On the Relationship Between Process

The relation between process algebra and I/O au-tomata models is investigated in a general setting of structured operational semantics (SOS). For a series of (approximations of) key properties of I/O au-tomata, syntactic constraints on inference rules are proposed which guarantee these properties. A rst result is that, in a setting without assumptions about actions, the well-known trace and failure preorders are substitutive for any set of rules in a format due to De Simone. Next additional constraints are imposed which capture the notion of internal actions and guarantee substitutivity of the testing preorders of De Nicola and Hennessy, and also of a preorder related to the failure semantics with fair abstraction of unstable divergence of Bergstra, Klop and Olderog. Subsequent constraints guarantee that input actions are always enabled and output actions cannot be blocked, two key features of input/output automata. The main result is that for any I/O calculus, i.e. a De Simone calculus which combines the constraints for internal, input and output actions, the quiescent trace preorder and the fair trace preorder are substi-tutive. A simple I/O calculus is presented which is suuciently expressive to specify all nitely branching I/O automata over a given, nite action signature.