## 4 Results
### Routing Performance Evaluation
#### 4.4.2 Impact of Routing Frequency  
In this experiment, a similar urban transportation scenario with 40 automobiles and 20 RSUs in the simulation and a maximum vehicle speed set to 60 km/h were used. We set \(\mathcal{C}_{th}=0.5\) and \(T_{th}=0.2\), as experimentally found in Section 4.4.1. We set the data collection frequency (\(f\)), the nominal optimization frequency (\(f^{\prime}\)), and the routing frequency (\(f^{\prime\prime}\)) equal to each other. Therefore, the variables in this experiment for all architectures were \(f\), \(f^{\prime}\), and \(f^{\prime\prime}\), which were synchronized together. The results obtained accordingly are shown in Figure 17.  
The proposed link stability-based routing method had the least communication cost, compared to Dijkstra and AODV for any routing frequency, as evident from Figure 17a. The average communication costs of proposed routing and Dijkstra increase with increasing routing frequency. However, the gradient of cost increment in increasing order is the proposed routing and Dijkstra, as evident from Figure 17a. The gradient of cost increment with frequency is much lower in magnitude in Dijkstra and the proposed routing compared to the magnitude of the gradient of cost reduction in AODV with increment of frequency. The communication cost per data gathering cycle is much higher in AODV at lower frequencies. That is due to the fact that AODV is a routing protocol in VANET that sends packets for discovering routes during the entire routing cycle, which results in very high communication costs when the routing cycle period is long (routing frequency is low). On the other hand, for routing in SDVNs (Dijkstra and proposed routing), metadata are collected by the controller only once for the entire routing period, such that the communication cost is much lower than AODV. However, the average communication cost of AODV approaches routing in SDVNs when the routing frequency is higher. The average communication cost of Dijkstra is higher than proposed routing, as Dijkstra uses a proactive approach for flow rule installation, while the proposed method uses an adaptive approach that causes a lower communication cost due to the lesser number of FlowMod packets unicasted from controller to switches. Figure 17: Performance evaluation of the routing frameworks under various routing frequencies. (**a**) Communication cost fluctuation for diverse routing frequencies. (**b**) Channel utilization fluctuation for various routing frequencies. (**c**) Latency fluctuation for diverse routing frequencies. (**d**) Packet delivery ratio fluctuation for diverse routing frequencies. Next, we inspected the variation in channel utilization with respect to routing frequency. As evident from Figure 17b, all channel utilization for routing in SDVNs increases with frequency, while DSRC channel utilization for routing using AODV decreases with routing frequency. Ethernet and LTE channel utilization is always zero for AODV; thus, it does not vary with routing frequency. The DSRC utilization of AODV is closer to 100% at low routing frequencies, and it drops to 30.5% at 5 Hz. The reason for decreasing DSRC utilization with frequency in AODV is that, under low frequencies, it has enough time for route discovery and forwarding packets in the discovered routes. On the other hand, for the proposed routing method, the DSRC utilization was as low as 1.2% at 0.02 routing frequency, and gradually increased to 78.6% at 5 Hz routing frequency. For routing using Dijkstra, the DSRC utilization was very low at low routing frequencies, and it gradually grew to 12.4% by 5 Hz routing frequency. The reason for growing DSRC utilization with routing frequency in routing methods for SDVN is that the percentage of time during which the data plane routing takes place increases when the routing cycle period decreases (routing frequency increases).