimissions to the sink, which are
more expensive but “bypass” the sensors lying close to the
sink. Note that, in most protocols, these close to the sink
sensors tend to be overused and die out early.
By a detailed analysis we precisely estimate the probabilities
for each propagation choice in order to guarantee
energy balance. The needed estimation can easily be performed
by current sensors using simple to obtain information.
Under some assumptions, we also derive a closed form
for these probabilities.
The fact (shown by our analysis) that direct (expensive)
transmissions to the sink are needed only rarely, shows that
our protocol, besides energybalanced, is also energy efficient.
Abstract: We study the problem of energybalanced data propagation in wireless sensor networks. The energy balance property guarantees that the average per sensor energy dissipation is the same for all sensors in the network, during the entire execution of the data propagation protocol. This property is important since it prolongs the network¢:s lifetime by avoiding early energy depletion of sensors.
We propose a new algorithm that in each step decides whether to propagate data onehop towards the final destination (the sink), or to send data directly to the sink. This randomized choice balances the (cheap) onehop transimssions with the direct transimissions to the sink, which are more expensive but “bypass” the sensors lying close to the sink. Note that, in most protocols, these close to the sink sensors tend to be overused and die out early.
By a detailed analysis we precisely estimate the probabilities for each propagation choice in order to guarantee energy balance. The needed estimation can easily be performed by current sensors using simple to obtain information. Under some assumptions, we also derive a closed form for these probabilities.
The fact (shown by our analysis) that direct (expensive) transmissions to the sink are needed only rarely, shows that our protocol, besides energybalanced, is also energy efficient.
Abstract: The energy balance property (i.e., all nodes having the same energy throughout the network evolution) contributes significantly (along with energy efficiency) to the maximization of the network lifespan and network connectivity. The problem of achieving energy balanced propagation is well studied in static networks, as it has attracted a lot of research attention.
Recent technological advances have enabled sensor devices to be attached to mobile entities of our every day life (e.g. smartphones, cars, PDAs etc), thus introducing the formation of highly mobile sensor networks.
Inspired by the aforementioned applications, this work is (to the best of our knowledge) the first studying the energy balance property in wireless networks where the nodes are highly and dynamically mobile. In particular, in this paper we propose a new diverse mobility model which is easily parameterized and we also present a new protocol which tries to adaptively exploit the inherent node mobility in order to achieve energy balance in the network in an efficient way.
Abstract: In this work we study energy efficient routing strategies
for wireless adhoc networks. In this kind of networks,
energy is a scarce resource and its conservation
and efficient use is a major issue. Our strategy follows
the multicost routing approach, according to which a
cost vector of various parameters is assigned to each
link. The parameters of interest are the number of hops
on a path, and the residual energy and the transmission
power of the nodes on the path. These parameters
are combined in various optimization functions,
corresponding to different routing algorithms, for selecting
the optimal path. We evaluate the routing algorithms
proposed in a number of scenarios, with respect
to energy consumption, throughput and other performance
parameters of interest. From the experiments
conducted we conclude that routing algorithms that take
into account energy related parameters, increase the
lifetime of the network, while achieving better performance
than other approaches, such as minimum hop
routing.
Abstract: In this work, we propose an energyefficient multicasting algorithm
for wireless networks for the case where the transmission
powers of the nodes are fixed. Our algorithm is
based on the multicost approach and selects an optimal
energyefficient set of nodes for multicasting, taking into account:
i) the node residual energies, ii) the transmission
powers used by the nodes, and iii) the set of nodes covered.
Our algorithm is optimal, in the sense that it can
optimize any desired function of the total power consumed
by the multicasting task and the minimum of the current
residual energies of the nodes, provided that the optimization
function is monotonic in each of these parameters. Our
optimal algorithm has nonpolynomial complexity, thus, we
propose a relaxation producing a nearoptimal solution in
polynomial time. The performance results obtained show
that the proposed algorithms outperform established solutions
for energyaware multicasting, with respect to both
energy consumption and network lifetime. Moreover, it is
shown that the nearoptimal multicost algorithm obtains
most of the performance benefits of the optimal multicost
algorithm at a smaller computational overhead.
Abstract: A crucial issue in wireless networks is to support efficiently communication patterns that are typical in traditional (wired) networks. These include broadcasting, multicasting, and gossiping (alltoall communication). In this work we study such problems in static ad hoc networks. Since, in ad hoc networks, energy is a scarce resource, the important engineering question to be solved is to guarantee a desired communication pattern minimizing the total energy consumption. Motivated by this question, we study a series of wireless network design problems and present new approximation algorithms and inapproximability results.
Abstract: We consider classical lineartime planar separator algorithms, determining for a given planar graph a small subset of the nodes whose removal separates the graph into two components of similar size. These algorithms are based upon Planar Separator Theorems, which guarantee separators of size MediaObjects/InlineFigure1.png and remaining components of size less than 2n/3. In this work, we present a comprehensive experimental study of the algorithms applied to a large variety of graphs, where the main goal is to find separators that do not only satisfy upper bounds but also possess other desirable qualities with respect to separator size and component balance. We propose the usage of fundamental cycles, whose size is at most twice the diameter of the graph, as planar separators: For graphs of small diameter the guaranteed bound is better than the MediaObjects/InlineFigure2.png bounds, and it turns out that this simple strategy almost always outperforms the other algorithms, even for graphs with large diameter.
Abstract: The presentwork considers the following computational problem:
Given any finite game in normal form G and the corresponding
infinitely repeated game G∞, determine in polynomial time (wrt1 the representation
ofG) a profile of strategies for the players inG∞ that is an equilibrium
point wrt the limitofmeans payoff. The problem has been solved
for two players [10], based mainly on the implementability of the threats
for this case. Nevertheless, [4] demonstrated that the traditional notion of
threats is a computationally hard problem for games with at least 3 players
(see also [8]). Our results are the following: (i) We propose an alternative
notion of correlated threats, which is polynomial time computable
(and therefore credible). Our correlated threats are also more severe than
the traditional notion of threats, but not overwhelming for any individual
player. (ii) When for the underlying game G there is a correlated strategy
with payoff vector strictly larger than the correlated threats vector,
we efficiently compute a polynomial–size (wrt the description of G) equilibrium
point for G∞, for any constant number of players. (iii) Otherwise,
we demonstrate the construction of an equilibrium point for an arbitrary
number of players and up to 2 concurrently positive payoff coordinates in
any payoff vector of G. This completely resolves the cases of 3 players, and
provides a direction towards handling the cases of more than 3 players. It
is mentioned that our construction is not a Nash equilibrium point, because
the correlated threats we use are implemented via, not only full synchrony
(as in [10]), but also coordination of the other players¢ actions. But
this seems to be a fair tradeoff between efficiency of the construction and
players¢ coordination, in particular because it only affects the punishments
(which are anticipated never to be used).
Abstract: In this paper, we demonstrate the significant impact of (a) the mobility rate and (b) the user density on the performance of routing protocols in adhoc mobile networks. In particular, we study the effect of these parameters on two different approaches for designing routing protocols: (a) the route creation and maintenance approach and (b) the support approach that forces few hosts to move, acting as helpers for message delivery. We study one representative protocol for each approach, i.e. AODV for the first approach and RUNNERS for the second. We have implemented the two protocols and performed a large scale and detailed simulation study of their performance. The main findings are: the AODV protocol behaves well in networks of high user density and low mobility rate, while its performance drops for sparse networks of highly mobile users. On the other hand, the RUNNERS protocol seems to tolerate well (and in fact benefit from) high mobility rates and low densities.
Abstract: Evolutionary Game Theory is the study of strategic interactions
among large populations of agents who base their decisions on simple,
myopic rules. A major goal of the theory is to determine broad classes
of decision procedures which both provide plausible descriptions of selfish
behaviour and include appealing forms of aggregate behaviour. For example,
properties such as the correlation between strategies¢ growth rates
and payoffs, the connection between stationary states and the wellknown
game theoretic notion of Nash equilibria, as well as global guarantees of
convergence to equilibrium, are widely studied in the literature.
Our paper can be seen as a quick introduction to Evolutionary Game
Theory, together with a new research result and a discussion of many
algorithmic and complexity open problems in the area. In particular, we
discuss some algorithmic and complexity aspects of the theory, which
we prefer to view more as Game Theoretic Aspects of Evolution rather
than as Evolutionary Game Theory, since the term “evolution” actually
refers to strategic adaptation of individuals¢ behaviour through a
dynamic process and not the traditional evolution of populations. We
consider this dynamic process as a selforganization procedure which,
under certain conditions, leads to some kind of stability and assures robustness
against invasion. In particular, we concentrate on the notion of
the Evolutionary Stable Strategies (ESS). We demonstrate their qualitative
difference from Nash Equilibria by showing that symmetric 2person
games with random payoffs have on average exponentially less ESS than
Nash Equilibria. We conclude this article with some interesting areas of
future research concerning the synergy of Evolutionary Game Theory
and Algorithms.
Abstract: Evolutionary Game Theory is the study of strategic interactions among large populations of agents who base their decisions on simple, myopic rules. A major goal of the theory is to determine broad classes of decision procedures which both provide plausible descriptions of selfish behaviour and include appealing forms of aggregate behaviour. For example, properties such as the correlation between strategies' growth rates and payoffs, the connection between stationary states and Nash equilibria and global guarantees of convergence to equilibrium, are widely studied in the literature. In this paper we discuss some computational aspects of the theory, which we prefer to view more as Game Theoretic Aspects of Evolution than Evolutionary Game Theory, since the term "evolution" actually refers to strategic adaptation of individuals ' behaviour through a dynamic process and not the traditional evolution of populations. We consider this dynamic process as a selforganization procedure, which under certain conditions leads to some kind of stability and assures robustness against invasion.
Abstract: Raising awareness among young people, and especially students, on the relevance of behavior change for achieving energy savings is increasingly being considered as a key enabler towards longterm and costeffective energy efficiency policies. However, the way to successfully apply educational interventions focused on such targets inside schools is still an open question. In this paper, we present our approach for enabling IoTbased energy savings and sustainability awareness lectures and promoting datadriven energysaving behaviors focused on a high school audience. We present our experiences toward the successful application of sets of educational tools and software over a realworld Internet of Things (IoT) deployment. We discuss the use of gamification and competition as a very effective enduser engagement mechanism for school audiences. We also present the design of an IoTbased handson lab activity, integrated within a high school computer science curriculum utilizing IoT devices and data produced inside the school building, along with the NodeRED platform. We describe the tools used, the organization of the educational activities and related goals. We report on the experience carried out in both directions in a high school in Italy and conclude by discussing the results in terms of achieved energy savings within an observation period.
Abstract: Core optical networks using reconfigurable optical
switches and tunable lasers appear to be on the road towards
widespread deployment and could evolve to alloptical mesh
networks in the coming future. Considering the impact of physical
layer impairments in the planning and operation of alloptical
(and translucent) networks is the main focus of the DICONET
project. The impairment aware network planning and operation
tool (NPOT) is the main outcome of DICONET project, which
is explained in detail in this paper. The key building blocks of
the NPOT, consisting of network description repositories, the
physical layer performance evaluator, the impairment aware
routing and wavelength assignment engines, the component
placement modules, failure handling and the integration of
NPOT in the control plane are the main contributions of this
work. Besides, the experimental result of DICONET proposal for
centralized and distributed control plane integration schemes and
the performance of the failure handling in terms of restoration
time is presented in this work.
Abstract: Online and Realtime counting and estimating the cardinality of sets is highly desirable for a large variety of applications, representing a foundational block for the efficient deployment and access of emerging internet scale information systems. In this work we implement three well known duplicate
insensitive counting algorithms and evaluate their performance in a testbed of resourcelimited commercial offtheshelf hardware devices. We focus on devices that can be used in wireless mobile and sensor applications and evaluate the memory complexity, time complexity and absolute error of the algorithms under different realistic scenaria. Our findings indicate the suitability of each algorithm depending on the application characteristics.
Abstract: We present key aspects (hardware, software, topology, networking) of SenseWall, an experimental sensor network testbed we have created for the implementation and engineering of distributed sensor network algorithms. We then describe how SenseWall has been in particular used to implement two recent state of the art algorithms for energy balanced sensor data propagation. We elaborate on the issues and challenges created by the restrictions and particularities of the experimental testbed and how we dealt with them. We also carry out a detailed performance evaluation comparing the energy balance protocols to two baseline protocols that include only either single hop or direct data transmissions.
Abstract: This paper presents experimental measurements of bulk data
transfer in a wireless multihop sensor network environment. We
investigate the effect of the number of the hops and the
conditions of the surrounding environment on the performance of
the network in terms of achieved transfer rates. Our findings
validate the theoretically established results on the relation
between the throughput and the network diameter, i.e.~is inversely
proportional to the network diameter and in particular the number
of hops needed for data to reach its destination. Furthermore, we
indicate how throughput is (significantly) affected by the type of
the physical environment, i.e.~it drops as the harshness of the
ambient conditions increases.
Abstract: We investigate the impact of multiple, mobile sinks on
efficient data collection in wireless sensor networks. To
improve performance, our protocol design focuses on minimizing
overlaps of sink trajectories and balancing the service load
among the sinks. To cope with high network dynamics, placement
irregularities and limited network knowledge we propose three different
protocols: a) a centralized one, that explicitly equalizes spatial coverage;
this protocol assumes strong modeling assumptions, and also serves as a kind
of performance lower bound in uniform networks of low dynamics b)
a distributed protocol based on mutual avoidance of sinks c) a clustering
protocol that distributively groups service areas towards balancing the load per sink.
Our simulation findings demonstrate significant gains in latency, while keeping the success
rate and the energy dissipation at very satisfactory levels even under
high network dynamics and deployment heterogeneity.
Abstract: ManyWSN algorithms and applications are based on knowledge
regarding the position of nodes inside the network area.
However, the solution of using GPS based modules in order
to perform localization in WSNs is a rather expensive solution
and in the case of indoor applications, such as smart
buildings, is also not applicable. Therefore, several techniques
have been studied in order to perform relative localization
in WSNs; that is, to compute the position of
a node inside the network area relatively to the position
of other nodes. Many such techniques are based on indicators
like the Radio Signal Strength Indicator (RSSI)
and the Link Quality Indicator (LQI). These techniques are
based on the assumption that there is strong correlation between
the Euclidian distance of the communicating motes
and these indicators. Therefore, high values of RSSI and
LQI should indicate physical proximity of two communicating
nodes. However, these indicators do not depend solely on
distance. Physical obstacles, ambient electromagnetic noise
and interferences from other wireless transmissions also affect
the quality of wireless communication in a stochastic
way. In this paper we propose, implement, experimentally
fine tune and evaluate a localization algorithm that exploits
the stochastic nature of interferences during wireless communications
in order to perform localization in WSNs. Our
algorithm is particularly designed for indoor localisation of
moving people in smart buildings. The localisation achieved
is finegrained, i.e. the position of the target mote is successfully
computed with approximately one meter accuracy.
This finegrained localisation can be used by smart Building
Management Systems in many applications such as room
adaptation to presence. In our scenario, our proposed algorithm is used by a smart room in order to localise the
position of people inside the room and adapt room illumination
accordingly.
Abstract: The Internet and the Web have arguably surpassed the von Neumann Computer as the most complex computational artifacts of our times. Out of all the formidable characteristics of the Internet/Web, it seems that the most novel is its socioeconomic complexity. The Internet and the Web are built, operated and used by a multitude of very diverse economic interests, which compete or collaborate in various degrees. In fact, this suggests that some very important insights about the Web may come from a fusion of ideas from Algorithms with concepts and techniques from Mathematical Economics and Game Theory. Since 2000, a new field has emerged (Algorithmic Game Theory and Computational Internet Economics), which examines exactly such ideas. We feel that this field belongs to Web Science. Some of the main topics of that field examined so far are Internet equilibria, the so called “Price of Anarchy” (a measure of loss of optimality due to selfishness [Koutsoupias,Papadimitriou (1999)], electronic markets and their equilibria, auctions, and algorithmic mechanisms design (inverse game theory or how to design a game in the net in such a clever way that individual players, motivated solely by their selfish interests, actually end up meeting the goals of the game designer!). Our paper here surveys the main concepts and results of this very promising subfield.
Abstract: We consider the following distributed optimization problem: three agents i =
1; 2; 3 are each presented with a load drawn independently from the same known prior distribution.
Then each agent decides on which of two available bins to put her load. Each bin has
capacity �, and the objective is to find a distributed protocol that minimizes the probability
that an overflow occurs (or, equivalently, maximizes the winning probability).
In this work, we focus on the cases of full information and local information, depending on
whether each agent knows the loads of both other agents or not. Furthermore, we distinguish
between the cases where the agents are allowed to follow different decision rules (eponymous
model) or not (anonymous model ). We assume no communication among agents.
First, we present optimal protocols for the full information case, for both the anonymous and
the eponymous model.
For the local information, anonymous case, we show that the winning probability is upper
bounded by 0.622 in the case where the input loads are drawn from the uniform distribution.
Motivated by [3], we present a general method for computing the optimal singlethreshold protocol
for any continuous distribution, and we apply this method to the case of the exponential
distribution.
Finally, we show how to compute, in exponential time, an optimal protocol for the local
information, eponymous model for the case where the input loads are drawn from a discretevalued,
bounded distribution.
Abstract: In this paper we present the design of a simulator platform called FUSE (Fast Universal Simulator Engine). The term Universal means that the Engine can be adapted easily to different domains and be used for varying simulation needs, although our main target is simulation of distributed algorithms in distributed computing environments. The Engine is Fast in the sense that the simulation overhead is minimal and very large systems can be simulated. We discuss the architecture and the design decisions that form the basis of these features. We also describe the functionality that is provided to its users (e.g., monitoring, statistics, etc.).
Abstract: We discuss key findings and technological challenges related to SmartSantander, an EU project that is developing a cityscale experimental facility for Internet of Things and Future Internet experimentation. The main goal of the project is to design and construct a city scale lab for experimentation and provide an integrated framework for implementing Smart City services.
Abstract: Distributed algorithm designers often assume that system processes execute the same predefined software. Alternatively, when they do not assume that, designers turn to noncooperative games and seek an outcome that corresponds to a rough consensus when no coordination is allowed. We argue that both assumptions are inapplicable in many real distributed systems, e.g., the Internet, and propose designing selfstabilizing and Byzantine faulttolerant distributed game authorities. Once established, the game authority can secure the execution of any complete information game. As a result, we reduce costs that are due to the processes¢ freedom of choice. Namely, we reduce the price of malice.
Abstract: In this work we experimentally study the min order Radiocoloring problem (RCP) on Chordal, Split and Permutation graphs, which are three basic families of perfect graphs. This problem asks to find an assignment using the minimum number of colors to the vertices of a given graph G, so that each pair of vertices which are at distance at most two apart in G have different colors. RCP is an NPComplete problem on chordal and split graphs [4]. For each of the three families, there are upper bounds or/and approximation algorithms known for minimum number of colors needed to radiocolor such a graph [4,10].
We design and implement radiocoloring heuristics for graphs of above families, which are based on the greedy heuristic. Also, for each one of the above families, we investigate whether there exists graph instances requiring a number of colors in order to be radiocolored, close to the best known upper bound for the family. Towards this goal, we present a number generators that produce graphs of the above families that require either (i) a large number of colors (compared to the best upper bound), in order to be radiocolored, called ldquoextremalrdquo graphs or (ii) a small number of colors, called ldquononextremalrdquoinstances. The experimental evaluation showed that random generated graph instances are in the most of the cases ldquononextremalrdquo graphs. Also, that greedy like heuristics performs very well in the most of the cases, especially for ldquononextremalrdquo graphs.
Abstract: Geographic routing is becoming the protocol of choice for many sensor network applications. Some very efficient geographic routing algorithms exist, however they require a preliminary planarization of the communication graph. Planarization induces overhead which makes this approach not optimal when lightweight protocols are required. On the other hand, georouting algorithms which do not rely on planarization have fairly low success rates and either fail to route messages around all but the simplest obstacles or have a high topology control overhead (e.g. contour detection algorithms). In this entry we describe the GRIC algorithm which was designed to overcome some of those limitations. The GRIC algorithm was proposed in [PN07a]. It is the first lightweight and efficient on demand (i.e. alltoall) geographic routing algorithm which does not require planarization, has almost 100% delivery rates (when no obstacles are added), and behaves well in the presence of large communication blocking obstacles.
Abstract: The technological as well as software advances in
microelectronics and embedded component design have led to the
development of low cost, smallsized devices capable of forming
wireless, adhoc networks and sensing a number of qualities of
their environment, while performing computations that depend
on the sensed qualities as well as information received by their
peers. These sensor networks rely on the collective power of
the separate devices as well as their computational and sensing
capabilities to understand "global" environmental states through
locally sampled information and local sensor interactions. Due
to the locality of the sensor networks, that naturally arises due
to the locality of their communications capabilities, a number
of interesting connections exist between these networks and
geometrical concepts and problems. In this paper we study two
simple problems that pertain to the formation of low power
and low interference communication patterns in fixed topology
sensor networks. We study the problem of using multihop
communication links instead of direct ones as well as the problem
of forming a communication ring of sensor networks so as to
reduce power consumption as well as interference from other
nodes. Our focus is on the connection between sensor networks
and geometrical concepts, rather than on practicality, so as to
highlight their interrelationship.
Abstract: The Internet of Things is shaping up to be the ideal vehicle for introducing pervasive computing in our everyday lives, especially in the form of smart home and building management systems. However, although such technologies are gradually becoming more mainstream, there is still a lot of ground to be covered with respect to public buildings and specifically ones in the educational sector. We discuss here \Green Mindset", an action focusing on energy efficiency and
sustainability in Greek public schools. A largescale sensor infrastructure has been deployed to 12 public school buildings across diverse settings. We report on the overall design and implementation of the system, as well as on some first results coming from the data produced. Our system provides a flexible and efficient basis for realizing a unified approach to monitoring energy consumption and environmental parameters,
that can be used both for building administration
and educational purposes.
Abstract: The simplex method has been successfully used in solving linear programming problems for many years. Parallel approaches have also extensively been studied due to the intensive computations required, especially for the solution of large linear problems (LPs). In this paper we present a highly scalable parallel implementation framework of the standard full tableau simplex method on a highly parallel (distributed memory) environment. Speciﬁcally, we have designed and implemented a suitable column distribution scheme as well as a row distribution scheme and we have entirely tested our implementations over a considerably powerful distributed platform (linux cluster with myrinet interface). We then compare our approaches (a) among each other for variable number of problem size (number of rows and columns) and (b) to other recent and valuable corresponding eﬀorts in the literature. In most cases, the column distribution scheme performs quite/much better than the row distribution scheme. Moreover, both schemes (even the row distribution scheme over largescale problems) lead to particularly high speedup and eﬃciency values, which are considerably better in all cases than the ones achieved in other similar research eﬀorts and implementations. Moreover, we further evaluate our basic parallelization scheme over very large LPs in order to validate more reliably the high eﬃciency and scalability achieved.
Abstract: One of the major problems algorithm designers usually face is to know in advance whether a proposed optimization algorithm is going to behave as planned, and if not, what changes are to be made to the way new solutions are examined so that the algorithm performs nicely. In this work we develop a methodology for differentiating good neighborhoods from bad ones. As a case study we consider the structure of the space of assignments for random 3SAT formulas and we compare two neighborhoods, a simple and a more refined one that we already know the corresponding algorithm behaves extremely well. We give evidence that it is possible to tell in advance what neighborhood structure will give rise to a good search algorithm and we show how our methodology could have been used to discover some recent results on the structure of the SAT space of solutions. We use as a tool Go with the winners, an optimization heuristic that uses many particles that independently search the space of all possible solutions. By gathering statistics, we compare the combinatorial characteristics of the different neighborhoods and we show that there are certain features that make a neighborhood better than another, thus giving rise to good search algorithms.
Abstract: We consider the offline version of the routing and
wavelength assignment (RWA) problem in transparent alloptical networks. In such networks and in the absence of regenerators, the signal quality of transmission degrades due to physical layer
impairments. We initially present an algorithm for solving the static RWA problem based on an LP relaxation formulation that tends to yield integer solutions. To account for signal degradation due to physical impairments, we model the effects of the path length, the path hop count, and the interference among ligthpaths by imposing additional (soft) constraints on RWA. The objective of the resulting optimization problem is not only to serve the
connection requests using the available wavelengths, but also to minimize the total accumulated signal degradation on the selected lightpaths. Our simulation studies indicate that the proposed RWA algorithms select the lightpaths for the requested connections so as to avoid impairment generating sources, thus dramatically reducing the overall physicallayer blocking when compared to RWA algorithms that do not account for impairments.
Abstract: Dynamic graph algorithms have been extensively studied in the last two
decades due to their wide applicabilityin manycon texts. Recently, several
implementations and experimental studies have been conducted investigating
the practical merits of fundamental techniques and algorithms. In most
cases, these algorithms required sophisticated engineering and finetuning
to be turned into efficient implementations. In this paper, we surveysev 
eral implementations along with their experimental studies for dynamic
problems on undirected and directed graphs. The former case includes
dynamic connectivity, dynamic minimum spanning trees, and the sparsification
technique. The latter case includes dynamic transitive closure and
dynamic shortest paths. We also discuss the design and implementation of
a software libraryfor dynamic graph algorithms.
Abstract: In this work we study the implementation of multicost rout
ing in a distributed way in wireless mobile ad hoc networks.
In contrast to traditional singlecost routing, where each
path is characterized by a scalar, in multicost routing a
vector of cost parameters is assigned to each network link,
from which the cost vectors of candidate paths are calcu
lated. These parameters are combined in various optimiza
tion functions, corresponding to diﬀerent routing algorithms,
for selecting the optimal path. Up until now the performance
of multicost and multiconstrained routing in wireless ad hoc
networks has been evaluated either at a theoretical level or
by assuming that nodes are static and have full knowledge
of the network topology and nodes� state. In the present
paper we assess the performance of multicost routing based
on energyrelated parameters in mobile ad hoc networks by
embedding its logic in the Dynamic Source Routing (DSR)
algorithm, which is a wellknown fully distributed routing
algorithm. We use simulations to compare the performance
of the multicostDSR algorithm to that of the original DSR
algorithm and examine their behavior under various node
mobility scenarios. The results conﬁrm that the multicost
DSR algorithm improves the performance of the network in
comparison to the original DSR algorithm in terms of energy eﬃciency. The multicostDSR algorithm enhances the
performance of the network not only by reducing energy
consumption overall in the network, but also by spreading
energy consumption more uniformly across the network, pro
longing the network lifetime and reducing the packet drop
probability. Furthermore the delay suﬀered by the packets
reaching their destination for the case of the multicostDSR
algorithm is shown to be lower than in the case of the orig
inal DSR algorithm.
Abstract: In this work we discuss Fun in Numbers, a software platform for implementing multiplayer games and interactive installations, that are based on the use of ad hoc mobile sensing devices. We utilize a detailed log of a threeday long public showcase as a basis to discuss the implementation issues related to a set of games and installations, which are examples of this unique category of applications, utilizing a blend of technologies. We discuss their fundamental concepts and features, also arguing that they have many aspects and potential uses. The architecture of the platform and implementation details are highlighted in this work, along with detailed descriptions of the protocols used. Our experiments shed light on a number of key issues, such as network scaling and realtime performance, and we provide experiments regarding crosslayer software issues. We additionally provide data showing that such games and installations can be efficiently supported by our platform, with as many as 50 concurrent players in the same physical space. These results are backed up by a user evaluation study from a large sample of 136 visitors, which shows that such applications can be seriously fun.
Abstract: We describe algorithms for finding shortest paths and distances in outerplanar and planar digraphs
that exploit the particular topology of the input graph. An important feature of our algorithms is that they can
work in a dynamic environment, where the cost of any edge can be changed or the edge can be deleted. In the
case of outerplanar digraphs, our data structures can be updated after any such change in only logarithmic time.
A distance query is also answered in logarithmic time. In the case of planar digraphs, we give an interesting
tradeoff between preprocessing, query, and update times depending on the value of a certain topological
parameter of the graph. Our results can be extended to nvertex digraphs of genus O.n1¡"/ for any " > 0.
Abstract: We present improved methods for computing a set of alternative sourcetodestination routes
in road networks in the form of an alternative graph. The resulting alternative graphs are
characterized by minimum path overlap, small stretch factor, as well as low size and complexity.
Our approach improves upon a previous one by introducing a new pruning stage preceding any
other heuristic method and by introducing a new filtering and finetuning of two existing methods.
Our accompanying experimental study shows that the entire alternative graph can be computed
pretty fast even in continental size networks.
Abstract: The problem of robust line planning requests for a set of
origindestination paths (lines) along with their frequencies in an underlying
railway network infrastructure, which are robust to
uctuations of
realtime parameters of the solution.
In this work, we investigate a variant of robust line planning stemming
from recent regulations in the railway sector that introduce competition
and free railway markets, and set up a new application scenario: there is
a (potentially large) number of line operators that have their lines xed
and operate as competing entities struggling to exploit the underlying
network infrastructure via frequency requests, while the management of
the infrastructure itself remains the responsibility of a single (typically
governmental) entity, the network operator.
The line operators are typically unwilling to reveal their true incentives.
Nevertheless, the network operator would like to ensure a fair (or, socially
optimal) usage of the infrastructure, e.g., by maximizing the (unknown to
him) aggregate incentives of the line operators. We show that this can be
accomplished in certain situations via a (possibly anonymous) incentivecompatible
pricing scheme for the usage of the shared resources, that is
robust against the unknown incentives and the changes in the demands
of the entities. This brings up a new notion of robustness, which we
call incentivecompatible robustness, that considers as robustness of the
system its tolerance to the entities' unknown incentives and elasticity
of demands, aiming at an eventual stabilization to an equilibrium point
that is as close as possible to the social optimum.
Abstract: We present a unique locationbased application of sensor networks. The ``Magnetize Words'' installation produces word clouds following visitors around, interacting with each other as the visitors meet in physical space. The installation can be based on either of two localizationproviding platforms. The first achieves high localization accuracy while requiring visitors to carry identifying tags. The other provides low accuracy, but is fully nonintrusive.
Abstract: We propose efficient schemes for informationtheoretically secure
key exchange in the Bounded Storage Model (BSM), where the adversary
is assumed to have limited storage. Our schemes generate a
secret One Time Pad (OTP) shared by the sender and the receiver,
from a large number of public random bits produced by the sender
or by an external source. Our schemes initially generate a small
number of shared secret bits, using known techniques. We introduce
a new method to expand a small number of shared bits to a
much longer, shared key.
Our schemes are tailored to the requirements of sensor nodes
and wireless networks. They are simple, efficient to implement and
take advantage of the fact that practical wireless protocols transmit
data in frames, unlike previous protocols, which assume access to
specific bits in a stream of data. Indeed, our main contribution is
twofold.
On the one hand, we construct schemes that are attractive in
terms of simplicity, computational complexity, number of bits read
from the shared random source and expansion factor of the initial
key to the final shared key.
On the other hand, we show how to transformany existing scheme
for key exchange in BSM into a more efficient scheme in the number
of bits it reads from the shared source, given that the source is
transmitted in frames.
Abstract: In this work, we study the impact of the dynamic changing of the network link capacities on the stability properties of packetswitched networks. Especially, we consider the Adversarial, QuasiStatic Queuing Theory model, where each link capacity may take on only two possible (integer) values, namely 1 and C>1 under a (w,\~{n})adversary. We obtain the following results:
• Allowing such dynamic changes to the link capacities of a network with just ten nodes that uses the LIS (LongestinSystem) protocol for contention–resolution results in instability at rates View the MathML source and for large enough values of C.
• The combination of dynamically changing link capacities with compositions of contention–resolution protocols on network queues suffices for similar instability bounds: The composition of LIS with any of SIS (ShortestinSystem), NTS (NearesttoSource), and FTG (FurthesttoGo) protocols is unstable at rates View the MathML source for large enough values of C.
• The instability bound of the network subgraphs that are forbidden for stability is affected by the dynamic changes to the link capacities: we present improved instability bounds for all the directed subgraphs that were known to be forbidden for stability on networks running a certain greedy protocol.
Abstract: A number of infrastructures are being deployed for Future Internet experimentation purposes, providing access to largescale IoT resources to researchers and industry. SmartSantander is among the largest ones, deployed at the city center of Santander. We discuss SmartSantander¢s augmentation using smartphones provided by volunteers, in order to increase sensing resources and ubiquity. Our system allows developers to write code for Android and automatically deploy their experiments to Android devices, alongside the SmartSantander platform. Initial results produced by experiments with a small number of volunteers show that the system provides meaningful extensions to the existing platform.
Abstract: Wireless networks are nowadays widely used and constantly evolving; we are constantly surrounded by numerous such networks. A modern idea is to exploit this variety and aggregate all available connections together towards improving Internet connectivity. In this paper we present our design of a networking system that enables participating entities to connect with each other and share their possible broadband connections. Every connected entity will gain broadband connection access from the ones that have and share one, independently from how many connections are available and how fast they are. Our goal is to improve the broadband connections' utilization and aim towards fault tolerance on these connections. To this end, the system is built on top of a peertopeer network, where the participating entities share their connections according to various scheduling algorithms.
Abstract: With this work we aim to make a threefold contribution.
We first address the issue of supporting efficiently queries
over stringattributes involving prefix, suffix, containment,
and equality operators in largescale data networks. Our
first design decision is to employ distributed hash tables
(DHTs) for the data network?s topology, harnessing their
desirable properties. Our next design decision is to derive
DHTindependent solutions, treating DHT as a black box.
Second, we exploit this infrastructure to develop efficient
content based publish/subscribe systems. The main con
tribution here are algorithms for the efficient processing of
queries (subscriptions) and events (publications). Specifi
cally, we show that our subscription processing algorithms
require O(logN) messages for a Nnode network, and our
event processing algorithms require O(l ? logN) messages
(with l being the average string length).
Third, we develop algorithms for optimizing the proces
sing of multidimensional events, involving several string at
tributes. Further to our analysis, we provide simulation
based experiments showing promising performance results
in terms of number of messages, required bandwidth, load
balancing, and response times.
Abstract: This special issue of Computer Science Review features seven papers on foundations of adaptive networked societies of tiny artefacts. The introduction describes the motivation for the special issue and briefly
Abstract: This work provides a jitter analysis of sizebased
burst assembly algorithms and also discusses other burst assembly
algorithms that use the packet delay as the assembly threshold
to provide a bound on jitter.
Abstract: In this work we study the combination of multicost
routing and adjustable transmission power in wireless
ad hoc networks, so as to obtain dynamic energy and
interferenceefficient routes to optimize network performance.
In multicost routing, a vector of cost parameters is
assigned to each network link, from which the cost vectors
of candidate paths are calculated. Only at the end these
parameters are combined in various optimization functions,
corresponding to different routing algorithms, for selecting
the optimal path. The multicost routing problem is a
generalization of the multiconstrained problem, where no
constraints exist, and is also significantly more powerful
than singlecost routing. Since energy is an important
limitation of wireless communications, the cost parameters
considered are the number of hops, the interference caused,
the residual energy and the transmission power of the
nodes on the path; other parameters could also be included,
as desired. We assume that nodes can use power control to
adjust their transmission power to the desired level. The
experiments conducted show that the combination of multicost
routing and adjustable transmission power can lead to
reduced interference and energy consumption, improving
network performance and lifetime.
Abstract: In translucent (or managed reach) WDM optical
networks, regenerators are employed at specific nodes. Some of
the connections in such networks are routed transparently, while
others have to go through a sequence of 3R regenerators that serve
as “refueling stations” to restore their quality of transmission
(QoT). We extend an online multicost algorithm for transparent
networks presented in our previous study [1], to obtain an IARWA
algorithm that works in translucent networks and makes use,
when required, of the regenerators present at certain locations
of the network. To characterize a path, the algorithm uses a
multicost formulation with several cost parameters, including the
set of available wavelengths, the length of the path, the number of
regenerators used, and noise variance parameters that account for
the physical layer impairments. Given a new connection request
and the current utilization state of the network, the algorithm calculates
a set of non dominated candidate paths, meaning that any
path in this set is not inferior with respect to all cost parameters
than any other path. This set consists of all the costeffective (in
terms of the domination relation) and feasible (in terms of QoT)
lightpaths for the given sourcedestination pair, including all the
possible combinations for the utilization of available regenerators
of the network. An optimization function or policy is then applied
to this set in order to select the optimal lightpath. Different optimization
policies correspond to different IARWA algorithms.
We propose and evaluate several optimization policies, such as the
most used wavelength, the best quality of transmission, the least
regeneration usage, or a combination of these rules. Our results
indicate that in a translucent network the employed IARWA
algorithm has to consider all problem parameters, namely, the
QoT of the lightpaths, the utilization of wavelengths and the
availability of regenerators, to efficiently serve the online traffic.
Abstract: Wireless sensor networks can be very useful in applications that require the detection of crucial events, in physical environments subjected to critical conditions, and the propagation of data reporting their realization to a control center. In this paper we propose jWebDust, a generic and modular application environment for developing and managing applications that are based on wireless sensor networks. Our software architecture provides a range of services that allow to create customized applications with minimum implementation effort that are easy to administrate. We move beyond the ?networkingcentric? view of sensor network research and focus on how the end user (administrator, control center supervisor, etc.) will visualize and interact with the system.
We here present its open architecture, the most important design decisions, and discuss its distinct features and functionalities. jWebDust allows heterogeneous components to interoperate (real world sensor networks will rarely be homogeneous) and allows the integrated management and control of multiple such networks by also defining webbased mechanisms to visualize the network state, the results of queries, and a means to inject queries in the network. The architecture also illustrates how existing protocols for various services can interoperate in a bigger framework  such as the tree construction, query routing, etc.
Abstract: In this work we discuss Kafebook, a system combining popular social networking platforms
with activity input from the physical domain inside public spaces. The system is envisioned as
a means for users to augment and communicate their activities to other people in their physical
proximity through a public display, while catering for anonymity issues. We developed a
client for Android smartphones that is used as the user interface and the enabling platform,
with which users connect to the system infrastructure and interact with it. Apart from
providing access to input from social networking sites, the Android client allows for chat
between the users and music selection polls. Wireless networking is based on Bluetooth, a
widespread technology in smartphones, which enables a more pervasive mode of operation,
while utilizing it also as a proximity sensor. We deployed our system in a twoday public
event as part of an undergraduate theses showcase, receiving positive feedback from visitors.
Abstract: In this book chapter we will consider key establishment protocols for wireless sensor networks.
Several protocols have been proposed in the literature for the establishment of a shared group key for wired networks.
The choice of a protocol depends whether the key is established by one of the participants (and then transported to the other(s)) or agreed among the participants, and on the underlying cryptographic mechanisms (symmetric or asymmetric). Clearly, the design of key establishment protocols for sensor networks must deal with different problems and challenges that do not exist in wired networks. To name a few, wireless links are particularly vulnerable to eavesdropping, and that sensor devices can be captured (and the secrets they contain can be compromised); in many upcoming wireless sensor networks, nodes cannot rely on the presence of an online trusted server (whereas most standardized authentication and key establishment protocols do rely on such a server).
In particular, we will consider five distributed group key establishment protocols. Each of these protocols applies a different algorithmic technique that makes it more suitable for (i) static sensor networks, (ii) sensor networks where nodes enter sleep mode (i.e. dynamic, with low rate of updates on the connectivity graph) and (iii) fully dynamic networks where nodes may even be mobile. On the other hand, the common factor for all five protocols is that they can be applied in dynamic groups (where members can be excluded or added) and provide forward and backward secrecy. All these protocols are based on the DiffieHellman key exchange algorithm and constitute natural extensions of it in the multiparty case.
Abstract: This paper addresses the efficient processing of
topk queries in widearea distributed data
repositories where the index lists for the attribute
values (or text terms) of a query are distributed
across a number of data peers and the
computational costs include network latency,
bandwidth consumption, and local peer work.
We present KLEE, a novel algorithmic
framework for distributed topk queries,
designed for high performance and flexibility.
KLEE makes a strong case for approximate topk
algorithms over widely distributed data sources.
It shows how great gains in efficiency can be
enjoyed at low resultquality penalties. Further,
KLEE affords the queryinitiating peer the
flexibility to tradeoff result quality and expected
performance and to tradeoff the number of
communication phases engaged during query
execution versus network bandwidth
performance. We have implemented KLEE and
related algorithms and conducted a
comprehensive performance evaluation. Our
evaluation employed realworld and synthetic
large, webdata collections, and query
benchmarks. Our experimental results show that
KLEE can achieve major performance gains in
terms of network bandwidth, query response
times, and much lighter peer loads, all with small
errors in result precision and other resultquality
measures
Abstract: Abstract— Numerous smart city testbeds and system deployments have surfaced around the world, aiming to provide services over unified large heterogeneous IoT infrastructures. Although we have achieved new scales in smart city installations and systems, so far the focus has been to provide diverse sources of data to smart city services consumers, while neglecting to provide ways to simplify making good use of them. We believe that knowledge creation in smart cities through data annotation, supported in both an automated and a crowdsourced manner, is an aspect that will bring additional value to smart cities. We present here our approach, aiming to utilize an existing smart city deployment and the OrganiCity software ecosystem. We discuss key challenges along with characteristic use cases, and report on our design and implementation, along with preliminary results.
Abstract: We investigate the existence and efficient algorithmic
construction of close to optimal independent sets in random models
of intersection graphs. In particular, (a) we propose \emph{a new model} for random intersection graphs
($G_{n, m, \vec{p}}$) which includes the model of
\cite{RIG} (the ``uniform" random intersection graphs model) as an
important special case. We also define an interesting variation of
the model of random intersection graphs, similar in spirit to
random regular graphs. (b) For this model we derive \emph{exact formulae} for the mean
and variance of the number of independent sets of size $k$ (for
any $k$) in the graph. (c) We then propose and analyse \emph{three algorithms} for the
efficient construction of large independent sets in this model.
The first two are variations of the greedy technique while the
third is a totally new algorithm. Our algorithms are analysed for
the special case of uniform random intersection graphs.
Our analyses show that these algorithms succeed in finding
\emph{close to optimal} independent sets for an interesting range
of graph parameters.
Abstract: A new social form of play taking place in public spaces, such as city parks, squares and streets that can scale up to involve large number of participants and engage players located simultaneously in dispersed areas is discussed here. This paper presents a platform that allows for the creation of interactive installations in public spaces. The FinN platform provides functionality for detecting and localizing people, by requiring players to carry identifying devices that offer many possibilities for gesture interaction (i.e., gestures such as shaking, facilitated by the sensorbouquet of the devices). One of the deployed example installations, where visitors can interact with words formations, is presented here as a case study.
Abstract: A number of Future Internet testbeds are being deployed around the world for research experimentation and development. SmartSantander
is an infrastructure of massive scale deployed inside a city centre. We argue that utilising the concept of participatory sensing can augment the functionality and potential usecases of such a system and be beneficiary in a number of scenarios. We discuss
the concept of extending SmartSantander with participatory sensing through the use of volunteers¢ smartphones. We report on our design and implementation, which allows for developers to write
their code for Android devices and then deploy and execute on the devices automatically through our system. We have tested our implementation in a number of scenarios in two cities with the help
of volunteers with promising results; the data collected enhance the ones by fixed infrastructure both quantitatively and qualitatively across the cities, while also engaging citizens more directly.
Abstract: Although we have reached new levels in smart city installations and systems, efforts so far have focused on providing diverse sources of data to smart city services consumers while neglecting to provide ways to simplify making good use of them. In this context, one first step that will bring added value to smart cities is knowledge creation in smart cities through anomaly detection and data annotation, supported in both an automated and a crowdsourced manner. We present here LearningCity, our solution that has been validated over an existing smart city deployment in Santander, and the OrganiCity experimentationasaservice ecosystem. We discuss key challenges along with characteristic use cases, and report on our design and implementation, together with some preliminary results derived from combining large smart city datasets with machine learning.
Abstract: Typical sensor nodes are resource constrained devices containing user level applications, operating system components, and device drivers in a single address space, with no form of memory protection. A malicious user could easily capture a node and tamper the applications running, in order to perform different types of attacks. In this paper, we propose a remote live forensics protection architecture that prevents the execution of tampered software while alarming the owners of the sensors network. Using sandboxing to restrict application memory accesses within the address space and forensic techniques to validate the authenticity of the running applications we prevent malicious code from being executed while specifying the intrusion.
Abstract: We consider the problem of searching for a piece of
information in a fully interconnected computer network
(also called a complete network or clique) by exploiting
advice about its location from the network nodes. Each
node contains a database that ?knows? what kind of
documents or information are stored in other nodes
(e.g., a node could be a Web server that answers queries
about documents stored on the Web). The databases in
each node, when queried, provide a pointer that leads to
the node that contains the information. However, this
information is uptodate (or correct) with some
bounded probability. While, in principle, one may always
locate the information by simply visiting the network
nodes in some prescribed ordering, this requires a time
complexity in the order of the number of nodes of the
network. In this paper, we provide algorithms for locating
an information node in the complete communication
network, which take advantage of advice given from
network nodes. The nodes may either give correct advice,
by pointing directly to the information node, or give
wrong advice, by pointing elsewhere. On the lowerbounds?
side, we show that no fixedmemory (i.e., with
memory independent of the network size) deterministic
algorithm may locate the information node in a constant
(independent of the network size) expected number of
steps. Moreover, if p (1/n) is the probability that a
node of an nnode clique gives correct advice, we show
that no algorithm may locate the information node in an
expected number of steps less than 1/p o(1). To study
how the expected number of steps is affected by the
amount of memory allowed to the algorithms, we give a
memoryless randomized algorithm with expected number
of steps 4/p o(1/p) o(1) and a 1bit randomized
algorithm requiring on the average at most 2/p o(1)
steps. In addition, in the memoryless case, we also
prove a 4/p lower bound for the expected number of
steps in the case where the nodes giving faulty advice
may decide on the content of this advice in any possible
way and not merely at random (adversarial fault model).
Finally, for the case where faulty nodes behave randomly,
we give an optimal, unlimited memory deterministic
algorithm with expected number of steps bounded
from above by 1/p o(1/p) 1.
Abstract: We consider the problem of searching for a piece of information in a fully interconnected computer network or clique by exploiting
advice about its location from the network nodes Each node contains a
database that knows what kind of documents or information are stored
in other nodes e.g. a node could be a Web server that answers queries
about documents stored on the Web. The databases in each node when
queried provide a pointer that leads to the node that contains the information. However this information is up to date or correct with some
bounded probability. While in principle one may always locate the information by simply visiting the network nodes in some prescribed ordering
this requires a time complexity in the order of the number of nodes of the
network. In this paper we provide algorithms for locating an information node in the complete communication network that take advantage
of advice given from network nodes The nodes may either give correct
advice by pointing directly to the information node or give wrong advice
by pointing elsewhere We show that on the averageif the probability that a node provides correct advice is asymptotically larger than
where is the number of the computer nodes then the average time complexity for locating the information node is asymptotically or depending on the available memory.The probability may in general be a function of the number of network nodes . On the lower bounds
side we prove that noxed memory deterministic algorithm can locate
the information node in nite expected number of steps. We also prove
a lower bound of
for the expected number of steps of any algorithm
that locates the information node in the complete network.
Abstract: In this paper we examine the problem of searching for some information item in the nodes of a fully
interconnected computer network, where each node contains information relevant to some topic
as well as links to other network nodes that also contain information, not necessarily related to
locally kept information. These links are used to facilitate the Internet users and mobile software
agents that try to locate specific pieces of information. However, the links do not necessarily point
to nodes containing information of interest to the user or relevant to the aims of the mobile agent.
Thus an element of uncertainty is introduced. For example, when an Internet user or some search
agent lands on a particular network node, they see a set of links that point to information that is,
supposedly, relevant to the current search. Therefore, we can assume that a link points to relevant
information with some unknown probability p that, in general, is related to the number of nodes
in the network (intuitively, as the network grows, this probability tends to zero since adding more
nodes to the network renders some extant links less accurate or obsolete). Consequently, since there
is uncertainty as to whether the links contained in a node?s Web page are correct or not, a search
algorithm cannot rely on following the links systematically since it may end up spending too much
time visiting nodes that contain irrelevant information. In this work, we will describe and analyze
a search algorithm that is only allowed to transfer a fixed amount of memory along communication
links as it visits the network nodes. The algorithm is, however, allowed to use one bit of memory at
each node as an ?already visited? flag. In this way the algorithm has its memory distributed to the
network nodes, avoiding overloading the network links as it moves from node to node searching for
the information. We work on fully interconnected networks for simplicity reasons and, moreover,
because according to some recent experimental evidence, such networks can be considered to be a
good approximation of the current structure of the World Wide Web.
Abstract: Internet of Things technologies are considered the next big
step in Smart Building installations. Although such technologies have
been widely studied in simulation and experimental scenarios it is not so
obvious how problems of real world installations should be dealt with. In
this work we deploy IoT devices for sensing and control in a multioffice
space and employ technologies such as CoAP, RESTful interfaces and
Semantic Descriptions to integrate them with the Web. We report our
research goals, the challenges we faced, the decisions we made and the
experience gained from the design, deployment and operation of all the
hardware and software components that compose our system.
Abstract: We briefly present the design and architecture of a system that aims to simplify the process of organizing, executing and administering crowdsensing campaigns in a smart city context over smartphones volunteered by citizens. We built our system on top of an Android app substrate on the enduser level, which enables us to utilize smartphone resources. Our system allows researchers and other developers to manage and distribute their “mini” smart city applications, gather data and publish their results through the Organicity smart city platform. We believe this is the first time such a tool is paired with a large scale IoT
infrastructure, to enable truly cityscale IoT and smart city experimentation.
Abstract: The promises inherent in users coming together to form data
sharing network communities, bring to the foreground new problems formulated
over such dynamic, ever growing, computing, storage, and networking
infrastructures. A key open challenge is to harness these highly
distributed resources toward the development of an ultra scalable, efficient
search engine. From a technical viewpoint, any acceptable solution
must fully exploit all available resources dictating the removal of any
centralized points of control, which can also readily lead to performance
bottlenecks and reliability/availability problems. Equally importantly,
however, a highly distributed solution can also facilitate pluralism in informing
users about internet content, which is crucial in order to preclude
the formation of informationresource monopolies and the biased visibility
of content from economicallypowerful sources. To meet these challenges,
the work described here puts forward MINERVA{\^a}{\"i}¿½{\"i}¿½, a novel search
engine architecture, designed for scalability and efficiency. MINERVA{\^a}{\"i}¿½{\"i}¿½
encompasses a suite of novel algorithms, including algorithms for creating
data networks of interest, placing data on network nodes, load balancing,
topk algorithms for retrieving data at query time, and replication algorithms
for expediting topk query processing. We have implemented the
proposed architecture and we report on our extensive experiments with
realworld, webcrawled, and synthetic data and queries, showcasing the
scalability and efficiency traits of MINERVA{\^a}{\"i}¿½{\"i}¿½.
Abstract: We present here, Fun in Numbers, a framework for developing multiplayer pervasive games that rely on the use of ad hoc mobile sensor networks. The unique feature in such games is that players interact with each other and their surrounding environment by using movement and presence as a means of performing gamerelated actions, utilizing sensor devices. We present the fundamental issues and challenges related to these type of games and the scenarios associated with them is provided. Our framework is developed using Java and is based on a multilayer architecture, which provides developers with a set of templates and services for building and operating new games. Our framework handles a number of challenging fundamental and practical issues, such as synchronization, network congestion, delaytolerant communication and neighbors discovery. We also present our platform and identify issues that arise in pervasive games which utilize sensor network nodes. The implemented games show how to use nonconventional user interface methods to breathe new life into familiar concepts, like the multiplayer games played out in open space.
Abstract: In this work, we propose an obstacle model to be used while simulating wireless sensor networks. To the best of our knowledge, this is the first time such an integrated and systematic obstacle model appears. We define several types of obstacles that can be found inside the deployment area of a wireless sensor network and provide a categorization of these obstacles, based on their nature (physical and communication obstacles), their shape, as well as
their nature to change over time. In light of this obstacle model we conduct extensive simulations in order to study the effects of obstacles on the performance of representative data propagation protocols for wireless sensor networks. Our findings
show that obstacle presence has a significant impact on protocol performance. Also, we demonstrate the effect of each obstacle type on different protocols, thus providing the network designer with advice on which protocol is best to use.
Abstract: Recent rapid developments in microelectromechanical systems
(MEMS), wireless communications and digital electronics have already
led to the development of tiny, lowpower, lowcost sensor devices.
Such devices integrate sensing, limited data processing and restricted
communication capabilities.
Each sensor device individually might have small utility, however the
effective distributed coordination of large numbers of such devices can
lead to the efficient accomplishment of large sensing tasks. Large numbers
of sensors can be deployed in areas of interest (such as inaccessible
terrains or disaster places) and use selforganization and collaborative
methods to form an adhoc network.
We note however that the efficient and robust realization of such large,
highlydynamic, complex, nonconventional networking environments is
a challenging technological and algorithmic task, because of the unique
characteristics and severe limitations of these devices.
This talk will present and discuss several important aspects of the
design, deployment and operation of sensor networks. In particular, we
provide a brief description of the technical specifications of stateoftheart
sensor, a discussion of possible models used to abstract such networks,
a discussion of some key algorithmic design techniques (like randomization,
adaptation and hybrid schemes), a presentation of representative
protocols for sensor networks, for important problems including data
propagation, collision avoidance and energy balance and an evaluation
of crucial performance properties (correctness, efficiency, faulttolerance)
of these protocols, both with analytic and simulation means.
Abstract: In this paper we propose an energyaware broadcast algorithm for wireless networks. Our algorithm is based on the multicost approach and selects the set of nodes that by transmitting implement broadcasting in an optimally energyefficient way. The energyrelated parameters taken into account are the node transmission power and the node residual energy. The algorithm{\^a}€™s complexity however is nonpolynomial, and therefore, we propose a relaxation producing a nearoptimal solution in polynomial time. We also consider a distributed information exchange scheme that can be coupled with the proposed algorithms and examine the overhead introduced by this integration. Using simulations we show that the proposed algorithms outperform other solutions in the literature in terms of energy efficiency. Moreover, it is shown that the nearoptimal algorithm obtains most of the performance benefits of the optimal algorithm at a smaller computational overhead.
Abstract: We propose a class of novel energyefficient multicost routing algorithms for wireless mesh networks, and evaluate their performance. In multicost routing, a vector of cost parameters is assigned to each network link, from which the cost vectors of candidate paths are calculated using appropriate operators. In the end these parameters are combined in various optimization functions, corresponding to different routing algorithms, for selecting the optimal path. We evaluate the performance of the proposed energyaware multicost routing algorithms under two models. In the network evacuation model, the network starts with a number of packets that have to be transmitted and an amount of energy per node, and the objective is to serve the packets in the smallest number of steps, or serve as many packets as possible before the energy is depleted. In the dynamic onetoone communication model, new data packets are generated continuously and nodes are capable of recharging their energy periodically, over an infinite time horizon, and we are interested in the maximum achievable steadystate throughput, the packet delay, and the energy consumption. Our results show that energyaware multicost routing increases the lifetime of the network and achieves better overall network performance than other approaches.
Abstract: In this work we study the combination of
multicost routing and adjustable transmission power
in wireless adhoc networks, so as to obtain dynamic
energy and interferenceefficient routes to optimize network performance. In multicost routing, a vector of
cost parameters is assigned to each network link, from
which the cost vectors of candidate paths are calcu
lated. Only at the end are these parameters combined in
various optimization functions, corresponding to different routing algorithms, for selecting the optimal path.
The multicost routing problem is a generalization of
the multiconstrained problem, where no constraints exist, and is also significantly more powerful than single
cost routing. Since energy is an important limitation of
wireless communications, the cost parameters consid
ered are the number of hops, the interference caused,
the residual energy and the transmission power of the
nodes on the path; other parameters could also be in
cluded, as desired.We assume that nodes can use power
control to adjust their transmission power to the desired
level. The experiments conducted show that the com
bination of multicost routing and adjustable transmis sion power can lead to reduced interference and energy
consumption, improving network performance and life
time.
Abstract: In this work we study the dynamic onetoone communica
tion problem in energy and capacityconstrained wireless adhoc net
works. The performance of such networks is evaluated under random
traﬃc generation and continuous energy recharging at the nodes over an
inﬁnitetime horizon.We are interested in the maximum throughput that
can be sustained by the network with the node queues being ﬁnite and in
the average packet delay for a given throughput. We propose a multicost
energyaware routing algorithm and compare its performance to that of
minimumhop routing. The results of our experiments show that gener
ally the energyaware algorithm achieves a higher maximum throughput
than the minimumhop algorithm. More speciﬁcally, when the network
is mainly energyconstrained and for the 2dimensional topology consid
ered, the throughput of the proposed energyaware routing algorithm is
found to be almost twice that of the minimumhop algorithm.
Abstract: We present an architecture for implementing optical
buffers, based on the feedforwardbuffer concept, that can truly
emulate input queuing and accommodate asynchronous packet
and burst operation. The architecture uses wavelength converters
and fixedlength delay lines that are combined to form either a
multipleinput buffer or a shared buffer. Both architectures are
modular, allowing the expansion of the buffer at a cost that grows
logarithmically with the buffer depth, where the cost is measured
in terms of the number of switching elements, and wavelength
converters are employed. The architectural design also provides
a tradeoff between the number of wavelength converters and their
tunability. The buffer architectures proposed are complemented
with scheduling algorithms that can guarantee lossless communication
and are evaluated using physicallayer simulations to
obtain their performance in terms of biterror rate and achievable
buffer size.
Abstract: We present an architecture for implementing optical
buffers, based on the feedforwardbuffer concept, that can truly
emulate input queuing and accommodate asynchronous packet
and burst operation. The architecture uses wavelength converters
and fixedlength delay lines that are combined to form either a
multipleinput buffer or a shared buffer. Both architectures are
modular, allowing the expansion of the buffer at a cost that grows
logarithmically with the buffer depth, where the cost is measured
in terms of the number of switching elements, and wavelength
converters are employed. The architectural design also provides
a tradeoff between the number of wavelength converters and their
tunability. The buffer architectures proposed are complemented
with scheduling algorithms that can guarantee lossless communication
and are evaluated using physicallayer simulations to
obtain their performance in terms of biterror rate and achievable
buffer size.
Abstract: We present and discuss challenges and solutions posed by the design of an
adaptable network infrastructure of tiny artifacts. Such artifacts are characterized by
severe limitations in computational power, communications capacity and energy;
nevertheless they must realize a communication infrastructure able to deliver
services to the endusers in a very dynamic and challenging environment. Namely
we present one unifying scenario for the activities of the FRONTS project
(www.fronts.cti.gr). The aim of the unifying scenario is to show how the results
achieved in the project can be exploited to build such a communication
infrastructure.
Abstract: In this work, we study the fundamental naming and counting problems (and some variations) in networks that are anonymous, unknown, and possibly dynamic. In counting, nodes must determine the size of the network n and in naming they must end up with unique identities. By anonymous we mean that all nodes begin from identical states
apart possibly from a unique leader node and by unknown that nodes
have no a priori knowledge of the network (apart from some minimal
knowledge when necessary) including ignorance of n. Network dynamicity is modeled by the 1interval connectivity model [KLO10], in which communication is synchronous and a (worstcase) adversary chooses the edges of every round subject to the condition that each instance is connected. We first focus on static networks with broadcast where we prove that, without a leader, counting is impossible to solve and that naming is impossible to solve even with a leader and even if nodes know n. These impossibilities carry over to dynamic networks as well. We also show that a unique leader suffices in order to solve counting in linear time.
Then we focus on dynamic networks with broadcast. We conjecture that
dynamicity renders nontrivial computation impossible. In view of this,
we let the nodes know an upper bound on the maximum degree that will
ever appear and show that in this case the nodes can obtain an upper
bound on n. Finally, we replace broadcast with onetoeach, in which a
node may send a different message to each of its neighbors. Interestingly,
this natural variation is proved to be computationally equivalent to a
fullknowledge model, in which unique names exist and the size of the
network is known.
Abstract: Evolutionary dynamics have been traditionally studied in the context of homogeneous populations, mainly described by the Moran process [15]. Recently, this approach has been generalized in [13] by arranging individuals on the nodes of a network (in general, directed). In this setting, the existence of directed arcs enables the simulation of extreme phenomena, where the fixation probability of a randomly placed mutant (i.e. the probability that the offsprings of the mutant eventually spread over the whole population) is arbitrarily small or large. On the other hand, undirected networks (i.e. undirected graphs) seem to have a smoother behavior, and thus it is more challenging to find suppressors/amplifiers of selection, that is, graphs with smaller/greater fixation probability than the complete graph (i.e. the homogeneous population). In this paper we focus on undirected graphs. We present the first class of undirected graphs which act as suppressors of selection, by achieving a fixation probability that is at most one half of that of the complete graph, as the number of vertices increases. Moreover, we provide some generic upper and lower bounds for the fixation
probability of general undirected graphs. As our main contribution, we introduce the natural alternative of the model proposed in [13]. In our new evolutionary model, all individuals interact simultaneously and the result is a compromise between aggressive and nonaggressive individuals. That is, the behavior of the individuals in our new model and in the model of [13] can be interpreted as an “aggregation” vs. an “allornothing” strategy, respectively. We prove that our new model of mutual influences admits a potential function, which guarantees the convergence of the system for any graph topology and any initial fitness vector of the individuals. Furthermore, we prove fast convergence to the stable state for the case of the complete graph, as well as we provide almost tight bounds on the limit fitness of the individuals. Apart from being important on its own, this new evolutionary model appears to be useful also in the abstract modeling of control mechanisms over invading populations in networks. We demonstrate this by introducing and analyzing two alternative control approaches, for which we bound the time needed to stabilize to the “healthy” state of the system.
Abstract: Geographic routing scales well in sensor networks, mainly
due to its stateless nature. Still, most of the algorithms are
concerned with finding some path, while the optimality of
the path is difficult to achieve. In this paper we are presenting
a novel geographic routing algorithm with obstacle
avoidance properties. It aims at finding the optimal path
from a source to a destination when some areas of the network
are unavailable for routing due to low local density or
obstacle presence. It locally and gradually with time (but,
as we show, quite fast) evaluates and updates the suitability
of the previously used paths and ignores non optimal paths
for further routing. By means of extensive simulations, we
are comparing its performance to existing state of the art
protocols, showing that it performs much better in terms of
path length thus minimizing latency, space, overall traffic
and energy consumption.
Abstract: We study network load games, a class of routing games in
networks which generalize sel{\^A}¯sh routing games on networks consisting
of parallel links. In these games, each user aims to route some tra{\^A}±c from
a source to a destination so that the maximum load she experiences in the
links of the network she occupies is minimum given the routing decisions
of other users. We present results related to the existence, complexity,
and price of anarchy of Pure Nash Equilibria for several network load
games. As corollaries, we present interesting new statements related to
the complexity of computing equilibria for sel{\^A}¯sh routing games in net
works of restricted parallel links.
Abstract: We propose new burst assembly schemes and fast reservation (FR) protocols for Optical Burst Switched (OBS) networks that are based on traffic prediction. The burst assembly schemes aim at minimizing (for a given burst size) the average delay of the packets incurred during the burst assembly process, while the fast reservation protocols aim at further reducing the endtoend delay of the data bursts. The burst assembly techniques use a linear prediction filter to estimate the number of packet arrivals at the ingress node in the following interval, and launch a new burst into the network when a certain criterion, different for each proposed scheme, is met. The fast reservation protocols use prediction filters to estimate the expected length of the burst and the time needed for the burst assembly process to complete. A Burst Header Packet (BHP) packet carrying these estimates is sent before the burst is completed, in order to reserve bandwidth at intermediate nodes for the time interval the burst is expected to pass from these nodes. Reducing the packet aggregation delay and the time required to perform the reservations, reduces the total time needed for a packet to be transported over an OBS network and is especially important for realtime applications. We evaluate the performance of the proposed burst assembly schemes and show that a number of them outperform the previously proposed timerbased, lengthbased and average delaybased burst assembly schemes. We also look at the performance of the fast reservation (FR) protocols in terms of the probability of successfully establishing the reservations required to transport the burst.
Abstract: We propose new burst assembly techniques that aim at reducing the average delay experienced by the packets during the burstification process in optical burst switched (OBS) networks, for a given average size of the bursts produced. These techniques use a linear prediction filter to estimate the number of packet arrivals at the ingress node in the following interval, and launch a new burst into the network when a certain criterion, which is different for each proposed scheme, is met. Reducing the packet burstification delay, for a given average burst size, is essential for realtime applications; correspondingly, increasing the average burst size for a given packet burstification delay is important for reducing the number of bursts injected into the network and the associated overhead imposed on the core nodes. We evaluate the performance of the proposed schemes and show that two of them outperform the previously proposed timer  based, length  based and average delaybased burst aggregation schemes in terms of the average packet burstification delay for a given average burst size.
Abstract: Wireless sensor networks are about to be part of everyday life. Homes and workplaces capable of selfcontrolling and adapting airconditioning for different temperature and humidity levels, sleepless forests ready to detect and react in case of a fire, vehicles able to avoid sudden obstacles or possibly able to selforganize routes to avoid congestion, and so on, will probably be commonplace in the very near future. Mobility plays a central role in such systems and so does passive mobility, that is, mobility of the network stemming from the environment itself. The population protocol model was an intellectual invention aiming to describe such systems in a minimalistic and analysisfriendly way. Having as a startingpoint the inherent limitations but also the fundamental establishments of the population protocol model, we try in this monograph to present some realistic and practical enhancements that give birth to some new and surprisingly powerful (for this kind of systems) computational models.
Abstract: Wireless Sensor Networks (WSNs) constitute a recent and promising new
technology that is widely applicable. Due to the applicability of this
technology and its obvious importance for the modern distributed
computational world, the formal scientific foundation of its inherent laws
becomes essential. As a result, many new computational models for WSNs
have been proposed. Population Protocols (PPs) are a special category of
such systems. These are mainly identified by three distinctive
characteristics: the sensor nodes (agents) move passively, that is, they
cannot control the underlying mobility pattern, the available memory to
each agent is restricted, and the agents interact in pairs. It has been
proven that a predicate is computable by the PP model iff it is
semilinear. The class of semilinear predicates is a fairly small class. In
this work, our basic goal is to enhance the PP model in order to improve
the computational power. We first make the assumption that not only the
nodes but also the edges of the communication graph can store restricted
states. In a complete graph of n nodes it is like having added O(n2)
additional memory cells which are only read and written by the endpoints
of the corresponding edge. We prove that the new model, called Mediated
Population Protocol model, can operate as a distributed nondeterministic
Turing machine (TM) that uses all the available memory. The only
difference from a usual TM is that this one computes only symmetric
languages. More formally, we establish that a predicate is computable by
the new model iff it is symmetric and belongs to NSPACE(n2). Moreover, we
study the ability of the new model to decide graph languages (for general
graphs). The next step is to ignore the states of the edges and provide
another enhancement straight away from the PP model. The assumption now is
that the agents are multitape TMs equipped with infinite memory, that can
perform internal computation and interact with other agents, and we define
spacebounded computations. We call this the Passively mobile Machines
model. We prove that if each agent uses at most f(n) memory for f(n)={\`U}(log
n) then a predicate is computable iff it is symmetric and belongs to
NSPACE(nf(n)). We also show that this is not the case for f(n)=o(log n).
Based on these, we show that for f(n)={\`U}(log n) there exists a space
hierarchy like the one for classical symmetric TMs. We also show that the
latter is not the case for f(n)=o(loglog n), since here the corresponding
class collapses in the class of semilinear predicates and finally that for
f(n)={\`U}(loglog n) the class becomes a proper superset of semilinear
predicates. We leave open the problem of characterizing the classes for
f(n)={\`U}(loglog n) and f(n)=o(log n).
Abstract: Web services are becoming an important enabler of the Semantic Web. Besides the need for a rich description mechanism, Web Service information should be made available in an accessible way for machine processing. In this paper, we propose a new P2P basedapproach for Web Services discovery. Peers that store Web Services information, such as data item descriptions, are efficiently located using a scalable and robust data indexing structure for PeertoPeer data networks, NIPPERS. We present a theoretical analysis which shows that the communication cost of the query and update operations scale doublelogarithmically with the number of NIPPERS nodes. Furthermore, we show that the network is robust with respect to failures fulfilling quality of web services requirements.
Abstract: In this paper we discussed different switch architectures. We focus mainly on optical buffering. We investigate an alloptical buffer architecture comprising of cascaded stages of quantumdot semiconductor optical amplifier based tunable wavelength converters, at 160 Gb/s. We also propose the optical buffer with multiwavelength converters based on quantumdot semiconductor optical amplifiers. We present multistage switching fabrics with optical buffers, where optical buffers are based on fibre delay lines and are located in the first stage. Finally, we describe a photonic asynchronous packet switch and show that the employment of a few optical buffer stages to complement the electronic ones significantly improves the switch performance. We also propose two asynchronous optical packet switching node architectures, where an efficient contention resolution is based on controllable optical buffers and tunable wavelength converters TWCs.
Abstract: We consider the offline version of the routing and
wavelength assignment (RWA) problem in transparent alloptical
networks. In such networks and in the absence of regenerators,
the signal quality of transmission degrades due to physical layer
impairments. Because of certain physical effects, routing choices
made for one lightpath affect and are affected by the choices made
for the other lightpaths. This interference among the lightpaths
is particularly difficult to formulate in an offline algorithm since,
in this version of the problem, we start without any established
connections and the utilization of lightpaths are the variables of
the problem.We initially present an algorithm for solving the pure
(without impairments) RWA problem based on a LPrelaxation
formulation that tends to yield integer solutions. Then, we extend
this algorithm and present two impairmentaware (IA) RWA algorithms
that account for the interference among lightpaths in their
formulation. The first algorithm takes the physical layer indirectly
into account by limiting the impairmentgenerating sources. The
second algorithm uses noise variancerelated parameters to directly
account for the most important physical impairments. The
objective of the resulting crosslayer optimization problem is not
only to serve the connections using a small number of wavelengths
(network layer objective), but also to select lightpaths that have
acceptable quality of transmission (physical layer objective).
Simulations experiments using realistic network, physical layer,
and traffic parameters indicate that the proposed algorithms can
solve real problems within acceptable time.
Abstract: The voting rules proposed by Dodgson and Young are both
designed to nd the alternative closest to being a Condorcet
winner, according to two dierent notions of proximity; the
score of a given alternative is known to be hard to compute
under either rule.
In this paper, we put forward two algorithms for ap
proximating the Dodgson score: an LPbased randomized
rounding algorithm and a deterministic greedy algorithm,
both of which yield an O(logm) approximation ratio, where
m is the number of alternatives; we observe that this result
is asymptotically optimal, and further prove that our greedy
algorithm is optimal up to a factor of 2, unless problems in
NP have quasipolynomial time algorithms. Although the
greedy algorithm is computationally superior, we argue that
the randomized rounding algorithm has an advantage from
a social choice point of view.
Further, we demonstrate that computing any reasonable
approximation of the ranking produced by Dodgson's rule
is NPhard. This result provides a complexitytheoretic
explanation of sharp discrepancies that have been observed
in the Social Choice Theory literature when comparing
Dodgson elections with simpler voting rules.
Finally, we show that the problem of calculating the
Young score is NPhard to approximate by any factor. This
leads to an inapproximability result for the Young ranking.
Abstract: An adhoc mobile network is a collection of mobile hosts, with
wireless communication capabilities, forming a temporary network
without the aid of any established fixed infrastructure.
In such networks, topological connectivity is subject to frequent,
unpredictable change. Our work focuses on networks with high
rate of such changes to connectivity. For such dynamic changing
networks we propose protocols which exploit the coordinated
(by the protocol) motion of a small part of the network.
We show that such protocols can be designed to work
correctly and efficiently even in the case of arbitrary (but not
malicious) movements of the hosts not affected by the protocol.
We also propose a methodology for the analysis of the expected
behaviour of protocols for such networks, based on the assumption that mobile hosts (whose motion is not guided by
the protocol) conduct concurrent random walks in their
motion space.
Our work examines some fundamental problems such as pairwise
communication, election of a leader and counting, and proposes
distributed algorithms for each of them. We provide their
proofs of correctness, and also give rigorous analysis by
combinatorial tools and also via experiments.
Abstract: Understanding the graph structure of the Internet is a crucial step for building accurate
network models and designing efﬁcient algorithms for Internet applications.Yet,obtaining this graph
structure can be a surprisingly difﬁcult task,as edges cannot be explicitly queried.For instance,
empirical studies of the network of InternetProtocol (IP) addresses typically rely on indirect methods
like trace route to build what are approximately singlesource,alldestinations,shortestpath trees.
These trees only sample a fraction of the network’s edges,and a paper by Lakhinaetal.[2003]found
empirically that there sulting sample is intrinsically biased.Further,in simulations,they observed that the degree distribution under trace route sampling exhibits a power law even when the underlying
degree distribution is Poisson.
Abstract: It was only recently shown by Shi and Wormald, using the differential equation method to analyze an appropriate algorithm, that a random 5regular graph asymptotically almost surely has chromatic number at most 4. Here, we show that the chromatic number of a random 5regular graph is asymptotically almost surely equal to 3, provided a certain fourvariable function has a unique maximum at a given point in
a bounded domain. We also describe extensive numerical evidence that strongly suggests that the latter condition holds. The proof applies the small subgraph conditioning method to the number of locally rainbow balanced 3colorings, where a coloring is balanced if the number of vertices of each color is equal, and locally rainbow if every vertex is adjacent to at least one vertex of each of the other colors.
Abstract: In this paper we demonstrate the significant impact of (a) the mobility rate and (b) the user density on the performance of routing protocols in adhoc mobile networks. In particular, we study the effect of these parameters on two different approaches for designing routing protocols: (a) the route creation and maintenance approach and (b) the "support" approach, that forces few hosts to move acting as "helpers" for message delivery. We study one representative protocol for each approach, i.e., AODV for the first approach and RUNNERS for the second. We have implemented the two protocols and performed a large scale and detailed simulation study of their performance. For the first time, we study AODV (and RUNNERS) in the 3D case. The main findings are: the AODV protocol behaves well in networks of high user density and low mobility rate, while its performance drops for sparse networks of highly mobile users. On the other hand, the RUNNERS protocol seems to tolerate well (and in fact benefit from) high mobility rates and low densities. Thus, we are able to partially answer an important conjecture of [Chatzigiannakis, I et al. 2003].
Abstract: In sponsored search auctions, advertisers compete for a number
of available advertisement slots of different quality. The
auctioneer decides the allocation of advertisers to slots using
bids provided by them. Since the advertisers may act
strategically and submit their bids in order to maximize their
individual objectives, such an auction naturally defines a
strategic game among the advertisers. In order to quantify
the efficiency of outcomes in generalized second price auctions,
we study the corresponding games and present new
bounds on their price of anarchy, improving the recent results
of Paes Leme and Tardos [16] and Lucier and Paes
Leme [13]. For the full information setting, we prove a surprisingly
low upper bound of 1.282 on the price of anarchy
over pure Nash equilibria. Given the existing lower bounds,
this bound denotes that the number of advertisers has almost
no impact on the price of anarchy. The proof exploits
the equilibrium conditions developed in [16] and follows by
a detailed reasoning about the structure of equilibria and a
novel relation of the price of anarchy to the objective value
of a compact mathematical program. For more general equilibrium
classes (i.e., mixed Nash, correlated, and coarse correlated
equilibria), we present an upper bound of 2.310 on
the price of anarchy. We also consider the setting where
advertisers have incomplete information about their competitors
and prove a price of anarchy upper bound of 3.037
over BayesNash equilibria. In order to obtain the last two
bounds, we adapt techniques of Lucier and Paes Leme [13]
and significantly extend them with new arguments
Abstract: We investigate the problem of how to achieve energy balanced data propagation in distributed wireless sensor networks. The energy balance property guarantees that the average per sensor energy dissipation is the same for all sensors in the network, throughout the execution of the data propagation protocol. This property is crucial for prolonging the network lifetime, by avoiding early energy depletion of sensors.
We survey representative solutions from the state of the art. We first present a basic algorithm that in each step probabilistically decides whether to propagate data onehop towards the final destination (the sink), or to send it directly to the sink. This randomized choice tradesoff the (cheap, but slow) onehop transmissions with the direct transmissions to the sink, which are more expensive but bypass the bottleneck region around the sink and propagate data fast. By a detailed analysis using properties of stochastic processes and recurrence relations we precisely estimate (even in closed form) the probability for each propagation option necessary for energy balance.
The fact (shown by our analysis) that direct (expensive) transmissions to the sink are needed only rarely, shows that our protocol, besides energy balanced, is also energy efficient. We then enhance this basic result by surveying some recent findings including a generalized algorithm and demonstrating the optimality of this twoway probabilistic data propagation, as well as providing formal proofs of the energy optimality of the energy balance property.
Abstract: The Moran process models the spread of genetic mutations through a population. A mutant with relative fitness r is introduced into a population and the system evolves, either reaching fixation (in which every individual is a mutant) or extinction (in which none is). In a widely cited paper (Nature, 2005), Lieberman, Hauert and Nowak generalize the model to populations on the vertices of graphs. They describe a class of graphs (called "superstars"), with a parameter k. Superstars are designed to have an increasing fixation probability as k increases. They state that the probability of fixation tends to 1−r−k as graphs get larger but we show that this claim is untrue as stated. Specifically, for k=5, we show that the true fixation probability (in the limit, as graphs get larger) is at most 1−1/j(r) where j(r)=Θ(r4), contrary to the claimed result. We do believe that the qualitative claim of Lieberman et al.\  that the fixation probability of superstars tends to 1 as k increases  is correct, and that it can probably be proved along the lines of their sketch. We were able to run larger computer simulations than the ones presented in their paper. However, simulations on graphs of around 40,000 vertices do not support their claim. Perhaps these graphs are too small to exhibit the limiting behaviour.
Abstract: In routing games, the network performance at equilibrium can be significantly improved if we remove some edges from the network. This counterintuitive fact, widely known as Braess's paradox, gives rise to the (selfish) network design problem, where we seek to recognize routing games suffering from the paradox, and to improve the equilibrium performance by edge removal. In this work, we investigate the computational complexity and the approximability of the network design problem for nonatomic bottleneck routing games, where the individual cost of each player is the bottleneck cost of her path, and the social cost is the bottleneck cost of the network. We first show that bottleneck routing games do not suffer from Braess's paradox either if the network is seriesparallel, or if we consider only subpathoptimal Nash flows. On the negative side, we prove that even for games with strictly increasing linear latencies, it is NPhard not only to recognize instances suffering from the paradox, but also to distinguish between instances for which the Price of Anarchy (PoA) can decrease to 1 and instances for which the PoA is as large as \Omega(n^{0.121}) and cannot improve by edge removal. Thus, the network design problem for such games is NPhard to approximate within a factor of O(n^{0.121\eps}), for any constant \eps > 0. On the positive side, we show how to compute an almost optimal subnetwork w.r.t. the bottleneck cost of its worst Nash flow, when the worst Nash flow in the best subnetwork routes a nonnegligible amount of flow on all used edges. The running time is determined by the total number of paths, and is quasipolynomial when the number of paths is quasipolynomial.
Abstract: We investigate the practical merits of a parallel priority queue
through its use in the development of a fast and workefficient parallel
shortest path algorithm, originally designed for an EREW PRAM. Our
study reveals that an efficient implementation on a real supercomputer
requires considerable effort to reduce the communication performance
(which in theory is assumed to take constant time). It turns out that the
most crucial part of the implementation is the mapping of the logical
processors to the physical processing nodes of the supercomputer. We
achieve the requested efficient mapping through a new graphtheoretic
result of independent interest: computing a Hamiltonian cycle on a directed
hypertorus. No such algorithm was known before for the case of
directed hypertori. Our Hamiltonian cycle algorithm allows us to considerably
improve the communication cost and thus the overall performance
of our implementation.
Abstract: We study network connection games where the nodes of a networ
k perform edge swaps
in order to improve their communication costs. For the model
proposed by [2], in which the selfish
cost of a node is the sum of all shortest path distances to the o
ther nodes, we use the probabilistic
method to provide a new, structural characterization of equ
ilibrium graphs. We show how to use this
characterization in order to prove upper bounds on the diame
ter of equilibrium graphs in terms of the
size of the largest
k
vicinity (defined as the the set of vertices within distance
k
from a vertex), for
any
k
≥
1 and in terms of the number of edges, thus settling positivel
y a conjecture of [2] in the cases
of graphs of large
k
vicinity size (including graphs of large maximum degree) a
nd of graphs which are
dense enough.
Next, we present a new swapbased network creation game, in w
hich selfish costs depend on the imme
diate neighborhood of each node; in particular, the profit of
a node is defined as the sum of the degrees
of its neighbors. We prove that, in contrast to the previous m
odel, this network creation game admits
an exact potential, and also that any equilibrium graph cont
ains an induced star. The existence of the
potential function is exploited in order to show that an equi
librium can be reached in expected polyno
mial time even in the case where nodes can only acquire limite
d knowledge concerning nonneighboring
nodes.
Abstract: In many cryptographic applications it is necessary to generate
elliptic curves (ECs) with certain security properties. These curves
are commonly constructed using the Complex Multiplication method
which typically uses the roots of Hilbert or Weber polynomials. The former
generate the EC directly, but have high computational demands,
while the latter are faster to construct but they do not lead, directly, to
the desired EC. In this paper we present in a simple and unifying manner
a complete set of transformations of the roots of a Weber polynomial to
the roots of its corresponding Hilbert polynomial for all discriminant values
on which they are defined. Moreover, we prove a theoretical estimate
of the precision required for the computation of Weber polynomials. Finally,
we experimentally assess the computational efficiency of theWeber
polynomials along with their precision requirements for various discriminant
values and compare the results with the theoretical estimates. Our
experimental results may be used as a guide for the selection of the most
efficient curves in applications residing in resource limited devices such as
smart cards that support secure and efficient Public Key Infrastructure
(PKI) services.
Abstract: We propose and evaluate an impairmentaware multiparametric routing and wavelength assignment algorithm for online traffic in transparent optical networks. In such networks the signal quality of transmission degrades due to physical layer impairments. In the multiparametric approach, a vector of cost parameters is assigned to each link, from which the cost vectors of candidate lightpaths are calculated. In the proposed scheme the cost vector includes impairment generating source parameters, such as the path length, the number of hops, the number of crosstalk sources and other interlightpath interfering parameters, so as to indirectly account for the physical layer effects. For a requested connection the algorithm calculates a set of candidate lightpaths, whose quality of transmission is validated using a function that combines the impairment generating parameters. For selecting the lightpath we propose and evaluate various optimization functions that correspond to different IARWA algorithms. Our performance results indicate that the proposed algorithms utilize efficiently the available resources and minimize the total accumulated signal degradation on the selected lightpaths, while having low execution times.
Abstract: One oftcited strategy towards sustainability is improving energy efficiency inside public buildings. In this context, the educational buildings sector presents a very interesting and important case for the monitoring and management of buildings, since it addresses both energy and educational issues. In this work, we present and discuss the hardware IoT infrastructure substrate that provides realtime monitoring in multiple school buildings. We believe that such a system needs to follow an open design approach: rely on hardwareagnostic components that communicate over welldefined open interfaces. We present in detail the design of our hardware components, while also providing insights to the overall system design and a first set of results on their operation. The presented hardware components are utilized as the core hardware devices for GAIA, an EU research project aimed at the educational community. As our system has been deployed and tested in several public school buildings in Greece, we also report on its validation.
Abstract: The objective of this research is to propose two new optical procedures for packet routing and forwarding in the framework of transparent optical networks. The singlewavelength labelrecognition and packetforwarding unit, which represents the central physical constituent of the switching node, is fully described in both cases. The first architecture is a hybrid optoelectronic structure relying on an optical serialtoparallel converter designed to slow down the label processing. The remaining switching operations are done electronically. The routing system remains transparent for the packet payloads. The second architecture is an alloptical architecture and is based on the implementation of alloptical decoding of the parallelized label. The packetforwarding operations are done optically. The major subsystems required in both of the proposed architectures are described on the basis of nonlinear effects in semiconductor optical amplifiers. The experimental results are compatible with the integration of the whole architecture. Those subsystems are a 4bit timetowavelength converter, a pulse extraction circuit, a an optical wavelength generator, a 3 x 8 alloptical decoder and a packet envelope detector.
Abstract: We report a polarization insensitive fiberbased NOLM scheme that incorporates nonPM fibers as the nonlinear
medium. The circuit is configured to operate as a fiber compressor and produces narrow and pedestalfree optical
pulses. The FRM stabilization ensures the excellent performance regardless of any externally applied perturbations.
Abstract: In this paper we propose an energyefficient broadcast algorithm for wireless networks for the case where the transmission powers of the nodes are fixed. Our algorithm is based on the multicost approach and selects an optimal energyefficient set of nodes for broadcasting, taking into account: i) the node residual energies, ii) the transmission powers used by the nodes, and iii) the set of nodes that are covered by a specific schedule. Our algorithm is optimal, in the sense that it can optimize any desired function of the total power consumed by the broadcasting task and the minimum of the current residual energies of the nodes, provided that the optimization function is monotonic in each of these parameters. Our algorithm has nonpolynomial complexity, thus, we propose a relaxation producing a nearoptimal solution in polynomial time. Using simulations we show that the proposed algorithms outperform other established solutions for energyaware broadcasting with respect to both energy consumption and network lifetime. Moreover, it is shown that the nearoptimal multicost algorithm obtains most of the performance benefits of the optimal multicost algorithm at a smaller computational overhead.
Abstract: This paper studies the data gathering problem in wireless networks, where data generated at the nodes has to be collected at a single sink. We investigate the relationship between routing optimality and fair resource management. In particular, we prove that for energy balanced data propagation, Pareto optimal routing and flow maximization are equivalent, and also prove that flow maximization is equivalent to maximizing the network lifetime. We algebraically characterize the network structures in which energy balanced data flows are maximal. Moreover, we algebraically characterize communication links which are not used by an optimal flow. This leads to the characterization of minimal network structures supporting the maximal flows.
We note that energy balance, although implying global optimality, is a local property that can be computed efficiently and in a distributed manner. We suggest online distributed algorithms for energy balance in different optimal network structures and numerically show their stability in particular setting. We remark that although the results obtained in this paper have a direct consequence in energy saving for wireless networks they do not limit themselves to this type of networks neither to energy as a resource. As a matter of fact, the results are much more general and can be used for any type of network and different type of resources.
Abstract: This paper studies the data gathering problem in wireless networks, where data generated at the nodes has to be collected at a single sink. We investigate the relationship between routing optimality and fair resource management. In particular, we prove that for energybalanced data propagation, Pareto optimal routing and flow maximization are equivalent, and also prove that flow maximization is equivalent to maximizing the network lifetime. We algebraically characterize the network structures in which energybalanced data flows are maximal. Moreover, we algebraically characterize communication links which are not used by an optimal flow. This leads to the characterization of minimal network structures supporting the maximal flows.
We note that energybalance, although implying global optimality, is a local property that can be computed efficiently and in a distributed manner. We suggest online distributed algorithms for energybalance in different optimal network structures and numerically show their stability in particular setting. We remark that although the results obtained in this paper have a direct consequence in energy saving for wireless networks they do not limit themselves to this type of networks neither to energy as a resource. As a matter of fact, the results are much more general and can be used for any type of network and different types of resources.
Abstract: In this paper, we demonstrate optical transparency
in packet formatting and network traffic offered by alloptical
switching devices. Exploiting the bitwise processing capabilities
of these “optical transistors,” simple optical circuits are designed
verifying the independency to packet length, synchronization
and packettopacket power fluctuations. Devices with these attributes
are key elements for achieving network flexibility, fine
granularity and efficient bandwidthondemand use. To this end, a
header/payload separation circuit operating with IPlike packets,
a clock and data recovery circuit handling asynchronous packets
and a burstmode receiver for bursty traffic are presented. These
network subsystems can find application in future high capacity
datacentric photonic packet switched networks.
Abstract: In this paper, we describe the implementation of
applying and testing the ”Lightweight Target Tracking using
Passive Traces algorithm” [1] on a FIRE wireless sensors testbed
located in the Theoretical Computer Science/Sensors Lab in
Geneva, Switzerland. We provide information about the hardware
installation and configuration, the changes we did to the
algorithm to adapt it to a real testbed as well as the tools we
implemented to operate the network and receive feedback from
the algorithm’s operation. Finally, we discuss the performance
evaluation findings of our implementation.
Abstract: We propose a new theoretical model for passively mobile Wireless Sensor Networks. We
call it the PALOMA model, standing for PAssively mobile LOgarithmic space MAchines. The main
modification w.r.t. the Population Protocol model [2] is that agents now, instead of being automata, are
Turing Machines whose memory is logarithmic in the population size n. Note that the new model is still
easily implementable with current technology. We focus on complete communication graphs. We define
the complexity class PLM, consisting of all symmetric predicates on input assignments that are stably
computable by the PALOMA model. We assume that the agents are initially identical. Surprisingly, it
turns out that the PALOMA model can assign unique consecutive ids to the agents and inform them
of the population size! This allows us to give a direct simulation of a Deterministic Turing Machine
of O(n log n) space, thus, establishing that any symmetric predicate in SPACE(n log n) also belongs
to PLM. We next prove that the PALOMA model can simulate the Community Protocol model [15],
thus, improving the previous lower bound to all symmetric predicates in NSPACE(n log n). Going
one step further, we generalize the simulation of the deterministic TM to prove that the PALOMA
model can simulate a Nondeterministic TM of O(n log n) space. Although providing the same lower
bound, the important remark here is that the bound is now obtained in a direct manner, in the sense
that it does not depend on the simulation of a TM by a Pointer Machine. Finally, by showing that a
Nondeterministic TM of O(n log n) space decides any language stably computable by the PALOMA
model, we end up with an exact characterization for PLM: it is precisely the class of all symmetric
predicates in NSPACE(n log n).
Abstract: We consider path protection in the routing and
wavelength assignment (RWA) problem for impairment
constrained WDM optical networks. The proposed multicost
RWA algorithms select the primary and the backup lightpaths by
accounting for physical layer impairments. The backup lightpath
may either be activated (1+1 protection) or it may be reserved and
not activated, with activation taking place when/if needed (1:1
protection). In case of 1:1 protection the period of time where the
quality of its transmission (QoT) is valid, despite the possible
establishment of future connections, should be preserved, so as to
be used in case the primary lightpath fails. We show that, by using
the multicost approach for solving the RWA with protection
problem, great benefits can be achieved both in terms of the
connection blocking rate and in terms of the validity period of the
backup lightpath. Moreover the multicost approach, by providing
a set of candidate lightpaths for each source destination pair,
instead of a single one, offers ease and flexibility in selecting the
primary and the backup lightpaths.
Abstract: — This work discusses PatrasSense, a system utilizing a range of technologies for monitoring environmental conditions, based on the concept of participatory monitoring. We discuss here the main issues in this category of applications, and its relation to the Real World Internet vision. We are interested in the use of sensors for collecting data related to the quality of life in urban areas, making it publicly available. We describe a possible architecture for such a system and a plan for deployment in the city of Patras, Greece. We have thus far implemented a set of features and conducted an initial set of smallscale experiments using sensor devices mounted on vehicles. We report on the respective results
Abstract: In this work, we study the impact of dynamically changing link capacities on the delay bounds of LIS (LongestInSystem) and SIS (ShortestInSystem) protocols on specific networks (that can be modelled as Directed Acyclic Graphs (DAGs)) and stability bounds of greedy contention–resolution protocols running on arbitrary networks under the Adversarial Queueing Theory. Especially, we consider the model of dynamic capacities, where each link capacity may take on integer values from [1,C] with C>1, under a (w,\~{n})adversary. We show that the packet delay on DAGs for LIS is upper bounded by O(iw\~{n}C) and lower bounded by {\`U}(iw\~{n}C) where i is the level of a node in a DAG (the length of the longest path leading to node v when nodes are ordered by the topological order induced by the graph). In a similar way, we show that the performance of SIS on DAGs is lower bounded by {\`U}(iw\~{n}C), while the existence of a polynomial upper bound for packet delay on DAGs when SIS is used for contention–resolution remains an open problem. We prove that every queueing network running a greedy contention–resolution protocol is stable for a rate not exceeding a particular stability threshold, depending on C and the length of the longest path in the network.
Abstract: In this paper we present an implementation and performance evaluation of a descent algorithm that was proposed in \cite{tsaspi} for the computation of approximate Nash equilibria of noncooperative bimatrix games. This algorithm, which achieves the best polynomially computable \epsilonapproximate equilibria till now, is applied here to several problem instances designed so as to avoid the existence of easy solutions. Its performance is analyzed in terms of quality of approximation and speed of convergence. The results demonstrate significantly better performance than the theoretical worst case bounds, both for the quality of approximation and for the speed of convergence. This motivates further investigation into the intrinsic characteristics of descent algorithms applied to bimatrix games. We discuss these issues and provide some insights about possible variations and extensions of the algorithmic concept that could lead to further understanding of the complexity of computing equilibria. We also prove here a new significantly better bound on the number of loops required for convergence of the descent algorithm.
Abstract: Pervasive games represent a radically new game form that extends
gaming experiences out into the physical world, weaving ICTs
into the fabric of players’ real environment. This emerging type of
games is rather challenging for developers who are engaged in
exploring new technologies and methods to achieve high quality
interactive experience for players. This paper follows a systematic
approach to explore the landscape of pervasive gaming. First, we
present ten representative pervasive game projects, classified in
five genres based on their playing environment and features.
Then, we present a comparative view of those projects with
respect to several design aspects. Last, we shed light on current
trends, design principles and future directions for pervasive games
development.
Abstract: This paper describes recent research activities and results in the area of photonic switching
carried out within the Virtual Department on Switching (VDS) of the European ePhoton/
ONe Network of Excellence. Contributions from outstanding European research groups in
this field are collected to offer a platform for future research in optical switching. The paper
contains the main topics related to network scenarios, switch architectures and experiments,
with an effort to investigate synergies and challenging opportunities for collaboration
and integration of research expertise in the field.
Abstract: In this work we extend the population protocol model of Angluin et al., in
order to model more powerful networks of very small resource limited
artefacts (agents) that is possible to follow some unpredictable passive
movement. These agents communicate in pairs according to the commands of
an adversary scheduler. A directed (or undirected) communication graph
encodes the following information: each edge (u,\~{o}) denotes that during the
computation it is possible for an interaction between u and \~{o} to happen in
which u is the initiator and \~{o} the responder. The new characteristic of
the proposed mediated population protocol model is the existance of a
passive communication provider that we call mediator. The mediator is a
simple database with communication capabilities. Its main purpose is to
maintain the permissible interactions in communication classes, whose
number is constant and independent of the population size. For this reason
we assume that each agent has a unique identifier for whose existence the
agent itself is not informed and thus cannot store it in its working
memory. When two agents are about to interact they send their ids to the
mediator. The mediator searches for that ordered pair in its database and
if it exists in some communication class it sends back to the agents the
state corresponding to that class. If this interaction is not permitted to
the agents, or, in other words, if this specific pair does not exist in
the database, the agents are informed to abord the interaction. Note that
in this manner for the first time we obtain some control on the safety of
the network and moreover the mediator provides us at any time with the
network topology. Equivalently, we can model the mediator by communication
links that are capable of keeping states from a edge state set of constant
cardinality. This alternative way of thinking of the new model has many
advantages concerning the formal modeling and the design of protocols,
since it enables us to abstract away the implementation details of the
mediator. Moreover, we extend further the new model by allowing the edges
to keep readable only costs, whose values also belong to a constant size
set. We then allow the protocol rules for pairwise interactions to modify
the corresponding edge state by also taking into account the costs. Thus,
our protocol descriptions are still independent of the population size and
do not use agent ids, i.e. they preserve scalability, uniformity and
anonymity. The proposed Mediated Population Protocols (MPP) can stably
compute graph properties of the communication graph. We show this for the
properties of maximal matchings (in undirected communication graphs), also
for finding the transitive closure of directed graphs and for finding all
edges of small cost. We demonstrate that our mediated protocols are
stronger than the classical population protocols. First of all we notice
an obvious fact: the classical model is a special case of the new model,
that is, the new model can compute at least the same things with the
classical one. We then present a mediated protocol that stably computes
the product of two nonnegative integers in the case where G is complete
directed and connected. Such kind of predicates are not semilinear and it
has been proven that classical population protocols in complete graphs can
compute precisely the semilinear predicates, thus in this manner we show
that there is at least one predicate that our model computes and which the
classical model cannot compute. To show this fact, we state and prove a
general Theorem about the composition of two mediated population
protocols, where the first one has stabilizing inputs. We also show that
all predicates stably computable in our model are (nonuniformly) in the
class NSPACE(m), where m is the number of edges of the communication
graph. Finally, we define Randomized MPP and show that, any Peano
predicate accepted by a Randomized MPP, can be verified in deterministic
polynomial time.
Abstract: With a rapidly aging population, the healthcare community will
soon face a severe medical personnel shortage. It is imperative that automated
health monitoring technologies be developed to help meet this
shortage. In this direction, we are developing Ayushman, a health monitoring
infrastructure and testbed. The vision behind its development
is twofold: first, to develop a wireless sensorbased automated health
monitoring system that can be used in diverse situations, from homebased
care, to disaster situations, without much customization; second,
to provide a testbed for implementing and testing communication protocols
and systems. Ayushman provides a collection of services which
enables it to perform this dual role. It possess a hierarchical cluster
topology which provides a faulttolerant and reliable system by ensuring
that each tier in the hierarchy is selfcontained and can survive on its
own in case of network partition. Ayushman is being implemented using
offtheshelf and diverse hardware and software components, which
presents many challenges in system integration and operational reliability.
This is an ongoing project at the IMPACT lab at Arizona State
University1, and in this paper, we present our system¢s architecture and
some of our experiences in the development of its initial prototype.
Abstract: In this paper we study the problem of assigning transmission ranges to the nodes of a multihop
packet radio network so as to minimize the total power consumed under the constraint
that adequate power is provided to the nodes to ensure that the network is strongly connected
(i.e., each node can communicate along some path in the network to every other node). Such
assignment of transmission ranges is called complete. We also consider the problem of achieving
strongly connected bounded diameter networks.
For the case of n + 1 colinear points at unit distance apart (the unit chain) we give a tight
asymptotic bound for the minimum cost of a range assignment of diameter h when h is a xed
constant and when h>(1 + ) log n, for some constant > 0. When the distances between the
colinear points are arbitrary, we give an O(n4) time dynamic programming algorithm for nding
a minimum cost complete range assignment.
For points in three dimensions we show that the problem of deciding whether a complete
range assignment of a given cost exists, is NPhard. For the same problem we give an O(n2)
time approximation algorithm which provides a complete range assignment with cost within a
factor of two of the minimum. The complexity of this problem in two dimensions remains open,
while the approximation algorithm works in this case as well.
Abstract: In this work we focus on the energy efficiency challenge in wireless sensor networks, from both an online perspective (related to routing), as well as a network design perspective (related to tracking). We investigate a few representative, important aspects of energy efficiency: a) the robust and fast data propagation b) the problem of balancing the energy
dissipation among all sensors in the network and c) the problem of efficiently tracking moving
entities in sensor networks. Our work here is a methodological survey of selected results that
have alre dy appeared in the related literature.
In particular, we investigate important issues of energy optimization, like minimizing the total
energy dissipation, minimizing the number of transmissions as well as balancing the energy
load to prolong the system¢s lifetime. We review characteristic protocols and techniques in the recent literature, including probabilistic forwarding and local optimization methods. We study the problem of localizing and tracking multiple moving targets from a network design perspective i.e. towards estimating the least possible number of sensors, their positions and operation characteristics needed to efficiently perform the tracking task. To avoid an expensive massive deployment, we try to take advantage of possible coverage overlaps over space and time, by introducing a novel combinatorial model that captures such overlaps. Under this model, we abstract the tracking network design problem by a covering combinatorial problem and then design and analyze an efficient approximate method for sensor placement
and operation.
Abstract: In this work we present three new distributed, probabilistic data propagation protocols for Wireless Sensor Networks which aim at maximizing the network's operational life and improve its performance. The keystone of these protocols' design is fairness which declares that fair portions of network's work load should be assigned to each node, depending on their role in the system. All the three protocols, EFPFR, MPFR and TWIST, emerged from the study of the rigorously analyzed protocol PFR. Its design elements were identified and improvements were suggested and incorporated into the introduced protocols. The experiments conducted show that our proposals manage to improve PFR's performance in terms of success rate, total amount of energy saved, number of alive sensors and standard deviation of the energy left. Indicatively we note that while PFR's success rate is 69.5%, TWIST is achieving 97.5% and its standard deviation of energy is almost half of that of PFR.
Abstract: The efficient delivery of web content has been identified
as a key issue of research for some time. Forward (or
reverse) proxies, which are positioned along the request
route from the users' browsers to the origin content servers,
maintain a cache with copies of content from their origin
servers. The strategic placement of proxies across the
backbone ISP or the Content Delivery Network can
drastically improve the performance of the system (in terms
of network bandwidth savings, origin server load, and
userrequest latency).
The ultimate goal of this work is to develop a tool that
decides on the position and the number of proxies required
in order to achieve given performance improvements
(expressed in terms of network bandwidth, origin server
load, and userlatency). We believe such a tool will be very
helpful to ISPs/CDNs, content providers, and endusers
and is, thus, very much lacking.
Abstract: The efficient delivery of internet content has been identified as a key issue of research for some time. Forward (or reverse) proxies, which are positioned along the request route from the users' browsers to the origin content servers, maintain a cache with copies of content from their origin servers. The strategic placement of proxies across the backbone ISP network (or the Content Delivery Network) can drastically improve the performance of the system (in terms of network bandwidth savings, origin server load, and userrequest latency).
The ultimate goal of this work is to develop a tool that decides on the position and the number of proxies required in order to achieve given performance improvements (expressed in terms of network bandwidth, origin server load, and userlatency). We believe such a tool will be very helpful to ISPs/CDNs, content providers, and endusers and is, thus, very much lacking.
Abstract: The contentbased publish/subscribe (pub/sub) paradigm for system design is becoming increasingly popular, offering
unique benefits for a large number of dataintensive applications. Coupled with the peertopeer technology, it can serve as
a central building block for developing datadissemination applications deployed over a largescale network infrastructure.
A key open problem towards the creation of largescale contentbased pub/sub infrastructures relates to efficiently and accu
rately matching subscriptions with substring predicates to incoming events. This work addresses this issue.
Abstract: The contentbased publish/subscribe (pub/sub)paradigm for system design is becoming increasingly popular, offering unique benefits for a large number of dataintensive applications. Coupled with the peertopeer technology, it can serve as a central building block for such applications
deployed over a largescale network infrastructure. A key problem toward the creation of largescale contentbased pub/sub infrastructures relates to dealing efficiently with continuous queries (subscriptions) with rich predicates on string attributes; In particular, efficiently and accurately
matching substring queries to incoming events is an open problem. In this work we study this problem. We provide and analyze novel algorithms for processing subscriptions with substring predicates and events in a variety of environments. We provide experimental data demonstrating the
relative performance behavior of the proposed algorithms using as key metrics the network bandwidth requirements, number of messages, load balancing, as well as requirements for extra routing state (and related maintenance) and design flexibility.
Abstract: This research attempts a first step towards investigating the aspect of radiation awareness in environments with abundant heterogeneous wireless networking. We call radiation at a point of a 3D wireless network the total amount of electromagnetic quantity the point is exposed to, our definition incorporates the effect of topology as well as the time domain, data traffic and environment aspects. Even if the impact of radiation to human health remains largely unexplored and controversial, we believe it is worth trying to understand and control. We first analyze radiation in well known topologies (random and grids), randomness is meant to capture not only node placement but also uncertainty of the wireless propagation model. This initial understanding of how radiation adds (over space and time) can be useful in network design, to reduce health risks. We then focus on the minimum radiation path problem of finding the lowest radiation trajectory of a person moving from a source to a destination point of the network region. We propose three heuristics which provide low radiation paths while keeping path length low, one heuristic gets in fact quite close to the offline solution we compute by a shortest path algorithm. Finally, we investigate the interesting impact on the heuristics' performance of diverse node mobility.
Abstract: In this work we present an efficient algorithm which, with high probability, provides
an almost uniform sample from the set of proper $\chi$colourings of an instance of sparse
random graphs $G_{n,d/n}$, where $\chi=\chi(d)$ is a sufficiently large constant.
This work improves, asymptotically, the result of Dyer, Flaxman Frieze and Vigoda
in \cite{previousresult} where the algorithm proposed there needs at least
$\Theta(\frac{\log \log n}{\log \log \log n})$ colours.
Abstract: The sensor devices are battery powered thus energy is the most precious resource of a wireless sensor
network since periodically replacing the battery of the nodes in large scale deployments is infeasible. The
collected data is disseminated to a static control point { data sink in the network, using node to node
{ multihop data propagation, [4, 6]. However, sensor devices consume signicant amounts of energy in
addition to increased implementation complexity since a routing protocol is executed. Also, a point of
failure emerges in the area near the control center where nodes relay the data from nodes that are farther
away
Abstract: In this work we address the issue of efficient processing of range queries in DHTbased P2P data networks. The novelty of the proposed approach lies on architectures, algorithms, and mechanisms for identifying and appropriately exploiting powerful nodes in such networks. The existence of such nodes has been well documented in the literature and plays a key role in the architecture of most successful realworld P2P applications. However, till now, this heterogeneity has not been taken into account when architecting solutions for complex query processing, especially in DHT networks. With this work we attempt to fill this gap for optimizing the processing of range queries. Significant performance improvements are achieved due to (i) ensuring a much smaller hop count performance for range queries, and (ii) avoiding the dangers and inefficiencies of relying for range query processing on weak nodes, with respect to processing, storage, and communication capacities, and with intermittent connectivity. We present detailed experimental results validating our performance claims.
Abstract: A collection of pervasive street games is presented in this paper, that constitute a new social form of play taking place in public spaces, such as city parks, public spaces and streets. The main characteristic of these games is the ability to scale to a large number of players (in some cases involving more than 40 players) and can engage players located simultaneously in dispersed areas. Players interact with each other using a wide range of hardware devices that are either generic (such as smart phones) or specific (such as wireless sensor devices). We discuss a set of fundamental issues related to game design emphasizing on the one hand the interaction of the players with the ubiquitous computing environment and on the other hand the embedding of the game rules within the environment. The games are developed using open source technologies and evaluated in a series of events such as the Athens Plaython 2012 festival. The feedback received from the players indicates that this new form of gaming is indeed very promising.
Abstract: The population protocol model (PP) proposed by Angluin et al. [2] describes sensor networks consisting of passively mobile finitestate agents. The agents sense their environment and communicate in pairs to carry out some computation on the sensed values. The mediated population protocol model (MPP) [13] extended the PP model by communication links equipped with a constant size buffer. The MPP model was proved in [13] to be stronger than the PP model. However, its most important contribution is that it provides us with the ability to devise optimizing protocols, approximation protocols and protocols that decide properties of the communication graph on which they run. The latter case, suggests a simplified model, the GDM model, that was formally defined and studied in [11]. GDM is a special case of MPP that captures MPP's ability to decide properties of the communication graph. Here we survey recent advances in the area initiated by the proposal of the PP model and at the same time we provide new protocols, novel ideas and results.
Abstract: In this paper we present a signaling protocol for
QoS differentiation suitable for optical burst switching networks.
The proposed protocol is a twoway reservation scheme that
employs delayed and inadvance reservation of resources. In this
scheme delayed reservations may be relaxed, introducing a
reservation duration parameter that is negotiated during call
setup phase. This feature allows bursts to reserve resources
beyond their actual size to increase their successful forwarding
probability and is used to provide QoS differentiation. The
proposed signaling protocol offers a low blocking probability for
bursts that can tolerate the roundtrip delay required for the
reservations. We present the main features of the protocol and
describe in detail timing considerations regarding the call setup
and the reservation process. We also describe several methods
for choosing the protocol parameters so as to optimize
performance and present corresponding evaluation results.
Furthermore, we compare the performance of the proposed
protocol against that of two other typical reservation protocols, a
TellandWait and a TellandGo protocol.
Abstract: This paper presents a summary of Optical Burst Switching (OBS) research within the VI framework program ePhoton/ONe network of excellence. The paper includes network aspects such as routing techniques, resilience and contention resolution, together with burst switch architectures. On the other hand, we also discuss traffic analysis issues, Quality of Service (QoS) schemes, TCP/IP over OBS and physical layer aspects for OBS.
Abstract: This paper presents a summary of Optical Burst Switching (OBS) research within the VI framework program ePhoton/ONe
network of excellence. The paper includes network aspects such as routing techniques, resilience and contention resolution, together
with burst switch architectures. On the other hand, we also discuss traffic analysis issues, Quality of Service (QoS) schemes, TCP/IP
over OBS and physical layer aspects for OBS.
Abstract: We propose information aggregation as a method for summarizing the resourcerelated information, used by the task scheduler. Through this method the information of a set of resources can be uniformly represented, reducing at the same time the amount of information transferred in a Grid network. A number of techniques are described for aggregating the information of the resources belonging to a hierarchical Grid domain. This information includes the cpu and storage capacities at a site, the number of tasks queued, and other resourcerelated parameters. The quality of the aggregation scheme affects the efficiency of the scheduler{\^a}€™s decisions. We use as a metric of aggregation efficiency the Stretch Factor (SF), defined as the ratio of the task delay when the task is scheduled using complete resource information over the task delay when an aggregation scheme is used. The simulation experiments performed show that the proposed aggregation schemes achieve large information reduction, while enabling good task scheduling decisions as indicated by the SF achieved.
Abstract: Distributed algorithm designers often assume that system processes execute the same predefined software. Alternatively, when they do not assume that, designers turn to noncooperative games and seek an outcome that corresponds to a rough consensus when no coordination is allowed. We argue that both assumptions are inapplicable in many real distributed systems, e.g., the Internet, and propose designing selfstabilizing and Byzantine faulttolerant distributed game authorities. Once established, the game authority can secure the execution of any complete information game. As a result, we reduce costs that are due to the processes¢ freedom of choice. Namely, we reduce the price of malice.
Abstract: The problem of robust line planning requests for a set of
origindestination paths (lines) along with their tra±c rates (frequencies)
in an underlying railway network infrastructure, which are robust to
°uctuations of realtime parameters of the solution.
In this work, we investigate a variant of robust line planning stemming
from recent regulations in the railway sector that introduce competition
and free railway markets, and set up a new application scenario: there is
a (potentially large) number of line operators that have their lines ¯xed
and operate as competing entities struggling to exploit the underlying
network infrastructure via frequency requests, while the management of
the infrastructure itself remains the responsibility of a single (typically
governmental) entity, the network operator.
The line operators are typically unwilling to reveal their true incentives.
Nevertheless, the network operator would like to ensure a fair (or, socially
optimal) usage of the infrastructure, e.g., by maximizing the (unknown to
him) aggregate incentives of the line operators. We show that this can be
accomplished in certain situations via a (possibly anonymous) incentive
compatible pricing scheme for the usage of the shared resources, that is
robust against the unknown incentives and the changes in the demands
of the entities. This brings up a new notion of robustness, which we
call incentivecompatible robustness, that considers as robustness of the
system its tolerance to the entities' unknown incentives and elasticity
of demands, aiming at an eventual stabilization to an equilibrium point
that is as close as possible to the social optimum.
Abstract: In this paper we present an efficient general simulation strategy for
computations designed for fully operational BSP machines of n ideal processors,
on nprocessor dynamicfaultprone BSP machines. The fault occurrences are failstop
and fully dynamic, i.e., they are allowed to happen online at any point of the
computation, subject to the constraint that the total number of faulty processors
may never exceed a known fraction. The computational paradigm can be exploited
for robust computations over virtual parallel settings with a volatile underlying
infrastructure, such as a NETWORK OF WORKSTATIONS (where workstations may be
taken out of the virtual parallel machine by their owner).
Our simulation strategy is Las Vegas (i.e., it may never fail, due to backtracking
operations to robustly stored instances of the computation, in case of locally
unrecoverable situations). It adopts an adaptive balancing scheme of the workload
among the currently live processors of the BSP machine.
Our strategy is efficient in the sense that, compared with an optimal offline
adversarial computation under the same sequence of fault occurrences, it achieves an O
¡
.log n ¢ log log n/2¢
multiplicative factor times the optimal work (namely, this
measure is in the sense of the “competitive ratio” of online analysis). In addition,
our scheme is modular, integrated, and considers many implementation points.
We comment that, to our knowledge, no previous work on robust parallel computations
has considered fully dynamic faults in the BSP model, or in general distributed
memory systems. Furthermore, this is the first time an efficient Las Vegas
simulation in this area is achieved.
Abstract: A key problem in networks that support advance reservations is the routing and time scheduling of connections with flexible starting time and known data transfer size. In this paper we present a multicost routing and scheduling algorithm for selecting the path to be followed by such a connection and the time the data should start and end transmission at each link so as to minimize the reception time at the destination, or optimize some other performance criterion. The utilization profiles of the network links, the link propagation delays, and the parameters of the connection to be scheduled form the inputs to the algorithm. We initially present a scheme of nonpolynomial complexity to compute a set of so called nondominated candidate paths, from which the optimal path can be found. We then propose two mechanisms to appropriately prune the set of candidate paths in order to find multicost routing and scheduling algorithms of polynomial complexity. We examine the performance of the algorithms in the special case of an Optical Burst Switched network. Our results indicate that the proposed polynomialtime algorithms have performance that is very close to that of the optimal algorithm. We also study the effects network propagation delays and linkstate update policies have on performance.
Abstract: A key problem in networks that support advance reservations is the routing and time scheduling of
connections with flexible starting time and known data transfer size. In this paper we present a multicost
routing and scheduling algorithm for selecting the path to be followed by such a connection and the time the
data should start and end transmission at each link so as to minimize the reception time at the destination,
or optimize some other performance criterion. The utilization profiles of the network links, the link
propagation delays, and the parameters of the connection to be scheduled form the inputs to the algorithm.
We initially present a scheme of nonpolynomial complexity to compute a set of socalled nondominated
candidate paths, from which the optimal path can be found. We then propose two mechanisms to
appropriately prune the set of candidate paths in order to find multicost routing and scheduling algorithms of
polynomial complexity. We examine the performance of the algorithms in the special case of an Optical
Burst Switched network. Our results indicate that the proposed polynomial time algorithms have performance that is very close to that of the optimal algorithm. We also study the effects network
propagation delays and linkstate update policies have on performance.
Abstract: A key problem in networks that support advance
reservations is the routing and time scheduling of connections
with flexible starting time. In this paper we present a multicost
routing and scheduling algorithm for selecting the path to be
followed by such a connection and the time the data should start
so as to minimize the reception time at the destination, or some
other QoS requirement. The utilization profiles of the network
links, the link propagation delays, and the parameters of the
connection to be scheduled form the inputs to the algorithm. We
initially present a scheme of nonpolynomial complexity to
compute a set of socalled nondominated candidate paths, from
which the optimal path can be found. By appropriately pruning
the set of candidate paths using path pseudodomination
relationships, we also find multicost routing and scheduling
algorithms of polynomial complexity. We examine the
performance of the algorithms in the special case of an Optical
Burst Switched network. Our results indicate that the proposed
polynomial time algorithms have performance that it is very close
to that of the optimal algorithm.
Abstract: Orthogonal Frequency Division Multiplexing (OFDM)
has been recently proposed as a modulation technique for optical
networks, due to its good spectral efficiency and impairment
tolerance. Optical OFDM is much more flexible compared to
traditional WDM systems, enabling elastic bandwidth
transmissions. We consider the planning problem of an OFDMbased optical network where we are given a traffic matrix that
includes the requested transmission rates of the connections to be
served. Connections are provisioned for their requested rate by
elastically allocating spectrum using a variable number of OFDM
subcarriers. We introduce the Routing and Spectrum Allocation
(RSA) problem, as opposed to the typical Routing and
Wavelength Assignment (RWA) problem of traditional WDM
networks, and present various algorithms to solve the RSA. We
start by presenting an optimal ILP RSA algorithm that minimizes
the spectrum used to serve the traffic matrix, and also present a
decomposition method that breaks RSA into two substituent
subproblems, namely, (i) routing and (ii) spectrum allocation
(R+SA) and solves them sequentially. We also propose a heuristic
algorithm that serves connections onebyone and use it to solve
the planning problem by sequentially serving all traffic matrix
connections. To feed the sequential algorithm, two ordering
policies are proposed; a simulated annealing metaheuristic is also
proposed to obtain even better orderings. Our results indicate
that the proposed sequential heuristic with appropriate ordering
yields close to optimal solutions in low running times.
Abstract: In this paper we demonstrate the significant impact of the user mobility rates on the performance on two different approaches for designing routing protocols for adhoc mobile networks: (a) the route creation and maintenance approach and (b) the "support" approach, that forces few hosts to move acting as
"helpers" for message delivery. We study a set of representative protocols for each approach, i.e.~DSR and ZRP for the first approach and RUNNERS for the second. We have implemented the three protocols and performed a large scale and detailed simulation study of their performance. Our findings are: (i) DSR achieves low message delivery rates but manages to deliver messages very fast; (ii) ZRP behaves well in networks of low mobility rate, while its performance drops for networks of highly mobile users; (iii) RUNNERS seem to tolerate well (and in fact benefit from) high mobility rates.
Based on our investigation, we design and implement two new protocols that result from the synthesis of the investigated routing approaches. We conducted an extensive, comparative simulation study of their performance. The new protocols behave well both in networks of diverse mobility motion rates, and in some cases they even outperform the original ones by achieving lower message delivery delays.
Abstract: Data propagation in wireless sensor
networks is usually performed as a multihop process.
Thus,
To deliver a single
message, the resources of many sensor nodes are used and
a lot of energy is spent.
Recently, a novel approach is catching momentum because of important applications;
that of having a mobile sink move inside the network area and collect
the data with low energy cost.
Here we extend this line of research by proposing and evaluating three new protocols.
Our protocols are novel in
a) investigating the impact of having {many} mobile sinks
b) in weak models with restricted mobility, proposing and evaluating
a mix of static and mobile sinks and c) proposing a distributed
protocol that tends to {equally spread the sinks} in the network to
further improve performance.
Our protocols are simple, based on randomization and assume locally
obtainable information. We perform an extensive evaluation via simulation; our
findings demonstrate that our solutions scale very well with respect to the number of sinks
and significantly reduce energy consumption and delivery delay.
Abstract: We consider information aggregation as a method for reducing the information exchanged in a Grid network and used by the resource manager in order to make scheduling decisions. In this way, information is summarized across nodes and sensitive or detailed information can be kept private, while resources are still publicly available for use. We present a general framework for information aggregation, trying to identify issues that relate to aggregation in Grids. In this context, we describe a number of techniques, including single point and intradomain aggregation, define appropriate gridspecific domination relations and operators for aggregating static and dynamic resource information, and discuss resource selection optimization functions. The quality of an aggregation scheme is measured both by its effects on the efficiency of the scheduler¢s decisions and also by the reduction it brings on the amount of resource information recorded, a tradeoff that we examine in detail. Simulation experiments demonstrate that the proposed schemes achieve significant information reduction, either in the amount of information exchanged, or in the frequency of the updates, while at the same time maintaining most of the value of the original information as expressed by a stretch factor metric we introduce.
Abstract: We present SeAl1, a novel data/resource and dataaccess management infrastructure designed for the purpose of addressing a key problem in P2P data sharing networks, namely the problem of widescale selfish peer behavior. Selfish behavior has been manifested and well documented and it is widely accepted that unless this is dealt with, the scalability, efficiency, and the usefulness of P2P sharing networks will be diminished. SeAl essentially consists of a monitoring/accounting subsystem, an auditing/verification subsystem, and incentive mechanisms. The monitoring subsystem facilitates the classification of peers into selfish/altruistic. The auditing/verification layer provides a shield against perjurer/slandering and colluding peers that may try to cheat the monitoring subsystem. The incentives mechanisms efectively utilize these layers so to increase the computational/networking and data resources that are available to the community. Our extensive performance results show that SeAl performs its tasks swiftly, while the overhead introduced by our accounting and auditing mechanisms in terms of response time, network, and storage overheads are very small.
Abstract: We present SeAl, a novel data/resource and dataaccess management infrastructure designed for the purpose of addressing a key problem in P2P data sharing networks, namely the problem of widescale selfish peer behavior. Selfish behavior has been manifested and well documented and it is widely accepted that unless this is dealt with, the scalability, efficiency, and the usefulness of P2P sharing networks will be diminished. SeAl essentially consists of a monitoring/accounting subsystem, an auditing/verification subsystem, and incentive mechanisms. The monitoring subsystem facilitates the classification of peers into selfish/altruistic. The auditing/verification layer provides a shield against perjurer/slandering and colluding peers that may try to cheat the monitoring subsystem. The incentives mechanisms effectively utilize these layers so to increase the computational/networking and data resources that are available to the community. Our extensive performance results show that SeAl performs its tasks swiftly, while the overhead introduced by our accounting and auditing mechanisms in terms of response time, network, and storage overheads are very small.
Abstract: During the last years web search engines have moved from the simple but inefficient syntactical analysis (first generation) to the more robust and usable web graph analysis (second generation). Much of the current research is focussed on the socalled third generation search engines that, in principle, inject human characteristics on how results are obtained and presented to the end user. Approaches exploited towards this direction include (among others): an alteration of PageRank [1] that takes into account user specific characteristics and bias the page ordering using the user preferences (an approach, though, that does not scale well with the number of users). The approach is further exploited in [3], where several PageRanks are computed for a given number of distinct search topics. A similar idea is used in [6], where the PageRank computation takes into account the content of the pages and the query terms the surfer is looking for. In [4], a decomposition of PageRank to basic components is suggested that may be able to scale the different PageRank computations to a bigger number of topics or even distinct users. Another approach to web search is presented in [2], where a rich extension of the web, called semantic web, and the application of searching over this new setting is described.
Abstract: Embedded computing devices dominate our everyday activities, from cell phones to wireless sensors that collect and process data for various applications. Although desktop and highend server security seems to be under control by the use of current security technology, securing the lowend embedded computing systems is a difficult longterm problem. This is mainly due to the fact that the embedded systems are constrained by their operational environment and the limited resources they are equipped with. Recent research activities focus on the deployment of lightweight cryptographic algorithms and security protocols that are well suited to the limited resources of lowend embedded systems. Elliptic Curve Cryptography (ECC) offers an interesting alternative to the classical public key cryptography for embedded systems (e.g., RSA and ElGamal), since it uses smaller key sizes for achieving the same security level, thus making ECC an attractive and efficient alternative for deployment in embedded systems. In this chapter, the processing requirements and architectures for secure network access, communication functions, storage, and high availability of embedded devices are discussed. In addition, ECCbased stateoftheart lightweight cryptographic primitives for the deployment of security protocols in embedded systems that fulfill the requirements are presented.
Abstract: In this work we tackle the open problem of selfjoin size (SJS) estimation in a largescale distributed data system, where tuples of a relation are distributed over data nodes which comprise an overlay network. Our contributions include adaptations of five wellknown SJS estimation centralized techniques (coined sequential, crosssampling, adaptive, bifocal, and samplecount) to the network environment and a novel technique which is based on the use of the Gini coefficient. We develop analyses showing how Gini estimations can lead to estimations of the underlying Zipfian or powerlaw value distributions. We further contribute distributed sampling algorithms that can estimate accurately and efficiently the Gini coefficient. Finally, we provide detailed experimental evidence testifying for the claimed increased accuracy, precision, and efficiency of the proposed SJS estimation method, compared to the other methods. The proposed approach is the only one to ensure high efficiency, precision, and accuracy regardless of the skew of the underlying data.
Abstract: Our position is that a key to research efforts on ensuring high
performance in very large scale sharing networks is the idea of
volunteering; recognizing that such networks are comprised of
largely heterogeneous nodes in terms of their capacity and
behaviour, and that, in many realworld manifestations, a few
nodes carry the bulk of the request service load. In this paper we
outline how we employ volunteering as the basic idea using
which we develop altruismendowed selforganizing sharing
networks to help solve two open problems in largescale peertopeer
networks: (i) to develop an overlay topology structure that
enjoys better performance than DHTstructured networks and,
specifically, to offer O(log log N) routing performance in a
network of N nodes, instead of O(log N), and (ii) to efficiently
process complex queries and range queries, in particular.
Abstract: Geographic routing is becoming the protocol of choice for
many sensor network applications. The current state of the art is unsatisfactory:
some algorithms are very efficient, however they require a
preliminary planarization of the communication graph. Planarization induces
overhead and is thus not realistic for some scenarios such as the
case of highly dynamic network topologies. On the other hand, georouting
algorithms which do not rely on planarization have fairly low success
rates and fail to route messages around all but the simplest obstacles.
To overcome these limitations, we propose the GRIC geographic routing
algorithm. It has absolutely no topology maintenance overhead, almost
100% delivery rates (when no obstacles are added), bypasses large convex
obstacles, finds short paths to the destination, resists link failure
and is fairly simple to implement. The case of hard concave obstacles
is also studied; such obstacles are hard instances for which performance
diminishes.
Abstract: In this work, we study protocols so that populations of distributed processes can construct networks. In order to highlight the basic principles of distributed network construction, we keep the model minimal in all respects. In particular, we assume finitestate processes that all begin from the same initial state and all execute the same protocol. Moreover, we assume pairwise interactions between the processes that are scheduled by a fair adversary. In order to allow processes to construct networks, we let them activate and deactivate their pairwise connections. When two processes interact, the protocol takes as input the states of the processes and the state of their connection and updates all of them. Initially all connections are inactive and the goal is for the processes, after interacting and activating/deactivating connections for a while, to end up with a desired stable network. We give protocols (optimal in some cases) and lower bounds for several basic network construction problems such as spanning line, spanning ring, spanning star, and regular network. The expected time to convergence of our protocols is analyzed under a uniform random scheduler. Finally, we prove several universality results by presenting generic protocols that are capable of simulating a Turing Machine (TM) and exploiting it in order to construct a large class of networks. We additionally show how to partition the population into k supernodes, each being a line of log k nodes, for the largest such k. This amount of local memory is sufficient for the supernodes to obtain unique names and exploit their names and their memory to realize nontrivial constructions.
Abstract: In this work, we study protocols (i.e. distributed algorithms) so that populations of distributed processes can construct networks. In order to highlight the basic principles of distributed network construction we keep the model minimal in all respects. In particular, we assume finitestate processes that all begin from the same initial state and all execute the same protocol (i.e. the system is homogeneous). Moreover, we assume pairwise interactions between the processes that are scheduled by an adversary. The only constraint on the adversary scheduler is that it must be fair, intuitively meaning that it must assign to every reachable configuration of the system a nonzero probability to occur. In order to allow processes to construct networks, we let them activate and deactivate their pairwise connections. When two processes interact, the protocol takes as input the states of the processes and the state of their connection and updates all of them. In particular, in every interaction, the protocol may activate an inactive connection, deactivate an active one, or leave the state of a connection unchanged. Initially all connections are inactive and the goal is for the processes, after interacting and activating/deactivating connections for a while, to end up with a desired stable network (i.e. one that does not change any more). We give protocols (optimal in some cases) and lower bounds for several basic network construction problems such as spanning line, spanning ring, spanning star, and regular network. We provide proofs of correctness for all of our protocols and analyze the expected time to convergence of most of them under a uniform random scheduler that selects the next pair of interacting processes uniformly at random from all such pairs. Finally, we prove several universality results by presenting generic protocols that are capable of simulating a Turing Machine (TM) and exploiting it in order to construct a large class of networks. Our universality protocols use a subset of the population (waste) in order to distributedly construct there a TM able to decide a graph class in some given space. Then, the protocols repeatedly construct in the rest of the population (useful space) a graph equiprobably drawn from all possible graphs. The TM works on this and accepts if the presented graph is in the class. We additionally show how to partition the population into k supernodes, each being a line of log k nodes, for the largest such k. This amount of local memory is sufficient for the supernodes to obtain unique names and exploit their names and their memory to realize nontrivial constructions. Delicate composition and reinitialization issues have to be solved for these general constructions to work.
Abstract: Buildings are among the largest consumers of electricity with a significant portion of this energy use is wasted in unoccupied areas or improperly installed devices.
Identifying such power leaks is hard especially in large office and enterprise buildings.
In this paper we present the design and implementation of a system that uses an underlying sensor network to provide accurate real time information about various characteristics like occupancy, lighting, temperature and power consumption at different levels of granularity.
All sensor devices require minimal installation and maintenance.
Using an experimental installation we evaluate a number of applications and services that achieve energy savings by applying different power conservation policies.
Furthermore we provide energy measurements to users and occupants to show how various choices and behaviors affect their personal energy savings.
Abstract: In 1876 Charles Lutwidge Dodgson suggested the intriguing voting rule that today bears his name. Although Dodgson's rule is one of the most wellstudied voting rules, it suffers from serious deciencies, both from the computational point of viewit is NPhard even to approximate the Dodgson score within sublogarithmic factorsand from the social choice point of viewit fails basic social choice desiderata such as monotonicity and homogeneity.
In a previous paper [Caragiannis et al., SODA 2009] we have asked whether there are approximation algorithms for Dodgson's rule that are monotonic or homogeneous. In this paper we give denitive answers to these questions. We design a monotonic exponentialtime algorithm that yields a 2approximation to the Dodgson score, while matching this result with a tight lower bound. We also present a monotonic polynomialtime O(logm)approximation algorithm (where m is the number of alternatives); this result is tight as well due to a complexitytheoretic lower bound. Furthermore, we show that a slight variation of a known voting rule yields a monotonic, homogeneous, polynomialtime O(mlogm)approximation algorithm, and establish that it is impossible to achieve a better approximation ratio even if one just asks for homogeneity. We complete the picture by studying several additional social choice properties; for these properties, we prove that algorithms with an approximation ratio that depends only on m do not exist.
Abstract: In 1876 Charles Lutwidge Dodgson suggested the intriguing
voting rule that today bears his name. Although Dodg
son's rule is one of the most wellstudied voting rules, it suf
fers from serious deciencies, both from the computational
point of viewit is NPhard even to approximate the Dodg
son score within sublogarithmic factorsand from the social
choice point of viewit fails basic social choice desiderata
such as monotonicity and homogeneity.
In a previous paper [Caragiannis et al., SODA 2009] we
have asked whether there are approximation algorithms for
Dodgson's rule that are monotonic or homogeneous. In this
paper we give denitive answers to these questions. We de
sign a monotonic exponentialtime algorithm that yields a
2approximation to the Dodgson score, while matching this
result with a tight lower bound. We also present a monotonic
polynomialtime O(logm)approximation algorithm (where
m is the number of alternatives); this result is tight as well
due to a complexitytheoretic lower bound. Furthermore,
we show that a slight variation of a known voting rule yields
a monotonic, homogeneous, polynomialtime O(mlogm)
approximation algorithm, and establish that it is impossible
to achieve a better approximation ratio even if one just asks
for homogeneity. We complete the picture by studying sev
eral additional social choice properties; for these properties,
we prove that algorithms with an approximation ratio that
depends only on m do not exist.
Abstract: We generalize Cuckoo Hashing [16] to dary Cuckoo Hashing and show how this yields a simple hash table data structure that stores n elements in (1 + ∈) n memory cells, for any constant ∈ > 0. Assuming uniform hashing, accessing or deleting table entries takes at most d = O(ln 1/∈ ) probes and the expected amortized insertion time is constant. This is the first dictionary that has worst case constant access time and expected constant update time, works with (1+∈) n space, and supports satellite information. Experiments indicate that d = 4 choices suffice for ∈ ≈ 0.03. We also describe a hash table data structure using explicit constant time hash functions, using at most d = O(ln2 1/∈ ) probes in the worst case.
A corollary is an expected linear time algorithm for finding maximum cardinality matchings in a rather natural model of sparse random bipartite graphs.
This work was partially supported by DFG grant SA 933/11 and the Future and Emerging Technologies programme of the EU under contract number IST1999 14186 (ALCOMFT).
The present work was initiated while this author was at BRICS, Aarhus University, Denmark.
Part of this work was done while the author was at MPII. 1 In this paper “whp.” will mean “with probability 1  O(1/n)”.
Abstract: We generalize Cuckoo Hashing [16] to dary Cuckoo Hashing and show how this yields a simple hash table data structure that stores n elements in (1 + ∈) n memory cells, for any constant ∈ > 0. Assuming uniform hashing, accessing or deleting table entries takes at most d = O(ln 1/∈ ) probes and the expected amortized insertion time is constant. This is the first dictionary that has worst case constant access time and expected constant update time, works with (1+∈) n space, and supports satellite information. Experiments indicate that d = 4 choices suffice for ∈ ≈ 0.03. We also describe a hash table data structure using explicit constant time hash functions, using at most d = O(ln2 1/∈ ) probes in the worst case.
A corollary is an expected linear time algorithm for finding maximum cardinality matchings in a rather natural model of sparse random bipartite graphs.
This work was partially supported by DFG grant SA 933/11 and the Future and Emerging Technologies programme of the EU under contract number IST1999 14186 (ALCOMFT).
The present work was initiated while this author was at BRICS, Aarhus University, Denmark.
Part of this work was done while the author was at MPII. 1 In this paper “whp.” will mean “with probability 1  O(1/n)”.
Abstract: Efficient task scheduling is fundamental for the success of the Grids,
since it directly affects the Quality of Service (QoS) offered to the users. Efficient
scheduling policies should be evaluated based not only on performance
metrics that are of interest to the infrastructure side, such as the Grid resources
utilization efficiency, but also on user satisfaction metrics, such as the percentage
of tasks served by the Grid without violating their QoS requirements. In this
paper, we propose a scheduling algorithm for tasks with strict timing requirements,
given in the form of a desired start and finish time. Our algorithm aims
at minimizing the violations of the time constraints, while at the same time
minimizing the number of processors used. The proposed scheduling method
exploits concepts derived from spectral clustering, and groups together for assignment
to a computing resource the tasks so to a) minimize the time overlapping
of the tasks assigned to a given processor and b) maximize the degree of
time overlapping among tasks assigned to different processors. Experimental
results show that our proposed strategy outperforms greedy scheduling algorithms
for different values of the task load submitted.
Abstract: Efficient task scheduling is fundamental for the success of the Grids,
since it directly affects the Quality of Service (QoS) offered to the users. Efficient
scheduling policies should be evaluated based not only on performance
metrics that are of interest to the infrastructure side, such as the Grid resources
utilization efficiency, but also on user satisfaction metrics, such as the percentage
of tasks served by the Grid without violating their QoS requirements. In this
paper, we propose a scheduling algorithm for tasks with strict timing requirements,
given in the form of a desired start and finish time. Our algorithm aims
at minimizing the violations of the time constraints, while at the same time
minimizing the number of processors used. The proposed scheduling method
exploits concepts derived from spectral clustering, and groups together for assignment
to a computing resource the tasks so to a) minimize the time overlapping
of the tasks assigned to a given processor and b) maximize the degree of
time overlapping among tasks assigned to different processors. Experimental
results show that our proposed strategy outperforms greedy scheduling algorithms
for different values of the task load submitted.
Abstract: One of the most eminent problems in sensor networks is the
routing of data to a central destination in a robust and e±cient manner.
In this work we propose a new scalable protocol for propagating infor
mation about a sensed event towards a receiving center. Using only local
information and total absence of coordination between sensors our pro
tocol achieves to propagate the sensed data to a receiving center by ac
tivating only those nodes that lie very close to the optimal path between
the source of the event and the destination, resulting in low activation of
the network's sensors. Thus the protocol is very energy e±cient. Further
more, our protocol is robust as it manages to propagate the information
even when sensors fail with certain probability.
Abstract: In this paper, we analyze the stability properties of the FIFO protocol in the Adversarial Queueing model for packet routing. We show a graph for which FIFO is stable for any adversary with injection rate r ≰ 0.1428. We generalize this results to show upper bound for stability of any network under FIFO protocol, answering partially an open question raised by Andrews et al. in [2]. We also design a network and an adversary for which FIFO is nonstable for any r ≱ 0.8357, improving the previous known bounds of [2].
Abstract: In wireless communication, the signal of a typical broadcast station is transmittes from a broadvast center p and reaches objects at a distance,say , r from it. In addition there is a radius ro, ro < r, such that the signal originating from the center p should be avoided. In other words, points within distance ro from the station compise a hazardous zone. We consider the following station layout problem: Cover a given planar region that includes a collection of buildings with a minimum number of astations so that every point in the region is within the reach of a station, while at the same time no interior point of any building is within the hazardous zone of a station. We give algorithms for computing such station layouts in both the one and two  dimensional cases
Abstract: In emerging pervasive scenarios, data is collected by sensing devices in streams that occur at several distributed points of observation. The size of the data typically far exceeds the storage and computational capabilities of the tiny devices that have to collect and process them. A general and challenging task is to allow (some of) the nodes of a pervasive network to collectively perform monitoring of a neighbourhood of interest by issuing continuous aggregate queries on the streams observed in its vicinity. This class of algorithms is fully decentralized and diffusive in nature: collecting all the data at a few central nodes of the network is unfeasible in networks of low capability devices or in the presence of massive data sets. Two main problems arise in this scenario: (i) the intrinsic complexity of maintaining statistics over a data stream whose size greatly exceeds the capabilities of the device that performs the computation; (ii) composing the partial outcomes computed at different points of observation into an accurate, global statistic over a neighbourhood of interest, which entails coping with several problems, last but not least the receipt of duplicate information along multiple paths of diffusion.
Streaming techniques have emerged as powerful tools to achieve the general goals described above, in the first place because they assume a computational model in which computational and storage resources are assumed to be far exceeded by the amount of data on which computation occurs. In this contribution, we review the main streaming techniques and provide a classification of the computational problems and the applications they effectively address, with an emphasis on decentralized scenarios, which are of particular interest in pervasive networks
Abstract: In this paper, we present and study solutions for the efficient processing of queries over string attributes in a large P2P data network implemented with DHTs. The proposed solutions support queries with equality, prefix, suffix, and containment predicates over string attributes. Currently, no known solution to this problem exists.
We propose and study algorithms for processing such queries and their optimizations. As eventbased, Publish/Subscribe information systems are a champion application class where string attribute (continuous) queries are very common, we pay particular attention to this type of data networks, formulating our solution in terms of this environment. A major design decision behind the proposed solution is our intention to provide a solution that is general (DHTindependent), capable of being implemented on top of any particular DHT.
Abstract: This work extends what is known so far for a basic model of
evolutionary antagonis
m in undirected ne
tworks (graphs).
More specif
ically, this work studies the generalized Moran process, as introduced
by Lieberman, Hauert, and Nowak [Nature, 433:312316, 2005], where
the individuals of a population reside on the vertices of an undirected
connected graph. The initial population has a single
mutant
of a
fitness
value
r
(typically
r>
1), residing at some vertex
v
of the graph, while
every other vertex is initially occupied by an individual of fitness 1. At
every step of this process, an individual (i.e. vertex) is randomly chosen
for reproduction with probability proportional to its fitness, and then it
places a copy of itself on a random neighbor, thus replacing the individ
ual that was residing there. The main quantity of interest is the
fixation
probability
, i.e. the probability that eventually the whole graph is occu
pied by descendants of the mutant. In this work we concentrate on the
fixation probability when the mutant is initially on a specific vertex
v
,
thus refining the older notion of Lieberman et al. which studied the fix
ation probability when the initial mutant is placed at a random vertex.
We then aim at finding graphs that have many “strong starts” (or many
“weak starts”) for the mutant. Thus we introduce a parameterized no
tion of
selective amplifiers
(resp.
selective suppressors
)ofevolution.We
prove the existence of
strong
selective amplifiers (i.e. for
h
(
n
)=
Θ
(
n
)
vertices
v
the fixation probability of
v
is at least 1
−
c
(
r
)
n
for a func
tion
c
(
r
) that depends only on
r
), and the existence of quite strong
selective suppressors. Regarding the traditional notion of fixation prob
ability from a random start, we provi
de strong upper and lower bounds:
first we demonstrate the nonexistence of “strong universal” amplifiers,
and second we prove the
Thermal Theorem
which states that for any
undirected graph, when the mutant starts at vertex
v
, the fixation prob
ability at least (
r
−
1)
/
(
r
+
deg
v
deg
min
). This theorem (which extends the
“Isothermal Theorem” of Lieberman et al. for regular graphs) implies
an almost tight lower bound for the usual notion of fixation probability.
Our proof techniques are original and are based on new domination ar
guments which may be of general interest in Markov Processes that are
of the general birthdeath type.
Abstract: A key issue when designing and implementing largescale publish/subscribe systems is how to efficiently propagate subscriptions among the brokers of the system. Brokers require this information in order to forward incoming events only to interested users, filtering out unrelated events, which can save significant overheads (particularly network bandwidth and processing time at the brokers). In this paper we contribute the notion of subscription summaries, a mechanism appropriately compacting subscription information. We develop the associated data structures and matching algorithms. The proposed mechanism can handle event/subscription schemata that are rich in terms of their attribute types and powerful in terms of the allowed operations on them. Our major results are that the proposed mechanism (i) is scalable, with the bandwidth required to propagate subscriptions increasing only slightly, even at hugescales, and (ii) is significantly more efficient, up to orders of magnitude, depending on the scale, with respect to the bandwidth requirements for propagating subscriptions.
Abstract: The contentbased publish/subscribe (pub/sub)
paradigm for system design is becoming increasingly
popular, offering unique benefits for a large number of
dataintensive applications. Coupled with the peertopeer
technology, it can serve as a central building block for
such applications deployed over a largescale network
infrastructure. A key problem toward the creation of
largescale contentbased pub/sub infrastructures relates to
dealing efficiently with continuous queries (subscriptions)
with rich predicates on string attributes; in this work we
study the problem of efficiently and accurately matching
substring queries to incoming events.
Abstract: Prompt availability of critical information to the right people is the main factor for the management of any
emergency situation. However, despite the numerous investments in the domain of public safety and
security, recent events demonstrate that, both in Europe and US, this is still an open issue.
Abstract: We investigate the impact of different mobility rates on the
performance of routing protocols in adhoc mobile networks. Based
on our investigation, we design a new protocol that results from
the synthesis of the well known protocols: ZRP and RUNNERS. We have implemented the new protocol as well as
the original two protocols and conducted an extensive, comparative
simulation study of their performance. The new protocol behaves
well both in networks of diverse mobility motion rates, and in
some cases even outperforms the original ones by achieving lower
message delivery delays.
Abstract: We study congestion games where players aim to access a set of resources. Each player has a set of possible strategies and each resource has a function associating the latency it incurs to the players using it. Players are non–cooperative and each wishes to follow strategies that minimize her own latency with no regard to the global optimum. Previous work has studied the impact of this selfish behavior to system performance. In this paper, we study the question of how much the performance can be improved if players are forced to pay taxes for using resources. Our objective is to extend the original game so that selfish behavior does not deteriorate performance. We consider atomic congestion games with linear latency functions and present both negative and positive results. Our negative results show that optimal system performance cannot be achieved even in very simple games. On the positive side, we show that there are ways to assign taxes that can improve the performance of linear congestion games by forcing players to follow strategies where the total latency suffered is within a factor of 2 of the minimum possible; this result is shown to be tight. Furthermore, even in cases where in the absence of taxes the system behavior may be very poor, we show that the total disutility of players (latency plus taxes) is not much larger than the optimal total latency. Besides existential results, we show how to compute taxes in time polynomial in the size of the game by solving convex quadratic programs. Similar questions have been extensively studied in the model of nonatomic congestion games. To the best of our knowledge, this is the first study of the efficiency of taxes in atomic congestion games.
Abstract: In this work we consider temporal networks, i.e. networks defined by a labeling $\lambda$ assigning to each edge of an underlying graph G a set of discrete timelabels. The labels of an edge, which are natural numbers, indicate the discrete time moments at which the edge is available. We focus on path problems of temporal networks. In particular, we consider timerespecting paths, i.e. paths whose edges are assigned by $\lambda$ a strictly increasing sequence of labels. We begin by giving two efficient algorithms for computing shortest timerespecting paths on a temporal network. We then prove that there is a natural analogue of Menger’s theorem holding for arbitrary temporal networks. Finally, we propose two cost minimization parameters for temporal network design. One is the temporality of G, in which the goal is to minimize the maximum number of labels of an edge, and the other is the temporal cost of G, in which the goal is to minimize the total number of labels used. Optimization of these parameters is performed subject to some connectivity constraint. We prove several lower and upper bounds for the temporality and the temporal cost of some very basic graph families such as rings, directed acyclic graphs, and trees.
Abstract: In this work, we consider a \emph{solution of automata} similar to \emph{Population Protocols} and \emph{Network Constructors}. The automata (also called \emph{nodes}) move passively in a wellmixed solution without being capable of controlling their movement. However, the nodes can \emph{cooperate} by interacting in pairs. Every such interaction may result in an update of the local states of the nodes. Additionally, the nodes may also choose to connect to each other in order to start forming some required structure. We may think of such nodes as the \emph{smallest possible programmable pieces of matter}, like tiny nanorobots or programmable molecules. The model that we introduce here is a more applied version of Network Constructors, imposing \emph{physical} (or \emph{geometrical}) \emph{constraints} on the connections that the nodes are allowed to form. Each node can connect to other nodes only via a very limited number of \emph{local ports}, which implies that at any given time it has only a \emph{bounded number of neighbors}. Connections are always made at \emph{unit distance} and are \emph{perpendicular to connections of neighboring ports}. Though such a model cannot form abstract networks like Network Constructors, it is still capable of forming very practical \emph{2D or 3D shapes}. We provide direct constructors for some basic shape construction problems, like \emph{spanning line}, \emph{spanning square}, and \emph{selfreplication}. We then develop \emph{new techniques} for determining the computational and constructive capabilities of our model. One of the main novelties of our approach, concerns our attempt to overcome the inability of such systems to detect termination. In particular, we exploit the assumptions that the system is wellmixed and has a unique leader, in order to \emph{give terminating protocols that are correct with high probability}. This allows us to develop terminating subroutines that can be \emph{sequentially composed} to form larger \emph{modular protocols} (which has not been the case in the relevant literature). One of our main results is a \emph{terminating protocol counting the size $n$ of the system} with high probability. We then use this protocol as a subroutine in order to develop our \emph{universal constructors}, establishing that \emph{it is possible for the nodes to become selforganized with high probability into arbitrarily complex shapes while still detecting termination of the construction}.
Abstract: We extend the population protocol model with a covertime service that informs a walking state every time it covers the whole network. This represents a known upper bound on the cover time of a random walk. The covertime service allows us to introduce termination into population protocols, a capability that is crucial for any distributed system. By reduction to an oraclemodel we arrive at a very satisfactory lower bound on the computational power of the model: we prove that it is at least as strong as a Turing Machine of space log n with input commutativity, where n is the number of nodes in the network. We also give a log nspace, but nondeterministic this time, upper bound. Finally, we prove interesting similarities of this model to linear bounded automata.
Abstract: We extend the population protocol model with a covertime service that informs a walking state every time it covers the whole network. This is simply a known upper bound on the cover time of a random walk. This allows us to introduce termination into population protocols, a capability that is crucial for any distributed system. By reduction to an oraclemodel we arrive at a very satisfactory lower bound on the computational power of the model: we prove that it is at least as strong as a Turing Machine of space logn with input commutativity, where n is the number of nodes in the network. We also give a lognspace, but nondeterministic this time, upper bound. Finally, we prove interesting similarities of this model to linear bounded automata.
Abstract: In this work, we discuss multiplayer pervasive
games that rely on the use of ad hoc mobile sensor networks.
The unique feature in such games is that players interact
with each other and their surrounding environment by using
movement and presence as a means of performing gamerelated
actions, utilizing sensor devices. We discuss the fundamental
issues and challenges related to these type of games and the
scenarios associated with them. We also present and evaluate
an example of such a game, called the “Hot Potato”, developed
using the Sun SPOT hardware platform. We provide a set of
experimental results, so as to both evaluate our implementation
and also to identify issues that arise in pervasive games which
utilize sensor network nodes, which show that there is great
potential in this type of games.
Abstract: In this paper we present the design of jWebDust, a generic and modular application environment for developing and managing applications based on wireless sensor networks that are accessible via the internet. Our software architecture provides a range of services that allow to create customized webbased applications with minimum implementation effort that are easy to administrate. We here present its open architecture, the most important design decisions, and discuss its distinct features and functionalities. jWebDust allows heterogeneous components to interoperate and the integrated management and control of multiple such networks by defining webbased mechanisms to visualize the network state, the results of queries, and a means to inject queries in the network.
Abstract: A lot of activity is being devoted to studying issues related to energy consumption and efficiency in our buildings, and especially on public buildings. In this context, the educational public buildings should bean important part of the equation. At the same time, there is an evident need for open datasets, which should be publicly available for researchers to use. We have implemented a realworld multisite Internet of Things (IoT) deployment, comprising 25 school buildings across Europe, primarily designed as a foundation for enabling IoTbased energy awareness and sustainability lectures and promoting datadriven energysaving behaviors. In this work, we present some of the basic aspects to producing datasets from this deployment and discuss its potential uses. We also provide a brief discussion on data derived from a preliminary analysis of thermal comfortrelated data produced from this infrastructure.
Abstract: We explore the capability of a network of extremely limited
computational entities to decide properties about any of its subnetworks.
We consider that the underlying network of the interacting
entities (devices, agents, processes etc.) is modeled by a complete in
teraction graph and we devise simple graph protocols that can decide
properties of some input subgraph provided by some preprocessing on
the network. The agents are modeled as nitestate automata and run
the same global graph protocol. Each protocol is a xed size grammar,
that is, its description is independent of the size (number of agents) of
the network. This size is not known by the agents. We propose a simple
model, the Mediated Graph Protocol (MGP) model, similar to the Population
Protocol model of Angluin et al., in which each network link is
characterized by a state taken from a nite set. This state can be used
and updated during each interaction between the corresponding agents.
We provide some interesting properties of the MGP model among which
is the ability to decide properties on stabilizing (initially changing for a
nite number of steps) input graphs and we show that the MGP model
has the ability to decide properties of disconnected input graphs. We
show that the computational power within the connected components is
fairly restricted. Finally, we give an exact characterization of the class
GMGP, of graph languages decidable by the MGP model: it is equal
to the class of graph languages decidable by a nondeterministic Turing
Machine of linear space that receives its input graph by its adjacency
matrix representation.
Abstract: We explore the capability of a network of extremely limited computational entities to decide properties about itself or any of its subnetworks. We consider that the underlying network of the interacting entities (devices, agents, processes etc.) is modeled by an interaction graph that reflects the network’s connectivity. We examine the following two cases: First, we consider the case where the input graph is the whole interaction graph and second where it is some subgraph of the interaction graph given by some preprocessing on the network. In each case, we devise simple graph protocols that can decide properties of the input graph. The computational entities, that are called agents, are modeled as finitestate automata and run the same global graph protocol. Each protocol is a fixed size grammar, that is, its description is independent of the size (number of agents) of the network. This size is not known by the agents. We present two simple models (one for each case), the Graph Decision Mediated Population Protocol (GDMPP) and the Mediated Graph Protocol (MGP) models, similar to the Population Protocol model of Angluin et al., where each network link (edge of the interaction graph) is characterized by a state taken from a finite set. This state can be used and updated during each interaction between the corresponding agents. We provide some example protocols and some interesting properties for the two models concerning the computability of graph languages in various settings (disconnected input graphs, stabilizing input graphs). We show that the computational power within the family of all (at least) weaklyconnected input graphs is fairly restricted. Finally, we give an exact characterization of the class of graph languages decidable by the MGP model in the case of complete interaction graphs: it is equal to the class of graph languages decidable by a nondeterministic Turing Machine of linear space that receives its input graph by its adjacency matrix representation.
Abstract: We address several recent developments in noncooperative as well as
evolutionary game theory, that give a new viewpoint to Complex Systems understanding.
In particular, we discuss notions like the anarchy cost, equilibria formation,
social costs and evolutionary stability. We indicate how such notions help
in understanding Complex Systems behaviour when the system includes selfish,
antagonistic entities of varying degrees of rationality. Our main motivation is the
Internet, perhaps the most complex artifact to date, as well as largescale systems
such as the highlevel P2P systems, where where the interaction is among
humans, programmes and machines and centralized approaches cannot apply.
Abstract: Recent advances in the alloptical signal processing
domain report highspeed and nontrivial
functionality directly implemented in the optical
layer. These developments mean that the alloptical
processing of packet headers has a future.
In this article we address various important control
plane issues that must be resolved when
designing networks based on alloptical packetswitched
nodes.
Abstract: In this work we present the design of jWebDust, a
software environment for monitoring and controlling sensor networks via a web interface. Our software architecture provides a range of services that allow to create customized applications with minimum implementation effort that are easy to administrate. We present its open architecture, the most important design decisions, and discuss its distinct features and functionalities. jWebDust will allow heterogeneous components to operate in the same sensor network, and the integrated management and control of multiple such networks by defining webbased mechanisms to visualize the network state, the results of queries, and a means to inject queries in the network.
Abstract: We present the conceptual basis and the initial planning for an open
source management architecture for wireless sensor networks (WSN). Although
there is an abundance of open source tools serving the administrative needs of
WSN deployments, there is a lack of tools or platforms for high level integrated
WSN management. The current work is, to our knowledge, the first effort to
conceptualize and design a remote, integrated management platform for the
support of WSN research laboratories. The platform is based on the integration
and extension of two innovative platforms: jWebDust, a WSN operation and
management platform, and OpenRSM, an open source integrated remote
systems and network management platform. The proposed system architecture
can support several levels of integration in order to cover to multiple,
qualitatively differentiated usecases.
Abstract: In this paper we attempt to contribute to the integration of the
mathematics and the technological developments and demonstrate
their interplay in realizing the concept of a Digital Territory.
We describe the main mathematical tools that can be exploited in
the study of properties that emerge as soon as a population size
reaches a certain threshold point. Our aim is to show that
nowadays we have reached a level of mathematical and technological
maturity sufficient to model and simulate any possible world
model.
Abstract: In the near future, it is reasonable to expect that new types of systems will appear, of massive scale that will operating in a constantly changing networked environment. We expect that most such systems will have the form of a large society of tiny networked artefacts. Angluin et al. introduced the notion of "Probabilistic Population Protocols'' (PPP) in order to model the behavior of such systems where extremely limited agents are represented as finite state machines that interact in pairs under the control of an adversary scheduler. We propose to study the dynamics of Probabilistic Population Protocols, via the differential equations approach. We provide a very general model that allows to examine the continuous dynamics of population protocols and we show that it includes the model of Angluin et. al., under certain conditions, with respect to the continuous dynamics of the two models. Our main proposal here is to exploit the powerful tools of continuous nonlinear dynamics in order to examine the behavior of such systems. We also provide a sufficient condition for stability.
Abstract: Angluin et al. [1] introduced the notion of ``Probabilistic Population Protocols'' where extremely limited agents are represented as finite state machines that interact in pairs under the control of an adversary scheduler. We provide a very general model that allows to examine the continuous dynamics of population protocols and we show that it includes the model of [1], under certain conditions, with respect to the continuous dynamics of the two models.
Abstract: We investigate the existence and efficient algorithmic construction
of close to optimal independent sets in random models of intersection
graphs. In particular, (a) we propose a new model for random
intersection graphs (Gn,m,p) which includes the model of [10] (the “uniform”
random intersection graphs model) as an important special case.
We also define an interesting variation of the model of random intersection
graphs, similar in spirit to random regular graphs. (b) For this
model we derive exact formulae for the mean and variance of the number
of independent sets of size k (for any k) in the graph. (c) We then propose
and analyse three algorithms for the efficient construction of large
independent sets in this model. The first two are variations of the greedy
technique while the third is a totally new algorithm. Our algorithms are
analysed for the special case of uniform random intersection graphs.
Our analyses show that these algorithms succeed in finding close to optimal
independent sets for an interesting range of graph parameters.
Abstract: We investigate the existence and efficient algorithmic construction of close to opti
mal independent sets in random models of intersection graphs. In particular, (a) we
propose a new model for random intersection graphs (Gn,m,~p) which includes the
model of [10] (the “uniform” random intersection graphs model) as an important
special case. We also define an interesting variation of the model of random intersec
tion graphs, similar in spirit to random regular graphs. (b) For this model we derive
exact formulae for the mean and variance of the number of independent sets of size
k (for any k) in the graph. (c) We then propose and analyse three algorithms for
the efficient construction of large independent sets in this model. The first two are
variations of the greedy technique while the third is a totally new algorithm. Our
algorithms are analysed for the special case of uniform random intersection graphs.
Our analyses show that these algorithms succeed in finding close to optimal in
dependent sets for an interesting range of graph parameters.
Abstract: The Greek School Network (GSN) is a closed nationwide
educational network that offers advanced telematic and
networking services to all primary/secondary education schools
and administration offices in Greece. The primary objective of
GSN is the provisioning of a network infrastructure for the interconnection
of school PC laboratories so that modern educational
methods and pedagogical models can be applied to the school
community. GSN has scaled in size, has reached maturity, and
is currently delivering a wide range of network and telematic
services to its users. The emerging power of opensource software
provides a sound technological basis for building cuttingedge
services, capable of meeting internal administrative and monitoring
needs, and modern pedagogical requirements for tools and
services. The current paper presents an overview of GSN and an
evaluation of its services based on the opinions of its users, and on
service utilization and traffic measurement statistics. The paper
reaches the conclusion that opensource solutions provide a sound
technological platform that can cover, to a great extent, the needs
for advanced educational services of the school community.
Abstract: Content integration of web data sources is becoming increasingly
important for the next generation information
systems. However, all proposed solutions are faced with
the same performance bottleneck: the network overhead
incurred when contacting the integrated esites. With this
demo paper we shall demonstrate the functionality of HyperHotel.
HyperHotel is used for finding appropriate hotel
rooms when travelling. Its novetlies are that it is designed
and implemented as an internet webhotel content integration
application and that it is built on top of D.I.C.E. and
Co.In.S.; a novel content integration infrastructure consisting
of a domainindependent COntent INtegration System
and its Data Integration Cache Engine. We¢ll show how the
infrastructure of D.I.C.E. and Co.In.S. can be applied and
exploited in HyperHotel in order to improve the response
time of complex user queries. This exemplifies the significance
of this infrastructure since HyperHotel is representative
of a large class of ecommerce, content integration
applications.
Abstract: In this work, we study the impact of the dynamic changing of the network link capacities on the stability properties of packetswitched networks. Especially, we consider the Adversarial, QuasiStatic Queuing Theory model, where each link capacity may take on only two possible (integer) values, namely 1 and C>1 under a (w,\~{n})adversary. We obtain the following results:
• Allowing such dynamic changes to the link capacities of a network with just ten nodes that uses the LIS (LongestinSystem) protocol for contention–resolution results in instability at rates View the MathML source and for large enough values of C.
• The combination of dynamically changing link capacities with compositions of contention–resolution protocols on network queues suffices for similar instability bounds: The composition of LIS with any of SIS (ShortestinSystem), NTS (NearesttoSource), and FTG (FurthesttoGo) protocols is unstable at rates View the MathML source for large enough values of C.
• The instability bound of the network subgraphs that are forbidden for stability is affected by the dynamic changes to the link capacities: we present improved instability bounds for all the directed subgraphs that were known to be forbidden for stability on networks running a certain greedy protocol.
Abstract: Consider k particles, 1 red and k1 white, chasing each other on the nodes of a graph G. If the red one catches one of the white, it “infects” it with its color. The newly red particles are now available to infect more white ones. When is it the case that all white will become red? It turns out that this simple question is an instance of information propagation between random walks and has important applications to mobile computing where a set of mobile hosts acts as an intermediary for the spread of information.
In this paper we model this problem by k concurrent random walks, one corresponding to the red particle and k1 to the white ones. The infection time Tk of infecting all the white particles with red color is then a random variable that depends on k, the initial position of the particles, the number of nodes and edges of the graph, as well as on the structure of the graph.
In this work we develop a set of probabilistic tools that we use to obtain upper bounds on the (worst case w.r.t. initial positions of particles) expected value of Tk for general graphs and important special cases. We easily get that an upper bound on the expected value of Tk is the worst case (over all initial positions) expected meeting time m* of two random walks multiplied by . We demonstrate that this is, indeed, a tight bound; i.e. there is a graph G (a special case of the “lollipop” graph), a range of values k
Abstract: We consider a security problem on a distributed network.
We assume a network whose nodes are vulnerable to infection
by threats (e.g. viruses), the attackers. A system security
software, the defender, is available in the system. However,
due to the network¢s size, economic and performance reasons,
it is capable to provide safety, i.e. clean nodes from
the possible presence of attackers, only to a limited part of
it. The objective of the defender is to place itself in such a
way as to maximize the number of attackers caught, while
each attacker aims not to be caught.
In [7], a basic case of this problem was modeled as a
noncooperative game, called the Edge model. There, the
defender could protect a single link of the network. Here,
we consider a more general case of the problem where the
defender is able to scan and protect a set of k links of the
network, which we call the Tuple model. It is natural to expect
that this increased power of the defender should result
in a better quality of protection for the network. Ideally,
this would be achieved at little expense on the existence and
complexity of Nash equilibria (profiles where no entity can
improve its local objective unilaterally by switching placements
on the network).
In this paper we study pure and mixed Nash equilibria
in the model. In particular, we propose algorithms for computing
such equilibria in polynomial time and we provide a
polynomialtime transformation of a special class of Nash
equilibria, called matching equilibria, between the Edge
model and the Tuple model, and vice versa. Finally, we
establish that the increased power of the defender results in
higherquality protection of the network.
Abstract: In this paper we examine spectral properties of random intersection graphs when the number of vertices is equal to the number of labels. We call this class symmetric random intersection graphs. We examine symmetric random intersection graphs when the probability that a vertex selects a label is close to the connectivity threshold $\tau_c$. In particular, we examine the size of the second eigenvalue of the transition matrix corresponding to the Markov Chain that describes a random walk on an instance of the symmetric random intersection graph $G_{n, n, p}$. We show that with high probability the second eigenvalue is upper bounded by some constant $\zeta < 1$.
Abstract: In this paper we examine spectral properties of random intersection graphs when the number
of vertices is equal to the number of labels. We call this class symmetric random intersection graphs.
We examine symmetric random intersection graphs when the probability that a vertex selects a label
is close to the connectivity threshold ¿c. In particular, we examine the size of the second eigenvalue of
the transition matrix corresponding to the Markov Chain that describes a random walk on an instance
of the symmetric random intersection graph Gn,n,p. We show that with high probability the second
eigenvalue is upper bounded by some constant ³ < 1.
Abstract: We propose and evaluate the performance of a new MAClayer protocol for mobile ad hoc networks, called the Slow Start Power Controlled (abbreviated SSPC) protocol. SSPC improves on IEEE 802.11 by using power control for the RTS/CTS and DATA frame transmissions, so as to reduce energy consumption and increase network throughput and lifetime. In our scheme the transmission power used for the RTS frames is not constant, but follows a slow start principle. The CTS frames, which are sent at maximum transmission power, prevent the neighbouring nodes from transmitting their DATA frames at power levels higher than a computed threshold, while allowing them to transmit at power levels less than that threshold. Reduced energy consumption is achieved by adjusting the node transmission power to the minimum required value for reliable reception at the receiving node, while increase in network throughput is achieved by allowing more transmissions to take place simultaneously. The slow start principle used for calculating the appropriate DATA frames transmission power and the possibility of more simultaneous collisionfree transmissions differentiate the SSPC protocol from the other MAC solutions proposed for IEEE 802.11. Simulation results indicate that the SSPC protocol achieves a significant reduction in power consumption, average packet delay and frequency of RTS frame collisions, and a significant increase in network throughput and receivedtosent packets ratio compared to IEEE 802.11 protocol.
Abstract: We consider random systems of linear equations over GF(2) in which every equation binds k variables. We obtain a precise description of the clustering of solutions in such systems. In particular, we prove that with probability that tends to 1 as the number of variables, n, grows: for every pair of solutions \sigma, \tau, either there exists a sequence of solutions \sigma,...,\tau, in which successive elements differ by O(log n) variables, or every sequence of solutions \sigma,...,\tau, contains a step requiring the simultaneous change of \Omega(n) variables. Furthermore, we determine precisely which pairs of solutions are in each category. Our results are tight and highly quantitative in nature. Moreover, our proof highlights the role of unique extendability as the driving force behind the success of Low Density Parity Check codes and our techniques also apply to the problem of socalled pseudocodewords in such codes.
Abstract: In this work, we study the combinatorial structure and the
computational complexity of Nash equilibria for a certain game that
models selfish routing over a network consisting of m parallel links. We
assume a collection of n users, each employing a mixed strategy, which
is a probability distribution over links, to control the routing of its own
assigned traffic. In a Nash equilibrium, each user selfishly routes its traffic
on those links that minimize its expected latency cost, given the network
congestion caused by the other users. The social cost of a Nash equilibrium
is the expectation, over all random choices of the users, of the
maximum, over all links, latency through a link.
We embark on a systematic study of several algorithmic problems related
to the computation of Nash equilibria for the selfish routing game we consider.
In a nutshell, these problems relate to deciding the existence of a
Nash equilibrium, constructing a Nash equilibrium with given support
characteristics, constructing the worst Nash equilibrium (the one with
maximum social cost), constructing the best Nash equilibrium (the one
with minimum social cost), or computing the social cost of a (given) Nash
equilibrium. Our work provides a comprehensive collection of efficient algorithms,
hardness results (both as NPhardness and #Pcompleteness
results), and structural results for these algorithmic problems. Our results
span and contrast a wide range of assumptions on the syntax of the
Nash equilibria and on the parameters of the system.
Abstract: In this paper we study the threshold behavior of the fixed radius random graph model and its applications to the key management problem of sensor networks and, generally, for mobile adhoc networks. We show that this random graph model can realistically model the placement of nodes within a certain region and their interaction/sensing capabilities (i.e. transmission range, light sensing sensitivity etc.). We also show that this model can be used to define key sets for the network nodes that satisfy a number of good properties, allowing to set up secure communication with each other depending on randomly created sets of keys related to their current location. Our work hopes to inaugurate a study of key management schemes whose properties are related to properties of an appropriate random graph model and, thus, use the rich theory developed in the random graph literature in order to transfer ?good? properties of the graph model to the key sets of the nodes.
Partially supported by the IST Programme of the European Union under contact number IST200515964 (AEOLUS) and the INTAS Programme under contract with Ref. No 04777173 (Data Flow Systems: Algorithms and Complexity (DFSAC)).
Abstract: The problem of determining the unsatisfiability threshold for random 3SAT formulas consists in determining the clause to variable
ratio that marks the experimentally observed abrupt change from almost surely satisfiable formulas to almost surely unsatisfiable. Up
to now, there have been rigorously established increasingly better lower and upper bounds to the actual threshold value. In this paper,
we consider the problem of bounding the threshold value from above using methods that, we believe, are of interest on their own
right. More specifically, we show how the method of local maximum satisfying truth assignments can be combined with results for
the occupancy problem in schemes of random allocation of balls into bins in order to achieve an upper bound for the unsatisfiability
threshold less than 4.571. In order to obtain this value, we establish a bound on the qbinomial coefficients (a generalization of the
binomial coefficients). No such bound was previously known, despite the extensive literature on qbinomial coefficients. Finally,
to prove our result we had to establish certain relations among the conditional probabilities of an event in various probabilistic
models for random formulas. It turned out that these relations were considerably harder to prove than the corresponding ones for
unconditional probabilities, which were previously known.
Abstract: In this paper, the impact of burstification delay on the TCP
traffic statistics is presented as well as a new assembly scheme that uses
flow window size as the threshold criterion. It is shown that short assembly
times are ideally suitable for sources with small congestion windows,
allowing for a speed up in their transmission. In addition, large assembly
times do not yield any throughput gain, despite the large number of
segments per burst transmitted, but result in a low throughput variation, and
thus a higher notion of fairness among the individual flows. To this end, in
this paper, we propose a new burst assembly scheme that dynamically
assigns flows to different assembly queues with different assembly timers,
based on their instant window size. Results show that the proposed scheme
with different timers provides a higher average throughput together with a
smaller variance which is a good compromise for bandwidth dimensioning.
Abstract: We give an overview of models and efficient algorithms for
optimally solving timetable information problems like “given a departure
and an arrival station as well as a departure time, which is the
connection that arrives as early as possible at the arrival station?” Two
main approaches that transform the problems into shortest path problems
are reviewed, including issues like the modeling of realistic details
(e.g., train transfers) and further optimization criteria (e.g., the number
of transfers). An important topic is also multicriteria optimization,
where in general all attractive connections with respect to several criteria
shall be determined. Finally, we discuss the performance of the described
algorithms, which is crucial for their application in a real system.
Abstract: We give an overview of models and efficient algorithms for optimally solving timetable information problems like “given a departure and an arrival station as well as a departure time, which is the connection that arrives as early as possible at the arrival station?” Two main approaches that transform the problems into shortest path problems are reviewed, including issues like the modeling of realistic details (e.g., train transfers) and further optimization criteria (e.g., the number of transfers). An important topic is also multicriteria optimization, where in general all attractive connections with respect to several criteria shall be determined. Finally, we discuss the performance of the described algorithms, which is crucial for their application in a real system.
Abstract: In this work we study how to process complex queries in DHTbased
PeertoPeer (P2P) data networks. Queries are made over tuples and relations
and are expressed in a query language, such as SQL. We describe existing
research approaches for query processing in P2P systems, we suggest
improvements and enhancements, and propose a unifying framework that
consists of a modified DHT architecture, data placement and search algorithms,
and provides efficient support for processing a variety of query types, including
queries with one or more attributes, queries with selection operators (involving
equality and range queries), and queries with join operators. To our knowledge,
this is the first work that puts forth a framework providing support for all these
query types.
Abstract: Peertopeer sharing systems are becoming
increasingly popular and an exciting new class of
innovative, internetbased data management
systems. In these systems, users contribute their
own resources (processing units and storage
devices) and content (i.e., documents) to the P2P
community. We focus on the management of
content and resources in such systems. Our goal
is to harness all available resources in the P2P
network so that the users can access all available
content efficiently. Efficiency is taken both from
(i) the point of view of the system, in that we
strive to ensure fair load distribution among all
peer nodes, and (ii) from the point of view of the
users, in that we strive to ensure low userrequest
response times.
We propose a novel architecture for this new
class of applications, which differs drastically
from what is either found currently in existing
products or proposed in academia. We contribute
and study novel solutions that achieve our goals,
while at the same time addressing the formidable
challenges due to the autonomy of peers, their
heterogeneous processing and storage capacities,
their different content contributions, the huge
system scale, and the highly dynamic system
environment.
Abstract: In this paper, we discuss the integration of Wireless
Sensor Networks (WSN) and smart objects with the Web. We present a set of research challenges which we believe are the most important ones rising from this integration and propose a prototype system, Uberdust, which addresses such challenges. Uberdust is a brokerage web service for connecting smart objects to the Internet of Things, providing storage, sharing and discovery of realtime and historical data from smart objects, devices & building installations around the world via the Web. Our system provides highlevel languageindependent APIs so IoT application developers may choose their favorite programming or scripting languages.
Abstract: In this work we present a new simulation toolkit that we call TRAILS (Toolkit for Realism and Adaptivity In Largescale Simulations), which extends the \NS simulator by adding several important functionalities and optimizing certain
critical simulator operations. The added features focus on providing the user with the necessary tools to better study wireless networks of high dynamics; in particular, to implement advanced mobility patterns, obstacle presence and disaster scenarios, and failures injection. These scenarios and patterns can dynamically change throughout the execution of the simulation based on network related parameters. Moreover, we define a set of utilities that can facilitate the use of \NS providing advanced statistics and easytouse logging mechanisms. This functionality is implemented in a simple and flexible architecture, that follows design patterns, object oriented and generic programming principles, maintaining a proper balance between reusability, extendability and ease of use. We evaluate the performance of TRAILS and show that it offers significant speedups (at least 4 times faster) regarding the execution time of \NS in certain important, common wireless settings. Our results also show that this is achieved with minimum overhead in terms of memory usage.
Abstract: We examine the problem of transmitting in minimum time a given amount of data between a
source and a destination in a network with finite channel capacities and nonzero propagation delays. In
the absence of delays, the problem has been shown to be solvable in polynomial time. In this paper, we
show that the general problem is NPcomplete. In addition, we examine transmissions along a single path,
called the quickest path, and present algorithms for general and special classes of networks that improve
upon previous approaches. The first dynamic algorithm for the quickest path problem is also
given
Abstract: In this work, we introduce the notion of time to some wellknown combinatorial optimization problems. In particular, we study problems defined on temporal graphs. A temporal graph D=(V,A) may be viewed as a timesequence G_1,G_2,...,G_l of static graphs over the same (static) set of nodes V. Each G_t = D(t) = (V,A(t)) is called the instance of D at time t and l is called the lifetime of D. Our main focus is on analogues of traveling salesman problems in temporal graphs. A sequence of timelabeled edges (e.g. a tour) is called temporal if its labels are strictly increasing. We begin by considering the problem of exploring the nodes of a temporal graph as soon as possible. In contrast to the positive results known for the static case, we prove that, it cannot be approximated within cn, for some constant c > 0, in general temporal graphs and within (2 − \varepsilon), for every constant \varepsilon > 0, in the special case in which D(t) is connected for all 1 <= t <= l, both unless P = NP. We then study the temporal analogue of TSP(1,2), abbreviated TTSP(1,2), where, for all 1 <= t <= l, D(t) is a complete weighted graph with edgecosts from {1,2} and the cost of an edge may vary from instance to instance. The goal is to find a minimum cost temporal TSP tour. We give several polynomialtime approximation algorithms for TTSP(1,2). Our best approximation is (1.7 + \varepsilon) for the generic TTSP(1,2) and (13/8 + \varepsilon) for its interesting special case in which the lifetime of the temporal graph is restricted to n. In the way, we also introduce temporal versions of Maximum Matching, Path Packing, MaxTSP, and Minimum Cycle Cover, for which we obtain polynomialtime approximation algorithms and hardness results.
Abstract: For the Internet of Things to ﬁnally become a reality, obstacles on different levels need to be overcome. This is especially true for the upcoming challenge of leaving the domain of technical experts and scientists. Devices need to connect to the Internet and be able to offer services. They have to announce and describe these services in machine understandable ways so that userfacing systems are able to ﬁnd and utilize them. They have to learn about their physical surroundings, so that they can serve sensing or acting purposes without explicit conﬁguration or programming. Finally, it must be possible to include IoT devices in complex systems that combine local and remote data, from different sources, in novel and surprising ways.
We show how all of that is possible today. Our solution uses open standards and stateofthe art protocols to achieve this. It is based on 6LowPAN and CoAP for the communications part, semantic web technologies for meaningful data exchange, autonomous sensor correlation to learn about the environment, and software built around the Linked Data principles to be open for novel and unforeseen applications.
Abstract: Today we are experiencing a major reconsideration of the computing
paradigm, as witnessed by the abundance and increasing frequency
of use of terms such as {\em ambient intelligence}, {\em ubiquitous computing}, {\em disappearing computer}, {\em grid
computer}, {\em global computing} and {\em mobile adhoc
networks}. Systems that can be described with such terms are of a
dynamic, with no clear physical boundary, nature and it seems that
it is impossible (or, at least, difficult) to define sharply a
number of important properties holding with certainty as well as
holding throughout the whole lifetime of the system.
%
One such system property, which is important for the viability of
a system, is {\em trust}. Our departure point is the assumption
that it seems very difficult to define static system properties
related to trust and expect that they hold eternally in the
rapidly changing systems falling under the new computing paradigm.
One should, rather, attempt to define trust in terms of properties
that hold with some limiting probability as the the system grows
and try to establish conditions that ensure that ``good''
properties hold {\em almost certainly}. Based on this viewpoint,
in this paper we provide a new framework for defining trust
through formally definable properties that hold, almost certainly,
in the limit in randomly growing combinatorial structures that
model ``shapeless'' computing systems (e.g. adhoc networks),
drawing on results that establish the threshold behavior of
predicates written in the first and second order logic.
Abstract: In this paper, we review recent advances in ultrafast optical timedomain technology with emphasis on the use in optical packet switching. In this respect, several key building blocks, including highrate laser sources applicable to any timedivisionmultiplexing (TDM) application, optical logic circuits for bitwise processing, and clockrecovery circuits for timing synchronization with both synchronous and asynchronous data traffic, are described in detail. The circuits take advantage of the ultrafast nonlinear transfer function of semiconductorbased devices to operate successfully at rates beyond 10 Gb/s. We also demonstrate two more complex circuitsa header extraction unit and an exchangebypass switchoperating at 10 Gb/s. These two units are key blocks for any generalpurpose packet routing/switching application. Finally, we discuss the system perspective of all these modules and propose their possible incorporation in a packet switch architecture to provide lowlevel but highspeed functionalities. The goal is to perform as many operations as possible in the optical domain to increase node throughput and to alleviate the network from unwanted and expensive opticalelectricaloptical conversions.
Abstract: Unattended surveillance of public transportation infrastructures that may generate alarms in the case of a destructive event, and provide information on the current status of the infrastructure as well as on the short interval before the event, can empower the Search and Rescue teams with useful information that may save lives increasing their overall effectiveness. This paper presents a novel system that aims at providing a scalable, unattended surveillance system with networked embedded systems, cameras and sensors for public transportations sector.
Abstract: The interpolation method, originally developed in statistical physics, transforms distributions of random CSPs to distributions of much simpler problems while bounding the change in a number of associated statistical quantities along the transformation path. After a number of further mathematical developments, it is now known that, in principle, the method can yield rigorous unsatisfiability bounds if one “plugs in an appropriate functional distribution”. A drawback of the method is that identifying appropriate distributions and plugging them in leads to major analytical challenges as the distributions required are, in fact, infinite dimensional objects. We develop a variant of the interpolation method for random CSPs on arbitrary sparse degree distributions which trades accuracy for tractability. In particular, our bounds only require the solution of a 1dimensional optimization problem (which typically turns out to be very easy) and as such can be used to compute explicit rigorous unsatisfiability bounds.
Abstract: Recent activity in the field of InternetofThings experimentation has focused on the federation of discrete testbeds, thus placing less effort in the integration of other related technologies, such as smartphones; also, while it is gradually moving to more applicationoriented paths, such as urban settings, it has not dealt in large with applications having social networking features. We argue here that current IoT infrastructure, testbeds and related software technologies should be used in such a context, capturing realworld human mobility and social networking interactions, for use in evaluating and finetuning realistic mobility models and designing humancentric applications. We discuss a system for producing traces for a new generation of humancentric applications, utilizing technologies such as Bluetooth and focusing on human interactions. We describe the architecture for this system and the respective implementation details presenting two distinct deployments; one in an office environment and another in an exhibition/conference event (FET'11, The European Future Technologies Conference and Exhibition) with 103 active participants combined, thus covering two popular scenarios for human centric applications. Our system provides online, almost realtime, feedback and statistics and its implementation allows for rapid and robust deployment, utilizing mainstream technologies and components.
Abstract: Experimentally driven research for wireless sensor networks is invaluable to provide benchmarking and comparison of new ideas. An increasingly common tool in support of this is a testbed composed of real hardware devices which increases the realism of evaluation. However, due to hardware costs the size and heterogeneity of these testbeds is usually limited. In addition, a testbed typically has a relatively static configuration in terms of its network topology and its software support infrastructure, which limits the utility of that testbed to specific casestudies. We propose a novel approach that can be used to (i) interconnect a large number of small testbeds to provide a federated testbed of very large size, (ii) support the interconnection of heterogeneous hardware into a single testbed, and (iii) virtualise the physical testbed topology and thus minimise the need to relocate devices. We present the most important design issues of our approach and evaluate its performance. Our results indicate that testbed virtualisation can be achieved with high efficiency and without hindering the realism of experiments.
Abstract: We present the basic concepts behind the design and implementation of WebDust, a peertopeer platform for organizing,
monitoring and controlling wireless sensor networks, along with a discussion of its application regarding an actual testbed.
Our software architecture provides a range of services that allow to create customized applications with relatively low
implementation overhead. WebDust aims to allow heterogeneous components to operate in the same sensor network, and
give the ability to manage and control large numbers of such networks, possibly on a global scale. We also give insight to
several applications that can be implemented using our platform, and a description of our current testbed.
Abstract: In view of the apparent intractability of constructing Nash Equilibria (NE
in short) in polynomial time, even for bimatrix games, understanding the limitations
of the approximability of the problem is an important challenge.
In this work we study the tractability of a notion of approximate equilibria in bimatrix
games, called well supported approximate Nash Equilibria (SuppNE in short).
Roughly speaking, while the typical notion of approximate NE demands that each
player gets a payoff at least an additive term less than the best possible payoff, in a
SuppNE each player is assumed to adopt with positive probability only approximate
pure best responses to the opponent¢s strategy.
As a first step, we demonstrate the existence of SuppNE with small supports and
at the same time good quality. This is a simple corollary of Alth{\"o}fer¢s Approximation
Lemma, and implies a subexponential time algorithm for constructing SuppNE of
arbitrary (constant) precision.
We then propose algorithms for constructing SuppNE in win lose and normalized
bimatrix games (i.e., whose payoff matrices take values from {0, 1} and [0, 1] respectively).
Our methodology for attacking the problem is based on the solvability of zero sum bimatrix games (via its connection to linear programming) and provides a
0.5SuppNE for win lose games and a 0.667SuppNE for normalized games.
To our knowledge, this paper provides the first polynomial time algorithms constructing
{\aa}SuppNE for normalized or win lose bimatrix games, for any nontrivial
constant 0 ≤{\aa} <1, bounded away from 1.
Abstract: Data propagation in wireless sensor networks can be performed either by hopbyhop single transmissions or by multipath broadcast of data. Although several energyaware MAC layer protocols exist that operate very well in the case of single pointtopoint transmissions, none is especially designed and suitable for multiple broadcast transmissions.In this paper we propose a family of new protocols suitable of multipath broadcast of data, and show, through a detailed and extended simulation evaluation, that our parameterbased protocols significantly reduce the number of collisions and thus increase the rate of successful message delivery (to above 90%) by trading off the average propagation delay. At the same time, our protocols are shown to be very energy efficient, in terms of the average energy dissipation per delivered message.
Abstract: In this paper we present an overview of WISEBED, a largescale wireless sensor network testbed, which is currently being built for research purposes. This project is led by a number of European Universities
and Research Institutes, hoping to provide scientists, researchers and companies with an environment to conduct experiments with, in order to evaluate and validate their sensor networkrelated work. The initial planning of the project includes a large, heterogeneous testbed, consisting of at least 9 geographically disparate networks that include both sensor and actuator nodes, and scaling in the order of thousands (currently being in total 550 nodes).We present here the overall architecture
of WISEBED, focusing on certain aspects of the software ecosystem surrounding the project, such as the Open Federation Alliance, which will enable a view of the whole testbed, or parts of it, as single entities, and the testbed's tight integration with the Shawn network simulator. We also present examples of the actual hardware used currently in the testbed and outline the architecture of two of the testbed's sites.
Abstract: There exists a great amount of algorithms for wireless sensor networks (WSNs) that have never been tried in practice. This is due to the fact that programming sensor nodes still happens on a very technical level. We remedy the situation by introducing our algorithm library Wiselib, which allows for simple implementations of algorithms. It can adopt to a large variety of hardware and software. This is achieved by employing advanced C++ techniques such as templates and inline functions, which allow to write generic code that is resolved and bound at compile time, resulting in virtually no memory or computation overhead at run time. The Wiselib runs on different host operating systems such as Contiki, iSense OS, and ScatterWeb. Furthermore, it runs on virtual nodes simulated by Shawn. The Wiselib provides an algorithm with data structures that suit the specific properties of the target platform. Algorithm code does not contain any platformspecific specializations, allowing a single implementation to run natively on heterogeneous networks. In this paper, we describe the building blocks of the Wiselib, analyze the overhead, and show how cryptographically secured routing algorithms can be implemented. We also report on results from experiments with real sensor node hardware.
Abstract: In this paper we describe a new simulation platform for complex wireless sensor networks that operate a collection of distributed algorithms and network protocols. Simulating such systems is complicated because of the need to coordinate different network layers and debug protocol stacks, often with very different interfaces, options, and fidelities. Our platform (which we call WSNGE) is a flexible and extensible environment that provides a highly scalable simulator with unique characteristics. It focuses on user friendliness, providing every function in both scriptable and visual way, allowing the researcher to define simulations and view results in an easy to use graphical environment. Unlike other solutions, WSNGE does not distinguish between different scenario types, allowing multiple different protocols to run at the same time. It enables rich online interaction with running simulations, allowing parameters, topologies or the whole scenario to be altered at any point in time.