We focus our paper on wireless local area networks such
as IEEE 802.11 that have become popular as access networks
to the wireless mobile Internet. They can be deployed in hot
spots areas and offer performance comparable to wired local
area networks. The question of the performance effectively
perceived by mobile hosts becomes increasingly important
as many new emerging applications such as mobile information
access, real-time multimedia communications, networked
games, immersion worlds, cooperative work require sufficient
bandwidth.
Although IEEE 802.11b provides a means for allocating a
part of the radio channel bandwidth to some hosts (PCF -
Point Coordination Function), the commonly available access
method (DCF - Distributed Coordination Function) uses the
CSMA/CA protocol to share the radio channel in a fair way.
However, we have observed that in some common situations
in a wireless environment, the method results in a considerable
performance degradation. In a typical wireless local area
network, some hosts may be far away from their access point
so that the quality of their radio transmissions is low. In this
case current 802.11b products degrade the bit rate from the
nominal 11 Mb/s rate to 5.5, 2, or 1 Mb/s – when a host
detects repeated unsuccessful frame transmissions, it decreases
its bit rate. If there is at least one host with a lower rate, a
802.11 cell presents a performance anomaly: the throughput
of all hosts transmitting at the higher rate is degraded belowthe level of the lower rate. Such a behavior penalizes fast hosts
and privileges the slow one. The reason for this anomaly is
the basic CSMA/CA channel access method which guarantees
that the long term channel access probability is equal for all
hosts. When one host captures the channel for a long time
because its bit rate is low, it penalizes other hosts that use the
higher rate.