A Study of Short-Term Fairness in Wireless MAC Protocols

In this paper, we study short-term fairness in distributed, contention-based wireless medium access control (MAC) protocols. While most MAC protocols provide fairness among contending stations in the long term, they are not fair over shorter time scales. We rst demonstrate short-term unfairness of CSMA/CA using packet-level traces collected from an in-bulding wireless LAN network. We also analyze these traces and show adverse interactions between TCP and CSMA/CA. We then model the system of contending stations as a Markov chain and deene the short-term fairness index for this model as the zeroth-order entropy rate of the underlying stochastic process. We argue that because this quantity is a good measure of uncertainty in channel accesses, it models short-term fairness well. We solve the analytical model for a CSMA/CA-based system with diierent numbers of stations and conclude that its short-term fairness is signiicantly lower than ideal.