# Intersection Routing Based on Fuzzy Multi-Factor Decision for VANETs
## 2 Related Work  
In this section, we review some existing routing protocols to indicate our motivations. This section describes the unicast routing protocols in which a single data packet is transported to the destination vehicle without any duplication due to the overhead concern. There are two categories of routing protocols: topology-based routing protocols and geographic (position-based) routing protocols [6; 7]. In topology-based routing protocols, the links' information that exists in the network is used to perform packet forwarding. They can also further be divided into proactive and reactive routing. The distinct feature of proactive routing is that the full routing information such as the next forwarding hop is maintained in the background regardless of communication requests. In reactive routing, a route will be established only when a vehicle must communicate with another vehicle. The two more famous topology-based routing protocols are ad hoc on-demand distance vector (AODV) [8] and the dynamic source routing (DSR) [9]. These two routing protocols maintain only current route information to reduce packet overhead. Nevertheless, increasing overhead size is inevitable with increasing network diameter results from vehicle mobility. Figure 1: The architecture of the routing protocol. Geographic routing (GR) determines forwarding route considering destination position and the positions of the vehicle's 1-hop neighbors. Vehicles that are within a vehicle's radio range will become neighbors of the vehicle. Geographic routing assumes that not only every vehicle knows its position, but also the sending vehicle knows the receiving vehicle's position by the global positioning system (GPS) unit. Since GR neither exchanges link state information nor maintains established routes as topology-based routing protocols, it is more robust and promising compared with VANETs which are highly dynamic. GR can be sub-classified into three categories, namely non-delay tolerant network (non-DTN), delay tolerant network (DTN), and hybrid [10]. DTN considers dysconnectivity, whereas non-DTN does not consider intermittent connectivity and is suitable for densely populated VANETs. Hybrid combines non-DTN and DTN routing protocols to exploit partial network connectivity. The key objective of non-DTN is to minimize packet delivery time from source to destination. Greedy forwarding is the commonly used technique in which the fundamental principle is to forward packets to a neighbor, which is geographically closer to the destination. However, the local maximum issue could appear when a vehicle reaches closer to the destination and finds no neighbors closer to the destination than the vehicle itself. Greedy perimeter stateless routing (GPSR) [11] is the most well-known geographic routing protocol in VANETs. A vehicle forwards a packet to a 1-hop neighbor which is geographically closest to the destination vehicle. This mode is called greedy mode, and a recovery mode is switched to when a packet reaches the local maximum. The packet is forwarded along the perimeter of a planar graph without crossing edges based on the right-hand rule. If the packet reaches a vehicle whose distance to the destination is closer than the vehicle at the local maximum to the destination, the packet resumes forwarding in greedy mode. Greedy perimeter coordinator routing (GPCR) [12] enhances GPSR by using the Dijkstra shortest path algorithm to determine the junctions that have to be traversed based on a static street map. A packet has to be forwarded to a vehicle that is located on the junction because junctions are the only places where routing decisions are made. Packets can be forwarded between the junctions in greedy mode. And GPCR consists of two parts. One is a restricted greedy forwarding procedure and another is a repair strategy that is based on the topology of real-world streets and junctions. Hence, GPCR not only eliminates the inaccuracy of vehicle planarization but also improves routing performance as packets travel shorter hops in the perimeter mode.