SIGMA S Simulation Graph Window and the PE System Model

SIGMA

SIGMA is based onthe concept of an event graph. Event Graphs are relationshipgraphs. Thus, a simple graph can represent a large and complexsystem.In SIGMA, stochastic discrete event systems are represented assimple dynamic graphs. Pictorially, SIGMA uses the vertices (nodes)of a graph to represent state changes that are associated with thevarious events in the simulation.

SIGMA’s simulation graph window and the PE system model

A simulation graph window is similar to the above. This is the primary window for a SIGMA modeling session. Linked to this modeling session are simulation plot windows and output windows.

The three elements of a discrete event system model are the statevariables, the events that change the values of these statevariables, and the relationships between the events (one eventcausing another to occur). An event graph organizes sets of thesethree objects into a simulation model.

State Variables

Click the mouse on the Create/Edit Variables command under Variables menu.We can see the State Variable Editor dialog box. Any state variable we define can be used anywhere in our SIGMA model. The dialog box for state variables has a line for a brief description of the object.

Vertices

Double-click on the RUN vertex. This vertex was the first vertex created in the model. SIGMA calls the first vertex created vertex number 1; it always be executed first when the model is run. This first vertex is colored green.In the vertex dialog box, there is also a line for a brief description. In State Change text, we can edit the values of statevariables which are changed by this event.

Edges

Basically, edges represent the conditions (edit in Condition text) under whichone event will cause another event to occur, perhaps after a timedelay (edit in Delay text). There is also a line for a brief description in the box.

Running SIGMA simulation

The Run Option dialog box controls how a simulation of a SIGMA model will run. The Output File text box allows us to name the file in which we want the numerical output recorded. Four run modes available in SIGMA can be found in the Run Mode drop-down list. They are Single Step, Graphics, High Speed, and Time Steps. Two methods are available within the Run Options dialog box to terminate a run, Stop on Time or Event. In the Trace Variable text box, we select the state variable to be recorded and displayed during the run. In the initial Attributes text box, we can specify the initial values for some state variables. These are the values for the parameters of the first vertex we created in the model. Another starting condition for a simulation is the seed for the pseudo-random number stream; this is any valid integer between 1 and 65000.