Refining Interfaces of Cummunicating Systems

There are now several theories for describing and reasoning about the behavior of communicating systems, where the behavior of a communicating system is described in terms of its capabilities to perform communication actions in cooperation with its environment. In such theories, preorders or equivalences are defined as criteria for when one system is an acceptable substitute or implementation of another. Existing theories of communicating systems define preorders or equivalence relations only between systems with identical sets of communication actions. In many practical design situations, however, it may be desirable to refine a system by changing its set of communication actions. We present a simple method for carrying out such refinements. The method is first formulated in a general setting, and then elaborated in more detail in the trace model and a simple version of the failure model. We illustrate the usefulness of our method by an application to I.451, an ISDN access protocol.