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 Resource Type:
 Article
 Creator:
 Urrutia, J., Opatrny, J., Chávez, E., Dobrev, S., Stacho, L., and Kranakis, Evangelos
 Abstract:
 We address the problem of discovering routes in strongly connected planar geometric networks with directed links. Motivated by the necessity for establishing communication in wireless ad hoc networks in which the only information available to a vertex is its immediate neighborhood, we are considering routing algorithms that use the neighborhood information of a vertex for routing with constant memory only. We solve the problem for three types of directed planar geometric networks: Eulerian (in which every vertex has the same number of incoming and outgoing edges), Outerplanar (in which a single face contains all vertices of the network), and Strongly Face Connected, a new class of geometric networks that we define in the article, consisting of several faces, each face being a strongly connected outerplanar graph.
 Date Created:
 20060801

 Resource Type:
 Conference Proceeding
 Creator:
 Krizanc, Danny, Kranakis, Evangelos, and Kirousis, Lefteris M.
 Abstract:
 Let φ be a random Boolean formula that is an instance of 3SAT. We consider the problem of computing the least real number such that if the ratio of the number of clauses over the number of variables of φ strictly exceeds κ, then φ is almost certainly unsatisfiable. By a well known and more or less straightforward argument, it can be shown that κ 3.
 Date Created:
 19960101

 Resource Type:
 Conference Proceeding
 Creator:
 Peleg, David, Krizanc, Danny, Kirousis, Lefteris M., Kranakis, Evangelos, Kaklamanis, Christos, and Bose, Prosenjit
 Abstract:
 In wireless communication, the signal of a typical broadcast station is transmited from a broadcast center p and reaches objects at a distance, say, R from it. In addition there is a radius r, r < R, such that the signal originating from the center of the station is so strong that human habitation within distance r from the center p should be avoided. Thus every station determines a region which is an “annulus of permissible habitation". We consider the following station layout (SL) problem: Cover a given (say, rectangular) planar region which includes a collection of orthogonal buildings with a minimum number of stations so that every point in the region is within the reach of a station, while at the same time no building is within the dangerous range of a station. We give algorithms for computing such station layouts in both the oneand twodimensional cases.
 Date Created:
 19990101

 Resource Type:
 Conference Proceeding
 Creator:
 Cervera, Gimer, Barbeau, Michel, GarciaAlfaro, Joaquin, and Kranakis, Evangelos
 Abstract:
 The Hierarchical Optimized Link State Routing (HOLSR) protocol enhances the scalability and heterogeneity of traditional OLSRbased Mobile AdHoc Networks (MANETs). It organizes the network in logical levels and nodes in clusters. In every cluster, it implements the mechanisms and algorithms of the original OLSR to generate and to distribute control traffic information. However, the HOLSR protocol was designed with no security in mind. Indeed, it both inherits, from OLSR, and adds new security threats. For instance, the existence of misbehaving nodes can highly affect important HOLSR operations, such as the cluster formation. Cluster IDentification (CID) messages are implemented to organize a HOLSR network in clusters. In every message, the hop count field indicates to the receiver the distance in hops to the originator. An attacker may maliciously alter the hop count field. As a consequence, a receiver node may join a cluster head farther away than it appears. Then, the scalability properties in a HOLSR network is affected by an unbalanced distribution of nodes per cluster. We present a solution based on the use of hash chains to protect mutable fields in CID messages. As a consequence, when a misbehaving node alters the hop count field in a CID message, the receiver nodes are able of detecting and discarding the invalid message.
 Date Created:
 20120127

 Resource Type:
 Conference Proceeding
 Creator:
 Czyzowicz, Jurek, Opatrny, Jaroslav, Kranakis, Evangelos, Narayanan, Lata, Krizanc, Danny, Stacho, Ladislav, Urrutia, Jorge, Yazdani, Mohammadreza, and Lambadaris, Ioannis
 Abstract:
 A set of sensors establishes barrier coverage of a given line segment if every point of the segment is within the sensing range of a sensor. Given a line segment I, n mobile sensors in arbitrary initial positions on the line (not necessarily inside I) and the sensing ranges of the sensors, we are interested in finding final positions of sensors which establish a barrier coverage of I so that the sum of the distances traveled by all sensors from initial to final positions is minimized. It is shown that the problem is NP complete even to approximate up to constant factor when the sensors may have different sensing ranges. When the sensors have an identical sensing range we give several efficient algorithms to calculate the final destinations so that the sensors either establish a barrier coverage or maximize the coverage of the segment if complete coverage is not feasible while at the same time the sum of the distances traveled by all sensors is minimized. Some open problems are also mentioned.
 Date Created:
 20101213

 Resource Type:
 Conference Proceeding
 Creator:
 Barbeau, Michel, Kranakis, Evangelos, and GarciaAlfaro, Joaquin
 Abstract:
 The design and implementation of security threat mitigation mechanisms in RFID systems, specially in lowcost RFID tags, are gaining great attention in both industry and academia. One main focus of research interests is the authentication and privacy techniques to prevent attacks targeting the insecure wireless channel of these systems. Cryptography is a key tool to address these threats. Nevertheless, strong hardware constraints, such as production costs, power consumption, time of response, and regulations compliance, makes the use of traditional cryptography in these systems a very challenging problem. The use of lowoverhead procedures becomes the main approach to solve these challenging problems where traditional cryptography cannot fit. Recent results and trends, with an emphasis on lightweight techniques for addressing critical threats against lowcost RFID systems, are surveyed.
 Date Created:
 20100503

 Resource Type:
 Conference Proceeding
 Creator:
 Kranakis, Evangelos, Krizanc, Danny, Narayanan, Lata, and Keane, Michael
 Abstract:
 Delay (or disruption) tolerant sensor networks may be modeled as Markovian evolving graphs [1]. We present experimental evidence showing that considering multiple (possibly not shortest) paths instead of one fixed (greedy) path can decrease the expected time to deliver a packet on such a network by as much as 65 per cent depending on the probability that an edge exists in a given time interval. We provide theoretical justification for this result by studying a special case of the Markovian evolving grid graph. We analyze a natural algorithm for routing on such networks and show that it is possible to improve the expected time of delivery by up to a factor of two depending upon the probability of an edge being up during a time step and the relative positions of the source and destination. Furthermore we show that this is optimal, i.e., no other algorithm can achieve a better expected running time. As an aside, our results give high probability bounds for Knuth's toilet paper problem [11].
 Date Created:
 20091201

 Resource Type:
 Conference Proceeding
 Creator:
 Krizanc, D., Yazdani, M., Stacho, L., Narayanan, L., Lambadaris, Ioannis, Opatrny, J., Czyzowicz, J., Kranakis, Evangelos, and Urrutia, J.
 Abstract:
 We consider n mobile sensors located on a line containing a barrier represented by a finite line segment. Sensors form a wireless sensor network and are able to move within the line. An intruder traversing the barrier can be detected only when it is within the sensing range of at least one sensor. The sensor network establishes barrier coverage of the segment if no intruder can penetrate the barrier from any direction in the plane without being detected. Starting from arbitrary initial positions of sensors on the line we are interested in finding final positions of sensors that establish barrier coverage and minimize the maximum distance traversed by any sensor. We distinguish several variants of the problem, based on (a) whether or not the sensors have identical ranges, (b) whether or not complete coverage is possible and (c) in the case when complete coverage is impossible, whether or not the maximal coverage is required to be contiguous. For the case of n sensors with identical range, when complete coverage is impossible, we give linear time optimal algorithms that achieve maximal coverage, both for the contiguous and noncontiguous case. When complete coverage is possible, we give an O(n 2) algorithm for an optimal solution, a linear time approximation scheme with approximation factor 2, and a (1∈+∈ε) PTAS. When the sensors have unequal ranges we show that a variation of the problem is NPcomplete and identify some instances which can be solved with our algorithms for sensors with unequal ranges.
 Date Created:
 20091019

 Resource Type:
 Conference Proceeding
 Creator:
 Kranakis, Evangelos and Wiese, Andreas
 Abstract:
 We investigate the problem of locally coloring and constructing special spanners of location aware Unit Disk Graphs (UDGs). First we present a local approximation algorithm for the vertex coloring problem in UDGs which uses at most four times as many colors as required by an optimal solution. Then we look at the colorability of spanners of UDGs. In particular we present a local algorithm for constructing a 4colorable spanner of a unit disk graph. The output consists of the spanner and the 4coloring. The computed spanner also has the properties that it is planar, the degree of a vertex in the spanner is at most 5 and the angles between two edges are at least π/3. By enlarging the locality distance (i.e. the size of the neighborhood which a vertex has to explore in order to compute its color) we can ensure the total weight of the spanner to be arbitrarily close to the weight of a minimum spanning tree. We prove that a local algorithm cannot compute a bipartite spanner of a unit disk graph and therefore our algorithm needs at most one color more than any local algorithm for the task requires. Moreover, we prove that there is no local algorithm for 3coloring UDGs or spanners of UDGs, even if the 3colorability of the graph (or the spanner respectively) is guaranteed in advance.
 Date Created:
 20081201

 Resource Type:
 Conference Proceeding
 Creator:
 Kranakis, Evangelos, Shi, Q., Bhattacharya, B., Wiese, A., Burmester, B., and Hu, Y.
 Abstract:
 Intrusion detection, area coverage and border surveillance are important applications of wireless sensor networks today. They can be (and are being) used to monitor large unprotected areas so as to detect intruders as they cross a border or as they penetrate a protected area. We consider the problem of how to optimally move mobile sensors to the fence (perimeter) of a region delimited by a simple polygon in order to detect intruders from either entering its interior or exiting from it. We discuss several related issues and problems, propose two models, provide algorithms and analyze their optimal mobility behavior.
 Date Created:
 20080922