An Opportunistic Approach to Enhance the Geographical Source Routing Protocol for Vehicular Ad Hoc Networks

Geographical source routing is a promising routing technique for VANETs, due to its adaptability for network dynamics and ability to handle topology holes. In traditional geographical source routing algorithms a best-known neighbor, typically the neighbor closest to the next junction in a greedy fashion, is designated as the next hop. This approach may cause two drawbacks: (1) the designated neighbor might not receive the packet correctly and (2) non-neighbor nodes are never given opportunities to do forwarding. Due to the broadcast nature of wireless media, non-neighbor nodes can overhear the transmission and may offer better forwarding progress than the designated neighbor. The above two factors both degrades the end-to-end delivery ratio. In this paper, we resolve the degradation of delivery ratio by introducing the concept of opportunistic routing to geographical source routing. A new routing protocol, named Geographical Opportunistic Source Routing (GOSR), is developed. GOSR allows non-neighbor nodes as well as the bestknown neighbor to become a forwarder. The notification cost of opportunistic routing is minimized by enforcing a scope from which candidate forwarders are selected. Defer timers are adopted to avoid conflicts caused by simultaneous transmissions by nodes in the designated scope. Extensive simulation studies are performed and the results show that GOSR can substantially reduce hop count and also improve end-to-end delivery ratio remarkably.