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Medium Access Control, Fairness and Phase Transitions in Multihop Wireless Networks

Thiran, P (Lausanne)
Tuesday 06 April 2010, 15:30-16:30

Seminar Room 1, Newton Institute


The last couple of years have seen considerable progress in the design of new throughput-optimal Medium Access Control (MAC) protocols for wireless networks. They are however not compatible in general with the IEEE 802.11 protocol, which is the most widely used protocol in current wireless networks. We adopt a dual point a view, which is to find models of 802.11 protocols which capture rigorously all the essential features of this protocol, and remain simple enough to provide insight in the context of multi-hop wireless networks.

We consider two scenarios. In the first one, all nodes are saturated with fresh traffic (all queues are thus infinite), and we explore systematically the trade-off between spatial reuse (and thus throughput) and fairness, which is inherent to decentralized CSMA MAC protocols, such as IEEE 802.11. We show in particular that the widely observed unfairness of the protocol in small network topologies does not always persist in large topologies. In large 1-dim. networks, nodes sufficiently far away from the border of the network have equal access to the channel, but not in 2-dim networks, where we observe a phase transition implying a strong unfairness of the protocol.

In the second scenario, one node only (the source) is saturated with fresh traffic, whereas all other nodes act as relays or destination for the traffic originated at the source node. We show theoretically and experimentally that IEEE 802.11 can become unstable if the network diameter is larger than 3 hops, and we propose a modification (EZ-flow) that stabilizes the network while staying backward compatible with the former protocol. We evaluate its performance on an experimental testbed.

This is a joint work with Adel Aziz, Mathilde Durvy and Olivier Dousse.


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