# Data Collection Based on Opportunistic Node Connections in Wireless Sensor Networks
## 5 Performance Evaluation
### Evaluation Results
#### 5.2.2 Energy Consumption  
In this section, two scenarios are designed to evaluate the performance of EC in network. In the first scenario, we evaluate the EC for a source node to forward its sensing data to the mobile sink successfully. In the second scenario, we evaluate the total EC in network varies by time. Figure 12 shows the EC for a source node to forward its sensing data to the mobile sink varies by the network density. In this work, we select the source node whose distance to the mobile sink is half of \(R_{l}\) to conduct this experiment and the EC in this work only reflects the energy consumed by the forwarding process when the source node sends its sensing data to the sink successfully. Lots of tests show that the EC increases as the network density expands and it increases more obviously in POFA than that in the proposed scheme, ExOR and RPRDC. The main reason is that the increased number of nodes increases the hops a source node takes in date delivering, as shown in Figure 13, which further increases the EC. Figure 11: The PDR varies with the network density, in addition, \(R_{l}=283\) m.  
Figure 12: The Energy Consumption (EC) varies by the network density, in addition, \(R_{l}=110\) m.  
The EC in the proposed scheme is slightly higher than that in ExOR, and they are much lower than that in POFA. The reason is that, in POFA, its data forwarding strategy increases its PDR by selecting multiple forwarders but at the expense of energy, as network density expands, the improved network connectivity increases the success rate of each hop of data forwarding, which further increases hops and the times of reliability calculation it takes, so that the EC in it is always high. In the proposed scheme, the neighbor of a node increases as the network density increases, when a node on the optimal path adjusts the path due to the next hop node is in sleeping status, it needs to select an optimal neighbor that can maximized the PC of the path from it to the sink, this process increases the times of calculation by using the PCs of all neighbors, which further increases the EC. Figure 14 shows the EC for a source node to forward its sensing data to the mobile sink grows with the long radio range \(R_{l}\) of the mobile sink increases. The node whose distance to the mobile sink is half of \(R_{l}\) is selected as the source node. With the increasing of \(R_{l}\), the distance from the source node to the mobile sink increases, so the hops it takes in data forwarding also increases, as shown in Figure 15, which further increases the EC. Figure 14: The EC varies by the long radio range \(R_{l}\) of the mobile sink, in addition, the number of nodes (N) is 1100. Figure 13: The Hops varies by the network density, in addition, \(R_{l}=110\) m.  
The EC in the proposed scheme is slightly higher than that in ExOR, but they are much lower than that in POFA. In POFA, each node selects multiple forwarders to achieve the target reliability until one of them connected to the mobile sink, the number of forwarders increases greatly as \(R_{l}\) expands, so the increased data delivering and reliability calculation increases its EC. In the proposed scheme, with the expansion of \(R_{l}\), the network density keeps unchanged but the distance from the source node to the mobile sink increases, the success rate of data delivering decreases, which further increases the times of calculating the PC to adjust the data forwarding path. This path adjustment scheme can improve its PDR but increase its EC. Figure 16 shows that the total EC increases by time, in which the mobile sink launches multiple data collection tasks anywhere in network randomly, the proposed scheme consumes 55%, 80% and 61% less energy than that in ExOR, POFA and RPRDC. Here, the energy consumption of nodes is calculated regardless of whether source nodes send their sensing data to the sink successfully or not. In the proposed scheme, the total EC is mainly consist of the energy consumption caused by the probe message forwarding and the sensing data delivering.