Reliable broadcast in vehicular ad-hoc networks

For most safety-related applications that are envisaged in VANETs, the provisioning of a real-time medium access control is of great importance. However, the ability to access the wireless medium in a collision-free and time-bounded manner is particularly challenging in networks with unreliable wireless channel and dynamic topologies. Existing CSMA protocols such as 802.11p are prone to high rates of transmission collision, whereas TDMA-based protocols are not able to cope with the highly dynamic nature of vehicular networks. In this paper, we propose a novel MAC protocol: Real-time Reservation Protocol (RRP) which supports reliable and timely broadcast in VANETs. We believe that the vision of context awareness and farsightedness which constitute the core of most safety applications in VANETs should be brought to the design of the MAC layer as well. In the proposed protocol, broadcast messages are transmitted with high success rates, and the maximum time to access the medium is bounded. In a simulation-based study we compare the performance of RRP with 802.11p and RR-ALOHA in terms of packet delivery rate and staleness - a novel application-level QoS metric. Results indicate that RRP achieved satisfactory performance, which demonstrates the feasibility and effectiveness of real-time medium access control in VANETs.