## 4 Implications and Limitations of the Study
By using it, the system with single- and two-resources considering the reliability of an underground mine communication system is formulated in ordinary differential form by Weibull distribution law and was calculated using the system dynamic modeling methodology in Vensim software, and the results were compared with each other and with the maintenance improvement method. This was an advantage of the study. Oyu Tolgoi communication system failure data were collected only between 2014 and 2018, and it was difficult to collect data after 2018, because the underground mine communication system was in the construction phase, and this process is still ongoing. In addition, since the monitoring system at that time was in the process of being updatedfrom 2018, the failure data after 2018 were not recorded correctly until 2021, which were the main limitations that occurred in the time and data collection process of our study. The following problems occurred when the study was conducted (Figures 7 and 8):  
* Simulating and testing in MATLAB requires a lot of programming code, and it was relatively easy to simulate and test in Vensim. One of the main advantages of system dynamic modeling was the ease of conducting experiments with different parameter values. * When the failure function (\(\lambda\)) and repair function (\(\mu\)) values of three parallel communication systems (communication system in a one-, two-, and three-shaft mine) were modeled by the Weibull distribution law, the reliability was not converged to a fixed number, that is, it was not stabilized. Therefore, for the case of three parallel communication systems, the Weibull distribution was left. * Moreover, for dual- and triple-parallel communication systems, we entered the failure function (\(\lambda\)) and correction function (\(\mu\)) mixed, or exponent on the failure function (\(\lambda\)), Poisson on the correction function (\(\mu\)), or failure function (\(\lambda\)) or Weibull on the correction function (\(\mu\)), exponential distribution on the correction function (\(\mu\)), etc. But this idea was not implemented because the reliability index was not stabilized. * While calculating reliability model of a communication system with a single standby device (Figure 7) and reliability model of a communication system with dual standby devices (Figure 8), we tried using the dsolve command in MATLAB, but it took a long time to receive the results. Therefore, the dsolve command was replaced by the ode23 command to complete the calculation. * When the values of failure function (\(\lambda\)) and repair function (\(\mu\)) were assumed to be governed by Poisson distribution law, the factorial of t (t!) was included our formula, which made it difficult to calculate differential equations. Therefore, the factorial of t (t!) was replaced by Stirling formula. * In the design of reliability model of a communication system with dual standby devices considering all options, some cases were left because differential equation systems were computationally and conceptually difficult. For example: it was left out that these three systems are working and all three stop at the same time, i.e., one device starts to stop, then the second device stops, and finally the third device stops, so all three parallel systems are modeled in such way that all of them stop.