| With the development of information technology and the popularity of wireless smart terminal, people’s need for accessing the Internet has been increasing rapidly. Wireless Local Area Networks (WLAN) have high flexibility, low cost of deployment and high data rate. For the reasons above, WLAN has been one of the most important access methods in recent years. IEEE802.11standard has been the most common WLAN standard after the development in nearly20years.As one of the two protocol layers described in IEEE802.11standards, medium access control (MAC) layer is the key point among researches on WLAN systems. The main problem which MAC layer solved is to assign the channel to several wireless stations in the network. Then the channel can be shared among multi-users and the overall performance of the WLAN system along with the channel utility will be improved. At present the share of the channel in IEEE802.11standards is realized mainly by distributed coordination function (DCF). The core algorithm used in DCF is the binary exponential backoff (BEB) algorithm. The IEEE802.11WLAN network performance deteriorates severely for the reason that BEB algorithm has some drawbacks. As a result, the service transmission cannot be guaranteed.In this paper, we start our research with the BEB algorithm. The first section of the paper introduces the IEEE802.11standards and the research backgrounds. Then the research situation of the channel access mechanism in WLAN is described in detail in the second chapter. In the next chapters, we summarize the reasons why the network performance deteriorates in saturation state by analyzing the BEB algorithm and propose the solutions to the problems. Based on the solutions we design a novel channel access algorithm. After that, the theoretical and simulation analysis are deployed. The primary works of this paper are as below:Firstly, the reasons why the network performance of BEB algorithm deteriorates in saturation state are summarized in the paper. One is that the backoff intervals of different stations are overlapped. Hence the collision among the stations in different backoff stages still exists. Moreover, it weakens the backoff effect of the exponential growth of the contention window (CW). The other one is the restoration of contention window to the smallest value upon a successful transmission. There exists the risk of experiencing multiple collisions and retransmissions until CW reaches high values again when there are a large number of stations in the network, wasting time and bandwidth. To overcome the disadvantages of the BEB algorithm, two mechanisms, backoff interval isolation (BII) and improved slow decrease (ISD), are proposed in this paper. BII decreases the collision probabilities among stations in different backoff stages by separating the backoff interval. ISD avoids the reset-restore process in BEB algorithm by sensing the channel and deploying slow decrement of CW. We design a novel channel contention access algorithm based BII and ISD and the algorithm are described in detail in the third chapter.Secondly, theoretical analysis and simulation analysis are deployed in this paper. We improve the classical two dimension Markov chains to make it describe the proposed algorithm accurately. Then we accomplish the theoretical analysis based on the new model and achieve the steady state distribution of the Markov chains along with the expression of the normalized system throughput. The software simulations are deployed based on the results of the theoretical analysis. We analyze the performance of the proposed algorithm by comparing with other algorithms.Finally, the conclusion of the paper is given. Besides, future researches are also put forward based on current situation of the WLAN technology development. |
