Cluster-based MAC in VANETs for safety applications

Recently, we are witnessing increased interest in the research of Vehicular Ad-hoc Networks (VANETs). Traffic safety applications using VANETs aim to improve traffic safety in the road using messages which have to meet high reliability, low and predictable delay constraints. Moreover, due to characteristics of VANETs, such as high speed, unstable communication link, and network partitioning, information transfer becomes inevitably challenging. The effectiveness of traffic safety applications using VANETs depends up on the performance of medium access protocol (MAC). Vehicles using CSMA/CA (IEEE 802.11p) MAC algorithm the time to channel access increases randomly every time when the channel is sensed busy. The main challenge for the design of MAC protocol for VANETs is to achieve reliable delivery of messages within the time limit even when the density of vehicles varies rapidly in the network. Furthermore, MAC algorithms with good scalability, fairness, and predictable delay are needed to fulfill the requirements of traffic safety applications. Therefore, cluster-based MAC protocols that can avoid or limit channel contention, provide fairness to channel access, increases radio capacity by the spatial reuse of network resources and effectively control the topology of the network. However, due to the high mobility characteristics of VANETS, vehicles frequently join and leave clusters affects the stability of network. Furthermore, the stability become worse if the cluster heads (CH) elected are not stable. Therefore, maintaining cluster stability can increase performance of safety applications in VANETs. Finally, we propose two cluster based MAC protocols (D-CBM) based on contention based carrier sense multiple access (CSMA) and collision free time division multiple access (TDMA) in order to achieve high stability, low communication overhead and real time delivery of safety messages. Simulation results indicate the proposed protocols are able to achieve the above requirements. Furthermore, we analyze the performance of the proposed MAC design with SBCA protocol.