A new MAC mechanism to resolve 802.11b performance anomaly

Since the IEEE 802.11 standard supports multiple data rates at the physical layer, many rate adaptation mechanisms have been proposed to utilize this multi-rate capability by dynamically switching rates in response to different channel conditions. In IEEE 802.11b networks, the guarantee of an equal transmission opportunity for all participating nodes results in performance anomaly. Initial contention window adaptation, as one of the solutions to solve performance anomaly, has some limitations, which come from inefficient back-off mechanism. In this paper, we propose a new multi-rate MAC mechanism, which allows nodes in two hops to get each other´s data rate information by introducing modifications to the control packet. In order to improve the channel utilization, nodes initialize their CWmin based on the ratio of their own data rate to their competitive nodes´ data rate. We evaluate the performance of our mechanism on the network simulator NS-2. Comparing with RBAR, the proposed mechanism performs better in throughput especially when the network load is heavy since the performance anomaly problem can be solved effectively. Significant improvements are also shown in both aggregate throughput and average delay when compared with initial contention window adaptation because the proposed mechanism can avoid unnecessary back off.