next up previous
Next: Execution Mechanism of Discrete-Event Up: General Principles Previous: General Principles

Concepts and Definitions

A collection of entities (e.g. people and machines) that interact together over time to accomplish one or more goals.

An abstract representation of a system, usually containing structural, logical or mathematical relationships which describe a system in terms of state, entities and their attributes, sets, processes, events, activities, and delays.

E.g. a bank operation can be modeled as a queueing system: tellers being the server(s), customers being the jobs in the queue. We ignore all other features of a bank in this model (doors, receptionist, waiting areas, etc.)

System state:
A collection of variables that contain all the information necessary to describe the system at any time.

E.g. In the Able-Baker carhop example, we may define the system state as (Able-busy, Baker-busy, number-of-cars-waiting-in-line).

Any object or component in the system which requires explicit representation in the model.

E.g. a server, a job on an assembly line, a machine.

The properties of a given entity.

E.g. the priority of a waiting customer, the routing distribution of a job through a workshop.

A collection of associated entities, ordered in some logical fashion.

E.g. when simulating a computer network with multiple computers, packets waiting to be transmitted at each computer form a list ordered by the time of their generation. There will be multiple instances of this kind of lists.

An instantaneous occurrence that changes the state of a system (such as an arrival or departure of a customer).

Event notice:
A record of an event to occur at the current or some future time, along with any associated information necessary to execute the event. At a minimum, the record includes the event type and the event time.

E.g. when simulating the operation of an airport, we may have two types of events, take-off and landing. With these two events, a possible event notice would take the following form.

Event list:
A list of event notices for future events, ordered by time of occurrence; also known as the future event list (FEL).

A duration of time of spcified length (e.g. a service time or an inter-arrival time), which is known when it begins (although it may be defined in terms of statistical distribution).

Note that the term time here is not necessarily a reading of the clock, rather it is a process (e.g. life-time of a bearing).

E.g. take-off time: an aircraft will complete its take-off in three minutes after it starts its engine.

E.g. a customer starts to be serviced at a barber shop, it may take the barber a random amount of time to finish.

A duration of time of unspecified indefinite length, which is not known until it ends.

E.g. a customer's delay in a last in, first out waiting queue which, when it begins, depends on future arrivals (e.g. procedure call stack).

A variable representing simulated time. CLOCK is used in the textbook examples.

The clock variable can be centralized or distributed.

next up previous
Next: Execution Mechanism of Discrete-Event Up: General Principles Previous: General Principles
Meng Xiannong 2002-10-18