Chapter 2
Discrete Event Simulation
The majority of modern computer simulation tools (simulators) implement a paradigm, called discrete-event simulation (DES). This paradigm is so general and powerful that it provides an implementation framework for most simulation languages, regardless of the user worldview supported by them. Because this paradigm is so pervasive, we will review and explain in this chapter its working in some detail.
2.1
ELEMENTS OF DISCRETE EVENT SIMULATION
In the DES paradigm, the simulation model possesses a state S (possibly vectorvalued) at any point in time. A system state is a set of data that captures the salient variables of the system and allows us to describe system evolution over time. In a computer simulation program, the state is stored in one or more program variables that represent various data structures (e.g., the number of customers in a queue, or their exact sequence in the queue). Thus, the state can be defined in various ways, depending on particular modeling needs, and the requisite level of detail is incorporated into the model. As an example, consider a machine, fed by a raw-material storage of jobs. A “coarse” state of the system is the number jobs in the storage; note, however, that this state definition does not permit the computation of waiting times, because the identity of individual jobs is not maintained. On the other hand, the more “refined” state consisting of customer identities in a queue and associated data (such as customer arrival times) does permit the computation of waiting times. In practice, the state definition of a system should be determined based on its modeling needs, particularly the statistics to be computed. The state trajectory over time, S(t), is abstracted as a step function, whose jumps (discontinuities) are triggered by discrete events, which induce state transactions (changes in the system state) at particular points in time. Although computer implementation of events varies among DES simulators, they are all conceptually similar: An event is a data structure that always has a field containing its time of occurrence, and any number of other fields. Furthermore, the “occurrence” of an event in a DES simulator is implemented as the execution of a corresponding procedure (computer code) at the scheduled event occurrence time. When that procedure is run, we say that the event is processed or executed. Simulation Modeling and Analysis with Arena Copyright 2007 by Academic Press, Inc. All rights of reproduction in any form reserved.
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