5.1: Introduction

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This chapter discusses the computations necessary to simulate a model on a computer. These computational tasks are performed by software that can be referred to as a simulation engine. The engine produces performance measure values as output. It does this transparently to the simulation user whose primary concern is performing the steps of the simulation process including model building and experiment design as well as the statistical analysis of performance measure values and drawing conclusions about system behavior. Nevertheless, a basic understanding of how a simulation engine does its computation tasks is fundamental.

All models are mapped, transparently to the modeler, into a set of events within the simulation engine. The mapping may be complex and not straightforward. An event is a point in simulation time when the value of one or more the state variables changes. In addition an event is used to specific when in simulated time, or under what conditions, other events, including itself, next occur.

The basic operations that a simulation engine must perform are presented in the context of the two workstation example model that was presented in previous chapters. Fundamentally, the engine must conduct the simulation step by step from start to finish. This requires

Sequencing the events.
Processing each event.
Organizing entities waiting for resources.
Generating individual samples from probability distributions to obtain values for entity attributes and times between entity arrivals as well as operation and transportation times.

A discussion of the events in the two workstation example will precede a discussion of each of the activities of the simulation engine.