MAC protocols with dynamic interval schemes for VANETs.

Under the dynamically changing topology of Vehicular Ad-hoc NETworks (VANETs), the restricted intervals in Medium Access Control (MAC) protocols cannot provide sufficient capacity to carry both safety and non-safety applications. One approach which can solve these issues is a dynamic MAC protocol that can adapt itself to the vehicle density or traffic conditions. In this survey, we, therefore, study various techniques for dynamic intervals used in MAC protocols, their advantages, and disadvantages. First, we classify these protocols into three following categories: 1) contention-based, 2) contention-free, and 3) a hybrid between contention-free and contention-based medium access methods. Second, in each medium access method, we classify the methodologies depending on their operating principles. The conclusions of our study are as follows: 1) Adaptive MAC protocols improve the channel utilization and adapt themselves to various traffic states, 2) Adaptive MAC protocols perform better than protocols using fixed intervals, 3) Currently, MAC protocols using dynamic intervals, which are suitable to VANET standards, are only applicable to optimize control channel interval; therefore, it is necessary to expand to both optimize control channel interval and service channel interval according to network and traffic condition. Finally, we discuss some open issues and whether designing MAC protocols using dynamic intervals can meet the QoS requirements for different applications in the future.