# A Review of Vehicle to Vehicle Communication Protocols for VANETs in the Urban Environment
## 3 Position-Based Routing in Vehicular Networks
GSR: Geographic Source Routing, GPCR: Greedy Perimeter Coordinator Routing, GPSRJ+: Greedy Perimeter Stateless Routing Junction+, A-STAR: Anchor based Street and Traffic Aware Routing (A-STAR), GyTAR: Greedy Traffic Aware Routing, E-GyTAR: Enhanced Greedy Traffic Aware Routing, TFOR: Traffic Flow Oriented Routing, DGSR: Directional Greedy Source Routing, E-GyTAR-D: Enhanced Greedy Traffic Aware Routing Directional, GPSR: Greedy Perimeter Stateless Routing, DGR: Directional Greedy Routing, PDGR: Predictive Directional Greedy Routing, SADV: Static-node-assisted Adaptive Datadissemination in Vehicular-networks, MIBR: Mobile Infrastructure Based VANET Routing Protocol, MGRP: Mobile Gateway Routing Protocol. Figure 6 shows the working of the perimeter-based forwarding strategy. The vehicular node is unable to locate the closest vehicular node to destination vehicle D than itself. It uses the right-hand rule of perimeter mode and select node B for forwarding packet. Similarly, at node B the packet is forwarded to node C; this will continue until the perimeter mode switches back to greedy mode. Although this protocol is designed for a high environment, some of the researchers implemented it in the city environment and found it inappropriate. The main drawback of this routing protocol in the city environment is that its graphical planarization fails in city scenarios due to obstacles. Secondly, the perimeter phase uses long routes in relaying packet from source to destination which increases end to end delay. It also creates routing loops, which increases routing overhead. These drawbacks motivated the researchers to design a separate and an appropriate routing strategy that is suitable to overcome routing issues in the city environment [22; 23; 24; 25; 26]. Geographic Source Routing (GSR) Protocol [22] is a location-based routing protocol which is designed for the urban environment. In GSR, the source node finds the destination location using reactive location service. It accomplishes shortest routing path between source and destination using Dijkstra shortest path algorithm. The shortest path is composed of a set of junctions that are arranged in sequential order. The packet sent by the source node passes through these junctions and reaches the destination. The packet is forwarded in between junctions using the greedy forwarding approach. The simulation outcomes, with the use of realistic vehicular traffic in the urban surroundings, illustrated that GSR performed better than topology based routing protocols like Dynamic Source  
Figure 5: Working of greedy mode. Figure 6: Working of perimeter phase of Greedy Perimeter Stateless Routing (GPSR). Routing (DSR) and Ad-hoc on Distance Vector (AODV) in terms of end to end delay and packet delivery ratio [8]. GSR selects junction statically without considering traffic density which is its drawback. Consideration of traffic density is very important which provides connectivity for relaying the packet toward destination [15]. Greedy Perimeter Coordinator Routing (GPCR) [23] is proposed for the city environment. It is composed of two phases, the restricted greedy forwarding, and the perimeter mode. The distinguishing feature of GPCR is that the coordinator node is responsible for making routing decision without considering digital map. The node that is located at the junction is called coordinator node. The restricted forwarding mode bound the packet carrier node to forward the packets to a node that is located at the junction, instead of sending them to a node across the junction as shown in Figure 7. When restricted greedy forwarding meets local optimum, it uses perimeter mode to overcome such problem. In perimeter mode, it is assumed that the graphs are naturally planner. So, it does not compute graph planarization which induces partition in the network. The perimeter phase uses the right-hand rule to relay the packet to the destination. The first problem with this protocol is that it is not traffic aware [2].