Research On The Medium Access Control Protocol For Multiple Load Vehicular Ad Hoc Network

Multiple load data traffic and high speed mobility bring a new challenge to the medium access control protocol of VANET in intelligent transportation system. It is hard for the traditional MAC protocols to meet the need of the new network. Thus it is very important to improve the performance of the traditional MAC protocol. A new MAC protocol which can keep efficient and reliable in both low and high load networks is needed, which should also keep robust in the network with high mobility nodes.A hybrid MAC mechanism is proposed in this paper, which takes the advantages of both TDMA and CSMA mechanism. This hybrid mechanism is based on TDMA, while CSMA mechanism is added in time slots to improve the slot utilization. The main contribution and innovation of this paper are as follows:(1) A conclusion about the development of VANET and analysis on the demand of the multiple-load VANET are made in this paper. The performances of CSMA and TDMA in the related scene are evaluated. The challenge and problems which brought by the new constraints of multiple load are especially concerned in this paper.(2) The key functional modules in the new mechanism are as follows:Slots are assigned to the nodes in the network with distributed slot assignment algorithm. Clock module is set for every node, and a hybrid protocol with backoff mechanism is carried out. Contention Notify Message is set to be the collision solution. Through these series of designs and access control rules, the new MAC mechanism becomes to be flexible and stable, which tend to be a TDMA mechanism when the load is high on the network, a CSMA mechanism in the low contention network.(3) Based on the data of simulation, the performance of the hybrid MAC protocol is evaluated in channel utilization, packet loss ratio and the overhead. It is can be concluded that the new protocol have a good performance in channel utilization whenever the load is high or low, though there are some packet lost because of the problem of contention-based mechanism, the overhead of the hybrid protocol has been reduced remarkably. The hybrid protocol also keeps a good robustness when the node speed changed. Through the research on the assistance mechanism, an idea for the future research is provided.