ERP: An Efficient and Reliable Protocol for Emergency Message Dissemination in Vehicular Ad Hoc Networks

Abstract—Many safety-related applications in Ve-hicular Ad Hoc Networks require fast and reliable emergency message dissemination through multi-hop broadcast. However, the conventional broadcast mech-anism is neither efficient nor reliable because it results in serious contention and collisions, which is usually referred to as the broadcast storm problem. In this paper, we propose ERP, a two-phase broadcast proto-col that improves both efficiency and reliability. The first phase, a “fast-propagation phase”, is designed to improve efficiency. We explicitly designate forwarders to relay the message and thus ensure both collision-free and quick propagation. The second phase, a “loss-recovery phase”, enhances reliability. In this phase, nodes overhear the message and repeatedly broadcast it for the benefit of nodes which have not received the message in the first phase. We analytically show that using a density-aware power control mechanism in the second phase can efficiently improve the recovery rate. We also demonstrate how to find the optimal trans-mission power. Simulation results illustrate that our protocol outperforms probabilistic forwarding, which is currently the most widely studied solution, by a factor of 2 to 3. I