## 2 Materials and Methods
Base station:
* The device shuts down due to power failure;
* The base station overheats, jams, and shuts down due to excessive room temperature;
* Damaging of internal parts;
* Turning off the base station's connection to the device;
* Disruption of the site link;
* Damage to the antenna cable. Transmission:
* Switching off the transmission due to a power failure;
* Overheating, jamming, or shutting down the connection due to excessive room temperature;
* Loss of transmission device configuration;
* Ignition of the transmission device power supply;
* Deterioration of connection quality due to contamination of the fiber optic cable connection module;
* Disconnection during fiber optic cable arrangement;
* Cutting the fiber optic cable during unpermitted excavation work. Repeater device:* Burning out the device due to a power supply voltage difference;
* Fiber optic cable breakage;
* Radiation cable breakage;
* Damage to internal parts. The causes of failure and the number of failures per communication system device at the Oyu Tolgoi mine in 2014-2018 were included in the statistical data, as shown in Table 1 (see Supplementary Materials). According to these statistics, base stations failed 151 times, connections failed 107 times, site link failed 19 times, the OMU failed 15 times, and failure of the core system occurred 13 times. However, according to the total hours of failure, core system failure was 24.5 h, failure of the base stations was 21 h, and connection line failure was 18 h. Considering the reasons for these failures, the main reason for the longest failure of the core \begin{table}
\begin{tabular}{c c c} \hline \hline
**Duration of** & **Device Name, Number of** & **Duration of Failure by Hour** \\
**Failure:** & **Failures:** & \\ \hline \hline \multirow{6}{*}{November 2014–December 2018} & Site link (transmission line)—107 & Site link (transmission line)—18 \\  & Base stations—151 & Base station—21 \\ \cline{1-1}  & Repeater—19 & Repeater—4 \\ \cline{1-1}  & OMU (Optic management unit)—15 & OMU—3 \\ \cline{1-1}  & Core system—13 & Core system—24.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Number of failures of communication system components. Figure 1: The general structure of an underground mine communication system with three shafts. system was that the system was too outdated in terms of hardware and software regarding system resources. According to the manufacturer's design, the core system of the Motorola Dimetra system is manufactured with a backup core system. Therefore, the standby core system was operating during the longest core system failure, and the communication system service was unaffected. In terms of the base stations, the total duration of the Oyu Tolgoi mine communication system's six base station failures was 21 h, and the base stations were mostly out of order due to power failure. Base stations have a standby device in their internal structure, just like the core system, but they can be interrupted by a power failure, no matter how many standby devices they have. From the general structure of the underground mine communication system shown in Figure 1, the reliable operation of the system is highly dependent on operational site link. The reason is that the core system of the underground mine communication system, the base station and the optical cable device, are located on the surface, and they are connected to the repeater devices located underground only using the optical cable connection line; therefore, the site link is a very important part of the underground mine communication system. It is important to install a communication system base station (whether it is located on the surface or underground), a site link, an OMU connected to the site link, and a repeater for each underground mining shaft. This study concludes that the installation of these system components can improve an underground mine's communication system reliability.