Achieving VoIP Guarantee in Wireless Local Area Networks

Recently, voice over Internet Protocol (VoIP) has gained much attention in wireless local area networks (WLANs). Although IEEE 802.11e enhanced distributed channel access (EDCA) has been standardized to provide voice traffic with quality of service (QoS) support, the voice quality could still be severely degraded by the traffic transmitted from low-priority stations (LP-STAs). Like 802.11b, the 802.11e EDCA also has the performance anomaly problem. Therefore, the voice quality of the high-rate stations (HR-STAs) could be severely degraded by the traffic transmitted from low-rate stations (LR-STAs). In fact, the LP-STAs and LR-STAs can be present simultaneously in practical WLAN environments. In this paper, we consider the effects of LP-STAs and/or LR-STAs on voice traffic and propose an extended EDCA scheme called voice differentiation-EDCA (VD-EDCA) to protect voice traffic from LP-STAs and/or LR-STAs. With VD-EDCA, the voice AC in a HR-STA has an absolute priority over LP-STAs and LR-STAs when they are notified that the wireless delay of a voice frame at the QoS access point (QAP) is larger than or equal to the predefined maximal delay threshold. We evaluate the performance of the proposed VD-EDCA scheme through extensive simulations. The simulation results show that, with appropriate maximal delay threshold settings, the proposed scheme well protects all of the VoIP calls transmitted by HR-STAs from LP-STAs and/or LR-STAs