## 4 Implications and Limitations of the Study  
The main contribution of our study is to develop a traditional differential equation system and system dynamic modeling method and suggested a methodology for calculating the reliability of the communication system using a combined method. The data about Oyu Tolgoi communication system failure was used in development this method. The most important result is that the interdependence model of the devices included in the communication system was established based on their conditional probabilities (Figure 11) on the example of the one-shaft underground mine communication system and it was possible to test the availability degree of each device and the entire system by simulating with the help of Oyu Tolgoi mine communication system failure and maintenance functions and availability information provided by Motorola Dimetra (TETRA) communication system manufacturer. The results in Figure 9 were results of comparing the availability degrees of the three-shaft underground mine communication system shown in Figure 1. But Figure 10 was the results for the model shown in Figure 5 and when there was not any condition and when doubling the preventive maintenance and after increasing the preventive maintenance by 50% and was considered a feature by simulating both corrective and preventive maintenance. Also, single- and dual standby system models are suggested in Figures 7 and 8. As for the single- and dual standby system the models shown in Figures 7 and 8 have been tested in MATLAB program only, given random data for the availability degree for the Exponential, Poisson, and Weibull distributions. In other words, the failure and maintenance functions of the Oyu Tolgoi mine communication system and the availability information provided by the Motorola Dimetra (TETRA) communication system manufacturer were not used in both models. Thus, the models shown in Figures 5 and 6 and the results shown in Figures 9-11 were varied from the models shown in Figures 7 and 8 by processing using TETRA communication system devices and their failure and manufacturer's availability data. The results for the Exponential, Poisson, and Weibull distributions of the models shown in Figures 7 and 8 varied according to the time, due to the availability degree was unstable across the three distributions. In this regard, the results of the models shown in Figures 7 and 8 were not included in this paper, and the problems faced during the study of this model were included detailly in this section. As for the methodology of the study, the system dynamic modeling methodology was not considered as the main research methodology, and we considered the system dynamic modeling methodology as a way of calculating the formulated calculation. In other words, we use a causal loop diagram, which is the basis of the system dynamic modeling methodology, formulated the basic model in the form of an ordinary differential equation without defining the feedback links, and performed its analysis using the Vensim program. This is another feature of this study. The work in [5] was based on the idea that one way to improve the reliability of any equipment in the research work is to increase the maintenance, using the failure data of Oyu Tolgoi underground mine communication system in 2014-2018, the probability of reliable operation was calculated for each device of the communication system in the mine. This is one of the distinguishing features of the work [5]--calculating the probability of reliable operation of the entire communication system by expanding it. The authors of [6] showed in a research paper that one way to improve the reliability of any equipment is to have a resource system available, and it is formulated as an ordinary differential equation for a system with single resource, by [PERSON]'s distribution law.