## 5 Performance Evaluation
### Simulation Results  
#### 5.3.1 Hot Spot Ratio  
In this simulation, we evaluate the hot spot ratio of two protocols varying traffic bit rate and hello packet interval. Hot sport ratio indicates how many nodes among all nodes become hot spots during simulation. The purpose of this simulation is to measure how effectively comparative protocols can prevent the temperature rise while maintaining the other performance metrics: packet delivery ratio and end-to-end delay. Thus, it is important to determine the weight of the temperature metric efficiently to prevent the performance degradation of other metrics.  
As illustrated in Figure 4a, the hot spot ratios of two protocols gradually increase as traffic bit rate increases. Even though two protocols perform load distribution based on the temperature of neighbor node, there are some performance differences. Especially, the hot spot ratio of MTARP becomes higher and higher as the traffic bit rate increases. The reason is that each source node tries to forward the data packet to a node which locates near to the destination rather than a node with lower temperature. It is brought that the weight of the hop count metric is higher than other metrics. On the other hand, if the temperature of the node with lower hop count is high, it will not be selected as a next hop. However, higher traffic load makes it difficult to prevent the temperature rise of the node with lower hop count.  
On the other hand, EMTARP determines the relative importance of the temperature metric according to the type of nodes constituting the network. From Figure 4, we can observe that the hot spot ratio of case 1 is lower than case 2 because the weight of the temperature metric is higher than that of case 2. Based on this result, it is required to increase the relative importance of the temperature metric if we want to prevent the temperature rise of the node. However, the objective of our protocol is to improve the overall performances by determining the relative importance of the metrics efficiently. Especially, if the network consists of critical medical sensors, we have to increase the weight of the link metric to prevent the packet loss due to node movement. However, the hot spot ratio may affect the packet delivery ratio and end-to-end delay because the number of available routing paths decrease as the number of hot spot increases. These side effects are described together in the following subsection.  
In addition, the hello packet interval also affects the hot spot ratio. In Figure 4b, we can see that all comparative protocols show the best performance when the hello packet interval is set to 5 s. Since the neighbor table is frequently updated as the hello packet interval is shorter, it is advantageous to be sensitive to the temperature rise of the neighbor node. However, as mentioned before, since the temperature rise of the node is most influenced by radio signals, the hot spot ratio increases as the  
Figure 4: Performance comparison for difference traffic bit rate and hello packet interval. (**a**) Impact of varying traffic bit rate on hot spot ratio, (**b**) Impact of varying hello packet interval on hot spot ratio.  
hello packet is transmitted frequently. On the other hand, if the hello packet interval is set to long, it is difficult to respond quickly to the temperature rise of the neighbor node, so that the hot spot ratio increases gradually.