State machine modeling: From synch states to synchronized state machines

Abstract

To synchronize concurrent regions of a state machine, the Unified Modeling Language (UML) provides the concept of so-called "synch states". Synch states insure that one region leaves a particular state or states before another region can enter a particular state or states. For some application areas, it is beneficial to synchronize not only regions but also state machines. For example, in data and telecommunications, a pure black box specification of communication interfaces via statechart diagrams gives no adequate means to describe their coordination and synchronization. To circumvent the limitations of the UML, this paper presents the concepts of Trigger Detection Points (TDP) and Trigger Initiation Points (TIP); it allows a modeler to couple state machines. The approach is generic, easy to extend and smoothly fits to the event model of the UML; it could also substitute the more specific concept of synch states.