# Pido: Predictive Delay Optimization for Intertidal Wireless Sensor Networks
## 3 System Deployment and Motivation
### Delay Problem in IT-WSN  
Unlike other WSN systems such as [28; 29], the communication in the IT-WSN is greatly impacted by environment. For example, radio signals in water strongly attenuate, creating asymmetric communication links, and the changes of high tide and low tide have tremendous effect on system performance as well, especially on end-to-end delay and PDR. Figure 2a,b provide an overview of  
Figure 1: The illustration of the intertidal wireless sensor network in Zhoushan. (**a**) Nodes deployment with low tide in Zhoushan; (**b**) High tide in Zhoushan, all nodes are submerged; (**c**) The hardware design of the sensor node. the statistical delay and PDR in the IT-WSN. We evaluate the average delay and PDR of all 26 nodes in the Zhoushan system, and we select eight nodes who cover a wide area with different altitudes as representatives. It is worth mentioning that we count delays in system cycles, which means if a packet is generated at n-th cycle and received by the sink at m-th cycle, the end-to-end delay of this packet is \((m-n)\) cycles. We classify delays in IT-WSNs into three categories: queuing delay caused by intermittent connections, queuing delay caused by buffer occupancy, and transmission delay caused by poor link quality. **Queuing delay caused by intermittent connections. ** Nodes are submerged by sea water for hours during the high tide period, and when nodes are under-water, the wireless channel is unavailable due to signal attenuation. The signal attenuation can be calculated by \(\alpha=0.0173\times\sqrt{f\times\sigma}\), where \(f\) is the radio frequency in Hertz, and \(\sigma\) is the conductivity of the seawater (typically 4 mhos/m). Consider a sensor node operating at 916 MHz, the attenuation under-water is 1046 dB/m, which results in a communication range of less than 12 cm at a transmission power of 10 dBm. Based on this, nodes can only send packets during the low tide period. Nodes are submerged during the high tide period, and packets can only be sent until the high tide period comes, and an extremely large delay is introduced. Though delay caused by intermittent connections cannot be avoided in the IT-WSN, it is possible to decrease it by choosing a better route considering the availability of nodes. **Queuing delay caused by buffer occupancy. ** Nodes store packets locally during the under-water phase, thus buffers are strongly possible occupied. Together with the poor link quality and accumulated packets, newly arrived packets have to wait for a long time to be sent. Figure 2c shows the average buffer occupations of all nodes and eight representative nodes in the Zhoushan system, and reveals that buffers are heavily occupied. The heavy buffer occupancy also brings about packet drop as shown in Figure 2b and severely impacts the system performance. Therefore, buffer condition should be considered for delay optimization. **Transmission delay caused by poor link quality. ** During the above-water period, the IT-WSN is supposed to be similar in stability and efficiency to a normal WSN. Unfortunately, the link quality of the IT-WSN is much worse than that in stable WSNs (e.g., terrestrial WSNs), and leads to non-negligible delays. To have a deep understanding of the link quality in the IT-WSN, we compare the ETX (expected transmissions) of routes in an indoor WSN and in the Zhoushan system. ETX proposed in CTP [16] measures the quality of a path, and the value of ETX reveals the expected times of transmissions of a packet from the source node to the destination node. A smaller value of the ETX represents a routing path with less cost. To obtain a similar deployment environment, we deploy 26 nodes in the indoor WSN, which work in the same operating mode with nodes in the Zhoushan system, and the distances between them vary from 3 m to 30 m. Figure 3 reveals the CDF (cumulative distribution function) of the  
Figure 2: The delay, PDR and buffer condition in an IT-WSN in Zhoushan, China.