## 4 Simulation and Evaluation
### Network Energy Consumption and Security  
The influence of nodes' speed on the death time of the network is shown in Figure 9.  
From Figure 9, it can be seen that as the speed of nodes increases in the range of 0-10 m/s, the round number of the first dead node's appearance will increase in the beginning, and then it will decrease in most of the interval. The round number of the 10% dead nodes appearing will decrease as the moving speed increases. The reason is that when the speed of movement of the nodes is too fast, the topology changes violently, and the percentage of the nodes moving across the boundary of clusters will increase. In the topology maintenance stage, the nodes which move across the boundary of the clusters need to maintain links with the new and the previous cluster head simultaneously, so the energy consumption for maintaining the safety of the link will be larger. If we define the round of the first dead node's appearance as the lifetime of the network, the best speed of movement of the nodes is 2 m/s, and at this speed the first dead node appears in round 201. A comparison of the survival nodes' numbers for the three schemes is shown in Figure 10.  
Figure 10: Comparison of the nodes’ survival numbers for three schemes.  
Figure 9: The influence of nodes’ speed on the network death time.  
Figure 10 shows the first dead node in the three methods occurs in the 197 th, 229 th and 255 th round, respectively. The proposed method can extend the lifetime of network as much as 58 rounds more than the LEACH method and 26 rounds more than the LEACH-M method. In addition, as Figure 10 shows, the black line falls faster than the red line and the blue line, that is to say the proposed TCM can make the energy more uniform during clustering. In the proposed method, we consider the residual energy of cluster heads, the distance from the sensor nodes to cluster heads and mobility of the nodes comprehensively in the clustering stage, and this can ensure the selected cluster heads satisfy that the condition that sum of distances between the sensor nodes and cluster heads is minimal and the energy left is maximal.  
In the topology maintenance phase, the TDMA time plot assignment and communication mechanism of soft handover are adopted. TDMA time slot assignment makes the nodes only work in their own time plots, and this wake mechanism can effectively decrease the energy consumption and extend the network lifetime. When entering a new cluster, the moving nodes will be authenticated by the digital signature based on ECC. By this step, the attacks from malicious nodes could be effectively detected. Without this mutual identity authentication, malicious nodes could pretend to be legitimate ones and communicate with the nodes to get critical information about the network, which could seriously affect the network security. After confirming the legitimacy, the moving nodes can send requests to join the clusters, and the nodes will maintain communication links with the previous cluster head and the new cluster head simultaneously until they establishes a stable link with the new cluster. This mechanism can ensure the moving nodes crossing the boundary quickly recover communication, and ensure the continuity of information transmission.