Research About The QoS Of The VoIP Applications In WLAN

With the rapid development of wireless communications technology and mobile terminal technology, the wireless local area network (WLAN) based on IEEE 802.11b is now widely used in our daily life, while the VoIP application becomes one of the fastest growing network applications due to its low cost and higher resource utilization rate. Driven by these two technologies, Voice over WLAN is becoming popular.IEEE 802.11 is the most widely used standard in current wireless local area network, which defines the technical specifications for the bottom two layers of the seven layer ISO model. The IEEE 802.11 wireless LANs standard specifies two medium access mechanisms - distributed coordination function (DCF) and point coordination function (PCF) in its medium access control sublayer architecture. DCF is the basic access control method in IEEE 802.11 and it is mandatory while PCF is an optional. DCF was developed for asynchronous data transmission, where all stations share the medium using the carrier sense multiple access with collision avoidance protocol and a random backoff mechanism. It provides a best effort service without any QoS guarantee. DCF is often employed for the non-real time data service, it is difficult to satisfy the QoS requirements of the voice service which is sensitive to the delay and delay jitter. PCF was developed for supporting time-sensitive services, where the point coordinator (PC) has the centralized control of the medium access and determine which station has the right to transmit. It is possible to give voice service high priority in order to improve the QoS of the voice service. Thus, it is important to study and research on how to improve the QoS of the VoIP application in PCF mechanism in wireless environment. Our research is carried out on this background.This paper first introduces the related knowledge about VoIP and WLAN, then focus on the PCF access model. Based on the study of the existing QoS enhancement methods, we propose a novel scheme - HAPS (Hybrid Adaptive Polling Scheme). The proposed scheme 1) differentiates the uplink and downlink by dividing the DCF period into Down period and Up period 2) differentiates the user's state by using talkspurt state and silence state in order to reduce the cost of poll frame. At the same time, it employs talkspurt detection algorithm to detect the change of the use's state and four polling list to manage the polling of the wireless nodes. Finally we use NS2 to evaluate the performance of the proposed scheme. The simulation results show that the proposed scheme can improve the QoS of VoIP application in wireless network by reducing the delay and packet loss rate effectively.