1Effect of Retransmissions on the Performance of the IEEE 802.11 MAC Protocol for DSRC

We consider a dedicated short range communication (DSRC) network based on the IEEE 802.11 distributed coordination function (DCF) protocol for broadcasting vehicle-to-vehicle safety messages. Reliable delivery of broadcast safety messages to all nearby vehicles is an important issue, and is challenging due to the unreliable wireless links, lack of acknowledgments, fast changing network topology and the prevalence of hidden stations. This is especially important for so-called event messages that have a more stringent requirement for fast and guaranteed delivery than routine safety messages. In this paper, we propose two mechanisms, which can be implemented on top of standard DCF or EDCA, that use “blind ” retransmissions with no feedback. We then develop a comprehensive analytical model to predict the behavior of the DCF protocol in unsaturated broadcast networks for both types of safety messages with and without retransmissions. In particular, we derive explicit expressions for the mean of the total packet delay and the packet delivery ratio (PDR). The model accuracy is validated using ns-2 simulations. We show that both the retransmission schemes improve the PDR of event messages, with acceptable degradation to that of routine safety messages