Performance analysis under finite load and improvements for multirate 802.11

Automatic rate adaptation in Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) wireless networks may cause drastic throughput degradation for high-speed bit rate stations (STAs). The CSMA/CA medium access method guarantees equal long-term channel access probability to all hosts when they are saturated. In previous work it has been shown that the saturation throughput of any STA is limited by the saturation throughput of the STA with the lowest bit rate in the same infrastructure. In order to overcome this problem, we first introduce in this article a new model for finite load sources with multirate capabilities. We use our model to investigate the throughput degradation outside and inside the saturation regime. We define a new fairness index based on the channel occupation time to have more suitable definition of fairness in multirate environments. Further, we propose two simple but powerful mechanisms to partly bypass the observed decline in performance and meet the proposed fairness. Finally we use our model for finite load sources to evaluate our proposed mechanisms in terms of total throughput and Medium Access Control layer delay for various network configurations.