II.3.1. Overview

After a model was drawn or retrieved from the disk, the user can simulate it. Three modes of simulation are implemented in the PN Toolbox, namely: Step, Run Slow and Run Fast, that are called from the Simulation menu or from the Simulation Panel.

The Step simulation mode ensures the progress of the simulation by a single step, when the status of a transition is changed from enabled to fired, or from fired to disabled. With the occurrence of each event two successive steps are associated, meaning the complete treatment of the firing transition.

The Step and Run Slow simulation modes are accompanied by animation. The animation displays the current number of tokens in the places of the net and colors transitions when they are fired. In the Run Slow mode, the speed of animation is adjustable from the Animation Speed option in the Simulation/Preferences dialogue box. The simulation can be stopped with a right-click.

In these two modes, the Status Panel placed in the bottom left-hand corner of the screen presents the number of events and the current time.

Also, the user can record the progress of the simulation by using the Log File command from the Simulation menu. This journal is automatically saved in the project directory as an HTML file and can be read by means of any Internet browser using the Simulation/View history command available in the PN Toolbox.

Note that in Step or Run Slow modes, the Scope and Diary facilities are available during the simulation. Their usage is relevant when studying timed PN. The Run Fast simulation mode is not accompanied by animation and the simulation speed depends on the computer performances. This mode is recommended when the user is interested in estimating global performance indices of the PN model over a large time horizon (or, equivalently, for the occurrence of a large number of events).

From the Simulation/Breakpoint submenu the user can control the end point of the simulation in the Run Fast mode. The simulation is stopped by default when 1000 events have occurred or if no transition can fire any longer (i.e. when deadlock appears).

In all simulation modes, immediately after the end of the simulation, the values of the Performance Indices which characterize the simulated dynamics are available on request, by accessing the menu Performance.

The simulation principle is different from one type of PN to another. Details about the simulation method implemented in the PN Toolbox for each type of PN model are given in the next pages.