Building an open communication application

The implementation of a series of protocol handlers is discussed in terms of the approach taken to design and implementation methods. The target was a communication sub-system insupport of terminal handling applications. The design of an OSI transport layer handler using the Jackson system design method is described, as are the reasons for opting to use a more conventional, finite-state machine approach to session layer and ECMA virtual terminal protocol handlers. The software engineering tools used to carry out these implementations are also described. In conclusion the paper considers the lessons learned both from the application of design methods and the efficiency of protocol implementation. The paper shows that the approach taken by ISO in defining its OSI protocols is effective and leads to viable implementations.