Vania Conan

The Accordion Phenomenon: Analysis, Characterization, and Impact on DTN Routing

We analyze the dynamics of a mobility dataset collected in a pipelined disruption-tolerant network (DTN), a particular class of intermittently-connected wireless networks characterized by a one-dimensional topology. First, we collected and investigated traces of contact times among a thousand participants of a rollerblading tour in Paris. The dataset 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 argue 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 disseminated in the network. We show that DA-SW leads to performance results that are close to the best case (obtained with an oracle).

Characterizing pairwise inter-contact patterns in delay tolerant networks

A good understanding of contact patterns in delay tolerant networks (DTNs) is elemental to the design of effective routing or content distribution schemes. Prior work has typically focused on inter-contact time patterns in the aggregate. In this paper, we argue that pairwise inter-contact patterns are a more refined and efficient tool for characterizing DTNs. First, we provide a detailed statistical analysis of pairwise contact and inter-contact times in three reference DTN data sets. We characterize heterogeneities in contact times and inter-contact times, and find that the empirical distributions of inter-contact times tend to be well fitted by log-normal curves, with exponential curves also fitting a significant portion of the distributions. Second, we investigate analytically the relationship between pairwise and aggregate inter-contact times. In particular, we consider both the exponential and log-normal cases and show analytically how the aggregation of pairwise inter-contacts may lead to aggregate inter-contacts with power laws of various degrees.

Data Offloading Techniques in Cellular Networks: A Survey

One of the most engaging challenges for mobile operators today is how to manage the exponential data traffic increase. Mobile data offloading stands out as a promising and low-cost solution to reduce the burden on the cellular network. To make this possible, we need a new hybrid network paradigm that leverages the existence of multiple alternative communication channels. This entails significant modifications in the way data are handled, affecting also the behavior of network protocols. In this paper, we present a comprehensive survey of data offloading techniques in cellular networks and extract the main requirements needed to integrate data offloading capabilities into todays mobile networks. We classify existing strategies into two main categories, according to their requirements in terms of content delivery guarantees: delayed and nondelayed offloading. We overview the technical aspects and discuss the state of the art in each category. Finally, we describe in detail the novel functionalities needed to implement mobile data offloading in the access network, as well as current and future research challenges in the field, with an eye toward the design of hybrid architectures.

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%.

HYMAD: Hybrid DTN-MANET routing for dense and highly dynamic wireless networks

In this paper we propose HYMAD, a Hybrid DTN-MANET routing protocol which uses DTN between disjoint groups of nodes while using MANET routing within these groups. HYMAD is fully decentralized and only makes use of topological information exchanges between the nodes. We evaluate the scheme in simulation by replaying real life traces which exhibit this highly dynamic connectivity. The results show that HYMAD outperforms the multi-copy Spray-and-Wait DTN routing protocol it extends, both in terms of delivery ratio and delay, for any number of message copies. Our conclusion is that such a Hybrid DTN-MANET approach offers a promising venue for the delivery of elastic data in mobile ad-hoc networks as it retains the resilience of a pure DTN protocol while significantly improving performance.

Fixed point opportunistic routing in delay tolerant networks

We propose in this work a single copy and multi-hop opportunistic routing scheme for sparse delay tolerant networks (DTNs). The scheme uses as only input the estimates of the average inter-contact times between the nodes in the network. Defined as the fixed point of a recursive process, it aims at minimizing delivery time in case of independent exponential pairwise inter-contacts. The two properties of loop-free forwarding and polynomial convergence make the scheme workable for routing in DTNs. The routing performances of the scheme are evaluated on three publicly available reference data sets. Comparisons with well known single-copy schemes, including MED and the two hop relay strategy, consistently demonstrate improvements for both delivery ratio and delay.

Fast track article: Push-and-track: Saving infrastructure bandwidth through opportunistic forwarding

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., WiFi). 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 re-injected into the network through the 3G interface. Push-and-Track is flexible and can be applied to a variety of scenarios, including periodic message flooding and floating data. For the former, this paper examines multiple strategies to determine how many copies of the content should be injected, when, and to whom; for the latter, it examines the achievable offload ratio depending on the freshness constraints. The short delay-tolerance of common content, such as news or road traffic updates, make them suitable for such a system. Use cases with a long delay-tolerance, such as software updates, are an even better fit. 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 about 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.

An efficient service oriented architecture for heterogeneous and dynamic wireless sensor networks

The purpose of this work is to bridge the gap between high-end networked devices and wireless networks of ubiquituous and resource-constrained sensors and actuators by extensively applying Service-Oriented Architecture (SOA) patterns. We present a multi-level approach that implements existing SOA standards on higher tiers, and propose a novel protocol stack, WSN-SOA, which brings the benefits of SOA to low capacity nodes without the overhead of XML-based technologies. This solution fully supports network dynamicity, auto-configuration, service discovery, device heterogeneity and interoperability with legacy architectures. As a proof-of-concept, we have studied a surveillance scenario in which the detection of an intruder, conducted within the range of a network of wireless sensors (e.g., MICAz from Crossbow), leads to the automatic triggering of tracking activities by a Linux-powered network camera and of alerts and video streams toward a control room.