Delay Guarantee and Service Interval Optimization for HCCA in IEEE 802.11e WLANs

IEEE 802.11e medium access control (MAC) is a supplement to the IEEE 802.11 wireless network (WLAN) standard to support quality of service (QoS). The 802.11e MAC defines a new coordination function, namely hybrid coordination function (HCF), which takes the QoS requirements of flows into account and allocates transmission opportunity (TXOP) to stations. On the basis of mean sending rate, delay of variable bit rate (VBR) traffic cannot be bounded with the reference HCF scheduling algorithm proposed in this supplement. In this paper, the authors propose a new scheduling algorithm that utilizes the token bucket and a modified latency-rate (LR) scheduling algorithm to guarantee a bounded delay for HCF controlled channel access (HCCA). The new service interval (SI) is calculated to optimize the number of stations accommodated and takes into account delay bound and token bucket parameters. The authors show that is possible to obtain worst-case performance guarantees on delay. First, the authors analyze the behavior of the proposed scheduler with a loss free wireless channel model and after this, with a burst loss model and the authors explain how it is possible to extend this scheduler for a multi-rate scheme. Properties of the proposal are investigated both theoretically and using ns-2 simulations. The authors present a set of simulations with both constant bit rate (CBR) and VBR flows and performance comparisons with HCF scheduling algorithm. The results show that the delay upper bound can be achieved for a large range of networks load with bandwidth optimization.