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 shown to be a more reliable and effective approach than implicit acknowledgements adopted in previous works. In addition, OppCast was extended to handle disconnected VANET scenarios, in which the optimal threshold density to switch between normal dissemination and store-carry-and-forward scheme is characterized. Our results revealed the intrinsic tradeoff and intricate interplay between WM reception reliability, dissemination rate and overheads. We believe it will provide valuable guidelines to protocol design in VANETs and mobile WMNs.
1 Although a recent technique (Dutta et al. 2009) can solve this problem in wireless networks with fixed topology; it is less suitable for VANETs with dynamic topology.
2 This will not cause a broadcast storm because the maximum level needed is small and bounded, and OBCF greatly reduces packet collisions.
3 The uniform distribution of vehicle positions is adopted in performance evaluation.
4 Because as E[Y] increases beyond these local minima points, the number of makeups per hop will first increase and then remain fixed.