Recovering VANET safety messages in transmission holes

The core concern in vehicular ad hoc networks (VANETs) is the reliable transfer of safety-related messages to all endangered vehicles on the road. The recent discovery of the presence of transmission holes in the VANET communication range poses a serious challenge in the reliable safety-message dissemination. In this work, a technique for recovering the safety message for vehicles located in transmission holes is proposed. Each vehicle that successfully receives the safety message actively estimates propagation loss for its immediate neighbors. When the receiving vehicle determines a neighbor located in a coverage hole, the safety message is rebroadcast by the receiving vehicle. The propagation-loss estimation makes use of the topology information appended in the periodic beaconing messages. Contention among multiple rebroadcasters is resolved by using the relay schedule mechanism. The proposed technique is evaluated using a detailed implementation in the ns-3 network simulator. The simulation results suggest that the proposed technique guarantees the safety-message dissemination with a minimal overhead delay of five milliseconds even in the dense-urban traffic scenario.