USENIX 2004 Annual Technical Conference, General Track Abstract
Pp. 269282 of the Proceedings
Time-based Fairness Improves Performance in Multi-Rate WLANs
Godfrey Tan and John Guttag, MIT
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. In access point (AP) based WLANs, the MAC
protocol and the queuing scheme at the AP dictate the allocation of
channel capacity among competing nodes.
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
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.
In this paper, 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
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