## 2 Materials and Methods  
Availability is the probability that the system will be operational at a given time. It combines aspects of reliability, maintainability, and maintenance support, and implies that the system is either in active operation or able to operate if required. This study determines the probability of the reliable operational availability of an underground communication system and considers its possibility for improvement through system maintenance and the use of a system with high resource [6]. In our study, we determine the availability probability of reliable operation of an underground mine communication system and consider the possibility of its improvement in terms of system maintenance and its standby system. To improve the reliability of the system, a cost-effective preventive and corrective maintenance plan was proposed [5], and in our paper, we discuss how the reliability of the system can be improved by using a standby system in the example of an underground mine communication system and present the study results. Maintenance improvement options:  
* Doubling the amount of money, manpower, and time spent on maintenance (during corrective maintenance);
* Carrying out regular preventative maintenance to reach 50 percent of total performance (during the scheduled repair);
* Using a mixed method in which abovementioned 2 options are taken simultaneously. The results are considered for one communication system device and for the entire communication system. Another way to increase the reliability of a communication system is to have a standby system available. Weibull, Poisson, exponential distribution laws are used to show the degree of availability or reliability of a single and dual standby (connected in parallel) system. To show how the system availability rate improves by taking the measures of the first two options for improving the system availability rate, system dynamics modeling is used to simulate the probability of availability in the Vensim program. The result confirms that when any system undergoes high-quality maintenance regularly and has a resource system, then its availability rate improves. The general structure of the underground mine communication system with three shafts is shown in Figure 1 below. An underground mine can have one or more shafts, and they are characterized by varying distances from each other. A shaft is an excavated engineering structure for the access of people and materials from the surface to the underground mine, and for the return of people and ore bodies from the underground mine to the surface. In other words, this shaft can be compared to a lift in a high-rise building. In terms of purpose, shafts are divided into those for access only by people, and only for ventilation systems and for transportation. For our study, we developed a reliability model for a two-reserve or three-parallel communication system for a three-shaft underground mine. Core systems, base stations and optical cable devices are located near the shaft and on the surface. The optical cable from the optical cable device, called the site link, is connected to the optical repeater located in the underground mine through the shaft tunnel, and the radiation cable from the optical repeater is attached to the side wall along the underground mining tunnel. In underground mine tunnels where radiation cables are laid, users or miners communicate with each other and with control rooms and people of other teams on the surface using radios. Factors that affect the probability of reliable operation of the communication system shown in Figure 1 below, or the causes of faults, using the information of the communication system of Oyu Tolgoi mine, if we determine each component (see Supplementary Materials):  
* Failure of electricity;
* Fiber optic cable breakage;
* Damage to the power supply battery. Core system:
* Database jams and crashes due to an outdated core system;
* Hard disk damage due to outdated core system hardware.