# A Review of Vehicle to Vehicle Communication Protocols for VANETs in the Urban Environment
## 3 Position-Based Routing in Vehicular Networks  
The position-based routing schemes are considered as suitable for tackling routing related issues in VANETs [8; 15; 16; 17]. The position-based routing protocols use the geographical position of source and destination to accomplish communication between them. Every node is aware of its position due to global positioning system (GPS). The position of the neighboring node is found through beacons exchange. The position of the destination node is found using location services. When source node or intermediate node wants to send data to the destination node, if the destination node is in its transmission range than it directly forwards packet to the destination node. If the destination node is not in the transmission range it will forward the packet to a neighbor node that is the nearest to the destination node. In this way, the packet is relayed to destination [8; 15; 16; 17; 18]. In position-based routing, every node maintains one-hope neighbor information. Existing position-based routing protocols are developed for highway environment and urban environment. The highway environment consists of straight roads architecture without obstacles. On the other hand, urban environment consists of obstacles in the form of buildings. It is composed of streets and junctions. The points where two or more streets meet each other are called junctions. The data packets are routed towards destination through a set of junctions [19]. Figure 3: Hybrid architecture. Figure 2: Pure ad-hoc networks. The routing of data in an urban environment is challenging because of obstacles. In the existing literature, there are many position-based routing protocols proposed for V2V and V2I communications considering the urban environment [15; 16; 17; 18; 19]. Some of these routing protocols are traffic aware. Consideration of traffic flow is very important in position-based routing for enhancing network performance in terms of packet delivery ratio, an end to end delay, and hop count [2]. The position based routing can be categorized based on the working environment as follows. In Figure 4, we have provided the most significant position-based routing protocols that are designed to provide V2V communication for the city environment. We describe them below. Greedy Perimeter Stateless Routing (GPSR) [20] finds the location for source node using GPS. The location of the neighbor node is found through beacons exchange. It locates destination using location services (likes Grid Location Service (GLS) and Hierarchical Location Service (HLS)) [21]. It consists of greedy mode and perimeter mode. In greedy mode source node or packet carrier node chooses a node among its neighbor that is closest to the destination and forward packet to it. When the forwarding node finds no neighbor node closest to the destination than itself and the destination is out of its reach. Packet meets local optimum. In such condition, GPSR uses perimeter mode to overcome a local optimum problem. Figure 5 shows the working of the greedy mode of GPSR where source node S selects neighbor node B among all its one-hop neighbors because it is the closest to the destination node D and forwards the packet to it. In the case of a local optimum problem, GPSR uses perimeter mode. Perimeter mode consists of two steps. In the first step, it creates graph planarization using relative neighborhood graph (RNG). In the second step, it uses right-hand rule for finding next neighbor node that relays the packet toward the destination. Figure 4: Categories of position based routing protocols.