5.3.1. ILC

Using RMVCA, we study and analyze the influence of different numbers of N-SDN nodes on the maximum intercept link coverage (i.e., max-ILC) and the accordingly needed minimum number of SDN nodes in the P set for realizing the maximum intercept link coverage (i.e., numP). Moreover, we take the operator's operation and maintenance cost (i.e., the minimum number of SDN nodes) and network intercept link coverage into account comprehensively, so as to find the best proportion of SDN nodes from the experimental results.

Randomly selecting the number of N-SDN nodes (node i ∈ (0,156)) in CRN topology, we conducted 10,000 experiments in the same proportion of N-SDN nodes. Due to the different network topologies under the same SDN node proportion, the results of each experiment are different. The statistical results of 10,000 experiments are shown in Figures 6 and 7.

**Figure 6.** The influence of different numbers of N-SDN nodes on max-ILC.

**Figure 7.** The influence of the number of N-SDN nodes on numP.

Figure 6 shows the influence of different numbers of N-SDN nodes on max-ILC. From the figure, we can see that the number of SDN links in hybrid SDN decreases gradually with the increase of the number of N-SDN nodes (the decrease of the number of SDN nodes), resulting in the gradual decline of the network intercept link coverage. And max-ILC = 0.00% denotes that all links in the whole network are N-SDN links that cannot be intercepted, namely, all nodes in the network are N-SDN nodes. Additionally, we can see that the intercept link coverage of the whole hybrid SDN can reach 80.53~100% when the number of N-SDN nodes is between 0 and 57 (i.e., the number of SDN nodes is between 99 and 156), namely, only when the number of SDN nodes in hybrid SDN is more than 99 can SDN nodes intercept more than 80% of the links of the whole network.

Figure 7 shows the influence of different numbers of N-SDN nodes on numP. From the figure, we can see that when the number of N-SDN is 0 (i.e., the number of SDN nodes is 156), 79 SDN nodes are required to achieve the maximum intercept link coverage; the number of SDN nodes required to intercept the whole network gradually increases first and then decreases gradually. This is because that when the number of N-SDN nodes is between 0 and 37 (i.e., the number of SDN nodes is between 119 and 156), though the increase of N-SDN links results in the decrease of the degree of some SDN nodes, the total number of SDN links does not decrease significantly. Thus, more SDN nodes are needed to intercept the same number of links. Accordingly, the number of SDN nodes required to intercept the whole network increases. While when the number of N-SDN is between 38 and 156 (i.e., the number of SDN is between 0 and 118), the number of SDN links greatly decreases with the increasing number of N-SDN nodes, so the number of SDN nodes needed to achieve maximum intercept link coverage also decreases gradually. Moreover, when all nodes in the network are N-SDN nodes, all links are N-SDN links, and thus the minimum vertex cover set *P* is empty (i.e., numP = 0). To sum up, according to Figures 4 and 5, we only need 69~95 SDN nodes to achieve 80.53~100% intercept link coverage of the whole interception system when the number of SDN nodes in the whole hybrid SDN is between 99~156.

Next, we will study and analyze the influence of three different approaches and three SDN interception models on intercept link coverage (ILC) as shown in Figure 8. From the figure, we can see that ILC of PA and HA with RMVCA is higher than that of RA without RMVCA in general, and ILC of PA and HA are relatively close, whether using T model, ECMP-T model or Fermat-point interception model. Additionally, compared with RA, PA and HA can significantly improve the intercept link coverage when the number of N-SDN nodes is between 0 and 60 (i.e., the number of SDN nodes is between 96 and 156). And this improvement decreases with the decrease of SDN nodes.

Meanwhile, another conclusion we can make is that the three SDN interception models have nearly the same intercept link coverage. In other words, the intercept link coverage (ILC) has no relationship with SDN interception models and the SDN interception models have little impact on ILC.
