Lavy Libman

Challenges in body area networks for healthcare: the MAC

Body area wireless sensor networks (BANs) are a key component to the ubiquitous healthcare revolution and perhaps one of its most challenging elements from a communications standpoint. The unique characteristics of the wireless channel, coupled with the need for extreme energy efficiency in many healthcare applications, require novel solutions in medium access control protocols. We present the main characteristics and challenges associated with BANs from a healthcare perspective, and present some MAC techniques based on studies of the BAN channel that could be used to address these challenges.

Atomic Resource Sharing in Noncooperative Networks

In noncooperative networks, resources are shared among selfish users, which optimize their individual performance measure. We consider the generic and practically important case of atomic resource sharing, in which traffic bifurcation is not implemented, hence each user allocates its whole traffic to one of the network resources. We analyze topologies of parallel resources within a game-theoretic framework and establish several fundamental properties.We prove the existence of and convergence to a Nash equilibrium. For a broad class of residual capacity performance functions, an upper bound on the number of iterations till convergence is derived. An algorithm is presented for testing the uniqueness of the equilibrium. Sufficient conditions for achieving a feasible equilibrium are obtained. We consider extensions to general network topologies. In particular, we show that, for a class of throughput-oriented cost functions, existence of and convergence to a Nash equilibrium is guaranteed in all topologies.

Energy-efficient data gathering with tour length-constrained mobile elements in wireless sensor networks

Several studies in recent years have considered the use of mobile elements for data gathering in wireless sensor networks, so as to reduce the need for multi-hop forwarding among the sensor nodes and thereby prolong the network lifetime. Since, typically, practical constraints preclude a mobile element from visiting all nodes in the sensor network, the solution must involve a combination of a mobile element visiting a subset of the nodes (cache points), while other nodes communicate their data to the cache points wirelessly. This leads to the optimization problem of minimizing the communication distance of the sensor nodes, while keeping the tour length of the mobile element below a given constraint. Several algorithms in existing literature have tackled this problem by separating the construction of the mobile element tour from the computation of the forwarding trees to the cache points. In this paper, we propose a new algorithm that alternates between these phases and iteratively improves the outcome of each phase, based on the result of the other. We compare the resulting performance of our algorithm with that of previous work, and show that it closes a considerable portion of the gap from the theoretical optimal solution.

Optimal Strategies for Cooperative MAC-Layer Retransmission in Wireless Networks

The concept of cooperative retransmission in wireless networks has attracted considerable research attention. The basic idea is that when a receiver cannot decode a frame, the retransmission is handled not by its original source but rather by a neighbour that overheard the transmission successfully, and may have a better channel to the destination. However, the majority of existing literature tackles the issue from the physical layer perspective, with either a single cooperating neighbour, or a multiple-neighbour setting where the receiver is capable of combining and decoding the signal from several simultaneous retransmissions. In this paper, we consider the case of multiple cooperating neighbours from a MAC-layer perspective. Thus, we assume a receiver that can only decode one transmission at a time, while multiple simultaneous retransmissions (by several neighbours that had overheard the frame successfully) will cause a collision. As a result, each neighbour with a successfully overheard copy of the frame faces a tradeoff between helping with a cooperative retransmission and possibly causing a collision. Accordingly, we pose the optimization problem of finding a distributed randomized strategy for the cooperating neighbours, which assigns a certain retransmission probability to every neighbour in each time slot, so as to minimize the expected latency until successful reception. We analyse the performance achieved by two approaches: one where the original source is silent while the neighbours conduct their cooperative retransmissions, and another where both the source and the neighbours may have a nonzero retransmission probability simultaneously. We show that the latter approach offers a significant performance improvement over the former one, as well as either traditional retransmission or two-hop routing to the destination.

The designer's perspective to atomic noncooperative networks

In noncooperative networks, resources are shared among selfish users, which optimize their individual performance measure. Traditional design methods tend to perform poorly in such networks, as they do not take into account the inherent noncooperative nature of the network users. Such networks require specialized design techniques in order to achieve efficient utilization of resources. We consider the generic and practically important class of atomic resource sharing networks, in which traffic bifurcation is not implemented, hence each user allocates its whole traffic to one of the network resources. We investigate topologies of parallel resources within a game-theoretic framework and establish the foundations of a design and management methodology that enables operation of such networks efficiently, despite both the lack of cooperation among users and the restrictions imposed by atomic resource sharing. We study various problems pertaining to capacity allocation, pricing, and admission control, and show that their solutions are substantially different from those corresponding to traditional networks.

Experiences and Lessons from Implementing a Wireless Sensor Network MAC Protocol in the Castalia Simulator

We describe our experience from the implementation of the T-MAC protocol for wireless sensor networks in the open-source Castalia simulator. Notwithstanding the popularity of the protocol in the research literature in recent years, we find several practical issues that are not addressed in the original protocol description, which lead to a degree of freedom in the protocol design and implementation and have an impact on its resulting performance. These issues include the ability of the underlying physical layer and hardware to efficiently detect the activation events in the protocol, and necessary changes to the collision resolution and clock synchronization procedures in the presence of varying sleep patterns. Our results highlight the need for rigorous detail in protocol descriptions in the research literature and provide important insights into some of the common pitfalls.

Adaptive delay-based congestion control for high bandwidth-delay product networks

The design of an end-to-end Internet congestion control protocol that could achieve high utilization, fair sharing of bottleneck bandwidth, and fast convergence while remaining TCP-friendly is an ongoing challenge that continues to attract considerable research attention. This paper presents ACP, an Adaptive end-to-end Congestion control Protocol that achieves the above goals in high bandwidth-delay product networks where TCP becomes inefficient. The main contribution of ACP is a new form of congestion window control, combining the estimation of the bottleneck queue size and a measure of fair sharing. Specifically, upon detecting congestion, ACP decreases the congestion window size by the exact amount required to empty the bottleneck queue while maintaining high utilization, while the increases of the congestion window are based on a “fairness ratio” metric of each flow, which ensures fast convergence to a fair equilibrium. We demonstrate the benefits of ACP using both ns-2 simulation and experimental measurements of a Linux prototype implementation. In particular, we show that the new protocol is TCP-friendly and allows TCP and ACP flows to coexist in various circumstances, and that ACP indeed behaves more fairly than other TCP variants under heterogeneous round-trip times (RTT).

Atomic resource sharing in noncooperative networks

In noncooperative networks, resources are shared among selfish users, which optimize their individual performance measure. We consider the generic and practically important case of atomic resource sharing, in which traffic bifurcation is not implemented, hence each user allocates its whole traffic to one of the network resources. We analyze topologies of parallel resources within a game-theoretic framework and establish several fundamental properties. We prove the existence of and convergence to a Nash equilibrium. For a broad class of residual capacity performance functions, an upper bound on the number of iterations till convergence is derived. An algorithm is presented for testing the uniqueness of the equilibrium. Sufficient conditions for achieving a feasible equilibrium are obtained. We consider extensions to general network topologies. In particular, we show that, for a class of throughput-oriented cost functions, existence of and convergence to a Nash equilibrium is guaranteed in all topologies. With these structural results at hand, we establish the foundations of a design and management methodology, that enables one to operate such networks efficiently, in spite of the lack of cooperation among users and the restrictions imposed by atomic resource sharing.

Conflicts and Incentives in Wireless Cooperative Relaying: A Distributed Market Pricing Framework

Extensive research in recent years has shown the benefits of cooperative relaying in wireless networks, where nodes overhear and cooperatively forward packets transmitted between their neighbors. Most existing studies focus on physical-layer optimization of the effective channel capacity for a given transmitter-receiver link; however, the interaction among simultaneous flows between different endpoint pairs, and the conflicts arising from their competition for a shared pool of relay nodes, are not yet well understood. In this paper, we study a distributed pricing framework, where sources pay relay nodes to forward their packets, and the payment is shared equally whenever a packet is successfully relayed by several nodes at once. We formulate this scenario as a Stackelberg (leader-follower) game, in which sources set the payment rates they offer, and relay nodes respond by choosing the flows to cooperate with. We provide a systematic analysis of the fundamental structural properties of this generic model. We show that multiple follower equilibria exist in general due to the nonconcave nature of their game, yet only one equilibrium possesses certain continuity properties that further lead to a unique system equilibrium among the leaders. We further demonstrate that the resulting equilibria are reasonably efficient in several typical scenarios.

SeAK: Secure Authentication and Key Generation Protocol Based on Dual Antennas for Wireless Body Area Networks

The increasing interest in the usage of wireless body area networks (WBAN) in healthcare and other critical applications underscores the importance of secure communication among the body sensor devices. Associating an unknown device with an existing network without prior knowledge of a secret key poses a major challenge. Existing authentication schemes in WBAN are typically based on received signal strength (RSS). However, RSS techniques using a single antenna are susceptible to environmental factors and are vulnerable to attacks that use variable transmission power. We present SeAK, the first secure light-weight device pairing protocol for WBAN based on RSS obtained by dual-antenna transceivers utilizing spatial diversity. With spatially separated antennas, the RSS values from a nearby device are large and distinct, as opposed to those from a far-away device. SeAK exploits this effect to accomplish authentication and shared secret key generation simultaneously. We have implemented a prototype of SeAK on the Opal sensor platform with a 2.4 GHz compatible RF231 radio. We demonstrate that our protocol is able to achieve a 100 % success acceptance rate, securely authenticate a nearby device and generate a 128-bit secret key in 640 ms, as opposed to 15.9 s in other recent RSS-based schemes (e.g. ASK-BAN).