Multi-hop wireless broadcast is a critical component in ad-hoc wireless networks. Vehicular Ad-hoc Networks (VANET) in particular utilize broadcast as a primary communication mechanism in many applications. Such networks exhibit situations of very high node densities so it is important that the broadcast protocols used to support applications on these networks scale well to high densities. This work evaluates existing broadcast protocols in terms of this scalability using both a high-level simulation assuming an ideal medium (WiBDAT) and a standard detailed wireless simulation (JiST/SWANS). Four broadcast methods and protocols, each having been used as a forwarding node selection algorithm in VANET, are chosen for evaluation: stochastic broadcast, distance method, Advanced Adaptive Gossiping (AAG), and Multi-Point Relaying (MPR). Results show a distinct performance difference in terms of rebroadcast efficiency at high densities. Protocols using network topology information, AAG and MPR, become inefficient at high densities. Stochastic broadcast and the distance method, which use neighbor count directly to calculate retransmit thresholds, are shown to be highly adaptive to node density.
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