ROFF: RObust and Fast Forwarding in Vehicular Ad-Hoc Networks

Many safety applications rely on multi-hop broadcasting to disseminate safety messages. In most existing multi-hop broadcasting protocols, one next forwarder is selected through contention among forwarder candidates based on their different waiting times. In this paper, we first analyze the latency and collision of the existing protocols, and point out two problems: 1) unnecessary delay occurs in the contention process due to the lack of considering the distribution of vehicles and 2) the short difference between waiting times of forwarder candidates may allow redundant broadcasts to collide with each other. Secondly, we propose a new multi-hop broadcast protocol called RObust and Fast Forwarding (ROFF) to mitigate both problems. ROFF solves the first problem of unnecessary delay by allowing a forwarder candidate to use the waiting time which is inversely proportional to its forwarding priority. A forwarder candidate acquires its forwarding priority using the novel concept of ESD bitmap, which describes the distribution of empty spaces between vehicles. In addition, ROFF prevents the waiting time difference from being shorter than the predefined lower bound in order to avoid collisions, thus solving the second problem. Our extensive simulations reveal that ROFF achieves faster and more reliable broadcasting as compared to the other protocols.