## 4 Results
### Routing Performance Evaluation
#### 4.4.2 Impact of Routing Frequency
Another important fact to note in the results given in Figure 17b is that the proposed routing method's Ethernet utilization is higher while the LTE channel utilization is lower than those of Dijkstra for any routing frequency. The reason for the preceding observation is because the proposed routing method utilizes adaptive flow rule computation, updating, and communicating flow rules to the switches along with an optimization framework for metadata collection, such that it reduces LTE utilization, which has a higher communication cost, while increasing Ethernet communication, since Ethernet communication has a relatively lower cost. As is evident from Figure 17c, the end-to-end latency of routing using Dijkstra slowly increases with the routing frequency, which has a similar gradient to the proposed routing. The reason for the latency increment with routing frequency in Dijkstra is due to more packets being delivered with the increment of routing frequency, as evident from Figure 17d, even though it is a shortest path algorithm that does not take delay into account for its routing decisions. For the proposed routing as well, an increasing trend in average latency can be observed with an increment in routing frequency. One of the reasons for observing an increasing trend with routing frequency for the proposed method is that it has control over delay in its algorithm. Specifically, if the network contention is less than the contention threshold, it switches to the mode with the highest stable least delay, and otherwise switches to the highest stable least distance mode. Thus, due to the selection of the least delay paths by the proposed routing, the latency of the proposed routing is always less than Dijkstra for any routing frequency. Note that routing frequency can be associated with network contention as follows. The network contention depends on other factors such as pending packet transmissions (how packet routing is scheduled) and network link entropy, as explained in the methodology section. Thus, with an increment in routing frequency, there can be an increment in pending transmissions, since pending transmissions from the previous routing cycle can exist in the present routing cycle when the routing frequency is high. Therefore, contention can increase with routing frequency. With high contention, the proposed routing method tends to select the highest stable least distance mode at higher frequencies. The other reason behind the increment in average latency for the proposed method is due to delivering more packets at higher frequencies, as is evident from Figure 17d. Thus, due to the combination of those two reasons, the latency of proposed routing increases with routing frequency. In contrast, for routing using AODV in VANET, latency tends to decrease with increments in routing frequency. When the routing frequency was as low as 0.02 Hz, the average latency of AODV could be as high as 1097 ms. This high latency is due to the fact that routes are discovered in AODV on demand, which costs AODV in terms of latency. Thus, when routes are discovered and then packets are routed in AODV, the latency is higher than routing in SDVN. However, the latency of AODV approaches that of the proposed method at higher frequencies, as the latency gap between the proposed routing and AODV reduces with the increment of routing frequency, as evident from Figure 17c. When analyzing the results for packet delivery ratio variation with routing frequency in Figure 17d, it is clear that both the proposed routing and Dijkstra's PDR increase with increasing routing frequency. This is because, for both the proposed method and Dijkstra, the routes are not computed at the time of routing (they are pre-computed by the controller using the collected metadata), and the increment in routing frequency enhances the PDR of those routing techniques. In contrast, for AODV, the increment in routing frequency degrades its PDR since, at high frequencies, there is less time for discovering routes in AODV.