6.1 Introduction

In Chapter 5, we explored how to develop models in the JSL for which time is not a significant factor. In the case of the news vendor problem, where we simulated each day’s demand, time advanced at regular intervals. In the case of the area estimation problem, time was not a factor in the simulation. These types of simulations are often termed static simulations. In the next section, we begin our exploration of simulation where time is an integral component in driving the behavior of the system. In addition, we will see that time will not necessarily advance at regular intervals (e.g. hour 1, hour 2, etc.). This will be the focus of the rest of the textbook.

In this chapter, we explore the JSL simulation software platform for developing and executing simulation models. We will begin our study of the major emphasis of this textbook: modeling discrete-event dynamic systems. Recall that a discrete-event dynamic system (DEDS) is a system that evolves dynamically through time. This chapter will introduce how time evolves for DEDSs and illustrate how the JSL can be used to develop models for simple discrete-event system.

We can think of a system as a set of inter-related objects that work together to accomplish a common objective. The objects within a system are the concepts, abstractions, things, and processes with definable boundaries and unique identity given our modeling perspective. Within the traditional simulation parlance, objects of particular interest within a system have often been called entities; however, this text takes a broader view to include other modeling elements of interest within the system (e.g. resources).

A discrete event system is a system in which the state of the system changes only at discrete points in time. There are essentially two fundamental viewpoints for modeling a discrete event system within simulation: the event view and the process view. These views are simply different representations for the same system. In the event view, the system and its elements are conceptualized as reacting to events. In the process view, the movement of entities through their processes implies the events within the system in the system. This chapter focuses on the event view.

The objects within the system change state because events occur within the system. In addition, the changes in state for an object may cause additional events to occur, affecting other objects within the system. Through the propagation of events and object state changes, the entire system state evolves through time. The state of the system is essentially the collection of states associated with all the objects in the system. The notion of modeling what happens to a system at particular events in time is the basis of discrete event modeling.