Towards guaranteed delivery of safety messages in VANETs

Reliability is a critical concern in disseminating safety messages in vehicular ad-hoc-networks (VANETs). Thus far, redundant broadcast (or next relay broadcast) has been used to ensure reliable transfer of safety messages. However, redundant broadcast fails to meet high-reliability requirement owing to lack of a feedback or acknowledgment (ACK) mechanism. In this work, a power controlled negative-acknowledgment (NACK) mechanism is introduced as a feedback technique to ensure reliable reception of safety messages. Significantly, this work also attempts to cover vehicles present in transmission holes in the broadcast region. Vehicles in the immediate neighborhood detect and recover safety message for a vehicle present in the transmission hole by estimating propagation loss for the vehicle. NACK together with hole detection and recovery mechanism nearly guarantees safety-message delivery in VANETs. Reliability of the proposed technique is formulated mathematically with packet reception ratio (PRR) as the reliability metric. Using theoretical analysis and simulation evaluation in ns-3 we establish that the proposed technique guarantees safety-message reception by mitigating packet loss caused by interference from hidden nodes, and at instances where vehicles are located in the transmission holes.