The Research On Qos Mechanism Of Ieee 802.11 Wlan

As the Wireless Local Area Networks (WLAN) technologies are improving rapidly and the service flow is growing everyday, how to ensure the quality of services becomes the focus of WLAN researches.After briefing on basic concepts of WLAN and Quality of Service (QoS), this paper points out the importance of modifications based on protocols of IEEE 802.11 standard to support QoS .Firstly, the paper introduces the operation principle and shortcoming to support QoS of DCF and PCF MAC access mechanism. Next, the paper introduces 802.11 MAC enhanced mechanism 802.11e, and mainly carries on research to EDCF. QoS supported in EDCF is satisfied, but there are some problems, such as when the network load is heavy, diversified traffics occur collision easily, the throughput reduce, particularly, the performance of the medium and lower priority traffics is worse. When the network load is light, the static establish of backoff cause delay access channel unnecessary. And the channel utility is decreased.In order to enhance the QoS performance of MAC further, this paper studied an improved adaptive EDCF (AR-EDCF) which depended on ACK rate factor (AR) and Network Load Threshold (TH) to change CW and backoff algorithm. And through the simulations using NS-2, we evaluate and compare the performance of AR-EDCF according to several pairs of TH and the achievement of some MAC protocols including DCF, EDCF, and AEDCF. We have confirmed that the value of TH has much effect on the QoS performance, the channel efficiency, and the fairness. Particularly the use of TH2 has made the estimation of network load state suitable, thus the overall performance has been strengthened. It is also seen that AR-EDCF adopting TH2 has improved a number of MAC properties, and enhanced channel efficiency, decreased collision rate, improved the fairness, increased the throughput of lower priority traffics.