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SUMMARY:Medium Access Control\, Fairness and Phase Transitions in Multihop
  Wireless Networks - Thiran\, P (Lausanne)
DTSTART:20100406T143000Z
DTEND:20100406T153000Z
UID:TALK24083@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:The last couple of years have seen considerable progress in th
 e design of new throughput-optimal Medium Access Control (MAC) protocols f
 or wireless networks. They are however not compatible in general with the 
 IEEE 802.11 protocol\, which is the most widely used protocol in current w
 ireless networks. We adopt a dual point a view\, which is to find models o
 f 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. \n\nWe consider two scenarios. In the fir
 st one\, all nodes are saturated with fresh traffic (all queues are thus i
 nfinite)\, and we explore systematically the trade-off between spatial reu
 se (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 doe
 s 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 transi
 tion implying a strong unfairness of the protocol. \n\nIn the second scena
 rio\, one node only (the source) is saturated with fresh traffic\, whereas
  all other nodes act as relays or destination for the traffic originated a
 t 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 sta
 ying backward compatible with the former protocol. We evaluate its perform
 ance on an experimental testbed. \n\nThis is a joint work with Adel Aziz\,
  Mathilde Durvy and Olivier Dousse.\n
LOCATION:Seminar Room 1\, Newton Institute
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