Establishing Strict Priorities in IEEE 802.11p WAVE Vehicular Networks

The WAVE (Wireless Access in Vehicular Environments) concept includes seven channels within the DSRC band. One of them, known as the Control Channel (CCH), is the one used to exchange all safety-related messages. Messages sent over the CCH have to be processed with different priorities depending on how critical they are for vehicle safety. However, the MAC protocol currently adopted for WAVE, namely EDCA, stops short of that requirement; it does not establish strict priorities, but only relative advantages for some types of messages over the others. Another problem is that, since messages are broadcasted on the CCH, there are no acknowledgments. This means that it is not possible to know whether a transmission was successful or not, which eliminates the possibility to use the binary exponential backoff technique to reduce congestion. In this paper, we propose a simple but effective solution to both of these problems. We use simulations to analyze the performance of the modified MAC protocol and compare it to that of the original EDCA. The results show that the proposed scheme outperforms EDCA. Our comparison focuses on the reduced probability of collision for high-priority frames (gain) and on the increased delays for lower-priority frames (price to pay).