# Pido: Predictive Delay Optimization for Intertidal Wireless Sensor Networks
## 4 Design of Pido
### Performance Evaluation of Pido
#### 4.4.4 Delay Evaluation  
We evaluate Pido in several aspects: delay distribution, delay vs. network size, delay vs. tidal periods and energy consumption. The results are compared with CTP and ORW. **Delay distribution:** We first compare the system delay distribution among Pido, ORW [10] and CTP [16] in Figure 7. We set the network size as 225 and the average under-water period of each node is 360 cycles (6 h with the system cycle as 1 min). We observe that 80% of delays in Pido is less than 170 cycles while nearly 400 cycles in ORW and 500 cycles in CTP. The result reveals Pido can effectively avoid selecting a submerging node as the next hop with the prediction of nodes future status. We also notice that there is an overlap among CTP, ORW and Pido. This is because one-hop \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Description** \\ \hline \multirow{2}{*}{Network Size} & The number of nodes in the network. \\  & In our simulation, we set the network sizes vary from 49, 81, 121, 169 and 225. \\ \hline \multirow{2}{*}{Tidal Period} & The time duration of a tidal period. \\  & In our simulation, we set the tidal period vary from 60, 120, 180, 240, 300, 360 cycles. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The summary of the simulation parameters. nodes are not affected by the buffer condition and node status, and they can send packets directly to base station. Considering the node status and buffer condition, Pido decreases the system latency in a significant level effectively. **Delay vs. Network sizes:** We then simulate the performance of Pido in different network sizes: 49, 81, 121, 169 and 225. We run the simulation for 10 times as well as CTP and ORW, and the average delay under each network size (average under-water time is set to 360 cycles) is shown in Figure 8a. With the increase of the network size, the average delay is also increased in CTP, ORW and Pido. The reason is that larger scale network also means a longer routing path, so packets will be passed through more hops, which leads to inevitable system delay. Besides this, we also notice Pido decreases an average delay of 69.94% and 43.23% when compared with CTP and ORW among variant network sizes. **Delay vs. Tidal periods:** To evaluate Pido in variant system tidal periods, we respectively simulate Pido with different average under-water periods for each node and the result is shown in Figure 8b. The result is based on two networks with different sizes: 81 and 225 and we set average disconnection time period between nodes from 60 to 360 cycles. The result in Figure 8b reveals the delay in Pido increases slower when compared with CTP and ORW in different network intermittent characteristics. This is because Pido utilizes more system knowledge and selects paths wisely. Notably, the average disconnection time in a real IT-WSN ranges from 240 to 360 cycles. During this time period, Pido can save up to 73% delay (69.72% on average) when compared with CTP. Even compared with ORW, a delay optimization routing protocol, Pido still shows better performance  
Figure 8: The average delay of Pido, ORW and CTP in different scenarios: network sizes and tidal periods. (**a**) The average delay of Pido, ORW and CTP in different network size (average under-water time: 360 cycles); (**b**) The average delay of Pido, ORW and CTP with different system tidal periods (network sizes are 81 and 225). Figure 7: The delay distribution of Pido, ORW and CTP (network size: 225; average under-water time: 360 cycles). in the IT-WSN, which coincides with the analysis in Section 4.4.1. The reason is that ORW is not designed for unpredictable environments like the intertidal zone, and does not take node status into consideration. We also notice when the disconnection time is 240 cycles, the delay in Pido is slightly higher than the 300 and 360 cases, and the delay in CTP also increases faster in this condition.