Reservation protocol: Towards collision-free transmission in vehicular ad-hoc networks (poster)

Collision-free transmission is important to achieving reliable and delay-bounded wireless communication, which is mandated by many safety-related applications in vehicular ad-hoc networks. A large number of evaluation studies have shown that CSMA-based approaches, e.g., 802.11p, are prone to high rates of transmission collision, especially in high node density scenarios. TDMA-based protocols, e.g., RR-ALOHA, are immune from collisions but were originally designed for stationary networks. The transmission schedules in these protocols must be constantly refreshed as the network topology evolves, which causes significant overhead and resulting degradation of communication quality. This paper proposes a near collision-free medium access control protocol specifically designed for networks with fast changing topologies. The protocol predicts the evolution of the network topology and estimates the collision probability if certain nodes transmit simultaneously. Reservation messages are exchanged between nodes that may conflict with each other, and disputes are eventually resolved. The paper compares the performance of the protocol, measured in terms of collision probability, average medium access delay, and packet delivery ratio with 802.11p by simulation. Results show that at the cost of longer average medium access delay, the collision probability of the protocol is considerably lower than 802.11p´s in various simulated scenarios.