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Abstract—We analyze the performance of the IEEE 802.11bwireless local area networks. We have observed that when somemobile hosts use a lower bit rate than the others, the performanceof all hosts is considerably degraded. Such a situaTIon is acommon case in wireless local area networks in which a host faraway from an Access Point is subject to important signal fadingand interference. To cope with this problem, the host changes itsmodulaTIon type, which degrades its bit rate to some lower value.Typically, 802.11b products degrade the bit rate from 11 Mb/s to5.5, 2, or 1 Mb/s when repeated unsuccessful frame transmissionsare detected. In such a case, a host transmitTIng for example at 1Mb/s reduces the throughput of all other hosts transmitTIng at 11Mb/s to a low value below 1 Mb/s. The basic CSMA/CA channelaccess method is at the root of this anomaly: it guarantees anequal long term channel access probability to all hosts. Whenone host captures the channel for a long time because its bitrate is low, it penalizes other hosts that use the higher rate. Weanalyze the anomaly theoretically by deriving simple expressionsfor the useful throughput, validate them by means of simulation,and compare with several performance measurements.
