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Simulation Concepts
Systems, models, and simulation
The first two chapters of the Tutorial showed how to build and run simulation models in ExtendSim. Since you have seen some of what can be accomplished with ExtendSim, now is a good time to explore some modeling and simulation concepts. The following discussion is meant to familiarize you with modeling and simulation terminology and concepts used throughout this guide. This chapter:
• Explainsmodelingconceptsandterminology
• Discusses model types and common approaches to modeling • Describesthemodelingprocess,includinggoalsandsteps
• Shows how to verify and validate a model
After reading this chapter you will have a better grasp of modeling concepts and will be ready to start using ExtendSim for your modeling needs. Note that an in-depth exploration of simula- tion is beyond the scope of this document. For more detailed definitions and theory, please refer to the numerous books on simulation.
☞ If you are already familiar with the concepts to be presented in this chapter, skip it and proceed to one of the other modules, as discussed in “About this User Guide” on page 6.
Systems, models, and simulation
All professions use models of one form or another. But the word “model” does not always have the same meaning to business professionals, managers, scientists, and engineers. Even within a specific discipline, such as manufacturing, modeling has many different definitions. The fol- lowing discussion serves to clarify what “modeling” means as it relates to ExtendSim.
The real world can be viewed as being composed of systems. A system is a set of related com- ponents or entities that interact with each other based on the rules or operating policies of the system:
• Entitiesaretheinternalcomponentsofthesystem.Entitiesareinvolvedinprocesses—activ- ities in which they interact with each other.
• Operating policies—the types of controls and availability of resources—are the external inputs to the system. They govern how the system operates and thus how the entities inter- act.
Over time, the activities and interactions of entities cause changes to the state of the system; this is called system behavior or dynamics. Systems can be mathematically straightforward, such as a flower growing in the soil and turning towards the sun to maximize photosynthesis. Or they can be more complex, such as supply chain operations composed of planning, selling, distribution, production, and sourcing subsystems.
A model is an abstracted and simplified representation of a system at one point in time. Models are an abstraction because they attempt to capture the realism of the system. They are a simpli- fication because, for efficiency, reliability, and ease of analysis, a model should capture only the most important aspects of the real system.
Most models can be classified into four basic types:

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