# Dynamic Multiple Junction Selection Based Routing Protocol for VANETs in City Environment
## 2 Related Work  
Routing protocols for VANETs can be categorized into two main types. The first type is topology-based routing protocols and the second type is position-based routing protocols. The topology-based routing protocols can further be categorized into reactive, proactive, and hybrid protocols [27; 28]. The reactive protocols like Dynamic Source Routing(DSR) [29] and Ad-hoc on Distance Vector Routing(AODV) [30] are based on demand routing because in these routing protocols only those routes that are currently in use are maintained [14; 21; 31]. Proactive routing protocols, like Optimized Link State Routing (OLSR) [32], are table driven routing protocols because in such protocols all the available routes in the network topology are maintained [2; 31; 33]. Due to frequently changing network topology in VANETs, the topology based routing protocols are not suitable. Such protocols are not scalable under highly mobile large-scale VANETs. These protocols incur high latency in finding a route because they store all the unused routing paths [3; 34; 35]. A protocol like AODV consumes additional bandwidth due to transmission of periodic heavy beacon messages in intermittently connected VANETs [14; 31]. In addition, the maintenance of routing paths in highly mobile environments results in large routing overhead for topology-based routing protocols [5; 6; 7; 8; 11]. In order to overcome the restrictions of topology-based routing, position-based routing which is based on location information is introduced [35]. In position-based routing protocols, the vehicular nodes are equipped with GPS for getting accurate information about their geographical locations, speeds, and moving directions. Destination node location can be found by using location services such as GLS (Grid Location Service) [36], RLS (Reactive Location Service) [37] and HLS(Hierarchical Location Service) [34]. Beacon messages are used to obtain one-hop neighbor information. There is no need to maintain the routing paths in position-based routing protocols. Due to all these features, position-based routing protocols overcome the major limitations of topology-based routing protocols [2; 3; 11; 12; 31; 33; 34]. In the literature, many position-based routing protocols are proposed in VANETs. As the focus of this work is city environments, therefore, we mention some of the well-known protocols specifically designed for city scenarios. Geographic Source Routing Protocol (GSR) [38] was designed for city scenarios. It uses Dijkstra shortest path algorithm [2] to accomplish the shortest path between sender and destination. The shortest path computed by GSR consists of road junctions that are ordered sequentially. The data packets have to transverse these junctions in order to move from source to destination. The source node makes use of Reactive Location Services (RLS) [14] for locating the destination. GSR uses greedy forwarding for relaying packets between the junctions. The simulation outcomes, with the use of realistic vehicular traffic in urban surroundings, illustrated that GSR performs better then topology based routing protocols like DSR and AODV in terms of end-to-end delay and packet delivery ratio [2; 31]. However, GSR is not a traffic aware routing protocol. Therefore, it does not make use of traffic information for making the routing decisions [3]. In [33], Greedy Perimeter Coordinator Routing (GPCR) was proposed for city environments. GPCR consists of greedy forwarding and perimeter mode. The unique feature of the protocol is that it makes routing decision at the coordinator node without considering digital maps. The node that is near the junction or at the junction is called coordinator. In GPCR the restricted greedy forwarding bound the packet carrier node to forward the packets to a node at the junction instead of dispatching them across the junction. The local optimum situation is overcome by perimeter mode. An assumption is made in perimeter mode that the graphs are naturally planner in city scenarios.