Godfrey Tan

The 802.11 MAC protocol leads to inefficient equilibria

Wireless local area networks (WLANs) based on the 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 hit error rate. In non-cooperative environments such as public hot-spots or WLANs operated by different enterprises that are physically close to each other, 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 this paper, we show both analytically using a game theoretic model and through simulation that the existing 802.11 distributed MAC protocol, DCF (for distributed coordination function), as well as its enhanced version, which is being standardized at part of 802.11e, can lead non-cooperative nodes to undesirable Nash equilibriums, in which the wireless channel is inefficiently used. We show that by establishing independence between the allocation of the shared channel resource and the transmission strategies used by individual nodes, an ideal MAC protocol can lead rational nodes to arrive at equilibriums in which all competing nodes achieve higher throughputs tan with DCF.

Divert: fine-grained path selection for wireless LANs

The performance of Wireless Local Area Networks (WLANs)often suffers from link-layer frame losses caused by noise, interference, multipath, attenuation, and user mobility. We observe that frame losses often occur in bursts and that three of the five main causes of frame losses -- multipath, attenuation, mobility--depends on the transmission path traversed between an access point (AP)and a client station.In a typical WLAN deployment, different transmission paths to a client exist in places where overlapping coverage is provided by a set of neighboring APs. Using experimental measurements and analysis on a 802.11b testbed, we show that fine-grained path selection among a set of neighboring APs can significantly reduce path-dependent losses in WLANs.We design and implement a WLAN distribution system called Divert, which supports fine-grained path selection for downlink communications, on an 802.11b testbed. Divert reduces frame losses without consuming any extra bandwidth in the wireless medium. Our experimental results show that Divert can reduce frame loss rates in realistic scenarios by as much as 26% compared to a fixed-path scheme that uses the best available transmitter.

A locally coordinated scatternet scheduling algorithm

There is growing interest in wireless personal area networks built from portable devices equipped with short-range radio interfaces such as Bluetooth. These small networks (called piconets) can be internetworked to form larger scatternets by means of bridge nodes that participate in more than one piconet on a time division basis. How well this works depends to a large part on the mechanism used to schedule communication across piconets. In this paper, we present a novel online scatternet scheduling algorithm, LCS, that effectively coordinates one-hop neighbors to converge to an efficient scatternet-wide communication schedule. Unlike previous work, LCS is robust and responsive to network conditions, dynamically adjusting the schedule based on varying workload conditions. We demonstrate that LCS has good performance on throughput, end-to-end packet latency and energy usage under various traffic loads.

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.