# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 6 Performance Analysis and Comparison
### Transmission Delay  
Since the hop count is adopted as a measure of delay in this paper, the delay of the node does not change without changing the network structure, which is determined by the transmission distance. That is, the COOR(R) method has an exactly the same delay as the traditional routing method. In fact, however, the COOR(R) method reduces the time for each hop and therefore has a smaller improvement effect on network delay. This section mainly shows the delay performance comparison between the COOR(P) method and the previous routing method. In Figure 19, the changes of the end-to-end delay of nodes after using COOR(P) method are compared at different distances. As shown in Figure 19a, the delay decreased from nine hops to five hops, and the decrease was 80% at 500 m to the sink. When the distance to the sink is 1000 m, the end-to-end delay of traditional routing method and COOR(P) method are respectively 17 hops and seven hops, which is 142% decrease rate. At the extreme edge of the network, the delay of the node may be lower than that of the near node, which results from the different levels of increase in transmission power. Similarly, when optimizing the unreliable network, the COOR(P) method will also have a limited improvement effect on delay. According to Figure 19b, the end-to-end delay of traditional routing method and COOR(P) method were respectively seven hops and six hops, which was a 16.7% decrease rate with a distance of 500 m to the sink. Meanwhile, when the distance to the sink reached 1000 m, the delay decreased from nine hops to five hops, and the decrease was 80%. Compared to the situation with \(r=60\)\(N=2\), the performance improvement is much smaller. As mentioned above, in the COOR(P) method, the farther the distance from the sink in the network is, the higher the transmission power of the node is. Meanwhile, because the transmission  
Figure 18: (**a**) End-to-end reliability of different region under \(r=60\), \(N=2\); (**b**) End-to-end reliability of different region under \(r=70\), \(N=3\); (**c**) End-to-end reliability of different region under \(r=80\), \(N=4\); (**d**) End-to-end reliability of different region under \(r=90\), \(N=4\). success rate is kept constant, the greater the transmission power, the farther the transmission distance will be. Consequently, the delay of the nodes in the far-sink region is greatly improved. Furthermore, the changes in the delay of different region in the network are shown in Figure 20.  
As shown in Figure 20a, the delay in traditional method increased linearly, while under COOR(P) method the delay increased more slowly and were kept at a certain value in the far-sink region. The end-to-end delay of nodes in T1 decreased from 1.968 to 1.553, which is 21.09% lower. As for the far-sink region (T5), the delay of traditional routing method and COOR(P) method were respectively 10.512, 4.994, which represent a 52.48% reduction rate. Figure 19: _Cont._  
Figure 19: (**a**) Delay of node at different distance under \(r=60\), \(N=2\); (**b**) Delay of node at different distance under \(r=70\), \(N=3\); (**c**) Delay of node at different distance under \(r=80\), \(N=4\); (**d**) Delay of node at different distance under \(r=90\), \(N=4\). Figure 20: _Cont._  
Similarly, the unreliable networks also limit the improvement of delay performance. In Figure 20b, the decrease rates are respectively 20.58%, 21.48%, and 35.68% in T1, T3 and T5. Compared to the circumstance in Figure 20a, the delay reduction in the traditional routing method naturally results from the longer transmission distance in each hop, while the delay increased (still lower than the traditional method) mainly due to the relatively small reference threshold energy. Specifically, when the transmission distance increased with the same transmission power, the reliability of communication links decreases, which renders a smaller data volume in the network. As a consequence, the small reference threshold energy limits the optimization of the COOR(P) method.