Most of the communication protocols always assume that the network is fully connected. The protocol design and its evaluation research also focuses on this kind of networks. The application for this type of networks has not yet witnessed mass market deployment. The ongoing research and users demand is mainly focused on implementing real-time applications where high bandwidth and open access to the public Internet. However, there are many realistic scenarios in which many areas are with no preexisting communication infrastructure (e.g., disaster recovery and rural environments) (Bruno et al., 2005). In a transportation system the vulnerability of the communication links of ITS is inevitable. Trade-off should be dealt with between high responsiveness and the flexibility of the network.
If certain delay is tolerant, partially connected routing protocol can achieve successful packet forwarding in non-fully connected network (Burgess et al., 2006). In DTN (Delay-Tolerant Network) architecture, routing protocol can use store-wait-forward process to provide communication even if the end-to-end fully connected path never exists between the source the destination.
In ITS, the communication link is highly dynamic and unstable. Due to the high mobility and low duty cycle of communication link, the delay could vary. This kind of network is named Vehicular Sensor Network (VSN), which is a type of VANET (Burgess et al., 2006). For some applications which are not delay sensitive, such as traffic information query, environmental monitoring and event reporting, certain delay is tolerable, because late massage is much better than no message. The goal of such a network is to increase the probability of finding a path through an intermittently connected network. Traditional protocols resist on immediate forwarding policy and may fail due to the unpredictable delay. In the routing of VSN such as proposed in paper (Zhao & Cao, 2008), messages can be buffered for a certain amount of delay for later forwarding and be routed to the destination in tolerant delay.