Fairness Research Of MAC Mechanism In Wireless Ad Hoc Networks

The Ad-hoc network is a collection of mobile nodes which equipped with radio transceivers without the aid of any existing network infrastructure. As a multi-hop temporary autonomous system, this network is highly flexible and destruction resistant which makes it suitable for the applications of military tactical communications, emergency rescue and office meeting. In these networks, the Medium Access Control (MAC) protocols are responsible for coordinating the access from active nodes to the shared medium, for collisions among packets would have been resulted while the terminals can communicate with several other nodes simultaneously.The present paper first briefs the development of ad-hoc network and discusses the current tendency of MAC protocols research at home and abroad, and then focuses on the Binary Exponential Backoff (BEB), Multiple Increase Linear Decrease (MILD) and Exponential Increase Exponential Decrease (EIED) algorithms to analysis the throughput and fairness based on network simulator-2 (NS-2).Then from the standpoint of the hidden and exposed terminal which affect the performance of MAC protocol. We modify the BEB algorithms. The new algorithms can reduce the serious unfairness problem caused by the exposed terminal, Based on which, a distributed fairness algorithm - Multi-windows Binary Exponential Backoff (MBEB) is proposed. This new algorithm set two contention window thresholds used to determine the channel holding time the mobile node occupied. If the contention window is larger than the specific upper threshold, it is planned to lower it to increase the probability accessing the channel next time; on the other side, when the contention window gets smaller than the other lower threshold, it is recommended to raise it to decrease the probability.In this paper, an extensive performance comparison discussion between the MBEB and BEB algorithm has been carried out base on the NS-2 simulator. The simulation results indicate that this new method is easier to realize and promote the fairness of the ad-hoc network, also, the TCP instability problem raised by the unfairness along with the BEB algorithm has been improved, which reveals the feasibility and advantage of this method.