# Development of Digital Device Using ZigBee for Environmental Monitoring in Underground Mines
## 5 Discussion  
In this study, data were acquired from all the sensor modules installed in an underground mine by constructing a WSN using ZigBee. After numerous experiments in the laboratory, and trial and error at the underground mining site, the applicability of the developed technology was improved. However, some modules still exhibited low network connection stability, depending on the installation location. To properly install a ZigBee-based sensor at an underground mining site and acquire stable data, we considered and improved on the following issues. 1. The vibration measurement module was set as an end device to minimize power consumption in the preliminary experiment. Unlike a router, which can send and receive data, the end device can only transmit the acquired data. Therefore, the number of routers was small compared to that of the end device; hence, the network connection was easily disconnected, and data acquisition was unstable in some vibration measurement modules. To address this, in this study, the number of router modules was increased by setting the vibration measurement module as a router, and the communication stability was improved. 2. In the preliminary experiment, by setting the vibration measurement module as an end device and applying the sleep mode, the battery life was increased, compared to supplying power at all times. However, when the power was turned off and on during the sleep mode, the connection to the WSN was weaker than when the power was constantly supplied. The stability of the network is generally low in underground mines compared to laboratories. Because the underground power supply is a crucial issue, if a permanent power supply is impossible, it is necessary to consider a method  
Figure 10: Spatial distribution of dust measurement results at 5 min intervals in the drift of Samdo Mine. of reducing battery consumption in sleep mode. However, for this, it is a prerequisite to maintain a stable network through the optimal arrangement of modules. 3. The sensors were arranged as linear or branched, considering the shape of a long mine shaft. Although the arrangement of the sensors appears to be a simple linear arrangement, in reality, multiple sensors can be directly connected to each other because the distance between adjacent modules was approximately 30-50 m. However, it is not possible to form a complex network. Increasing the number of router modules can be a solution to configure a more stable network. Thus, the stability of the network can be maintained even if the intermediate node does not function properly; however, this means that the required capital increases. Moreover, if continuous power supply through wires is difficult, it increases the number of devices to be replaced with batteries. Therefore, we must determine the optimal module arrangement by simultaneously considering the stability of the network, budget, and power supply possibility. 4. Owing to the nature of ZigBee, only one coordinator module can exist in a network, and the data transmitted from each node is concentrated in the coordinator module. Therefore, because there are many sensor modules in one network, the data load increases and may exceed the limit that can be processed by the coordinator. To solve this problem, there is a method for configuring multiple networks by increasing the number of coordinators, and the data load can be reduced by adjusting the sampling rate. Increasing the number of coordinators can be an effective method, but it requires multiple storage devices to be installed underground. Adjusting the sampling rate is useful when acquiring data at short intervals is unnecessary; however, in the opposite case, it is less useful. Therefore, we must select the optimal number of coordinators and sampling rate according to the purpose of monitoring and the environment of the underground mine. 5. Regardless of how well the network is configured, communication cannot be fully performed if the power of each device is insufficient.