Design and Analysis of a Prioritized Adaptive Multiple Access Scheme for VoIP over WLANs

Voice capacity over wireless local area networks (WLANs) can be increased by the statistical multiplexing among on/ off voice calls. However, in previously proposed schemes, the admitted voice calls that transit from silence state to talkspurt state are mixed up with the new voice calls to contend for the channel. The increase in the traffic load of new voice users could degrade the performance of ongoing voice calls. In this paper, we propose a novel MAC scheme for VoIP over WLANs, referred to as PAMA (Prioritized Adaptive Multiple Access). The key features of the proposed scheme are that 1) the admitted voice calls have higher priority access to the channel than the new voice calls, based on the fact that maintaining the required QoS of ongoing calls is more important than admitting new calls; 2) the dedicated contention window for admitted voice calls is dynamically adjusted according to the current estimation of the number of active ongoing voice calls to guarantee the QoS requirements; 3) a two-state Markov model is established to evaluate the system performance. Analytical and simulation results demonstrate that PAMA can increase the voice capacity while still satisfying the QoS of admitted voice calls.