The QoS Research Of VoIP Over WLAN

The combination technology of Wireless local area network (WLAN) and Voice over Internet Protocol (VoIP) is a very popular research topic in recent years. As users'requirements of mobile communications are increasing, demands of mobile IP voice communication are urgent; therefore, voice over WLAN (VoWLAN) is expected with significant growth in the near future. However, as an emerging technology, there are some inevitable problems. As VoWLAN, how to improve the Quality of Service (QoS) for real-time voice calls, has become a top priority. VoWLAN's problem of QoS is prominently more than ordinary wired line VoIP. Facing on these circumstances, the IEEE working group developed the draft IEEE802.l1e[1] protocol. It improved IEEE802.11MAC[2] layer, added a new media access function: Hybrid Coordination Function (HCF) and added some QoS improvement mechanisms and frame types. The Enhance Coordination of the Distribution Function (EDCF) is the base of HCF. However, the introduction of IEEE802.11e draft protocol have not totally resolved VoWLAN problem of quality of service. There is much for improvement.There are three mechanisms can be used to improve the quality of services VoWLAN: the first mechanism is the voice traffic multiplexing: In HCF, try to avoid polling of the voice stations which are in the silent period. Maintain a polling list dynamically in all access points (APs). Maintain every station's address or ID, and keep the voice stations which are in the silent period are not in the list; The second mechanism is to ensure the certain priority of voice traffic over data traffic in competition period. The competition mechanism which IEEE802.11e protocol stipulated is only based on window size settings. In practical using, not all the voice packets have priority over data packets to be sent. Such a mechanism could be improved, so that all the voice packets can be sent in real time in competition period; The third mechanism is to reduce the overhead of voice packets. Using compression technology, can reduce the size of voice packets without lowering the quality of voice. In addition, can use the AP buffer mechanism, in buffering a number of voice packets payload can be linked together, plus a common MAC layer for transmission. This can effectively reduce the overhead and improve quality of service. By using MATLAB to simulate these mechanisms and comparing the situation which is not to be improved by these mechanisms, it can confirm that these mechanisms have improved the quality of service of VoWLAN and are proved to have practical values.