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Cambridge, United Kingdom

Sastry N.,University of Cambridge | Manjunath D.,Indian Institute of Technology Bombay | Manjunath D.,Bharti Center for Communications | Sollins K.,Cambridge Intelligence | Crowcroft J.,University of Cambridge
IEEE Transactions on Mobile Computing | Year: 2011

Pocket Switched Networks take advantage of social contacts to opportunistically create data paths over time. This work employs empirical traces to examine the effect of the human contact process on data delivery in such networks. The contact occurrence distribution is found to be highly uneven: contacts between a few node pairs occur too frequently, leading to inadequate mixing in the network, while the majority of contacts occur rarely, but are essential for global connectivity. This distribution of contacts leads to a significant variation in the fraction of node pairs that can be connected over time windows of similar duration. Good time windows tend to have a large clique of nodes that can all reach each other. It is shown that the clustering coefficient of the contact graph over a time window is a good predictor of achievable connectivity. We then examine all successful paths found by flooding and show that though delivery times vary widely, randomly sampling a small number of paths between each source and destination is sufficient to yield a delivery time distribution close to that of flooding over all paths. This result suggests that the rate at which the network can deliver data is remarkably robust to path failures. © 2011 IEEE. Source

Benczur A.A.,Hungarian Academy of Sciences | Karger D.R.,Cambridge Intelligence
SIAM Journal on Computing | Year: 2015

We describe random sampling techniques for approximately solving problems that involve cuts and flows in graphs. We give a near-linear-time randomized combinatorial construction that transforms any graph on n vertices into an O(n log n)-edge graph on the same vertices whose cuts have approximately the same value as the original graph's. In this new graph, for example, we can run the Õ (m3/2)-time maximum flow algorithm of Goldberg and Rao to find an s-t minimum cut in Õ(n3/2) time. This corresponds to a (1 + ε)-times minimum s-t cut in the original graph. A related approach leads to a randomized divide-and-conquer algorithm producing an approximately maximum flow in Õ(m √n) time. Our algorithm can also be used to improve the running time of sparsest cut approximation algorithms from Õ(mn) to Õ (n2) and to accelerate several other recent cut and flow algorithms. Our algorithms are based on a general theorem analyzing the concentration of random graphs' cut values near their expectations. Our work draws only on elementary probability and graph theory. © 2015 Andras Benczúr and David R. Karger. Source

Montone A.,Polytechnic of Milan | Santambrogio M.D.,Cambridge Intelligence | Sciuto D.,Polytechnic of Milan | Memik S.O.,Northwestern University
ACM Transactions on Reconfigurable Technology and Systems | Year: 2010

The aim of this article is to describe a complete partitioning and floorplanning algorithm tailored for reconfigurable architectures deployable on FPGAs and considering communication infrastructure feasibility. This article proposes a novel approach for resource- and reconfiguration- aware floorplanning. Different from existing approaches, our floorplanning algorithm takes specific physical constraints such as resource distribution and the granularity of reconfiguration possible for a given FPGA device into account. Due to the introduction of constraints typical of other problems like partitioning and placement, the proposed approach is named floorplacer in order to underline the great differences with respect to traditional floorplanners. These physical constraints are typically considered at the later placement stage. Different aspects of the problems have been described, focusing particularly on the FPGAs resource heterogeneity and the temporal dimension typical of reconfigurable systems. Once the problem is introduced a comparison among related works has been provided and their limits have been pointed out. Experimental results proved the validity of the proposed approach. © 2010 ACM. Source

Kantor E.,Cambridge Intelligence | Lotker Z.,Ben - Gurion University of the Negev | Parter M.,Weizmann Institute of Science | Peleg D.,Weizmann Institute of Science
Journal of the ACM | Year: 2015

This article studies the topological properties of wireless communication maps and their usability in algorithmic design. We consider the SINR model, which compares the received power of a signal at a receiver against the sum of strengths of other interfering signals plus background noise. To describe the behavior of a multistation network, we use the convenient representation of a reception map, which partitions the plane into reception zones, one per station, and the complementary region of the plane where no station can be heard. SINR diagrams have been studied in Avin et al. [2009] for the specific case where all stations use the same power. It was shown that the reception zones are convex (hence connected) and fat, and this was used to devise an efficient algorithm for the fundamental problem of point location. Here we consider the more general (and common) case where transmission energies are arbitrary (or nonuniform). Under that setting, the reception zones are not necessarily convex or even connected. This poses the algorithmic challenge of designing efficient point location techniques for the nonuniform setting, as well as the theoretical challenge of understanding the geometry of SINR diagrams (e.g., the maximal number of connected components they might have). Our key result exhibits a striking contrast between d- and (d+ 1)-dimensional maps for a network embedded in d-dimensional space. Specifically, it is shown that whereas the d-dimensional map might be highly fractured, drawing the map in one dimension higher "heals" the zones, which become connected (in fact, hyperbolically connected). We also provide bounds for the fatness of reception zones. Subsequently, we consider algorithmic applications and propose a new variant of approximate point location. © 2015 ACM 0004-5411/2015/10-ART32 15.00. Source

News Article | March 20, 2014
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