The performance seen by individual clients on a wireless local area network (WLAN) is heavily influenced by the manner in which wireless channel capacity is allocated. The popular MAC protocol DCF (Distributed Coordination Function) used in 802.11 networks provides equal long-term transmission opportunities to competing nodes when all nodes experience similar channel conditions. When similar-sized packets are also used, DCF leads to equal achieved throughputs ( throughput-based fairness) among contending nodes.
Because of varying indoor channel conditions, the 802.11 standard supports multiple data transmission rates to exploit the trade-off between data rate and bit error rate. This leads to considerable rate diversity, particularly when the network is congested. Under such conditions, throughput-based fairness can lead to drastically reduced aggregate throughput.
We argue the advantages of time-based fairness, in which each competing node receives an equal share of the wireless channel occupancy time. We demonstrate that this notion of fairness can lead to significant improvements in aggregate performance while still guaranteeing that no node receives worse channel access than it would in a single-rate WLAN. We also describe our algorithm, TBR (Time-based Regulator), which runs on the AP and works with any MAC protocol to provide time-based fairness by regulating packets. Through experiments, we show that our practical and backward compatible implementation of TBR in conjunction with an existing implementation of DCF achieves time-based fairness.
Furthermore, in a series of experiments, we demonstrate that the DCF encourages non-cooperative nodes to use globally inefficient transmission strategies that lead to degraded aggregate throughputs. This is because 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.
We 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.
We have implemented TBR using Linux PCs equipped with standard PRISM II-based 802.11b wireless interfaces. The system allows long-term capacity allocation of competing nodes according to a desired allocation vector. Our experimental results agree with our analytical results that time-based fairness can significantly improve the performance of multi-rate WLANs.
The 802.11 MAC Protocol Leads to Inefficient Equilibria
Godfrey Tan and
John Guttag
Proc. of
IEEE INFOCOM,
Miami, Florida, March 2005 (To Appear)
Long-term Time-share Guarantees are Necessary for Wireless LANs
Godfrey Tan and
John Guttag
Proc. of
SIGOPS European Workshop,
Leuven, Belgium, September 2004
Time-based Fairness Improves Performance in Multi-rate Wireless LANs
Godfrey Tan and
John Guttag
Proc. of USENIX'04,
Boston, MA, June 2004
Capacity Allocation in Wireless LANs
Godfrey Tan and
John Guttag
MIT
LCS Technical Report, MIT-LCS-TR-973,
Cambridge, MA, November 2004
Coming soon.
Please send all questions and comments to Godfrey Tan (godfreyt at mit dot edu).
M. I. T. Computer Science and Artificial Intelligence Laboratory · The Stata Center · 32 Vassar Street · Cambridge, MA 02139 · USA