Browsing by Author "Delis, Alex"

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Citation - WoS: 4
Citation - Scopus: 6
Alleviating the Topology Mismatch Problem in Distributed Overlay Networks: a Survey
(Elsevier Science Inc, 2016) Moustakas, Vassilis; Akcan, Hüseyin; Roussopoulos, Mema; Delis, Alex
Peer-to-peer (P2P) systems have enjoyed immense attention and have been widely deployed on the Internet for well over a decade. They are often implemented via an overlay network abstraction atop the Internet's best-effort IP infrastructure. P2P systems support a plethora of desirable features to distributed applications including anonymity, high availability, robustness, load balancing, quality of service and scalability to name just a few. Unfortunately, inherent weaknesses of early deployments of P2P systems, prevented applications from leveraging the full potential of the paradigm. One major weakness, identified early on, is the topology mismatch problem between the overlay network and the underlying IP topology. This mismatch can impose an extraordinary amount of unnecessary stress on network resources and can adversely affect both the scalability and efficiency of the operating applications. In this paper, we survey over a decade's worth of research efforts aimed at alleviating the topology mismatch problem in both structured and unstructured P2P systems. We provide a fine-grained categorization of the suggested solutions by discussing their novelty, advantages and weaknesses. Finally, we offer an analysis as well as pictorial comparisons of the reviewed approaches since we aim to offer a comprehensive reference for developers, system architects and researchers in the field. (C) 2015 Published by Elsevier Inc.
Citation - WoS: 8
Citation - Scopus: 12
Managing Cohort Movement of Mobile Sensors Via Gps-Free and Compass-Free Node Localization
(Academic Press Inc Elsevier Science, 2010) Akcan, Hüseyin; Kriakov, Vassil; Bronnimann, Herve; Delis, Alex
A critical problem in mobile ad hoc wireless sensor networks is each node's awareness of its position relative to the network. This problem is known as localization. In this paper, we introduce a variant of this problem, directional localization, where each node must be aware of both its position and orientation relative to its neighbors. Directional localization is relevant for applications that require uniform area coverage and coherent movement. Using global positioning systems for localization in large scale sensor networks may be impractical in enclosed spaces, and might not be cost effective. In addition, a set of preexisting anchors with globally known positions may not always be available. In this context, we propose two distributed algorithms based on directional localization that facilitate the collaborative movement of nodes in a sensor network without the need for global positioning systems, seed nodes or a preexisting infrastructure such as anchors with known positions. Our first algorithm, GPS-free Directed Localization (GDL) assumes the availability of a simple digital. compass on each sensor node. We relax this requirement in our second algorithm termed GPS- and Compass-free Directed Localization (GCDL). Through experimentation, we demonstrate that our algorithms scale well for large numbers of nodes and provide convergent localization over time, despite errors introduced by motion actuators and distance measurements. In addition, we introduce mechanisms to preserve swarm formation during directed sensor network mobility. Our simulations confirm that, in a number of realistic scenarios, our algorithms provide for a mobile sensor network that preserves its formation over time, irrespective of speed and distance traveled. We also present our method to organize the sensor nodes in a polygonal geometric shape of our choice even in noisy environments, and investigate the possible uses of this approach in search-and-rescue type of missions. (C) 2010 Elsevier Inc. All rights reserved.