The modern way of life requires the exchange of large amounts of information via telecommunication networks. The demand for high transfer rates is steadily increasing, while the recent pandemic is weighing on the situation. Optical backbone networks serve much of the information being transferred. Although a reliable transmission medium, fibre optics are at risk of failure due to natural and technological disasters, with a large impact on the function of the state, businesses, and the needs of citizens. Passive optical networks are a fast-growing access technology for high-speed connection of end-users to backbone networks, having constant research interest. The purpose of the doctoral research is to study functional requirements of passive optical networks, namely availability, survivability, and energy efficiency, and to produce a methodology based on formal analysis and verification. The research will record possible failures of optical networks and examine the disasters that cause them. The formal analysis and verification of the failure consequences will be based on an initial model of functional requirements which will be refined later. Metrics for assessing the operation of passive optical networks will also be recorded. The results will be used to implement simulations and utilized to write proposals to improve the operational requirements of this research.