Service differentiation mechanisms for IEEE 802.11-based wireless networks

In this paper, we study the two medium access control (MAC) sublayer policies, namely, schedule after backoff (SAB) and schedule before backoff (SBB), which offer differentiated services in IEEE 802.11-based wireless networks. Both policies control channel access based on frame priorities. In the SAB policy, each node maintains concurrent backoff instances (one per priority class) with different parameter values to realize service differentiation. High-priority data frames use a shorter contention window, so that, in most cases, they are transmitted before the low-priority ones. The enhanced distributed coordination function (EDCF) scheme specified in the upcoming IEEE 802.11e standard is based on the SAB policy. Recent studies [A. Lindgren et al., June 2003] have shown that under high loads of high-priority traffic, EDCT starves low-priority frames. We propose SBB policy, which provides service differentiation without the need for maintaining concurrent backoff instances. In SBB, frame transmission proceeds in two sequential steps: (i) selecting the next frame to be transmitted, and (ii) channel access functionality until the selected frame is either successfully transmitted or its number of retransmissions exceeds the retry limit. These two steps are independent and can be performed using several alternatives. Depending on the way the two steps are carried out, we present four SBB-based MAC schemes. We evaluate SAB and SBB-based MAC schemes through simulation experiments. Our simulation results show that the SBB policy does not starve low-priority traffic and offers differentiated service on par with SAB. This suggests that maintenance of concurrent backoff instances contributes little towards performance improvement.