# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 6 Performance Analysis and Comparison
### End-to-End Reliability  
After the COOR scheme is applied, the _PAR_, one-hop reliability, and the end-to-end reliability have been significantly improved especially in far-sink region. The end-to-end reliability of node and partition are compared separately. As can be seen from Figure 17, both COOR(R) method and COOR(P) method have an improvement on end-to-end reliability, which means the data collection rate is increased. When the network itself has certain reliability, the performance of COOR method is significant. As shown in Figure 17a, when the distance to the sink is 200 m, the end-to-end reliabilities of the traditional routing method, COOR(P) method, and COOR(R) method were 0.802, 0.863, 0.923, respectively, which were 7.62% and 15.07% of the increase rate. As the distance to the sink reaches 1000 m, the end-to-end reliability gradually reaches 0.308, while the COOR(P) method and COOR(R) method maintain values of 0.643 and 0.927, which renders a 108.05% and 200.04% increase rate. Compared with Figure 17a, the information of improvement on low-reliability network can be obtained in Figure 17b. Due to the low reliability of the previous network, the selected threshold reference energy is low, which limits the improvement of the network by the COOR strategy. It can be seen that the COOR scheme begins to gradually improve the network reliability until the distance to sink reaching 240 m. However, with a distance of 900 m to the sink, the end-to-end reliability of traditional routing method, COOR(P) method, COOR(R) method were respectively 0.066, 0.166, 0.581, which represent a 149.78% and 774.41% increase rate. This is mainly due to the rapid decline in the reliability of the previous unreliable network and the fact the COOR scheme mainly improves the performance of the far-sink region nodes. Figure 17: _Cont._  
Obviously, in the COOR(R) method, the transmission success rate is significantly improved by the increased transmission power. When the transmission power exceeds a certain value, the reliability of the single hop link will reach 100%. At this point the end-to-end reliability of the current node depends on the end-to-end reliability of the next hop node. Therefore, the end-to-end reliability of the nodes in far-sink area appears periodic, and it will rise at the edge distance (k\(\cdot\)r). Since the reliability (\(\psi_{\mathrm{k}\cdot t+\varepsilon}\)) of the node at \(\mathrm{k}\cdot\mathrm{r}+\varepsilon\) is equal to the reliability (\(\psi_{(\mathrm{k}-1)\cdot\mathrm{r}+\varepsilon}\)) of the node at \((\mathrm{k}-1)\cdot\mathrm{r}+\varepsilon\) multiplied by the reliability of the first hop of the current node (\(\zeta_{1}\)). That is, when \(\zeta_{1}>\psi_{(\mathrm{k}-1)\cdot\mathrm{r}+\varepsilon}/\psi_{\mathrm{k} \cdot\mathrm{r}-\varepsilon}\), the reliability of the node is higher than the near one, but the reliability of the nodes in each cycle will be strictly not greater than the reliability of the nodes in the previous cycle. On the other hand, the COOR(P) method keeps the transmission success rate unchanged and improves the reliability by reducing the number of hops. Consequently, in the near-sink region, the increase of the transmission distance is small, and the improvement of reliability is not obvious. With the gradual increase of the transmission power, the reliability also becomes more and more obvious. Meanwhile, the reliability of the previous network has a greater impact on this method. In Figure 18, the changes in the reliability of different region in the network are shown. As can be seen from Figure 18a, the end-to-end reliabilities in tier1 (T1) are almost same. In the COOR(P) method, the end-to-end reliability of nodes in T1 increased from 0.874 to 0.893, which is 2.23% higher. As for the COOR(R) method, it maintained a reliability value of 0.937 with a 7.31% increase rate.