Wireless Lan Voip Service Qos Research

With the combination of VoIP and WLAN technology, wireless LAN, efficient and convenient VoIP calls through WLAN are possible. In VoIP research area, the main goal is to guarantee the QoS requirements for VoIP and improve the performance. The standard IEEE 802. l1e provides a solution for this. IEEE 802. 11e agreement aimed at WLAN to provide QoS for multimedia support. According to deference in media accessing, the function of hybrid coordination in IEEE 802. 11e includes enhanced distributed coordination mechanism to control access (EDCA) and hybrid coordination control access mechanism (HCCA).In this thesis, we focus on the MAC layer channel access technology, and aim to improve VoIP performance in WLAN under IEEE 802. l1e standard. Based on the in-depth analysis of the two access mechanisms for supporting of VoIP services, we have compared DCF and EDCA through performance simulation. More ever, we do some analysis on the HCCA polling scheduling mechanism, and study the simple scheduling algorithm in IEEE 802. l1e on the supporting details for VoIP business. Simulation results show that, as variable bit rate service (VBR) types of data in each interval cycle (SI) and packets generated by the number of data frames is not fixed, HCCA polling scheduling algorithm can not correctly calculate the actual generated packets size and number. For the shortage of the simple scheduling algorithm, a new dynamic scheduling algorithm for VBR is presented. The new algorithm is based on the dynamic changes in network traffic. In the new algorithm, the first step is to calculate the minimum time intervals for all business streams, and one of the smallest of the SI is selected as each site service interval. Then, the next polling time is determined. If a polling site in the former SI transmission is completed, this site will send an empty frame to claim the next poll, and the resource will be re-allocated for the next poll. If the delay in the site is happened, the algorithm will adaptively adjust the difference between the actual time and expected time. The proposed algorithm also does to the secondary distribution for the TXOP duration in the data transmission, and to improve the transmission efficiency of the site.The proposed algorithm can generate a reasonable distribution of transmission opportunity for data frame with different size. Therefore, the system performance would be improved with more throughput and less packet loss rate, and the VoIP service quality satisfied.