To date, most analysis of WLANs has been focused on their operation under saturation condition. This work is an attempt to understand the fundamental performance of WLANs under unsaturated condition. In particular, we are interested in the delay performance when collisions of packets are resolved by an exponential backoff mechanism. Using a multiple vacation queueing model, we derive an explicit expression for packet delay distribution. It is found that under some circumstances, mean delay and delay jitter may approach infinity even when the traffic load is way below the saturation throughput. Saturation throughput is therefore not a sound measure of WLAN capacity when the underlying applications are delay sensitive. To bridge the gap, we define safe bounded mean delay (SBMD) throughput and safe bounded delay jitter (SBDJ) throughput that reflect the actual network capacity users can enjoy when they require bounded mean delay and delay jitter, respectively. The analytical model in this paper is general enough to cover both single packet reception (SPR) and multi packet reception (MPR) WLANs, as well as carrier sensing and non carrier sensing networks. We show that the SBMD and SBDJ through puts scale super linearly with the MPR capability of a network. Together with our earlier work that proves super linear through put scaling under saturation condition, our results here complete the demonstration of MPR as a powerful capacity enhancement technique for both delay sensitive and delay tolerant applications.
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