Effect of Fragment Size and Contention Window on the Performance of IEEE 802.11 WLANs

Abstract

Wireless communications is, by any measure, the fastest growing segment of the communications industry. The IEEE has standardized the 802.11 protocol for wireless local area networks. The IEEE 802.11 standard has defined two different access mechanisms in order to allow multiple users to access a common channel, the distributed coordination function (DCF) and a centrally controlled access mechanism called the point coordination function (PCF). DCF is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme with binary exponential backoff algorithm (BEB). DCF describes two techniques to employ for packet transmission: the two-way handshaking technique called basic access mechanism, and an optional four way handshaking technique, known as request-to send/clear-to-send (RTS/CTS) mechanism. In noisy channel, data packets become erroneous, and retransmission reduces the throughput. IEEE 802.11 allows for fragmentation tuning and rate selection to achieve highest throughput in bad channel conditions. If an error rate is known, the parameters like fragment size and contention window can be adjusted to obtain the maximum throughput. In this paper, an analytic model is developed to evaluate the throughput of IEEE 802.11 wireless networks over noisy channels using constant backoff window. The optimal contention window and fragment size are calculated using this model.