# LESS: Link Estimation with Sparse Sampling in Intertidal WSNs
## 6 System Implementation
### Sensor Design  
In Figure 9a, we show that the MAC protocol for our system is CSMA and PHY layer specification is IEEE 802.15.4. LESS is proposed as a link quality metric estimation method, which can be incorporated into existing routing protocols. Figure 8: An example of routing path update process with three methods. The directed links represent the information transmission flow. The value on links (including dashed lines) indicate the transmission cost and nodes select the routing paths with the minimum total transmission cost. The dashed lines with arrows are the transmission paths of update packets. (**a**) A simple network topology; (**b**) sending update packets individually to _nodeB_ and _nodeC_; (**c**) sending update packets with a decreased packet’s length; and (**d**) sending compressed update packets with a decreased packet’s length. CSMA illustrated in 802.15.4 is easily implemented and widely utilized in environmental monitoring WSNs, including the soil moisture sensing system [35], carbon dioxide monitoring system CitySee [61], and the ocean monitoring system Oceansense [28]. Besides CSMA, TSCH (proposed in 802.15.4e) combines time slotted access and channel hopping and is widely utilized in industrial and vehicular applications. Though TSCH provides energy efficiency services, utilizing TSCH in IT-WSNs in our scenario is challenging. First, TSCH requires tight system synchronization [62]. The IT-WSN can only provide a rough synchronization service with a calibration error around 100 ms due to different clock skew. Intermittent connections caused by tide waves magnify the condition of the time drift among sensors and make time synchronization mechanism deficiency. To implement TSCH in IT-WSN, an elaborately designed time synchronization mechanism is required. On the other hand, TSCH needs re-schedule the time slots when a new node appears, or an existed node disappears, and brings a considerable overhear, especially in IT-WSNs. In IT-WSNs, the changes of available nodes are frequent, thus nodes need to re-schedule TX slots and RX slots frequently and cause extra energy. Based on this, we select CSMA as MAC layer in IT-WSNs. Nevertheless, we wish to investigate the possibility of TSCH in IT-WSNs in future study. Figure 9b shows a mussel-shaped sensor we designed and deployed near a bed of real mussels. As shown in Figure 9c, the sensor board is encapsulated by black epoxy for waterproof. There are several reasons for us to keep sensor nodes under water:  
* Several existing methods keep radio transceivers above water using buoys while keeping sensors underwater. For example, SmartCoast [63] is a wireless sensor network for monitoring water quality utilizing such a method. However, these methods are supposed to operate in lakes, rivers, or estuaries, instead of in intertidal zones where strong waves appear constantly. The strong waves may destroy the cables between the above-water transceivers and the under-water sensors. Besides, buoys will run aground during low-tide periods, and there are high risks that buoys will not float up successfully when high tide comes. * Intertidal WSNs are designed for monitoring the environmental parameters such as temperature precisely. As a result, sensors should be as close to the creature's microhabitat as possible, e.g., attached to the rock substrata wherein mussels live. In this condition, connecting sensor nodes to transceivers with wobbly cables will destroy the natural environments of the creatures. Note that to minimize the impact of sensor nodes in our system, we specially designed the sensor nodes as shown in Figure 9c. Figure 9: The design of the sensor node: (**a**) the diagram of the sensor node; (**b**) how we deploy sensor nodes in the intertidal area; (**c**) a customized mussel-shape sensor node which is designed for environmental monitoring, and the node is covered by epoxy for waterproof; and (**d**) the electrical components of the sensor node.