Abstract: The efficient use of resources and the lossless transfer of data bursts in future optical
networks requires the accurate knowledge of the available bandwidth for each network
link. Such information is important in monitoring congestions and can be used by
appropriate load balancing and congestion avoidance mechanisms. In this paper we
propose a mechanism for monitoring and subsequently managing bandwidth resources,
using the Simple NetworkManagement Protocol (SNMP). In the proposed mechanism,
link bandwidth availability is not a scalar parameter, but a function of time that records
the future utilization of the link. For every output port, each agent-node maintains a
simple data structure in the form of a table that records the utilization profile of that
outgoing link. With the addition of new objects in the Management Information Base
(MIB) of each agent-node and proper synchronization, SNMP can be used to update
and retrieve the reservations made on the links in order to obtain an instant picture of
the network traffic situation.
Abstract: The management of Grid resources requires scheduling of both computation and communication tasks at various levels. In this study, we consider the two constituent sub-problems of Grid scheduling, namely: (i) the scheduling of computation tasks to processing resources and (ii) the routing and scheduling of the data movement in a Grid network. Regarding computation tasks, we examine two typical online task scheduling algorithms that employ advance reservations and perform full network simulation experiments to measure their performance when implemented in a centralized or distributed manner. Similarly, for communication tasks, we compare two routing and data scheduling algorithms that are implemented in a centralized or a distributed manner. We examine the effect network propagation delay has on the performance of these algorithms. Our simulation results indicate that a distributed architecture with an exhaustive resource utilization update strategy yields better average end-to-end delay performance than a centralized architecture.
Abstract: The efficient use of resources and the lossless transfer of data bursts in future optical
networks requires the accurate knowledge of the available bandwidth for each network
link. Such information is important in monitoring congestions and can be used by
appropriate load balancing and congestion avoidance mechanisms. In this paper we
propose a mechanism for monitoring and subsequently managing bandwidth resources,
using the Simple NetworkManagement Protocol (SNMP). In the proposed mechanism,
link bandwidth availability is not a scalar parameter, but a function of time that records
the future utilization of the link. For every output port, each agent-node maintains a
simple data structure in the form of a table that records the utilization profile of that
outgoing link. With the addition of new objects in the Management Information Base
(MIB) of each agent-node and proper synchronization, SNMP can be used to update
and retrieve the reservations made on the links in order to obtain an instant picture of
the network traffic situation.
Abstract: We describe the design and implementation
of a secure and robust architectural datamanagement
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 on-line collaboration environments.
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: In this work, we present a concrete framework, based on web services-oriented 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: An ever growing emphasis is put nowadays in developing personalized journey planning and renewable mobility services in smart cities. These services combine means of scheduled-based public transport and electric vehicles or bikes, using crowdsourcing techniques for collecting real-time traffic information and for assessing the recommended routes. The goal is to develop an information system that will allow the fast, real-time 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 time-dependent, 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 real-time) and "door-to-door'' personalized mobility, in urban scheduled-based public transport environments. Scheduled-based public transport information systems should not only compute in real-time end-user 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 scheduled-based 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 array-based modeling (where the timetable is represented as an array) and the graph-based modeling (where the timetable is represented as a graph). Experimental results have shown so far that the array-based approaches are faster in terms of query time than graph-based ones, as they are able to better exploit data locality and do not rely on priority queues. On the other hand, the array-based 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 graph-based models are being developed that solve efficiently the aforementioned fundamental algorithmic mobility problems in urban scheduled-based public transport information systems, along with a mobile application (journey planner) running on Android-based 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 graph-based dynamic models that represent big data volumes regarding their suitability for representing timetable information. The study confirmed that the realistic time-expanded model is the most suitable for representing timetable information.
(b) Two new graph-based models have been developed for representing timetable information (in a timetable information system), the reduced time-expanded model and the dynamic timetable model (DTM), both of which are more space-efficient with respect to the realistic time-expanded 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 graph-based models and their associated query and update algorithms on a set of 14 real-world scheduled-based transportation systems, including the metropolitan areas of Berlin, Athens, London, Rome, and Madrid. The experimental results showed that the query algorithms of the reduced time-expanded 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 graph-based models compete favorably with those of the best array-based models.
(d) A mobile, cloud-based, journey planner (information system) was developed whose core algorithmic engine builds upon the new graph-based 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 graph-based models and their associated query and update algorithms.
Abstract: In this paper, we present BAD, an application-level 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: This chapter aims at presenting certain important aspects of the design of lightweight, event-driven algorithmic solutions for data dissemination in wireless sensor networks that provide support for reliable, efficient and concurrency-intensive 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 trade-off efficiency and fault-tolerance) and adaptation (to adjust to high network dynamics towards improved operation). In particular, we provide a) a brief description of the technical specifications of state-of-the-art 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: 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 non-homogeneous 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 peer-to-peer 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: Building efficient internet-scale datamanagement services is the main focus of this chapter. In particular, we aim to show how to leverage DHT technology and extend it with novel algorithms and architectures in order to (i) improve efficiency and reliability for traditional DHT (exact-match) queries, particularly exploiting the abundance of altruism witnessed in real-life P2P networks, (ii) speedup range queries for data stored on DHTs, and (iii) support efficiently and scalably the publish/subscribe paradigm over DHTs, which crucially depends on algorithms for supporting rich queries on string-attribute data.
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 large-scale 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: 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 ?networking-centric? 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 web-based 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: We address the issue of measuring storage, or query load distribution fairness in peer-to-peer datamanagement systems. Existing metrics may look promising from the point of view of specific peers, while in reality being far from optimal from a global perspective. Thus, first we define the requirements and study the appropriateness of various statistical metrics for measuring load distribution fairness towards these requirements. The metric proposed as most appropriate is the Gini coefficient (G). Second, we develop novel distributed sampling algorithms to compute G on-line, with high precision, efficiently, and scalably. Third, we show how G can readily be utilized on-line by higher-level algorithms which can now know when to best intervene to correct load imbalances. Our analysis and experiments testify for the efficiency and accuracy of these algorithms, permitting the online use of a rich and reliable metric, conveying a global perspective of the distribution.
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 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 types of resources.
Abstract: We present SeAl1, a novel data/resource and data-access management infrastructure designed for the purpose of addressing a key problem in P2P data sharing networks, namely the problem of wide-scale 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 data-access management infrastructure designed for the purpose of addressing a key problem in P2P data sharing networks, namely the problem of wide-scale 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: Efficient query processing in traditional database
management systems relies on statistics on base data. For centralized systems, there is a rich body of research results on such statistics, from simple aggregates to more elaborate synopses such as sketches and histograms. For Internet-scale distributed systems, on the other hand, statisticsmanagement still poses major challenges. With the work in this paper we aim to endow peer-to-peer datamanagement over structured
overlays with the power associated with such statistical information, with emphasis on meeting the scalability challenge.
To this end, we first contribute efficient, accurate, and decentralized algorithms that can compute key aggregates such as Count, CountDistinct, Sum, and Average. We show how to construct several types of histograms, such as simple Equi-Width, Average Shifted Equi-Width, and Equi-Depth histograms. We present a full-fledged open-source implementation
of these tools for distributed statistical synopses,
and report on a comprehensive experimental performance evaluation, evaluating our contributions in terms of efficiency, accuracy, and scalability.
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 ad-hoc 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 IST-2005-15964 (AEOLUS) and the INTAS Programme under contract with Ref. No 04-77-7173 (Data Flow Systems: Algorithms and Complexity (DFS-AC)).
Abstract: Peer-to-peer sharing systems are becoming
increasingly popular and an exciting new class of
innovative, internet-based datamanagement
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 user-request
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.