The physical systems due to inherent component variation and change in the surrounding environment are not completely failure free. There always exists the probability of failure that may cause unwanted and sometimes unexpected system behavior. It poses the requirement of detailed analysis of issues like availability, reliability, maintainability, and failure of a system. The availability of the system can be estimated though the analysis of system outcomes in the surrounding environment. In this chapter, the availability estimation has been performed for an optical wireless communication system through Monte Carlo simulation under different weather influences like fog, rain, and snow. The simulation has been supported by data measured for number of years. The measurement results have been compared with different theoretical models.