Modifying Backoff Freezing Mechanism to Optimize Dense IEEE 802.11 Networks

Abstract

From experience, it has been well known that the performance of IEEE 802.11 wireless local area networks significantly drops in dense settings, largely driven by poor capacity of the medium access control (MAC) layer. The current 802.11 MAC mechanism utilizes a contention window (CW) for random access, and the CW size is dictated to take discrete values from a bounded finite set. In this paper, with focus on a saturated network scenario, we closely investigate the impacts of the aforementioned constraint and show that the limitations on the CW size are the main MAC bottleneck in dense conditions. Then, in order to tackle this bottleneck, we propose a new backoff scheme that does not attempt to tune the CW but makes a small modification in the backoff freezing process to determine the optimal configuration. We prove that our backoff scheme can achieve the maximum network throughput even under densely populated networks (i.e., many contenders), while a maximal compliance with the mentioned constraints is maintained. Then, we turn our attention to minimizing the channel access delay, and propose a variant of our backoff algorithm to accomplish this aim. We further present a new result on the theoretical relation between the optimal configurations for which the throughput is maximized and the delay is minimized. Finally, we note that our modified backoff mechanism does not penalize legacy users and is, therefore, practical for implementation in existing networks, which removes co-existence concerns.