A survey on hybrid MAC protocols for vehicular ad-hoc networks

Intelligent Transportation System has become a vital technology in our lives. Among many applications observed in ITS, emergency message transfer between vehicles and roadside units is essentially used to prevent road accidents. The MAC protocols in vehicular networks are of three kinds - contention based, contention-free and hybrid. In this paper, a survey on the realizable methods in hybrid MAC protocols is provided, primarily to improve the performance of safety and non-safety message delivery. Challenges dealt with 802.11p MAC protocol are also highlighted. Hybrid MAC acquires the best characteristics of two channel access schemes, thereby improving the performance when traffic load increases. The design of MAC layer protocol in a vehicular network is relatively challenging due to frequent changes in topology, vehicle nodes at moving high speeds, lack of infrastructure, and numerous quality of service requirements. A hybrid MAC protocol ensures that all vehicles obtain a fair channel access for transmission of their messages. Moreover hybrid MAC reduces collision, overall delay and packet loss.