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An entity represents an item of interest in a discrete-event simulation. The meaning of an entity depends on what you are modeling. In this chapter, examples use entities to represent abstract customers in a queuing system and data packets from a remote controller to an actuator on the system being controlled.
An entity output port provides a way for an entity to depart from a block . An entity input port provides a way for an entity to arrive at a block.
A connection line indicates a path along which an entity can potentially advance. However, the connection line does not imply that any entities actually advance along that path during a simulation. For a given entity path and a given time instant during the simulation, any of the following could be true:
No entity is trying to advance along that path.
An entity has tried and failed to advance along that path. For some blocks, it is normal for an entity input port to be unavailable under certain conditions. This unavailability causes an entity to fail in its attempt to advance along that path, even though the path is intact (that is, even though the ports are connected). An entity that tries and fails to advance is called a pending entity.
An entity successfully advances along that path. This occurs only at a discrete set of times during a simulation.
Note: The simulation could also have one or more times at which one or more entities successfully advance along a given entity path and, simultaneously, one or more different entities try and fail to advance along that same entity path. For example, an entity departs from a queue and, simultaneously, the next entity in the queue tries and fails to depart.
In time-based dynamics, signals express the outputs of dynamic systems represented by blocks. Event-based blocks can also read and produce signals. One way to learn about signals is to plot them; the discussion in Explore the D/D/1 System Using Plots is about visualizing signals that reflect behavior of event-based blocks.
Time-based and event-based dynamics can interact via the data shared by both types of blocks. Attributes of entities provide a way for entities to carry data with them. The subsystem in Lesson 3: Add Event-Based Behavior illustrates the use of attributes in the interaction between time-based and event-based dynamics.
Although signals are common to both time-based and event-based dynamics, event-based dynamics can produce signals that have slightly different characteristics. For more information on event-based signals, see Role of Event-Based Signals in SimEvents.