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We propose a refinement of branching bisimulation equivalence that we call orthogonal bisimulation equivalence. Typically, internal activity (i.e., the performance of τ-steps) may be compressed, but not completely discarded. Hence, a process with τ-steps cannot be equivalent to one without τ-steps. Also, we present a modal characterization of orthogonal bisimulation equivalence. This equivalence is a congruence for ACP extended with abstraction and priority operations. We provide a complete axiomatization, and describe some expressiveness results. Finally, we present the verification of a PAR protocol that is specified with use of priorities.

The aim of this work is to provide a formal foundation for the unambiguous description of real-time, reactive, embedded systems in UML. For this application domain, we define the meaning of basic class diagrams where the behavior of objects is described by state machines. These reactive objects may communicate by means of asynchronous signals and synchronous operation calls. The notion of a thread of control is captured by a so-called activity group, which is a set of objects which contains exactly one active object and where at most one object may be executing. Explicit timing is realized via local clocks and an urgency predicate on transitions. We define a formal semantics for this kernel language, based on the run-to-completion paradigm. We show that this combination of communication primitives and execution mechanism gives rise to a large number of questions and discuss the decisions taken in the proposed semantics. The resulting semantics has been defined in the typed logic of the interactive theorem prover PVS.

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Abstract. We specify the tree identify protocol of the IEEE 1394 high performance serial multimedia bus at three different levels of detail using μCRL. We use the cones and foci verification technique of Groote and Springintveld to show that the descriptions are equivalent under branching bisimulation, thereby demonstrating that the protocol behaves as expected.