SIMSCRIPT II.5 Programming Language

properties of an event are: (1) it occurs at some instant of time, and (2) the occurrence is instantaneous.
Figure 5-1 illustrates an activity delimited by two events.
activity start
machining activity
activity end
start machining.
'start event' at time t1
stop machining.
'stop event' at time t2
Figure 5-1. An Activity Delimited by Two Events
To model an activity, using events, those events that delimit the activity must be identified. Any
necessary tests and conditional or unconditional state changes associated with the beginning or end
of the activity may be specified for each of these events. The duration of the activity may be specified
by scheduling the start event for a certain instant in simulated time, followed by the stop event
scheduled for some later time. When the initial event is called into activation, it alters the system
state in the specified manner. It may indeed be responsible for scheduling the activity terminating
event. After these state changes are performed, control is passed back to the SIMSCRIPT II.5
scheduling mechanism. After the apparent passage of the appropriate simulation time, the second
event, the stop event, is executed, performing the state-changes associated with the termination of
the activity. Either of these events may involve the scheduling of further events, perhaps of different
types, at suitable intervals in simulation time.
The changes in a system that occur when an activity starts or stops, i.e., in the instant of time an
activity begins or ends, are associated with events rather than activities. As these events comprise
all significant system state-changes, the passage of time between events need not be accurately followed.
Rather the passage of simulation time is driven by the sequence of events, advancing always
to the time of the next significant event. This is the crucial difference between discrete-event
and continuous-time simulation. In discrete-event simulation, state-changes take place at specified
points in time at which interactions between system components occur. In continuous-time simulation,
interactions and state-changes take place continuously. To model continuous changes, techniques
such as numeric integration must be employed. The choice of simulation methodology
depends on the characteristics of the system under study, and the way in which it must be understood.
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