Abstract: Core networks of the future will have a
translucent and eventually transparent optical
structure. Ultra-high-speed end-to-end connectiv-
ity with high quality of service and high reliability
will be realized through the exploitation of opti-
mized protocols and lightpath routing algorithms.
These algorithms will complement a flexible con-
trol and management plane integrated in the
proposed solution. Physical layer impairments
and optical performance are monitored and
incorporated in impairment-aware lightpath rout-
ing algorithms. These algorithms will be integrat-
ed into a novel dynamic network planning tool
that will consider dynamic traffic characteristics,
a reconfigurable optical layer, and varying physi-
cal impairment and component characteristics.
The network planning tool along with extended
control planes will make it possible to realize the
vision of optical transparency. This article pre-
sents a novel framework that addresses dynamic
cross-layer network planning and optimization
while considering the development of a future
transport network infrastructure.

Abstract: We discuss some new algorithmic and complexity issues in
coalitional and dynamic/evolutionary games, related to the understand-
ing of modern sel¯sh and Complex networks.
In particular: (a) We examine the achievement of equilibria via natural
distributed and greedy approaches in networks. (b) We present a model
of a coalitional game in order to capture the anarchy cost and complexity
of constructing equilibria in such situations. (c) We propose a stochastic
approach to some kinds of local interactions in networks, that can be
viewed also as extensions of the classical evolutionary game theoretic
setting.

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 consider the problem of energy balanced data propagation in wireless sensor networks and we generalise previous works by allowing realistic energy assignment. A new modelisation of the process of energy consumption as a random walk along with a new analysis are proposed. Two new algorithms are presented and analysed. The first one is easy to implement and fast to execute. However, it needs a priori assumptions on the process generating data to be propagated. The second algorithm overcomes this need by inferring information from the observation of the process. Furthermore, this algorithm is based on stochastic estimation methods and is adaptive to environmental changes. This represents an important contribution for propagating energy balanced data in wireless sensor netwoks due to their highly dynamic nature.

Abstract: We have conducted an extensive experimental study on algorithms for fully dynamic transitive
closure. We have implemented the recent fully dynamicalgorithms by King [1999], Roditty [2003],
Roditty and Zwick [2002, 2004], and Demetrescu and Italiano [2000, 2005] along with several variants
and compared them to pseudo fully dynamic and simple-minded algorithms developed in a
previous study [Frigioni et al. 2001].We tested and compared these implementations on random inputs,
synthetic (worst-case) inputs, and on inputs motivated by real-world graphs. Our experiments
reveal that some of the dynamicalgorithms can really be of practical value in many situations.

Abstract: We have conducted an extensive experimental study on algorithms for fully dynamic transitive
closure. We have implemented the recent fully dynamicalgorithms by King [1999], Roditty [2003],
Roditty and Zwick [2002, 2004], and Demetrescu and Italiano [2000, 2005] along with several variants
and compared them to pseudo fully dynamic and simple-minded algorithms developed in a
previous study [Frigioni et al. 2001].We tested and compared these implementations on random inputs,
synthetic (worst-case) inputs, and on inputs motivated by real-world graphs. Our experiments
reveal that some of the dynamicalgorithms can really be of practical value in many situations.

Abstract: We perform an extensive experimental study of several dynamicalgorithms for transitive closure. In particular, we implemented algorithms given by Italiano, Yellin, Cicerone et al., and two recent randomized algorithms by Henzinger and King. We propose a fine-tuned version of Italiano's algorithms as well as a new variant of them, both of which were always faster than any of the other implementations of the dynamicalgorithms. We also considered simple-minded algorithms that were easy to implement and likely to be fast in practice. Wetested and compared the above implementations on random inputs, on non-random inputs that are worst-case inputs for the dynamicalgorithms, and on an input motivated by a real-world graph.

Abstract: Avertical perspective, ranging from management
and routing to physical layer options, concerning dynamic
network monitoring and compensation of impairments
(M&C),is given.Feasibility, reliability,and performance
improvements on reconﬁgurable transparent networksare
expected to arise from the consolidated assessment of network management and control speciﬁcations, as a more accurate evaluation of available M&C techniques. In the network
layer,physical parameters aware algorithms are foreseen to
pursue reliable network performance. In the physical layer,
some new M&C methods were developed and rating of the state-of-the-art reported in literature is given. Optical monitoring implementation and viability is discussed.

Abstract: We consider algorithmic questions concerning the existence,
tractability and quality of atomic congestion games, among users that
are considered to participate in (static) selfish coalitions. We carefully
define a coalitional congestion model among atomic players.
Our findings in this model are quite interesting, in the sense that we
demonstrate many similarities with the non–cooperative case. For example,
there exist potentials proving the existence of Pure Nash Equilibria
(PNE) in the (even unrelated) parallel links setting; the Finite Improvement
Property collapses as soon as we depart from linear delays, but
there is an exact potential (and thus PNE) for the case of linear delays,
in the network setting; the Price of Anarchy on identical parallel
links demonstrates a quite surprising threshold behavior: it persists on
being asymptotically equal to that in the case of the non–cooperative
KP–model, unless we enforce a sublogarithmic number of coalitions.
We also show crucial differences, mainly concerning the hardness of algorithmic
problems that are solved efficiently in the non–cooperative case.
Although we demonstrate convergence to robust PNE, we also prove the
hardness of computing them. On the other hand, we can easily construct
a generalized fully mixed Nash Equilibrium. Finally, we propose a new
improvement policy that converges to PNE that are robust against (even
dynamically forming) coalitions of small size, in pseudo–polynomial time.
Keywords. Game Theory, Atomic Congestion Games, Coalitions, Convergence
to Equilibria, Price of Anarchy.

Abstract: We consider algorithmic questions concerning the existence, tractability and quality of Nash equi-
libria, in atomic congestion games among users participating in selsh coalitions.
We introduce a coalitional congestion model among atomic players and demonstrate many in-
teresting similarities with the non-cooperative case. For example, there exists a potential function
proving the existence of Pure Nash Equilibria (PNE) in the unrelated parallel links setting; in
the network setting, the Finite Improvement Property collapses as soon as we depart from linear
delays, but there is an exact potential (and thus PNE) for linear delays; the Price of Anarchy on
identical parallel links demonstrates a quite surprising threshold behavior: it persists on being
asymptotically equal to that in the case of the non-cooperative KP-model, unless the number of
coalitions is sublogarithmic.
We also show crucial dierences, mainly concerning the hardness of algorithmic problems that
are solved eciently in the non{cooperative case. Although we demonstrate convergence to robust
PNE, we also prove the hardness of computing them. On the other hand, we propose a generalized
fully mixed Nash Equilibrium, that can be eciently constructed in most cases. Finally, we
propose a natural improvement policy and prove its convergence in pseudo{polynomial time to
PNE which are robust against (even dynamically forming) coalitions of small size.

Abstract: We consider algorithmic questions concerning the existence,
tractability and quality of atomic congestion games, among users that
are considered to participate in (static) selfish coalitions. We carefully
define a coalitional congestion model among atomic players.
Our findings in this model are quite interesting, in the sense that we
demonstrate many similarities with the non–cooperative case. For example,
there exist potentials proving the existence of Pure Nash Equilibria
(PNE) in the (even unrelated) parallel links setting; the Finite Improvement
Property collapses as soon as we depart from linear delays, but
there is an exact potential (and thus PNE) for the case of linear delays,
in the network setting; the Price of Anarchy on identical parallel
links demonstrates a quite surprising threshold behavior: it persists on
being asymptotically equal to that in the case of the non–cooperative
KP–model, unless we enforce a sublogarithmic number of coalitions.
We also show crucial differences, mainly concerning the hardness of algorithmic
problems that are solved efficiently in the non–cooperative case.
Although we demonstrate convergence to robust PNE, we also prove the
hardness of computing them. On the other hand, we can easily construct
a generalized fully mixed Nash Equilibrium. Finally, we propose a new
improvement policy that converges to PNE that are robust against (even
dynamically forming) coalitions of small size, in pseudo–polynomial time.
Keywords. Game Theory, Atomic Congestion Games, Coalitions, Convergence
to Equilibria, Price of Anarchy.

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: 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 three-dimensional space with possible obstacles) as well as the motion that the nodes perform are \textit{input} to any distributed algorithm.

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 leader-based counting algorithms. Such algorithms are based on a technique that mimics an energy-transfer 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: We consider a synchronous distributed system with n processes that communicate through a dynamic network guaranteeing 1-interval connectivity i.e., the network topology graph might change at each interval while keeping the graph connected at any time. The processes belonging to the distributed system are identified through a set of labels L = {l1 , l2 . . . , lk } (with 1 ≤ k < n). In this challenging system model, the paper addresses the following problem: ”counting the number of processes with the same label”. We provide a distributed algorithm that is able solve the problem based on the notion of energy transfer. Each process owns a fixed energy charge, and tries to discharge itself exchanging, at each round, at most half of its charge with neighbors. The paper also discusses when such counting is possible in the presence of failures. Counting processes with the same label in dynamic networks with homonyms is of great importance because it is as difficult as computing generic aggregating functions.

Abstract: In this work we investigate the problem of communication among mobile hosts, one of the most fundamental problems in ad-hoc 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, a-priori 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 ad-hoc mobile networks: a) hierarchical ad-hoc networks, b) highly changing ad-hoc 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: Wireless sensor networks are comprised of a vast number of ultra-small fully autonomous computing, communication and sensing devices, with very restricted energy and computing capabilities, which co-operate to accomplish a large sensing task. Such networks can be very useful in practice in applications that require fine-grain 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 self-organization 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, highly-dynamic, complex, non-conventional 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: 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 co-ordinated (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 co-ordinated (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: 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 u-t-path 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 real-world data from six European countries.

Abstract: Many efforts have been done in the last years to model public transport timetables in order to
find optimal routes. The proposed models can be classified into two types: those representing the
timetable as an array, and those representing it as a graph. The array-based models have been
shown to be very effective in terms of query time, while the graph-based models usually answer
queries by computing shortest paths, and hence they are suitable to be used in combination with
speed-up techniques developed for road networks.
In this paper, we focus on the dynamic behavior of graph-based models considering the case
where transportation systems are subject to delays with respect to the given timetable. We
make three contributions: (i) we give a simplified and optimized update routine for the wellknown
time-expanded model along with an engineered query algorithm; (ii) we propose a new
graph-based model tailored for handling dynamic updates; (iii) we assess the effectiveness of
the proposed models and algorithms by an experimental study, which shows that both models
require negligible update time and a query time which is comparable to that required by some
array-based models.

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 well-known
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 self-organization 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 2-person
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: A fundamental approach in finding efficiently best routes or optimal itineraries in traffic information
systems is to reduce the search space (part of graph visited) of the most commonly used
shortest path routine (Dijkstra¢s algorithm) on a suitably defined graph. We investigate reduction
of the search space while simultaneously retaining data structures, created during a preprocessing
phase, of size linear (i.e., optimal) to the size of the graph. We show that the search space of
Dijkstra¢s algorithm can be significantly reduced by extracting geometric information from a given
layout of the graph and by encapsulating precomputed shortest-path information in resulted geometric
objects (containers). We present an extensive experimental study comparing the impact of
different types of geometric containers using test data from real-world traffic networks. We also
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 geometric containers have to be updated and show that
our new methods are two to three times faster than recomputing everything from scratch. Finally,
in an appendix, we discuss the software framework that we developed to realize the implementations
of all of our variants of Dijkstra¢s algorithm. Such a framework is not trivial to achieve as our
goal was to maintain a common code base that is, at the same time, small, efficient, and flexible,
as we wanted to enhance and combine several variants in any possible way.

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 fine-tuning
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 single-cost 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 multi-constrained 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 energy-related parameters in mobile ad hoc networks by
embedding its logic in the Dynamic Source Routing (DSR)
algorithm, which is a well-known fully distributed routing
algorithm. We use simulations to compare the performance
of the multicost-DSR 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 multicost-DSR 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 multicost-DSR
algorithm is shown to be lower than in the case of the orig
inal DSR algorithm.

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 n-vertex digraphs of genus O.n1¡"/ for any " > 0.

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
interference-efficient routes to optimize network performance.
In multi-cost 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 multi-cost routing problem is a
generalization of the multi-constrained 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
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 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 Diffie-Hellman key exchange algorithm and constitute natural extensions of it in the multiparty case.

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 information-resource monopolies and the biased visibility
of content from economically-powerful 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,
top-k algorithms for retrieving data at query time, and replication algorithms
for expediting top-k query processing. We have implemented the
proposed architecture and we report on our extensive experiments with
real-world, web-crawled, and synthetic data and queries, showcasing the
scalability and efficiency traits of MINERVA{\^a}{\"i}¿½{\"i}¿½.

Abstract: Recent rapid developments in micro-electro-mechanical systems
(MEMS), wireless communications and digital electronics have already
led to the development of tiny, low-power, low-cost 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 co-ordination 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 self-organization and collaborative
methods to form an ad-hoc network.
We note however that the efficient and robust realization of such large,
highly-dynamic, complex, non-conventional 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 state-of-theart
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, fault-tolerance)
of these protocols, both with analytic and simulation means.

Abstract: We propose a class of novel energy-efficient multi-cost routing algorithms for wireless mesh networks, and evaluate their performance. In multi-cost 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 energy-aware multi-cost 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 one-to-one 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 steady-state throughput, the packet delay, and the energy consumption. Our results show that energy-aware multi-cost 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 ad-hoc networks, so as to obtain dynamic
energy and interference-efficient routes to optimize network performance. In multi-cost 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 multi-cost routing problem is a generalization of
the multi-constrained 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 multi-cost 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 one-to-one communica-
tion problem in energy- and capacity-constrained wireless ad-hoc net-
works. The performance of such networks is evaluated under random
traﬃc generation and continuous energy recharging at the nodes over an
inﬁnite-time 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
energy-aware routing algorithm and compare its performance to that of
minimum-hop routing. The results of our experiments show that gener-
ally the energy-aware algorithm achieves a higher maximum throughput
than the minimum-hop algorithm. More speciﬁcally, when the network
is mainly energy-constrained and for the 2-dimensional topology consid-
ered, the throughput of the proposed energy-aware routing algorithm is
found to be almost twice that of the minimum-hop algorithm.

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 dynamic changing
networks we propose protocols which exploit the co-ordinated
(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 pair-wise
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: 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 NP-hard. 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: We consider the problem of preprocessing an n-vertex digraph with real edge weights so that
subsequent queries for the shortest path or distance between any two vertices can be efficiently answered. We
give algorithms that depend on the treewidth of the input graph. When the treewidth is a constant, our algorithms
can answer distance queries in O ({\'a}(n) ) time after O.n/ preprocessing. This improves upon previously known
results for the same problem.We also give a dynamicalgorithm which, after a change in an edge weight, updates
the data structure in time O.n¯ /, for any constant 0 < ¯ < 1. Furthermore, an algorithm of independent interest
is given: computing a shortest path tree, or finding a negative cycle in linear time.

Abstract: We consider the problem of preprocessing an n-vertex digraph with real edge weights so that subsequent queries for the shortest path or distance between any two vertices can be efficiently answered. We give parallel algorithms for the EREW PRAM model of computation that depend on the treewidth of the input graph. When the treewidth is a constant, our algorithms can answer distance queries in O({\'a}(n)) time using a single processor, after a preprocessing of O(log2n) time and O(n) work, where {\'a}(n) is the inverse of Ackermann's function. The class of constant treewidth graphs contains outerplanar graphs and series-parallel graphs, among others. To the best of our knowledge, these are the first parallel algorithms which achieve these bounds for any class of graphs except trees. We also give a dynamicalgorithm which, after a change in an edge weight, updates our data structures in O(log n) time using O(n{\^a}) work, for any constant 0 < {\^a} < 1. Moreover, we give an algorithm of independent interest: computing a shortest path tree, or finding a negative cycle in O(log2n) time using O(n) work.

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: 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 NP-complete. 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 dynamicalgorithm for the quickest path problem is also
given

Abstract: Numerous research efforts have produced a large number of algorithms and mechanisms for web proxy caches. In order to build powerful web proxies and understand their performance, one must be able to appreciate the impact and significance of earlier contributions and how they can be integrated. To do this we employ a cache replacement algorithm, 'CSP', which integrates key knowledge from previous work. CSP utilizes the communication Cost to fetch web objects, the objects' Sizes, their Popularities, an auxiliary cache and a cache admission control algorithm. We study the impact of these components with respect to hit ratio, latency, and bandwidth requirements.
Numerous research efforts have produced a large number of algorithms and mechanisms for web proxy caches. In order to build powerful web proxies and understand their performance, one must be able to appreciate the impact and significance of earlier contributions and how they can be integrated To do this we employ a cache replacement algorithm, 'CSP, which integrates key knowledge from previous work. CSP utilizes the communication Cost to fetch web objects, the objects' Sizes, their Popularifies, an auxiliary cache and a cache admission control algorithm. We study the impact of these components with respect to hit ratio, latency, and bandwidth requirements. Our results show that there are clear performance gains when utilizing the communication cost, the popularity of objects, and the auxiliary cache. In contrast, the size of objects and the admission controller have a negligible performance impact. Our major conclusions going against those in related work are that (i) LRU is preferable to CSP for important parameter values, (ii) accounting for the objects' sizes does not improve latency and/or bandwidth requirements, and (iii) the collaboration of nearby proxies is not very beneficial. Based on these results, we chart the problem solution space, identifying which algorithm is preferable and under which conditions. Finally, we develop a dynamic replacement algorithm that continuously utilizes the best algorithm as the problem-parameter values (e.g., the access distributions) change with time.