## 3 Results and Discussion
### Simulation Evaluation  
In this section, we use OPNET14.5 to analyze the performance of our routing algorithm, and compare it with efficient AODV without SDN [6], SVAO with SDN [24], and SDGR with SDN [23] in the same urban scenario. The simulations were carried out considering a roadway field having a \(10\times 5\) km rectangular shape, and the running time was set to 5 min. As shown in Figure 5, there are three \(4\) km \(\times\)\(100\) m one-way lanes named A, B and C, and assuming that A and B are in opposite directions while C is perpendicular to A and B. There are four RSUs placed near four corners of the rectangular simulation area, which ensures that all vehicles in the area are within the coverage of SDN network. To estimate the impact of density on the performance of the routing protocols, we varied the number of nodes from 15, 30, 50, and above 70 nodes. The network environment parameters are shown in Table 1. \begin{table}
\begin{tabular}{l l} \hline \hline
**Number of Nodes** & **15, 30, 50, above 70** \\ \hline Total simulation time & 300 s \\ Area size & 10 \(\times\) 5 km \\ Packet size & 1024 bytes \\ MAC protocol & IEEE 802.11 p \\ Mobility Model & Random Waypoint \\ Vehicle moving speed & 15–20 km/h \\ Vehicle max speed & 25 km/h \\ Radio range & 500 m \\ Link alive time & 5 s \\ Broadcasting cycle & 1–1.5 s \\ Max distance & 2 km \\ Protocols & Efficient AODV, SVAO, SDGR, the proposed algorithm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. Figure 5: Simulation scenario. The performance metrics that we considered were routing overhead, average end-to-end delay, packet drop ratio, and average throughput. _Routing overhead_: Ratio of the routing packets generated to the total data packets delivered to the receivers. _Average end-to-end delay_: Average delay experienced by a data packet across the inter-vehicle network from one source vehicle to another destination. _Packet drop ratio_: Ratio of the number of data packets lost at the destinations to the number of data packets generated by the source vehicle. _Average throughput_: Average rate of successful packets (including routing packets and data packets) delivery over the inter-vehicle network. Figure 6 shows the performance comparison of routing overhead for efficient AODV, SVAO, and SDGR and the proposed algorithm. From the figure, it can be seen that the proposed scheme outperforms the other three reference schemes, consistently maintains around 0.21 in all scenarios: 15, 30, 50 and above 70 nodes. The SDN controllers keep a global record of updated information due to the cooperation among the local controllers and vehicles. The selection of local controllers at different depths from the SDN controllers reduces unnecessary propagation of control messages; thereby the overhead ratio of SDN-based schemes shows better performance than the non-SDN-based approach i.e., efficient AODV. On the other hand, the distance-based selections in the reference protocols SVAO and SDGR, show performance degradation in terms of overhead ratio, as the stability is not assured. Figure 7 illustrates the comparison between average end-to-end delay as a function of the number of vehicles. As can be seen from the figure, with an increasing number of vehicular nodes in the network, the end-to-end delay decreases. The pattern is the same for all the schemes, since the connectivity in the network will increase with an increase in vehicular nodes. Obviously, SDN-based protocols show much less end-to-end delay than the efficient AODV because applying the local controllers can reduce the network delay. In addition, the SVAO and SDGR select the next node based on distance only, whereas the proposed scheme calculates metrics including forwarding probability, link duration, and wireless bandwidth allocation. Hence, our target is to maintain network stability by providing the optimized route between source and destination, as a result, the proposed scheme shows less end-to-end delay than other two SDN-based schemes.