CHAPTER ONE INTRODUCTION 1.1 Background

The family of IEEE 802.11 protocols has become the most popular access technologies in the world today. They provide an effective means of achieving wireless data connectivity in homes, public places and offices. In 802.11 protocols, the fundamental medium access method is called DCF (distributed coordination function), a form of carrier sense multiple access with collision avoidance (CSMA/CA). The DCF is based on listen before talk procedure

where the terminals first checks to see if the radio link is free before  transmitting and then initiates a random back off procedure to avoid collisions. The DCF can also use the RTS (request to send) and CTS (clear to send)  technique to further prevent collisions. 

Because of signal fading, transmission interference, and user mobility, wireless channels have time-varying characteristics [1].

This makes different mobile hosts to perceive different channel qualities at the same time. In order to obtain a better throughput, the hosts in a network need to use different transmission rates for different channel qualities. This multi-rate support is currently included in most protocols like IEEE 802.11b, 802.11a, 802.11g, and Hiper-LAN-11. Some rate adaptation techniques have been designed for 802.11 WLANs including ARF (Auto Rate fall-back) and RBAR (Receiver Based Auto Rate). 

Thus, it is necessary to evaluate and analyze the performance of IEEE 802.11 WLAN system under the fundamental access mechanism for medium access control (MAC) called DCF in a multirate WLAN.        Most of the analysis on DCF in the past were based on simulation and no particular IEEE 802.11 standard was used. Thus the need to carry out an empirical analysis of IEEE 802.11b in a multi-rate WLAN.     

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