Simscript II.5

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Simscript II.5

• Building simulation model with SIMSCRIPT II.5

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Simulation Model Structure

• Each simulation model has the following 3
  activities
• A mechanism for representing arrivals of new
  object (constant delay between successive
  arrivals, uniform, exponential, etc).
• A representation of what happens to the object in
  the model (objects may compete for a service,
  they will be put in queue if the server is busy)
• A mechanism for terminating the simulation

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Terminating the simulation

• Two approaches for terminating simulation
• The planned-termination-time method The
  simulation stops immediately at the planned time
  regardless the unfinished processes.
• The second method allows everything in the model
  to be finished. However the door will be closed
  and no new arrivals are allowed.

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The Process Concept

• The dynamic parts in the model that are created
• They become either immediately after creation or
  will be activated after some specific time.
• The description of its activity in the model is
  given by a process routine.

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The Resource Concept

• A resource is used to model an object that is
  needed by process objects.
• If the resource is not available (occupied by
  another process object) the process object will
  be placed in a waiting line.

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SIMSCRIPT II.5 Program Structure

• A SIMSCRIPT II.5 program consists of three
  modeling elements
• A preamble
• A main program
• A process routine for each process declared in
  the preamble

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1. Preamble

• The first part of each SIMSCRIPT II.5 program is
  the preamble.
• preamble is a declarative and does not include
  any executable statements.
• It declares all modeling elements including
• processes and resources in the program.
• It describes their static feature but does not
  describe how and why they interact.

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2. Main

• It contains executable statements.
• Execution begins with the first statement in the
  Main program.
• Resources must be created and initialized
• A simulation begins when the statement
• Start simulation is executed.

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Timing Routine

• All processes will be in the pending list.
• The timing routine check if there is a process in
  the pending list and select the one that has the
  earliest activation time.
• Then advance the simulation clock to this time
• Then determine the process type
• Remove this process from the pending list
• Execute the process routine.

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3. Process Routines

• Process (entity) is a dynamic object and the
  sequence of actions that experience through its
  life in the model.
• A process is created and becomes active
  immediately or later at some activation time.
• Processes interact either implicitly or
  explicitly through executing the statements
  activate, interrupt, or resume.
• Process routine is a subroutine, which
  corresponds to a particular process.
• Process notice this record contains information,
  which characterizes the particular realization.

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A Detailed Discussion of Processes and Resources

• 1- Processes
• Each type of process used in a SIMSCRIPT
  simulation model must be defined in the preamble,
•  Processes include ARRIVAL.GENERATOR and CUSTOMER
• Where ARRIVAL.GENERATOR and CUSTOMER are two
  processes.
• Processes can also have characterizing
  attributes, In our example, suppose that each
  CUSTOMER process entity has an associated age
  attribute called CU.AGE.

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process commands

• We now explain each of the process commands
•   Activate statement The activate statement is
  used to create a process notice for a particular
  process and then immediately place the notice in
  the event list.
• The reference variable for this type of process
  points to the notice.

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• Activate a CUSTOMER in 10 minutes
• This statement means that we create a new
  CUSTOMER process notice with an activation time,
  which is 10 minutes larger than the current value
  of the simulation clock, and then place this
  notice in the event list.

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Request/relinquish statement

• The request statement is used by a particular
  process entity (or process notice) in its process
  routine to request a certain number of units of a
  resource of a specified type.
• request 1 unit of SERVER(l)
• or
• request 1 unit of SERVER(l) with priority 5
• Note the one with the highest priority is
  served next

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Request/relinquish continue

• If no priority is specified (the usual case),
  then a priority of 0 is assumed.
• The relinquish statement is used by a particular
  process entity (or process notice) to relinquish
  (release) a certain number of units of a resource
  of a specified type.
• relinquish 1 unit of SERVER(l)
• Note 1 unit of may be shortened to 1.

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Work/wait

• The work/wait statements are used by a particular
  process entity, which is executing its process
  routine, to place its process notice back into
  the event list for a specified amount of time.
• A work statement represents "service" times and a
  wait statement represents "inter-event" (e.g.,
  inter-arrival) times.
• After one of these statements is executed,
  control is immediately returned to the timing
  routine.
• work 20 minutes

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Suspend/reactivate

• The suspend statement is used by a particular
  process entity to stop its own execution of the
  corresponding process routine. (for example when
  a more priority customer arrives then the current
  executable customer will be suspended)
• This results in its process notice moving from
  the "executing" state to the "created" state.
• The notice will stay in this latter state until
  "awakened" by a reactivate "the" (or resume)
  statement in a process or normal routine.
• reactivate the CUSTOMER now

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Interrupt/resume

• The interrupt statement is used in a process or
  normal routine to remove a process notice from
  the event list. (for example when the time of
  closing is reached the interrupt statement is
  used to prevent new customers form entering the
  system)
• This results in the process notice moving from
  the "pending" state to the "created" state.
• interrupt the CUSTOMER  
• resume the CUSTOMER
• Note resume statement can be used to move a
  suspended process notice from the "created" state
  to the "pending" state.

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Create/destroy

• The create statement is used to create a process
  notice for a particular type of process without
  placing it into the event list.
• create a CUSTOMER
• let CU.AGE (CUSTOMER) 35
• activate this CUSTOMER now
• The destroy statement is used to destroy (i.e.,
  to make non existent) a process notice which is
  in the "created" state.  
• destroy the CUSTOMER

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End/return

• The end, return statement is placed at the end of
  each process routine.

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2- Resources

• A resource is an object such as a person or
  machine which provides service to a process
  entity.
• A resource may have characterizing attributes and
  is defined in the preamble resources
• every WORK.STATION has a WS.NUM.MACHINES
• There are five system-defined variables, which
  are automatically associated with each defined
  resource.

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• The variable N.WORK.STATION is the number of
  types of workstations desired (i.e., 5).
• Q.WORK.STATION(I), The first set (or queue)
  contains those jobs waiting to be served by
  WORK.STATION type I, and the corresponding
  variable N.Q.WORK.STATION(I) is the number of
  jobs in the queue at a particular point in time.
• X.WORK.STATION(I), the second set contains those
  jobs executing WORK.STATION type I, and the
  variable
• type I at a particular point in time.

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• N.X.WORK.STATION(I) is the number of jobs that
  are executing WORK.STATION
• U.WORK.STATION(I) is the number of idle units or
  machines in WORK.STATION type I at a particular
  point in time.
• the variable WORK.STATION (i.e., it has the same
  name as the resource itself) is an index variable
  which is equal to one of the WORK.STATION types
  (i.e., 1, 2, 3, 4, or 5).

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• In order to allocate storage for the resource
  WORK.STATION, we can execute the following create
  statement
• create every WORK.STATION(5)
• (This statement implicitly sets the value of
  N.WORK.STATION to 5.)
• we can then specify how many units of each type
  are initially available.

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• let U.WORK.STATION(l) 3
• let U.WORK.STATION(2) 3
• let U.WORK.STATION(5) 1
• The variable U.WORK.STATION(I) will decrease by
  one each time a machine of type I becomes busy
  conversely, it will increase by one each time a
  machine of type I becomes idle.