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This paper describes experience with an implementation of the X25 communication protocols for accessing public data networks. Ihe implementation effort is characterized by: 1) the development of a formalized protocol specification on which all further implementation work is based, and 2) the use of Concurrent Pascal as the implementation language. The main features of the formalized protocol specification are given, and a method for deriving a protocol implementation based on parallel processes, monitors, and classes is explained. The overall structure of the system and the step-wise refinements leading to the complete implementation are discussed. Some comments on the possible implementation on multiple microprocessors are also given.