It is envisaged that supporting vehicle-to-vehicle and vehicle-to-infrastructure communications with a Vehicular Ad-Hoc Network (VANET) can improve road safety and increase transportation efficiency. Among the candidate applications of VANETs, cooperative collision avoidance (CCA) has attracted considerable interest as it can significantly improve road safety. Due to the ad hoc nature of these highly dynamic networks, no central coordination or handshaking protocol can be assumed and safety applications must broadcast information of interest to many surrounding cars by sharing a single channel in a distributed manner. This gives rise to one of the key challenges in vehicle-to-vehicle communication systems, namely, the development of an efficient and reliable medium access control (MAC) protocol for CCA. In this chapter, we provide an overview of proposed MAC protocols for VANETs and describe current standardization activities. We then focus on the performance of the IEEE 802.11 carrier sense multiple access (CSMA) based MAC protocol that is being standardized by the IEEE standards body for VANET applications. In particular, we review prominent existing analytical models and study their advantages, disadvantages and their suitability for performance evaluation of the MAC protocol for VANETs. After a discussion of the shortcomings of these models, we develop a new analytical model in the second half of the chapter. Explicit expressions are derived for the mean and standard deviation of the packet delay, as well as for the packet delivery ratio (PDR) at the MAC layer in an unsaturated network formed by moving vehicles on a highway. We validate the analytical results using extensive simulations and show that good accuracy can be achieved with the proposed model for a range of topologies and traffic load conditions. More importantly, using the model, we show that hidden terminals can have a severe, detrimental impact on the PDR, which may compromise the reliability required for safety applications.
Vehicle Density Based Forwarding Protocol for Safety Message Broadcast in VANET