## 5 Performance Evaluation and Discussion
### Study on Mobile Sink Sojourn Location  
We assume that mobile sink nodes are placed on public buses moving along the perimeter of the sensing field. Mobile sink nodes will start successively from the same position. Here, we analyze the performance of the sensor network with two mobile sink nodes in detail.  
The network topology is shown in Figure 8, where sensor nodes communicate with mobile sink nodes in a multi-hop manner based on the value of the link cost. Mobile sink nodes move along the  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Network Environment** & **Node ID** & **Round** \\ \hline Three mobile sink nodes & 78 & 4004 \\ Two mobile sink nodes & 5 & 2994 \\ One mobile sink node & 98 & 2490 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Round when the first node dies.  
Figure 7: Simulation results with different mobile sink number. (**a**) Residual energy comparison; (**b**) Network lifetime comparison.  
periphery of the rectangular region, and these sensor nodes can adjust their transmission range based on the value of \(d_{nn}\) and the value of \(d_{nn}\). Here, \(d_{nn}\) is the distance between sensor nodes, and \(d_{sn}\) is the distance between sensor nodes and mobile sink nodes. According to the different value of \(d_{sn}\) sensor nodes transmit data by multi-hop. Mobile sink nodes will stay at the special position long enough to collect data packets.  
Sojourn locations \(SL_{i}\) are selected from parking positions which are determined in the previous experiments. Each mobile sink node needs to choose one parking position as its sojourn location to sojourn and wait for data collection. The influence on performance of the sensor network where mobile sink nodes chose different sojourn locations is shown in Figure 9.  
In Figure 9a, we can observe the residual energy of the network over a period of time and find that the least energy is consumed when the interval of mobile sink nodes is two parking positions. Energy  
Figure 8: Network with two mobile sink nodes.  
Figure 9: Simulation results with different sojourn location. (**a**) Residual energy comparison; (**b**) Network lifetime comparison.  
is consumed most when there is no interval between parking positions, which can be viewed as a continuous movement pattern. After 71 rounds, the performance using three intervals is superior to that using one interval between parking positions. Thus, we choose the two intervals to improve energy efficiency. In Figure 9b, we compare the network lifetime using different parking positions intervals. Each mobile sink node will chose different parking positions to sojourn in one round. Similarly, it can be observed that network performance with a two parking position interval is the best.  
Table 2 shows the corresponding round and node ID when the first sensor node depletes its energy. The network lifetime is the longest when the mobile sink node interval is two parking positions. It is about two and half times the lifetime of the case when the sink node moves continuously. The first failed sensor node appears in the 7087\({}^{\text{th}}\) rounds, so it lasts longer than the others. Thus we can conclude that when the interval of parking positions is set to 2, the network performance will be superior to the others.