Resolving Intra-Class Unfairness in 802.11 EDCA

The de facto quality-of-service provisioning mechanism for wireless LANs is the Enhanced Distributed Channel Access (EDCA), which provides enhanced service differentiation based on four different prioritized Access Categories (ACs). In this paper, we show that the multiple category mechanism of EDCA causes a new type of unfairness problem among the ACs of same priority class when the numbers of active ACs in competing nodes are different. We present a simple analytical model to capture the impact of virtual collision, external collision, and Arbitration Interframe Space on the degree of this intra-class unfairness. Based on its analytical insight, we present a simple remedy scheme for resolving the unfairness and evaluate its efficiency using ns-2 simulations.