Documentation

SimEvents Terminology

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advance

To depart from one block and arrive immediately at another block. An entity advances from block to block during a simulation.

arrival

Entrance of an entity to a block via an entity input port. Arrival is the opposite of departure.

attribute

Data associated with an entity.

For example, an entity might be associated with a size, weight, speed, or part number.

available

The state of an entity input port that permits entities to arrive at the block.

For example, when a Entity Server block is empty, its entity input port is available. When the block is busy serving, its entity input port is unavailable.

blocked

The state of an entity output port when an entity is trying to depart via the port and the port connects to an unavailable entity input port of another block.

For example, consider a Entity Queue block whose entity output port is connected to the Entity Server block's entity input port. Suppose the queue contains one entity. The queue's entity output port is blocked if the server's entity input port is unavailable, and not blocked if the server's entity input port is available. If the queue is empty, then its entity output port is not blocked because no entity is trying to depart.

component entity

An entity that forms part of a composite entity.

composite entity

An entity that comprises one or more entities as subordinate parts. The parts are called component entities.

departure

Exit of an entity from a block via an entity output port. Departure is the opposite of arrival.

discrete-event system

A system in which state transitions depend on asynchronous discrete incidents called events. You typically construct a discrete-event system by adding a variety of blocks, such as generators, queues, and servers, from the SimEvents® block library.

One or more discrete-event systems can coexist with time-based systems in a Simulink® model. SimEvents software automatically handles signals transitioning from time-based components/systems to and from discrete-event components/systems and labels these signal lines with a capital E.

entity

An abstract representation of an item of interest in a discrete-event simulation. The specific interpretation of an entity depends on what you are modeling. Entities can carry data, known as attributes.

For example, an entity could represent a packet in a communication network, a person using a bank of elevators, or a part on a conveyor belt.

entity input port

An input port at which an entity can potentially arrive. An entity input port can be available or unavailable; this state, which can change during the simulation, helps determine whether the port actually accepts the arrivals of new entities.

entity output port

An output port from which an entity can potentially depart. An entity output port can have a state of blocked or not blocked; this state, which can change during the simulation, determines whether the port's attempt to output an entity is successful.

entity path

A connection from an entity output port to an entity input port, depicted as a line connecting the entity ports of two blocks. An entity path represents the equivalence between an entity's departure from the first block and arrival at the second block. The connection line depicts a relationship between the two blocks.

An entity path is in active use by an entity only at zero or more discrete times during the simulation. By contrast, a connection line between signal ports represents a signal that has a well-defined value at all times during the simulation.

entity port

An entity input port or an entity output port.

entity priority

A positive number associated with an entity, used to sequence its departure with regard to other simultaneous departures. A lower number indicates a higher priority.

event

An observation of an instantaneous incident that may change a state variable, an output, and/or the occurrence of other events. Examples of events are the generation of a new data packet in communications, the exit of a person from an elevator, and the placement of a new part on a conveyor belt.

event calendar

The internal list of events that are scheduled for the current time or future times.

For example, when a server begins its service time on a specific entity, the application inserts an entry into the event calendar for the completion of service on that entity at a future time. In a system representing elevator passengers, this event calendar entry might represent the event whereby a specific person in an elevator reaches the desired floor.

event-based simulation

A simulation that permits the system's state transitions to depend on asynchronous discrete incidents called events.

intergeneration time

The time interval between successive generations.

pending entity

An entity that has tried and failed to depart from the block in which the entity resides. The failure occurs because the entity output port through which the entity would depart is connected to an unavailable entity input port of another block.

preemption

The replacement of an entity in a server block by an entity that satisfies certain criteria.

signal port

An input or output port that represents a numerical quantity that changes over time and that is defined for all times during the simulation. Unlike an entity port, a signal port has no state and does not have entity arrivals or entity departures.

simultaneous events

Events that occur at the same value, or sufficiently close values, of the simulation clock. Events scheduled on the event calendar for times T and T+Δt are considered simultaneous if 0 ≤ Δt ≤ 128*eps*T, where eps is the floating-point relative accuracy in MATLAB® software and T is the simulation time.

For example, in a D/D/1 queuing system where the arrival rate equals the service rate, an entity generation event and a service completion event are simultaneous. Parameters in the model determine which of these events occurs first, though the clock has the same value in both cases.

time-based signal

A signal that can change only in response to the simulation clock.

time-based simulation

A simulation in which state transitions depend on time.

For example, a simulation based solely on differential equations in which time is an independent variable is a time-based simulation.

unavailable

The state of an entity input port that prevents entities from arriving at the block.

For example, when an Entity Server block is empty, its entity input port is available. When the block is busy serving, its entity input port is unavailable.

zero-duration value

A value that an event-based signal assumes at an instant in time but that does not persist for a positive duration.

For example, when a full N-server advances one entity to the next block, the statistical signal representing the number of entities in the block assumes the value N-1. However, if the departure causes another entity to arrive at the block at the same time instant, then the statistical signal assumes the value N. The value of N-1, which does not persist for a positive duration, is a zero-duration value.. This phenomenon occurs in many situations.

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