This paper describes the adaptation and certification process for switching equipment to be used in the Russian public switched telephone network (PSTN). The first part of the paper illustrates the basic certification procedure and its essential steps. Each step, whether it is described in a natural language or formally in a specification language SDL, is an integral part of the certification process. The described approach improves standardization in the way that specialists at different manufacturers as well as Russian telecommunications engineers use the same methods, the same specification language, and the same test environments. Even the documents for different switching equipment have standardized features. In the second part of this paper, the specification methodology based on formal methods has been shown to work in the adaptation and certification process. It does reduce the number of software errors in the resulting version of adopted switching equipment, and it does improve quality and productivity significantly.>

The article describes the main peculiarities of the former Soviet Union public switched telephone network (PSTN), signalling systems, call handling procedures, charging principles, etc. General information about the PSTN is also included. The primary goal of the paper is to show some specific PSTN features and give the classification of the existing interfaces rather than to describe existing exchanges in detail.>