# Data Collection Based on Opportunistic Node Connections in Wireless Sensor Networks
## 5 Performance Evaluation
### Evaluation Results  
#### 5.2.1 Packet Delivery Ratio  
In this work, the PDR is defined as the ratio of packets received by the mobile sink to packets sent by the sensor nodes, and three scenarios are designed to conduct simulations to evaluate the PDR based on the long radio range \(R_{l}\) of the mobile sink and the network density. In these experiments, we consider two cases that a node will drop the data packet, first, a node cannot find a forwarder due to the asynchronous working-sleeping cycle strategy for sensor nodes. Second, when data packets are forwarded between two adjacent nodes, a node enters sleeping status and causes transmission failure. As shown in Figure 9, the PDR in POFA increases as the long radio range \(R_{l}\) of the mobile sink increases while it decreases in the proposed scheme, ExOR and RPRDC. The reason is that when \(R_{l}\) increases, more nodes under the data collection scope will join the data collection process, and due to the network density keeps unchanged, the number of hops these newly joined nodes takes to forward their sensing data to the mobile sink increases correspondingly, which decreases the success rate of delivering the sensing data in the proposed scheme, ExOR and RPRDC. However, in POFA, the sensing data of a node will be forwarded along multiple paths until one of them connected to the mobile sink, as the number of nodes in this scope increase, the number of the possible paths increases, which improves the success rate of data delivering. From the figure it can be seen that, the proposed scheme performs better PDR (almost one time and 72%) than that in ExOR and RPRDC as \(R_{l}\) expands, the reason is that, on one hand, a node in the proposed scheme always forwards its sensing data along its optimal path that keeps the maximum PC. On the other hand, the routing protocol in the scheme can adjust the optimal path when it is disconnected due to node sleeping, which can further improve its PDR. As shown in Figure 10, the PDR increases as the network density increases, the proposed scheme achieves almost 130% and 102% improvement in PDR over ExOR and RPRDC, respectively. Since the number of working nodes in network increases with the increased network density, it can improve the network connectivity clearly and therefore increase the overall PDR. The proposed scheme outperforms POFA when the number of nodes in network is less than 900, but as the network density increases continuously, POFA performs better. This can be explained by that in POFA, a node needs to select multiple forwarders to achieve its target reliability, and the improvement of network connectivity increases the possibility of achieving this target reliability rapidly, which further improves its PDR. The main reason that the proposed scheme is always performs better than ExOR and RPRDC can be explained by the different routing strategies they employed. Figure 9: The Packet Delivery Ratio (PDR) varies with the long radio range \(R_{l}\) of the mobile sink, in addition, the number of nodes (N) is 1100. In RPRDC, the routing is divided into random dispersion and reliability path exploration, of which the packet reception rate, residual energy, and link quality of the node are considered, but it lacks the global information between the node and the sink. Furthermore, ExOR adopts the metric ETX associated with the link connectivity in network, to reflect the expected transmission times required to forward a data from a node to the mobile sink, and it selects a node with a minimum value of ETX as the forwarder during the data forwarding. However, in the proposed scheme, the metric PC reflects the connectivity of a path directly and the routing protocol can guarantee the data always be forwarded along a path with a max value of PC, so it achieves a higher success rate of data delivering than that in ExOR and RPRDC.