| In June of 1997, IEEE introduced the wireless LAN standard of the first generation ------IEEE 802.11. At present the work based on 802.11 MAC layer concentrates mainly on its throughput and delay, and two standards of the fundamental mechanism to access the medium, Point coordination function (PCF) and Distributed coordination function (DCF). In the paper, the two traditional backoff algorithms of IEEE 802.11 DCF which used to avoid collision,Binary Exponential Backoff (BEB) and more extreme Multiplicative Increase and Linear decrease (MILD),are analyzed. We can find the more bigger in network load due to the resume mechanism of BEB, the more collisions will be emerge, make the systematic throughput drop seriously. But the more smaller in network load, because of the linear decrease mechanism of MILD, the more slower changes in contention window size will reduce the utilization ratio of the channel, cause the deterioration of delay performance. Combined the backoff algorithms of two kinds of improvement types which some researchers have put forward at present, Exponential Increase Exponential Decrease (EIED) and Linear/Multiplicative Increase and Linear Decrease (LMILD), an outperformed backoff algorithm ------Linear/Multiplicative Increase and Linear/Multiplicative Decrease (LMILMD) is proposed. According to the concrete conditions of the collision node, it carries on rational adjustment to the size of contention window. With the help of simulator tool ------NS2, simulation has verify the performance of this algorithm, has improved delay and throughput of the wireless LAN, reach better performance to require. With more and more applications on the wireless LAN, our requirement to the performance of the wireless LAN will be higher and higher. Backoff algorithm LMILMD that this paper puts forward has certain improvement more than the existing algorithms, no matter its throughput or its delay, so has some higher using value.
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