- System:
- A collection of entities (e.g. people and machines)
that interact together over time to accomplish one or more goals.
- Model:
- 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).
- Entity:
- 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.
- Attributes:
- The properties of a given entity.
E.g. the priority of a waiting customer, the routing distribution of a
job through a workshop.
- List:
- 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.
- Event:
- 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 type (e.g. landing or take-off)
- Event time (e.g. 134)
- Flight number
- Aircraft type (e.g. Boeing 737-200, DC-10)
- Number of passengers on board (e.g. 125)
- Pointer to other flight information
- Pointer to the aircraft specification
- Pointer to the crew information
- Event list:
- A list of event notices for future events,
ordered by time of occurrence; also known as the future event list
(FEL).
- Activity:
- 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.
- Delay:
- 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).
- Clock:
- A variable representing simulated time. CLOCK
is used in the textbook examples.
The clock variable can be centralized or distributed.
