## 3 System Overview  
Dynamicity, well-distributed networking, and the simultaneous interaction of multiple devices for the collection, analysis, and sharing of information are well-known features of IoT [29]. These characteristics make IoT different from and more highly regarded than previous smart systems, and they also enable the development of IoT-based early-warning systems. To effectively prevent mine accidents and to enhance safety, the system must (i) provide useful monitoring data, (ii) track miners working inside the mine, and (iii) share information in real time. Mine safety systems based on IoT seamlessly integrate gas sensing monitoring, miner tracking, and cloud computing to create an intelligent loop of safety through analytics. In this study, IoT utilizes BLE for transmitting data from Arduino modular sensors. The cloud is helpful for determining the service state of the mine and for sharing information. Thus, seamless integration can result in a reduction in mine accidents. In short, a lack of closely-coupled information sharing between \"things\", miners, and mine management make it difficult to generate a quick response to emergencies occurring in underground mines. Therefore, integration based on the IoT concept is necessary for effective data collection, data analysis, automatically detecting abnormal events, and information sharing. Figure 1 shows the integration of different technologies used in early warning integrated system using IoT for underground coal mines.  
The mainframe of IoT has four characteristic layers: a perception layer, a network layer, a middleware layer, and an application layer. Each layer has its own functions in the main framework of IoT. The layered structure of IoT-based event detection and early-warning systems for underground coal mines is illustrated in Figure 2, which clearly indicates the function of each layer.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Previous Work**} & \multicolumn{2}{c}{**Standard**} & \multicolumn{2}{c}{**Intelligent Event**} & \multirow{2}{*}{\begin{tabular}{c} **Miner** \\ **Tracking** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Priority** \\ **Transmission for** \\ **Emergency** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Distributed** \\ **Processing** \\ \end{tabular} } \\ \cline{1-1} \cline{5-6}  & & & & & \\  
The architecture of IoT-based event detection and early-warning systems for underground coal mines is shown in Figure 3. The Arduino-based sensor network utilizes the Bluetooth communication protocol to cover the entire mine. In this network, the nodes are divided depending upon their function. Stationary nodes (SN) are the basic nodes for monitoring mine air parameters, and router nodes (RNs) are the parental nodes with a subordinate cluster consisting of a few SNs. Finally, both SNs and RNs transmit data to the base station (BS) through a gateway using a Bluetooth communication protocol. The base station is directly connected to the global internet and cloud computing to provide data analytics of the sensory data.  
Figure 3: The overall architecture of the proposed IoT system for underground mines.  
Figure 2: Layered structure of the IoT-based early warning system in underground mines.