A probabilistic discard congestion control for safety information in vehicle-to-infrastructure vehicular network

IEEE 802.11p provides a communication standard for Wireless Access in Vehicular Environment (WAVE), including both safety and non-safety communications. Safety-related communications can be from either event-driven or periodic-driven requests. The event-driven messages usually convey emergency information and, hence, are more critical than periodic-driven messages. IEEE 1609.3 specifies the operations under WAVE to provide network services and defines Wave Short Message Protocol (WSMP) for safety-related packets delivery. However, WSMP lacks a congestion control mechanism to avoid from packet lost. In this paper, we apply WSMP for communication among vehicles as well as the road site units (RSUs), and provide a discard-based congestion control scheme at RSU to mitigate the congestion problem. A Markov-chain model is also presented to compute the dropping probability and expected delay of safety-related packets. Numerical results show that the dropping probability of event-driven packets can be effectively controlled with the offset of discarding the periodic-driven packets using a threshold setting. Inter-relationship between traffic load and the value of threshold is also investigated.