A modified backoff algorithm for safety message delivery in IEEE 802.11p/WAVE networks

The alternating channel access scheme, as defined in the 802.11p/WAVE (Wireless Access in Vehicular Environments) standard, divides the entire channel access time into consecutive transmission intervals, called Sync Intervals. Each Sync Interval consists of two alternating intervals: control channel interval (CCHI) and service channel interval (SCHI), for delivering safety and non-safety applications, respectively. Broadcasting of safety-related data on 802.11p control channel is expected to be very crucial in vehicular environments. However, the flash crowd problem for safety messages is incurred by CCHI-SCHI interval switching. This paper presents a modified backoff algorithm (MBA) for safety message broadcasting in IEEE 802.11p/WAVE networks. The proposed MBA includes two phases: to decide the size of contention window and to generate a non-uniformly distributed backoff timer. Taking into account vehicle's travel information, such as location, speed, and direction, our proposed MBA scheme not only can adjust the contention window size according to the average speed among the neighboring vehicles, but also can randomly select a backoff value with a non-uniformly distributed function of vehicle's speed. The simulation results on ns-3 show that our proposed MBA scheme has a significant improvement in the probability of successful packet delivery and the packet loss ratio on 802.11p control channel.