# A Method for the Rapid Propagation of Emergency Event Notifications in a Long Vehicle Convoy
### Ad Hoc Routing Protocols  
Many would point to dedicated infrastructure, primarily cellular towers, as the solution to vehicular message dissemination. While current and future cellular infrastructure could be used for this task, it may not always have the capacity, latency, or (most importantly) coverage to guarantee safety-critical communication. A potential solution could be the use of ad hoc wireless networks, where participating devices take on the additional responsibility of routing messages for those that do not share a direct communication link.  
When applied to vehicles, this idea is known as a vehicular ad hoc network (VANET). While there is a large body of work on standardizing the wireless communication systems for vehicles (e.g., IEEE P1609), a standard for vehicular ad hoc routing has yet to be established. Thus, finding an effective and efficient routing scheme for VANETs has become the work of a large body of prior research. [PERSON] and [PERSON] wrote a survey paper discussing the various types of routing protocols for VANETs [16]. Their findings indicate that standard ad hoc routing protocols such as AODV and DSR are typically unsuitable for VANETs (see Section 3.3 for our tests on the matter). They mention geographic routing (using physical location to decide on the next hop) and cluster-based routing (recognizing groups of vehicles and assigning a \"head\" for each) among others as being attempts at solving the routing problem. A direct attempt at formulating a VANET routing protocol was made by [PERSON] et al. with their \"Vehicular Reactive Routing\" system [17]. The system was designed around the IEEE P1609 protocol stack and utilizes slightly modified versions of the existing messages. The relevant part of their system is in the retransmission choice mechanism. Every node that hears a packet being broadcast will decide on a backoff timer for retransmission. The timer was designed such that the node closest to and with the most similar motion to the destination will have the shortest timer and thus retransmit first. A similar backoff timer system was used by [PERSON] et al. for the purposes of removing neighbor discovery beacons in VANETs [18]. In their system, the rebroadcast delay is set purely based on proximity to the destination. The common thread in both systems is the use of \"eavesdropping\" to inform each node on the necessity of their retransmission, thus enhancing channel efficiency. [PERSON] primarily used the backoff timer as an arbitration mechanism, while [PERSON] at least mentioned the benefits of redundant transmission paths. Lastly, [PERSON] et al. approached the problem of safety message dissemination in an urban environment using clustering and vehicle movement prediction [19]. They recognized that infrastructure nodes will likely not be able to provide sufficient coverage to alert every vehicle of safety-critical information when and where it is needed. Their system constructs a \"core\" network of vehicles based on the vehicles' trajectories in order to improve communication coverage. To find this set of vehicles, the initiator sends a request indicating the region of interest for the safety message. The surrounding vehicles scale the delay of their responses to this request based on the similarity of their trajectories with respect to the region of interest. After the core vehicles are chosen by the initiator, those vehicles then repeat the process until as much of the region of interest is covered as possible. What many of the protocols mentioned here have in common is their use of backoff timers as more than just a message collision avoidance mechanism. Retransmission or response delays can be used to implicitly convey information about some relationship between the communicating vehicles, such as fitness toward a purpose.