Backoff Algorithm In Wireless Lan Research And Improvement

Wireless local area network as the extendibility of wired network has a lot of advantages such as supporting application flexibility, rapid installation, shortened construction period, breakthrough geographic limitations and so on. It has been widely used in every way of life. There is an essential technical issue which is haw to distribute the usufruct of channel when competition happens. In wireless local area network the function of distributing the channel usufruct is accomplished by Medium Access Control layer. Studying MAC mechanism of wireless local area network, establishing proper MAC layer rule, optimizing the channel allocation mechanism and improving the MAC layer quality of service are the important tasks in the field of wireless local area network.This paper discusses mainly DCF (Distributed Coordination Function). Through a analysis, we can see that, the main disadvantage of current MAC protocol is that, according to the number of collision, DCF can tune the value of CW. Due to the lack of a mechanism evaluating the channel condition, it can not tune the value of CW adaptively, according to the actual network load.According to current research, this paper has put forward a new improvement scheme, the principle of which is: A station has the ability to estimate the congested situation of the channel through accumulating the counter of frozen times before sending the data. The "busy" indicates that at least one station has transmitted a packet, no matter that the transmission is successful. If "busy" is often detected, it shows that the channel is more congested, otherwise, we can think the channel idler on the contrary.Current research to update CW is that, by the estimate of load MF, after the message has been sended successfully, MF×CW is to make replacement of CW for next message. The performance of the adaptive algorithm is better.Therefore, based on the current research, the principle of this paper is that: after the message has been sended,if successfully, MF×CW is to make replacement of CW for next message; if unsuccessfully due to collision, (MF + 1)×CW is to make replacement of CW for the message;if unsuccessfully due to error, 1×CW is to make replacement of CW for the message.So each station can tune the value of CW according to the congested degree of channel. In this scheme, no matter how the number of stations (n) changes, the back-off window can always makes corresponding adjustment exactly. Consequently the saturation throughput can be improved a lot.Afterwards, we employed OPNET to simulate the new back-off algorithm, the emulation experimental result shows: Comparing with the original standard, the performance, such as throughput and media access delay, has obviously improvement, so the improvement algorithm that this paper puts forward about DCF is effectual. In the next work we should pay more attention to the analysis of multiple scenarios, the selection of parameters and their effect on the performance.