Collision-Aware Backoff Mechanisms for IEEE 802, 11 WLANs

The distributed coordination function (DCF) is the most widely deployed medium access control (MAC) protocol for IEEE 802.11 WLANs. It is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme which employs a binary exponential backoff (BEB) algorithm to reduce the collision probability. In this paper we propose a novel backoff algorithm, collision-aware backoff mechanism (CABM), which dynamically selects a suitable contention window size according to the contention level of current network. A contention level is separately estimated by each station depending on unsuccessful frame transmission ratios. However, frame losses can result from not only collisions but also link errors. If the bit error rate (BER) of a channel is available, CABM can be enhanced by a more accurate estimation of collision probability. The enhanced variant, CABM+, is also proposed for improving the system performance in an error-prone channel. Simulation results show that our schemes outperform the standard protocol in terms of the collision probability and system throughput, especially when the number of contending stations is large.