II.3.5. Stochastic Petri Nets

For stochastic PNs (SPNs), only exponential type distributions can be used to assign the time durations of the transitions. For conflicting transitions, it is the shortest time duration that allows the choice of the transition to fire, without using priorities or probabilities. Multiple firing of the same transition is not permitted, even if the token content of its input places allow this; i.e. the transition fires once and after the allocated time elapses, it will fire again if the current marking is appropriate. (Notice that the mechanism for selecting the firing transition and the condition for firing only once make the difference between SPNs and T-timed PNs with exponential type distributions for the time durations).

The sequencing of the firing transitions is exclusively controlled by the time durations of exponential type, which ensures the equivalence of the SPNs with the Markov chains. The firing rate of the transitions (i.e. the inverse of the mean time-duration) is, by default, marking independent, but the user can select a marking-dependent operation (infinite server semantics).

The Diary facility opens a new window that displays (dynamically) the instant(s) when the next fireable transition(s) is (are) to fire.

The Scope facility opens a new figure window that displays (dynamically) the evolution of the selected performance indices. The final value of the global index shown by the Scope is identical to that displayed on request, at the end of simulation, when the user explores the Performance menu.