GENERALIZED STOCHASTIC PETRI NETS - A DEFINITION AT THE NET LEVEL AND ITS IMPLICATIONS

The original proposals of several stochastic Petri net modeling techniques and of generalized stochastic Petri nets (GSPN) in particular were based mainly on the characteristics of their underlying stochastic processes. This led to the use of GSPN only as a shortened notation for the description of stochastic models. Although already quite useful in practice, this approach did not fully exploit the benefits of a Petri net description; in particular, it did not use any of the results of classical net theory. The integration of qualitative net theory results, together with the probabilistic analysis approach, requires a deep structural foundation of the GSPN definition. In this paper, the class of Petri nets obtained by eliminating timing from GSPN models while preserving the qualitative behavior is identified. Structural results for those nets are also derived, thus obtaining the first structural analysis of Petri nets with priority and inhibitor arcs. A revision of the GSPN definition based on the structural properties of the models is then presented. The main advantage is that for a (wide) class of nets, the definition of firing probabilities of conflicting immediate transitions does not require the information on reachable markings (which was, instead, necessary with the original definition). Identification of the class of models for which the net-level specification is possible is also based on the structural analysis results. The new procedure for the model specification is illustrated by means of an example, which shows the usefulness of the new approach. A net level specification of the model associated with efficient structural analysis techniques can have a substantial impact on model analysis as well.