A Methodology for Variable Structure System Specification: Formalism, Framework, and Its Application to ATM-Based Network System

Executor Passive Model CompositeVSSS CompositeExecutor CompositeVSSS Model AtomicVSSS AtomicExecutor AtomicVSSS Model Fig. 3. Operational diagrams for abstract executors: CompositeExecutor and AtomicExecutor. transition functions and mapping functions. When the CompositeExecutor receives an inETRI Journal, volume 18, number 4, January 1997 Kyou H. Lee et al. 253 Fig. 4. Operational diagrams for VSSS Models: CompositeVSSS Model and AtomicVSSS Model. put, it activates the associated CompositeVSSS model to invoke the following function: state transition function: f; and with the new state updated by f-function, to invoke the following function: mapping function: g; The responsibilities of the AtomicExecutor is to request the associated AtomicVSSS model to execute its transition and output functions. When an AtomicExecutor receives an input, it activates the associated AtomicVSSS model to invoke the following function: state transition function: ı; output function: ; The complete algorithm for the two kinds of abstract executors will be described later, which are realized in the VSSS framework. 3. VSSS Framework Architecture and Environment There may be different ways to develop an object oriented environment for supporting system modeling within the VSSS formalism when the VSSS formalism and its associated abstract executors are given. However, We use the method in which an external agent operates on passive models in order to reuse through composition technologies in model development [14]. The execution sequence of VSSS models is controlled by abstract executors with associated models. Models have no executing thread of execution control that determines the order of models to be executed. As shown in Fig. 5, the VSSS environment defines classes for modeling and those for execution separately. A VSSS model is activated only when the associated executor requests to do so. Execution proceeds by means of messages passing between abstract executors, not between VSSS models. Thus, we identify VSSS models as passive components and associated abstract executors as external active agents. While the specification of a model is a developer’s responsibility, creation of an associated executor and pairing of the two to know each other must be managed by the execution environment implicitly. The VSSS environment provides the developer with modeling facilities 254 Kyou H. Lee et al. ETRI Journal, volume 18, number 4, January 1997 with the VSSS framework and execution of the models. For modeling, the VSSS environment provides developers with classes the definition of which are based on the VSSSL semantics. All components are built using class libraries. Fig. 5. System architecture of VSSS environment.