| As a convenient communication method, distributed wireless LAN (DWLAN) became more and more popular by the virtue of its inherent advantages on flexibly and rapidly networking. The main task of medium access control (MAC) protocol is to efficiently share limited wireless channel among distributed stations. IEEE 802.11 protocol is the widely-used wireless LAN Medium Access Contrl and Physical Layer Spercification. However, conventional MAC mechanism can hardly provide good performance in a complex environment of distributed networks. Consequently, based on performance analysis and optimization of system, this dissertation focuses on estimation of the number of competing terminals and fragmented transimission mechanism technologies for DWLAN.Chapter 1 introduces the features of wireless LAN (WLAN) and its related standards. Research works on MAC for WLAN are introduced, which includes objects, MAC mechanism in current standards, and recent research trends. Moreover, the main content is presented.Chapter 2 analyzes the estimation of the number of competing terminals in an IEEE 802.11 wireless LAN. The number of competing terminals has great influence on the performance of IEEE 802.11 MAC protocol, based on the distributed coordination function (DCF), which utilizes carrier sense multiple access with collision avoidance (CSMA/CA) and binary exponential backoff (BEB). To optimize the performance of DCF, it's significant to set the network's configuration, such as contention window, RTS thread etc, in the run-time by estimating the competing terminals accurately. Extended Kalman Filter (EKF) with change detection is an optional method, but it has some strict conditions to utilization. A new method is proposed in this paper to overcome the drawbacks and constrain of EKF. We apply the extended forgetting factor recursive least squares (EFRLS) filter to estimate the number of competing terminals in the wireless local area network. Moreover, two optional methods are given to determine the forgetting factor to satisfy different requirement on estimation accuracy and response speed of state changing. Simulation results show that, EFRLS can be used to estimate the number of competing terminals accurately, it also overcomes the disadvantages of EKF, and EFRLS has wider application than EKF in different kinds of networks situation.Chapter 3 focuses on the fragmented transimission mechanism in wireless noise channel. The necessity of fragmentation in wireless channel with transmission bit errors is introduced, as well as fragmentation/defragmentation specified in IEEE 802.11. The fragmentation method, which specified in IEEE 802.11, sets the fragmentation threshold as fixed value, but it doesn't take full advantage of fragmentation mechanism. Then some performance metrics have been produced from a mathimatics model which describes the states of a station in fragmentation transmission and their transitions, which include the probability of a packet to be dropped and the average retransmission times per packet. Then a method is proposed to dynamic set the fragmentation threshold by optimizing these two packet transimission target simultaneously. Since it also takes account of the bit-error-rate which varys with the networks environment, the new method has wider application background and capability of improving the transimission performance of the networks... |
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