## 2 Materials and Methods
### Proposed Wireless Network
(1200 m). This range depends on free-air terrain with few interference sources. The actual range varies depending on the transmitting power, orientation of the transmitter and receiver, height of the transmitting antenna, height of the receiving antenna, weather conditions, interference sources in the area, and terrain between the receiver and transmitter, including walls, trees, buildings, hills, and mountains. To obtain a strong communication signal and data transfer success rate for a practical multi-hop network deployment, we propose the following solution for deploying each sensor node in the test field. Since the receiver sensitivity of the Xbee3 micro-module is \(-103\) dBm with an acceptable packet error rate, the RSSI signal level at each hop will be monitored first. In this work, the RSSI of each link is kept higher than \(-93\) dBm (\(-10\) dBm from the receiver sensitivity) by adjusting and controlling the distance between two nodes that satisfy the RSSI level. Additionally, when all links are deployed, the RSSI levels of all links and the success rate of the data transmission from the transmitter to the receiver are again checked. We stop the process if the success rate reaches 100%. If not, the link with a lower RSSI level or high possibility of a weak signal will be rechecked and adjusted. This deployment process is also demonstrated in Figure 2, where two-way communications (i.e., transmitter-to-receiver (for data packet transmission) and receiver-to-transmitter (for command and acknowledgement packet transmission)) are considered, as in Figure 2a, and the deployment processes are illustrated in Figure 2b. The final deployment result is shown in Figure 2c, where all hop distances have been adjusted according to the environment of the test field, threshold requirements, and PDR achievement. We note that \(-93\) dBm is the illustrated threshold. It can differ depending on the test fields or applications, environmental/weather conditions, user needs, and the particular goal of each task. Finally, Figure 2d summarizes the flowchart for the node deployment process. We should keep a strong RSSI level since, at the border of the communication range, the RSSI can fluctuate and the packet can be lost [14, 15]; so, to guarantee reliable communications, this issue should be taken into consideration. Additionally, in this work, we also considered the placement of the transmitter, the relays, and the receiver, since the transmitter and the relays can be sensing sources, which should be at the optimal sensing positions, and the receiver should be placed at the optimal monitoring position to upload data to the cloud or be connected to the internet and IoT. Figure 2: _Cont._Figure 2: \(Cont\). ## 3 Experiments

As we mentioned before, our multi-hop WSN system was tested in two environments: mountainous forests and watercourses. The test environments were in a part of the Kho Hong Mountain in Hat-Yai City, Thailand. Test scenarios #1 and #2 are shown in Figures 3-5. In test scenario #1, we placed all nodes in the mountainous forest environment, where the transmitter was located at the top level of the mountain (above the sea level of 75 m) and the receiver was at the lower level (above the sea level of 5 m). In this test field, the location of the sensor nodes was on sloping terrain, and there were many small and big trees, which could affect the radio signal propagation. In test scenario #2, all nodes were placed at the watercourse. The transmitter node was located at the top, near the mountain, while the relay nodes were along the watercourse and the receiver was near the reservoir. Test scenario #1 can be referred to as the environmental monitoring application since, in such a test area, forest fire protection, landslide monitoring, and plant and animal reservations are the city's requirements. Test scenario #2 can be referred to as the flooding monitoring application since, during the rainy season, the huge amount of water from the mountain floods to the lower area, which has an effect on the city's population.