A modified 802.11-based MAC scheme to assure fair access for vehicle-to-roadside communications

Vehicular ad hoc networks (VANets) are special cases of mobile ad hoc networks (MANets) in which the mobility pattern is very different. Actually, the topology of the network in VANets changes more often and the variance of speeds is high. Such characteristics lead to fairness problem in VANets, considered in this paper. In fact, vehicles with different speeds have their different limited time to communicate with an access point (AP). This attribute adversely affects the chance of a fast moving vehicle in communicating with the access point. We propose a modified 802.11 DCF channel access scheme as a solution to the problem. The proposed scheme adjusts the probability of transmission at a time slot for each node according to its speed. To this end, we change the probability of transmission through changing the minimum contention window size. In this respect, we obtain some relations between speed and window size through analytical approximations. Simulation results indicate the validity of our approximations and the efficiency of the proposed MAC scheme in solving the fairness problem.