Contention Window Adaptation using the Busy-Idle Signal in 802.11 WLANs

In a wireless local area network (LAN), packets can be lost for a variety of reasons, including collisions due to high traffic and channel errors due to poor channel conditions. In practice, however, nodes cannot easily differentiate between these types of loss. As a result, adaptations based on packet loss alone can result in significantly degraded performance. In 802.11 networks, wireless nodes avoid collisions via the Binary Exponential Backoff (BEB) protocol. This performs well for moderate numbers of nodes and low channel error rates, but is inefficient for large numbers of nodes, high channel error rates, or in the presence of hidden terminals. In this paper, we propose a contention window adaptation scheme in which nodes use information shared by the AP to optimize contention window sizes in a distributed fashion to improve network utility. We show via NS-2 simulations that our method can improve throughput by as much as 24% in the high node count scenario, 35% in the high channel error scenario, and 350% in the presence of hidden terminals.