Quality of service provisioning for voice application over WLANs

Wireless Local Area Networks (WLANs) are widely deployed nowadays because of their low cost and convenient implementation. The Institute of Electrical and Electronics Engineers (IEEE) 802.11 series standards define the specifications for such networks. While the first proposed standard could support a data rate up to 2 Mbps, in the recent upgraded versions of the standard data rates up to 54 Mbps are achievable and up to 200 Mbps are said to be supported in next generation WLANs. An important concern in WLANs is the support of Quality of Service (QoS), specifically for multimedia applications. Because of the limited available bandwidth in wireless networks, bandwidth cannot be easily increased to support QoS. However, efficient protocols capable of providing QoS have to be designed to improve resource utilization in networks. The Medium Access Control (MAC) protocol crucially affects the QoS parameters. Admission control is also an essential element for QoS provisioning in WLANs.;The first contribution of this research is to propose an analytical framework which takes into account the traffic direction in non-saturated infrastructure mode of WLANs. Unlike previous work, in the proposed analysis the collision probability of a packet transmitted by each wireless station in the uplink direction is different from the probability of collision for the packets transmitted from the access point in the downlink direction. Our model differentiates between perstation backoff models in the uplink and downlink and is capable of expressing the MAC performance in terms of several system parameters such as contention window size, maximum number of backoff stages, size of buffer at the MAC layer, traffic parameters such as talk and silent durations and arrival rate, as well as the number of wireless stations. In contrast to the previous studies, we apply two groups of equations, one group is defined for the wireless station and the other one for the access point. These equations represent the transmission probability, probability of collision and the probability of being in the busy state in terms of the number of wireless stations, the traffic arrival rate and system parameters such as the size of the contention window and maximum number of retransmissions.;The second contribution is the derivation and evaluation of QoS metrics. After solving a set of nonlinear equations obtained from the proposed MAC model and by adopting a simple and accurate M/M/1/K queueing model for a tagged station, we evaluate the QoS metrics (packet delay, packet loss and throughput) of VoIP which is one of the fastest growing applications over WLANs. The packet service time which is required to solve the M/M/1/K queueing model, is calculated in the proposed analysis. We observe that the multiplexing at the MAC layer of the access point results in higher collision probability and service time for packets in the uplink direction in comparison with the downlink direction. Our analysis also reveals a significant downlink throughput degradation due to the asymmetric contention mechanism and a buffer over flow at the MAC layer of the access point.;In the area of admission control and QoS provisioning, we propose a simple centralized model-based admission control technique for provisioning QoS requirements to voice applications. The proposed admission control technique is based on the packet delay outage probability metric which plays an important role in fulfilling the QoS requirements of delay sensitive applications such as voice and video. We compute the packet delay outage probability by applying the packet service time calculated in the proposed analytical framework in the M/M/1/K queueing model. We also investigate the impact of the traffic specifications and system parameters on the maximum number of voice connections in 802.11b and 802.11g networks. The major advantage of this research is the analytical framework which can theoretically estimate the voice capacity without the need of numerous simulations. This is significantly helpful when we deploy new traffic specifications or different system parameters according to the new versions of the 802.11 networks. To validate the theoretical model, we performed simulations and found that there is a good agreement between the analysis and ns-2 simulation results which verifies the accuracy of the proposed model.;Our research covers mathematical modeling and analysis of the MAC layer and admission control considering Distributed Coordination Function (DCF) in infrastructure mode of IEEE 802.11-based WLANs. While our general goal is to guarantee the QoS parameters of multimedia applications over WLANs, we address several important issues such as MAC layer modeling, QoS evaluation and admission control.