Long-term time-share guarantees are necessary for wireless LANs

Wireless local area networks (WLANs) based on a family of 802.11 technologies are becoming ubiquitous. These technologies support multiple data transmission rates. Transmitting at a lower data rate (by using a more resilient modulation scheme) increases the frame transmission time but reduces the bit error rate. In non-cooperative environments such as public hot-spots, individual nodes attempt to maximize their achieved throughput by adjusting the data rate or frame size used, irrespective of the impact of this on overall system performance. In a series of experiments, we demonstrate that the existing distributed MAC protocol encourages non-cooperative nodes to use globally inefficient transmission strategies that lead to degraded aggregate throughputs. We also show that by establishing independence between the allocation of the shared channel time and the strategies used by individual nodes, an improved MAC protocol can lead rational but non-cooperative nodes to make choices that increase aggregate throughputs by as much as 30 % under some conditions.

Improving aggregate user utilities and providing fairness in multi-rate wireless LANs

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.Includes bibliographical references (p. 159-166).A distributed medium access control (MAC) protocol is responsible for allocating the shared spectrum efficiently and fairly among competing devices using a wireless local area network. Unfortunately, existing MAC protocols, including 802.11's DCF, achieve neither efficiency nor fairness under many realistic conditions. In this dissertation, we show that both bit and frame-based fairness,the most widely used notions, lead to drastically reduced aggregate throughput and increased average delay in typical environments, in which competing nodes transmit at different data transmission rates. We demonstrate the advantages of time-based fairness, in which each competing node receives an equal share of the wireless channel occupancy time. Through analysis, experiments on a Linux test bed, and simulation, we demonstrate that time-based fairness can lead to significant improvements in aggregate throughput and average delay. Through a game theoretic analysis and simulation, we also show that existing MAC protocols encourage non-cooperative nodes to employ globally inefficient transmission strategies that lead to low aggregate throughput. We show that providing long-term time share guarantees among competing nodes leads rational nodes to employ efficient transmission strategies at equilibriums.(cont.) We describe two novel solutions, TES (Time-fair Efficient and Scalable MAC protocol) and TBR (Time-based Regulator) that provide time-based fairness and long-term time share guarantees among competing nodes. TBR is a backward-compatible centralized solution that runs at the AP,works in conjunction with DCF, and requires no modifications to clients nor to DCF. TBR is appropriate for existing access point based networks, but not effective when nearby non-cooperative nodes fall under different administrative domains. Our evaluation of TBR on an 802.1lb/Linux test bed shows that TBR can improve aggregate TCP throughput by as much as 105% in rate diverse environments. TES is a non-backward compatible distributed contention-based MAC protocol that is effective in any environment, including non-cooperative environments. Furthermore, the aggregate throughputs sustained with increased loads. Through extensive simulation experiments, we demonstrate that TES is significantly more efficient(as much as 140% improvement in aggregate TCP throughput) and fairer than existing MAC protocols including DCF.by Godfrey Tan.Ph.D