7.1 Terminology of Simulation Modeling

When developing a simulation model using the event-view, there are a number of terms and concepts that are often used. Before learning some of these concepts in more detail, it is important that you begin with an understanding of some of the vocabulary used within simulation. The following terms will be used throughout the text:

System

A set of inter-related components that act together over time to achieve common objectives.

Parameters

Quantities that are properties of the system that do not change. These are typically quantities (variables) that are part of the environment that the modeler feels cannot be controlled or changed. Parameters are typically model inputs in the form of variables.

Variables

Quantities that are properties of the system (as a whole) that change or are determined by the relationships between the components of the system as it evolves through time.

System State

A "snap shot" of the system at a particular point in time characterized by the values of the variables that are necessary for determining the future evolution of the system from the present time. The minimum set of variables that are necessary to describe the future evolution of the system is called the system’s state variables.

Entity

An object of interest in the system whose movement or operation within the system may cause the occurrence of events.

Attribute

A property or variable that is associated with an entity.

Event

An instantaneous occurrence or action that changes the state of the system at a particular point in time.

Activity

An interval of time bounded by two events (start event and end event).

Resource

A limited quantity of items that are used (e.g. seized and released) by entities as they proceed through the system. A resource has a capacity that governs the total quantity of items that may be available. All the items in the resource are homogeneous, meaning that they are indistinguishable. If an entity attempts to seize a resource that does not have any units available it must wait in a queue.

Queue

A location that holds entities when their movement is constrained within the system.

Future Event List

A list that contains the time ordered sequence of events for the simulation.

When developing models, it will be useful to identify the elements of the system that fit some of these definitions. An excellent place to develop an understanding of these concepts is with entities because entities represent things that flow through and are processed by the system.