Multihop Wireless Networks: Opportunistic Routing

10.8 Conclusion

In this chapter, we studied the opportunistic broadcast of event-driven WMs in VANETs. The multihop broadcast of WMs needs to satisfy multiple stringent performance requirements such as high reliability (packet reception ratio), low end-to-end latency and low transmission overheads, which is especially difficult to achieve due to the lossy wireless environment and fast-changing topology in VANETs. Thus, a fully distributed opportunistic broadcast protocol (OppCast) for the multihop dissemination of event-driven warning messages in VANETs was proposed. Aiming at achieving high WM reception reliability and fast dissemination in a resource-efficient way, the concept of opportunistic routing was exploited in the link layer at each hop to enhance reception reliability and provide a small hop delay. Meanwhile, at the network layer, we propose a double-phase broadcast method, in which fast propagation is ensured by one phase, and the desired reliability level is ensured by the other. As a key idea in OppCast, we use explicit broadcast acknowledgements (BACK) in rebroadcast contention so that the optimal relays can always be selected with a high probability and the undesired redundant rebroadcasts are dramatically reduced. Through extensive simulations we have shown that, compared with state-of-the-art protocols, OppCast achieves a higher WM packet reception ratio and a higher dissemination rate using a smaller amount of transmissions. More importantly, the BACK has been ...