# Dynamic Multiple Junction Selection Based Routing Protocol for VANETs in City Environment
## 3 Dynamic Multiple Junction Based Source Routing Protocol  
### Limitations of Existing Protocols  
The protocols that are considered for performance comparison in this work are GyTAR, E-GyTAR and TFOR. The similarity among all these protocols is that all of these protocols are based on dynamic junction selection mechanism. In these routing protocols, one junction is selected at a time based on traffic density and shortest path to the destination. GSR is based on static junction selection mechanism while GyTAR, E-GyTAR and TFOR are based on dynamic junction selection mechanism. Existing literature shows that the protocols that are based on dynamic junction selection mechanism perform better as compared to protocols using static junction selection [2; 3; 11]. The dynamical junction selection protocols (GyTAR, E-GyTAR and TFOR) select one junction at a time for sending packets towards the destination. However, there are a few limitations of this approach. For instance, let us consider one of the scenarios shown in Figure 1. Figure 1 is added to this paper just to explain the possible shortcomings of the existing protocols and how the proposed protocol can solve these shortcomings. It is worth noting that the proposed routing protocol performs well for all the different simulation settings that were used for performance evaluation during our work.  
Suppose source vehicle S is at junction J1 and wants to forward a packet to the destination vehicle D. TFOR, GyTAR and E-GyTAR choose next junction on the basis of vehicular traffic density and shortest path to the destination node, therefore J3 will be selected as the next junction by all the protocols. At J3 GyTAR, E-GyTAR and TFOR will be unable to find next appropriate junction as a next junction through which packet can be routed toward the destination. Because the next shortest path providing streets that lead to destination contain no vehicular density to forward the packet towards the destination. When the packet is routed towards J3, all the protocols are unable to decide next path because from J3 to J6 there is no traffic density. Similarly, J3 to J5 and J3 to J4 there is no traffic density. All these streets are  
Figure 1: Limitations of Greedy Traffic Aware Routing Protocol (GyTAR), Enhanced Greedy Traffic Aware Routing Protocol (E-GyTAR), and Traffic Flow Oriented Routing Protocol (TFOR).  
out of traffic as there is no traffic density for routing packet further. In this case, routing performance is degraded here because of inappropriate selection of junctions due to consideration of a single junction only. The packet delivery ratio decrease and end-to-end delay increases despite the optimal paths (J1-J2, J2-J4, J4-J5, J5-J9) are available with rich traffic density. Consequently, all these protocols are inefficient in junction selection mechanism to decide optimal routing paths on the basis of vehicular traffic density and shortest distance to the destination. Furthermore, their current junction selection mechanism is very limited to move the packet progressively closer to the destination due to the consideration of just the immediate junction from the current junction. The probability of incurring a dead street (street without packet carriers/vehicles) along a selected path to the destination increases while considering just the immediate junction.  
We need a routing protocol that selects junctions efficiently by considering connectivity in such a way that it maximizes packet delivery ratio, minimizes routing overhead and end-to-end delay. We propose a novel routing strategy called dynamic multiple junction selection based routing mechanism, which selects multiple junctions based on shortest path and traffic density for tackling aforementioned like scenarios.