Distributed Periodic Access Scheme (DPAS) for the Periodic Safety Messages in the IEEE 802.11p WAVE

In WAVE applications, the safety information of neighboring vehicles must be collected as quickly and efficiently as possible. Safety applications broadcast their safety information periodically, but the delivery of the broadcast messages in IEEE 802.11p is unreliable. As the control channel has to share the Sync Interval (SI) with the service channels, access to the control channel by the safety messages is delayed during the Service CHannel Interval(SCHI). All safety messages generated during SCHI are rush to access as soon as the Control CHannel Interval (CCHI) starts. It causes the flash crowd problem for the safety applications. Safety messages, which are generated periodically and their periods overlap with SCHI, always face the latter problem; it causes unfairness among the safety messages. In this paper, a Distributed Periodic Access Scheme (DPAS) is proposed to solve the problem. DPAS uses a hashing function to distribute the access time of the safety messages into CCHI instead of SI. The simulation results show that DPAS drastically reduces the collision of the safety messages in a crowded environment and that choosing a hashing function is very important to reduce the collision in a crowded environment.