A dynamic safety interval protocol for VANETs

In IEEE 802.11p/1609-based vehicular networks, the channel access time of a synchronization interval is divided into two fixed-length intervals, i.e. control channel (CCH) and service channel (SCH) intervals. Since the fixed-length intervals cannot be effective for dynamically changing traffic load, some protocols have been recently proposed to support variable-length CCH intervals in order to improve channel utilization. In existing protocols, the CCH interval is subdivided into safety and non-safety intervals, and each interval is dynamically determined based on traffic load. However, they do not consider the presence of hidden nodes. Hence, each interval has no additional room for transmissions from hidden nodes. Consequently, messages transmitted in each interval are likely to overlap with simultaneous transmissions (i.e. interference) from hidden nodes. Particularly, life-critical safety messages can be unreliably delivered due to such interference, which deteriorates QoS of safety applications such as cooperative collision warning. In this paper, we therefore propose a new dynamic safety interval (DSI) protocol. DSI calculates the number of vehicles sharing the channel with the consideration of hidden nodes. The safety interval is derived based on the measured number of vehicles. From simulation study using the ns-2, we verified that DSI achieves better delivery ratio of periodic messages such as beacon.