Theoretical framework for quality of service analysis of differentiated traffic in 802.11 wireless local area networks

In this study, the authors provide an analytical framework to assess network-level quality of service (QoS) measures for differentiated, non-saturated traffic in infrastructure-mode 802.11 wireless local area networks using the distributed coordination function access mechanism. The authors build on a general analytical framework that takes into account the medium access control (MAC) access mechanism, the MAC layer packet buffer and the characteristics of the offered load to obtain the probability of a collision with M classes of traffic. The authors then derive analytical expressions for the throughput, the end-to-end packet delay and the packet delay outage probability for differentiated traffic. A case study of voice-over-IP (VoIP) traffic in 802.11b/g networks is used to validate the theoretical framework. The analytical results are in good agreement with the simulation results, showing that although the collision probability for packets transmitted at the access point (AP) is lower than for packets transmitted from the clients, the end-to-end delay in the downlink is much longer than that in the uplink because of the large queuing delay because of the multiplexing of several VoIP connections at the AP. The authors also compare the maximum number of VoIP connections that can be admitted in an 802.11 network while respecting their QoS constraints; these were computed with the proposed theoretical model, an ns-2 simulation model and other schemes previously proposed in the literature. The results indicate that their approach is more accurate over a wide range of parameter values, thus demonstrating the validity, the flexibility and the robustness of the proposed theoretical framework.