On MAC Access Delay Distribution for IEEE 802.11p Broadcast in Vehicular Networks

The inter-vehicle safety communication is generally considered as a promising technology to significantly enhance road safety. In fact, a lot of communications about inter-vehicle safety have real-time, rigorous requirements on broadcast messages in order to make sure that drivers have sufficient time for reaction towards emergencies. The procedure of the control for medium access is of significance for the delay-constrained communication systems. Therefore, in-depth investigation should be conducted for IEEE 802.11p covering the MAC and PHY protocol. Nonetheless, most existing studies only focus on the average delay analysis that includes insufficient information of inter-vehicle communications' real capacity. So as to evaluate the single-hop broadcast's performance under 802.11p, an analytical model is put forward in this paper. Both of simulation and real-world experiment results indicate that our model accurately estimates the probability distribution, deviation and mean of the MAC access delay. In addition, it can be proved that exponential distribution can be a good approximation to the distribution of MAC access delay with the usage of the K-S test. With such observation, it is convenient for us to conduct an analysis on the queuing delay in the MAC layer. As shown by the numerical analysis, the support of QoS in 802.11p provides sound guarantee about performance for higher priority messages. However, it cannot meet the real-time requirement of the applications with lower priority.