Research On Backoff Algorithm And Channel Reservation For Vehicular Networking MAC Layer

With the popularity of cars in the ordinary family, the safety and efficiency of traffic and driving experience slowly get people’s attentions. As an important part of intelligent transportation systems (ITS), Vehicular Ad Hoc Network (VANET), which provide the communication to vehicles, can make traffic system much safer, more efficient and more convenient. It can not only deliver safety information to reduce accidents, but also can provide non-safety information for user. Now, it has become an important research topic in ITS. VANET use the fixed contention window and backoff algorithm, which is not suitable for the different traffic. In a dense traffic environment, it can cause a lot of collisions in the transmission of data packets, and increases the time delay, which affect the transmission of the safety information seriously. And in a sparse traffic environment, it causes a lot of free time, which reduce the utilization of channel resources. In addition, on the aspect of multi-channel cooperation, VANET use a fixed cycle of switching in channels. In a sparse traffic environment, safety information is relatively less, so most of the time slot of CCH is idle, which causes the waste of channel resources.To solve the problem of channel access caused by fixed window and traditional backoff algorithm in VANET, this paper desgins a new backoff algorithm, which is based on the self-adaptive contention window. This paper establish one-dimensional markov model of broadcasting, and formulate the relationship beween the number of node and the optimal window by the saturation throughput, then design backoff algorithm of adaptive window based on the number of node. Simulation results show that the algorithm can effectively reduce the time delay, increase the throughput, and it is suitable for the different traffic. To solve the problem caused by channel switching,this paper proposes a RCMAC protocol, which is based on channel reservation. In order to improve the utilization of CCH in the sparse traffic environment, it divides the priority of business, establishes a channel status table, and uses the idle time of CCH to order SCH, then make a non-competitive media access method in SCH. Simulation results show that when the density of traffic is small, the RCMAC protocol can improve the performance of throughput and delay in SCH.