Performance Analysis And Improvement Of DCF Protocol In IEEE802.11Wireless Networks

With the improvement of information technology, IEEE802.11wireless local area network (WLAN) has been widely used in our work and daily life. WLAN involves only physical layer (PHY) and medium access control layer (MAC) of the air interface. The performance of MAC layer protocol becomes an important factor to affect WLAN performance. Thus, the performance analysis and improvement of IEEE802.11MAC protocol are of great importance and practical value. This thesis mainly includes the following researches:(1) The performance of DCF protocol for the multimedia services transmission is analysed in MAC layer. There are obvious differences between video and data frame transmission in WLAN, for the video streams need to provide people with consequent and distinct scene. Thus we should concern with the transmission delay and throughput for the video frame. In transmission, the long video frame would be segmented, and then fill into the SDU. Different frame rate and size lead to different MAC layer transmission performance such as delay and throughput. In this paper, a precise scheme is proposed to analyse the video transmission performance in IEEE802.11WLAN. With different frame rates, the relationship between frame size and video numbers has been conducted, and the video throughput has been preciously calculated. (2) The performance of DCF protocol with multi-rate transmission is analysed in MAC layer. The function of multi-rate support in PHY is prescribed in IEEE802.11protocol. To analyze the serious affects to the performance of whole networks caused by the low-rate access nodes, a precise algorithm is proposed based on the improved Markov model in DCF protocol. The throughput of WLAN is conducted with low-rate access nodes, and the simulation results with NS2prove the conclusion. After that the optimization methods are proposed for the low-rate access nodes, and all these are the effective references for WLAN planning and optimization.(3) The fairness of DCF protocol is analysed and improved in MAC layer. The data traffic needed to be transferred by AP is much more than the other STAs in the IEEE802.11DCF networks. However, AP should access the wireless link with the same priority as STAs, and thus the throughput between uplink and downlink is of great injustice. To solve the problem of unfainess, two schemes have been proposed. One sheme is acheieved by dynamically adjusting the minimum contention window of AP based on the network load. The AP minimum contention window value has been derived to ensure fairness under the conditions of different uplink and downlink data streams. AP monitors the growing real-time data traffic, and when the network system is in heavy traffic, AP could adaptively adjust the contention window to achieve fairness. And the other sheme is realized by adjusting the transmission probability of AP based on the method of transmission with probability. The optimal transmission probability of AP has been derived to ensure fairness under the conditions of different uplink and downlink data streams. The fairness could be acheieved by adjusting the transmission probability of AP. Detailed theoretical analysis and simulation results show that both schemes can effectively achieve fairness between uplink and downlink flows, with the total throughput increased.