Towards collision-free medium access control in vehicular ad-hoc networks

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 (VANETs). Contention-based MAC protocols, e.g., 802.11p, are prone to transmission collisions due to the absence of node coordination, while traditional TDMA-based protocols have difficulty coping with fast-changing network topology. We propose Reservation Protocol (RP) - a near collision-free MAC protocol that is specifically designed for VANETs. By disseminating position beacons, nodes in RP predict the trajectories of their neighbors and anticipate possible transmission collisions. Reservation messages are exchanged among rival nodes in order to avoid potential conflicts. Simulation-based evaluation shows that at the cost of longer average medium access delay, the collision probability in RP protocol is considerably lower than 802.11p in various load scenarios.