The next generation technology of Voice over Internet Protocol (VoIP) allows voice to be delivered over the Internet Protocol based networks. The integration of VoIP and Wireless Local Area Network (WLAN) has become popular. However, WLAN architecture was primarily designed to carry data and not for real-time applications, such as VoIP. Hence, there is a need to enhance the quality of VoIP transmission over the WLAN. On the other hand, IEEE 802.11 standard of WLAN provides support to link adaptive (LA) technique. LA leads to multi-rate transmissions, which causes degradation on voice quality. In this paper, cross-layer based adaptive approach is proposed to improve the quality of voice delivery over WLAN by minimizing the multi-rate effects on the quality of service (QoS) of VoIP. The approach in achieving this is by introducing a communication agent between MAC sub-layer and Application layer, by which congestion caused by the multi-rate could be reduced, if not eliminated. Moreover, Encoder adaptation is utilized at the Application layer as codec parameters are adapted according to the change of rate occurs at the MAC sub-layer. Simulation of different scenarios for different codec rates at different transmission rates has been implemented using OPNET. The result of QoS improvements is the evidence for the proposed cross-layer adaptation approach.

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In recent years, VoIP over Wireless Local Area Networks (VoWLAN) technology has been increasingly on demand due to its advantages such as it is cheaper and supports mobility. Yet, the required stringent service quality for VoIP applications particularly in WLAN is still a challenge. This paper investigates the effect of two key parameters on VoIP quality; transmission/data rate and packet size. In this study, the data rate value can be 11, 5.5, 2, or 1 Mbps according to IEEE802.11b WLAN, while packet size is determined by the number of frames inserted in the packet. A simulation study has been implemented using OPNET Modeler. The first part of the results compares VoIP quality values obtained for each data rate. It also shows how decreasing the data rate degrades the VoIP performance. On the other hand, the second part of the results demonstrates how changing voice packet size can significantly affect VoIP quality. Further, it is discussed and analyzed the effect of packet size on delay, packet loss and network bandwidth requirement which accordingly affects VoIP quality.