Mac Layer Access Technology And Qos Research In Broadband Wireless Networks

With the rising popularity of Wireless Local Area Networks (WLAN),the requirement of transmitting real-time multimedia traffic(such as video, audio) in the wireless networks becomes more and more important. Supporting Quality of Service (QoS) in WLAN has been an important challenging task. In comparison with the wire networks, the performance and QoS of wireless networks has great disparity. Real-time applications impose stricter requirements on service parameters, such data rate, delay and delay jitter. To provide reliable and guaranteed services for these applications, the medium access control (MAC) protocol is one of the important factors in performance and QoS problem of wireless networks. So it is significant to the performance of IEEE 802.11 MAC protocol, and the QoS problem that relate to MAC protocol in wireless networks.In this thesis the author introduces briefly the background, concepts, advantages, application future and many standards of WLAN. The author also introduces in detail the technique of transmission, basic network topology and protocol system. Then a more comprehensive discussion of the current DCF mechanism, EDCF mechanism for QoS support.Our research work bases on IEEE 802.11 standard. Through presenting the details of the DCF mechanism, we can get the conclusion that the traditional DCF mechanism can only provide the "best effort" service, those time-sensitive traffic such as video and audio require better quality of service, but it is because wireless LAN have the high mobility, high bit error rate and channel access scheme based on contention mode, to realize QoS mechanism becomes difficult. Then this paper research on the backoff algorithm, proposed an adaptive Slow CW Decrease backoff algorithm. With the backoff stage of n values to judgment the different degree of network congestion. Make each site to differentiate situations to adopt different ways of backoff, to reduce the collision probability. So we reduce the average delay and get higher throughput.At the same time, to supporting QoS for real-time service, we propose a priority-based access mechanism. In order to support the real-time traffic flows. The priority is differentiated mainly by the competition window size and the Retry Limit. And in combination with a fairness algorithm, order to be able to provide better QoS support for real-time service, at the same time makes the low-priority service can also be well services.The scheme relates to the throughput, end to end delay and packet drop rate of the traffic flows. On the QoS parameters consider a more comprehensive. Parameter adjustment strategy is simple and effective, easy to implement. Simulation results show that, the scheme not only able to provide a higher throughput and a smaller average delay for real-time service, but also to ensure the non-real-time service can get better services. Shows its great significance of enhance the QoS performance for wireless LANs. Finally we prospect the future of the QoS in WLAN.