Research Of Backoff Algorithm In MAC Protocol For Vehicular Ad Hoc Network

With the rapid development of wireless communication technology and the auto industry, Vehicular Ad Hoc Network has already become a hot topic of academic research, VANET is a particular kind of self-organizing network, which is the foundation of future intelligent transportation system. It communicates through Inter-vehicle (IVC) and Vehicle to Roadside (RVC). So that it can provide series of service for drivers. Such as Internet access, Real-time navigation and in-car entertainment, etc. In addition to that, it can provide the accident alarm information, so that it can make the traffic network more efficient and safer. This paper mainly researches the key technology of the VANET MAC layer protocol; the main contributions are as follows:First, MILD and EIED are researched on the basis of BEB, then the three kinds of algorithm are simulated with NS2and the fairness is compared. In order to improve network performance the latter two algorithms are optimized. And on the basis of newMILD a backoff algorithm which based on the statistical number are proposed. That is when the node sends success, the window of contention should be reduced, But in order to make the competition failed node could access channel, the algorithm sets a threshold value. When the number of nodes succeeding to send continuously is greater than the value, the contention window becomes the largest. Similarly, when the numbers of nodes failing to send continuously is greater than this value, the contention window will reduce to the minimum. Through the analysis of simulation, it is proved that the algorithm is effective to reduce the influence of hidden nodes and it has improved the fairness of node to access channel.Second, on the basis of the mobility characteristics of nodes, a model used to evaluate the VANET are proposed, and through simulating it has proved that the VANET node collision is based on the nodes speed and the number of the nodes. Then a backoff algorithm which is based on the relative distance is proposed. The algorithm is based on the relative distance and node access channel proportionally. It adjust the value of the contention window according to the average relative change of the distance that the node with it’s neighbor node. And city vehicle motion scene graph has been simulated using VanetMobiSim software, then the paper used the script that have generated and script made by cbrgen to generate scene in NS2. The simulation results show that the algorithm is effective to improve the preferment of network.