John Whitbeck

Relieving the wireless infrastructure: When opportunistic networks meet guaranteed delays

Major wireless operators are nowadays facing network capacity issues in striving to meet the growing demands of mobile users. At the same time, 3G-enabled devices increasingly benefit from ad hoc radio connectivity (e.g., Wi-Fi). In this context of hybrid connectivity, we propose Push-and-track, a content dissemination framework that harnesses ad hoc communication opportunities to minimize the load on the wireless infrastructure while guaranteeing tight delivery delays. It achieves this through a control loop that collects user-sent acknowledgements to determine if new copies need to be reinjected into the network through the 3G interface. Push-and-Track includes multiple strategies to determine how many copies of the content should be injected, when, and to whom. The short delay-tolerance of common content, such as news or road traffic updates, make them suitable for such a system. Based on a realistic large-scale vehicular dataset from the city of Bologna composed of more than 10,000 vehicles, we demonstrate that Push-and-Track consistently meets its delivery objectives while reducing the use of the 3G network by over 90%.

Density-Aware Routing in Highly Dynamic DTNs: The RollerNet Case

We analyze the dynamics of a mobility data set collected in a pipelined disruption-tolerant network (DTN), a particular class of intermittently-connected wireless networks characterized by a 1-D topology. First, we collected and investigated traces of contact times among thousands of participants of a rollerblading tour in Paris. The data set shows extreme dynamics in the mobility pattern of a large number of nodes. Most strikingly, fluctuations in the motion of the rollerbladers cause a typical accordion phenomenon - the topology expands and shrinks with time, thus influencing connection times and opportunities between participants. Second, we show through an analytical model that the accordion phenomenon, through the variation of the average node degree, has a major impact on the performance of epidemic dissemination. Finally, we test epidemic dissemination and other existing forwarding schemes on our traces, and conclude that routing should adapt to the varying, though predictable, nature of the network. To this end, we propose DA-SW (Density-Aware Spray-and-Wait), a measurement-oriented variant of the spray-and-wait algorithm that tunes, in a dynamic fashion, the number of a message copies to be disseminated in the network. The particularity of DA-SW is that it relies on a set of abaci that represents the three phases of the accordion phenomenon: aggregation, expansion, and stabilization. We show that DA-SW leads to performance results that are close to the best case (obtained with an oracle).

Temporal reachability graphs

While a natural fit for modeling and understanding mobile networks, time-varying graphs remain poorly understood. Indeed, many of the usual concepts of static graphs have no obvious counterpart in time-varying ones. In this paper, we introduce the notion of temporal reachability graphs. A (tau,delta)-reachability graph is a time-varying directed graph derived from an existing connectivity graph. An edge exists from one node to another in the reachability graph at time t if there exists a journey (i.e., a spatiotemporal path) in the connectivity graph from the first node to the second, leaving after t, with a positive edge traversal time tau, and arriving within a maximum delay delta. We make three contributions. First, we develop the theoretical framework around temporal reachability graphs. Second, we harness our theoretical findings to propose an algorithm for their efficient computation. Finally, we demonstrate the analytic power of the temporal reachability graph concept by applying it to synthetic and real-life datasets. On top of defining clear upper bounds on communication capabilities, reachability graphs highlight asymmetric communication opportunities and offloading potential.

Performance of Opportunistic Epidemic Routing on Edge-Markovian Dynamic Graphs

Connectivity patterns in intermittently-connected mobile networks (ICMN) can be modeled as edge-Markovian dynamic graphs. We propose a new model for epidemic propagation on such graphs and calculate a closed-form expression that links the best achievable delivery ratio to common ICMN parameters such as message size, maximum tolerated delay, and link lifetime. These theoretical results are compared to those obtained by replaying a real-life contact trace.

Vehicular carriers for big data transfers (Poster)

In the latest years, Internet traffic has increased at a significantly faster pace than its capacity, preventing efficient bulk data transfers such as data-center services and high-definition user-generated content applications. In this paper, we propose to take advantage of the existing worldwide road infrastructure as an offloading channel to help the legacy Internet assuage its burden. Our results suggest that piggybacking data on vehicles can easily lead to network capacity in the petabyte range.

Mobility trace breeding

Collecting real contact traces in disruption-tolerant networks is a complex procedure. From the experiment setup to the data cleaning, a lot of efforts is required. We propose a breeding system to derive possible contact traces from a single real experiment. We check the consistency of our system using synthetic mobility traces and show that bred traces do follow characteristics of the original trace. We apply then the system to a real-world contact dataset. Our results indicate that even when using incomplete or imperfect real contact traces, our system produces accurate traces.