The Research Of Back Off Algorithm In MAC Layer For Ad Hoc Network

The wireless ad hoc network is originated from the military communication which is a decentralized wireless network and does not rely on a preexisting infrastructure. Each node in the ad hoc network switches information using a transmit-receive device and participates in routing by forwarding data to other nodes. Minimal configuration and quick deployment make ad hoc networks suitable for emergency situations such as natural disasters or military conflicts.First, the different back off algorithms of wireless ad hoc networks and their advantage and disadvantage are introduced. Then the problem caused by 802.11DCF was analyzed deeply as an emphasis. The major contributions of the thesis are as follows:(1)Based on the capture effect and network congestion problem, a novel algorithm is put forward here. The contention window is adjusted according to logarithm back off as a part of network congestion avoidance. And if the node transmits a packet successfully, the contention window for the node would be decreased by back off factor r. As shown in the simulation results, the new algorithm improves the stability, fairness and reduces the packet loss ratio.(2)The traditional MAC protocol can't predict the congestion situation, it will cause frequent collisions. So we put forward a novel dynamic tuning of contention window for distributed coordination function. Each node computes the collision probability by checking slot number and adjusts contention window using capability and collision probability. The new algorithm reduces the collision and improves the network performance. We compare the dynamic algorithms with BEB using NS2 simulation software. The result shows that the algorithm descends the packet collision probability effectively and has obvious effect in throughput.