# A Review of Vehicle to Vehicle Communication Protocols for VANETs in the Urban Environment
## 3 Position-Based Routing in Vehicular Networks
Secondly, the perimeter phase also causes delay in relaying the packet toward the destination which degrades the throughput of the network [8]. The problem with the restricted greedy forwarding is that it takes more hops as compared to simple greedy forwarding as shown in Figure 8 which degrade the performance of the network [24]. Figure 7 shows that in case of greedy forwarding, node A forwards the packet to node C which is closest to the destination, while in case of restricted greedy forwarding, node A forwards the packet to a node on the junction instead of forwarding to node C.  
Figure 8 shows a scenario of the inefficiency of restricted greedy forwarding. In this figure, the number of hops from source to destination node that restricted greedy takes into account is 16 while simple greedy forwarding just takes 12. In case of restricted greedy forwarding of GPCR, the packets are always stopped at the junction and hence, packet traverses greater number of hops as compared to simple greedy forwarding. Therefore, restricted greedy forwarding is inefficient as compared to simple greedy forwarding [24; 25]. Figure 7: Greedy Forwarding vs. Restricted Greedy Forwarding of Greedy Perimeter Coordinate Routing (GPCR). GpsrJ+ [24] is an enhancement of GPCR. Unlike GPCR, it avoids unnecessary packet stop at intersection which increases hop count. It uses two-hop neighbor information to forecast which street its neighboring intersection vehicle will take. If forecast specifies that its neighboring intersection will forward the packet on to a street with different direction, it forwards the packet to the junction vehicle; else it avoids the intersection and forwards the packet to vehicular node that is closest to the destination. The simulation outcome shows that GPSRJ+ outperforms GPCR in terms of packet delivery ratio and hop count. Anchor based Street and Traffic Aware Routing (A-STAR) [25] establishes anchor path with high connectivity by using information based on statistically rated maps about city bus routes. When a packet faces local optimum situation then route recovery strategy is used to overcome a local optimum problem by calculating a new anchor path. The simulation results indicate that A-STAR outperforms GSR and GPSR because it has the capability of accomplishing end-to-end connected path even in case of low vehicular traffic density. However, one problem associated with this approach is that routes are along the anchor path that may not be optimal resulting in increased end-to-end delay [2]. Greedy Traffic Aware Routing (GyTAR) [15] is a junction-based geographic routing strategy that is capable of accomplishing robust routes in urban scenarios. It has two modes: (i) a dynamic junction selection mechanism for accomplishing shortest path based on traffic density; (ii) an improved greedy forwarding that forwards the packet in between two junctions. Because of these two mechanisms, packets move towards destination along the urban streets that present higher connectivity. GyTAR performs better than previous routing protocols in terms of packet delivery ratio, routing overhead and end-to-end delay. Problem with this approach is that it does not consider the direction of the vehicle while selecting next junction as a result in some city scenarios, protocol suffers from a local optimum problem which affects the performance of the network [8]. Enhanced Greedy Traffic Aware Routing Protocol (E-GyTAR) [8] is an enhanced version of GyTAR. It has also two phases: (i) dynamic junction selection mechanism based on directional density, (ii) improve greedy forwarding for routing in between junction. It accomplishes shortest routing path on the basis of directional density and thereby routes the packet toward the destination. It uses improved greedy packet forwarding strategy to forward the packet between the junctions. It avoids local optimum by using carry and forward approach.