Admission control for variable bit rate traffic in IEEE 802.11e WLANs

The IEEE 802.11 work group is working on the standard IEEE 802.11e and introduces the hybrid coordination function (HCF) to provide better QoS support to real-time traffic. Recently, a reference design of a simple scheduler and admission control unit is proposed in a TGe consensus proposal. This scheduler takes the QoS requirements of flows into account and allocates transmission opportunities (TXOP) duration to stations on the basis of the mean sending rate and mean packet size. However, the reference design is only efficient to the flows with constant bit rate (CBR) characteristic and does not take the rate mid packet size fluctuation of variable bit rate (VBR) traffic into account. The packet loss probability of VBR traffic may be very large. We enhance the simple admission control unit in the reference design to provide guarantee on the packet loss performance. The expression for packet loss probability of a single VBR flow in term of the TXOP duration is determined and the effective TXOP duration given the packet loss requirement can be found for admission control. Numerical results are presented to show that there is about 20 times improvement on the packet loss rate in our proposed admission control algorithm.