Achieving optimal performance in IEEE 802.11 wireless LANs with the combination of link adaptation and adaptive backoff
IEEE 802.11 is one of the most popular wireless LAN standards [Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, IEEE Standard 802.11, August 1999]. In the paper, we propose a simple analytical model, which helps one obtain deep insight into the mechanism of achieving optimal performance by using IEEE 802.11 DCF (Distributed Coordination Function) protocol. The first contribution of this paper is the analysis of the optimal operation point where maximum goodput can be achieved. Through the analysis, we answer some fundamental questions about the existence and the uniqueness of the optimal operation point; about the maximum system goodput can be achieved; about the existence of a simple rule to check out if the system operates under the optimal state or not; and how do the data transmission rates, which is dependent on the selected physical transmission mode, and packet transmission errors, caused by channel fading and (or) interference, affect the final system performance. Another contribution is the proposal of a simple distributed adaptive scheme ''LABS'' (i.e., Link adaptation and Adaptive Backoff Scheme), which makes the system operate under the optimal operation point and, at the same time, achieves some pre-defined target service differentiation ratio between different traffic flows. In the LABS, two adaptive schemes are combined: one is the so called ''Link Adaptation'' scheme, which dynamically selects an optimal modulation mode at a given time so as to improve the achieved system goodput; the other one is the so called ''Adaptive Backoff'' scheme, which adaptively adjusts the minimum contention window size of each sending node to guarantee that the system operates under (or near) the optimal operation point. Results obtained in the paper are relevant to both theoretical research and implementation of real systems.