The efficiency indicators of the functioning critical information infrastructures in the communication system are analyzed based on the architectural concept of future networks. The object of the study is hardware and software complexes critical information infrastructures for special purposes. Critical information infrastructure represents information and telecommunication communication systems, the maintenance, reliability and security which are necessary for the safe operation special-purpose enterprises. In order to avoid the occurrence of various security and reliability incidents, the studied critical infrastructures communication systems require constant analysis and updating operating rules. The subject of the research is a method for calculating quality indicators of the functioning of critical information infrastructures in communication systems. In this work, using the example of a communication system based on modern technologies, the sequence of actions for analyzing threats to the security of a critical information infrastructure facility is considered. The purpose of the study is to develop a new approach for creating methods for calculating indicators of efficiency, reliability and information security systems. Based on the analysis of the work, a method for calculating efficiency indicators critical information infrastructures of communication systems is proposed and important analytical expressions for further research are obtained. As a result of the study, the main conclusions of the study were obtained, which can be implemented and used in critical infrastructures of communication systems to calculate the quality of functioning public computer and telecommunication systems.