This document discusses concepts and algorithms in discrete-event simulation. It describes key concepts like events, activities, and the event scheduling algorithm. The event scheduling algorithm involves removing the imminent event from the future event list, advancing the clock, executing the event to update the system state, and generating future events. It also discusses world views like event scheduling and activity scanning. The document provides an example of simulating a single-channel queue and how simulation can be implemented in Java and GPSS.

1. GENERAL PRINCIPLES,
SIMULATION SOFTWARE
Unit 5

2. CONTENTS
• Concepts in Discrete-Event Simulation
• The Event-Scheduling / Time-Advance Algorithm
• World Views
• Manual simulation Using Event Scheduling
• List processing
• Simulation in Java
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3. CONCEPTS IN DISCRETE-EVENT
SIMULATION
• System: A collection of entities that interact together over time
• Model: A abstract representation of a system
• System State: A collection of variables that contain all the
information necessary to describe the system at any time
• Entity: Any object or component in the system that requires
explicit representation in the model
• Attributes: the properties of a given entity
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4. • List: A collection of associated entities , ordered in some
logical fashion
• Event: An instantaneous occurrence that changes the state
of a system
• Event Notice: A record of an event to occur at the current
and some future time , along with any associated data
necessary to execute an event
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5. • Event List: A list of event notices for future events ordered
by time of occurrence also known as FUTURE EVENT
LIST(FEL)
• Activity: A duration of time of specified length
• Delay: A duration of time of unspecified indefinite length
• Clock: A variable representing simulated time
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6. THE EVENT-SCHEDULING / TIME-ADVANCE
ALGORITHM
• The mechanism for advancing simulation time and guaranteeing that
all events occur in correct chronological order is based on the FEL
• The FEL is ordered by event time, in chronological order ,
t < t1 ≤ t2 ≤ t3 ≤ ….. tn
where t is the value of CLOCK , the current value of simulated time.
t1 is called the imminent event , i.e., the next event will occur.
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7. • After the system snapshot at simulation time,
CLOCK_t is updated , the CLOCK is advanced to simulation time
CLOCK t1 and the imminent event notice is removed from the FEL
and the event is executed.
• Sequence of actions which a simulator must perform to advance the
clock system snapshot is called the event scheduling/time advance
algorithm
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9. ALGORITHM
1. Remove the event notice for the imminent event
2. Advance the CLOCK to imminent event time
3. Execute imminent event: update system state , change entity ,
attributes and set membership as needed.
4. Generate future events and place their event notices ranked
by event time
5. Update cumulative statistics and counters
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12. WORLD VIEWS
• Orientation for developing a model.
the most prevalent world views are ,
• Event Scheduling world view
• Process interaction world view
• Activity scanning world view
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13. ACTIVITY SCANNING WORLD VIEW
• 3 phase approach , where events are considered to be activity duration-zero time
units which are categorized into 2 type of activities namely activities of type B & C
• B activities – bound to occur: all primary
• C activities: conditional , upon certain conditions being true
• 3 phases are
• Phase A
• Phase B
• Phase C
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14. MANUAL SIMULATION HANDLING USING
EVENT SCHEDULING
• Eg: single channel queue
• System state: LQ(t) , LS(t)
• Entities: server and the customers
• Events : Arrival (A) , Departure (D) , Stopping event(S)
• Event notices: (A,i) , (D,t) , (S , t)
• Activities: IAT , Service time
• Delay : customer spent in the waiting line
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17. LIST PROCESSING
• Lists: Basic properties and operations
• Using Arrays for list processing
• Using dynamic Allocations and Linked Lists
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18. SIMULATION IN JAVA
• A discrete event simulation model written in java contains
the components like System state, entities & attributes,
sets, events, activities, and delays.
• To facilitate the development and debugging , it is best to
organize the Java model in a modular fashion by using
methods
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19. • CLOCK: A variable defining simulated time
• INITIALIZATION METHOD: A method to define the system
state at time 0.
• MIN-TIME EVENT METHOD: A method that identifies the
imminent event , i.e, the element of the future event list that has
smallest time stamp.
• EVENT METHODS: for each type, a method to update system
state when that event occurs
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20. • RANDOM-VARIATE GENERATORS: Methods to generate samples from
desired probability distributions
• MAIN PROGRAM: to maintain overall control of the event-scheduling
algorithm.
• REPORT GENERATOR: A method that computes summary statistics from
cumulative statistics and prints a report at the end of the simulation.
• The overall structure of Java simulation program can be explained through
a flow chart which is an expansion of the Event scheduling algorithm
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22. SINGLE- SERVER QUEUE SIMULATION IN JAVA
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23. SIMULATION IN GPSS
• GPSS is a highly structured ,special purpose simulation
programming language based on the process interaction
approach and oriented towards queuing systems
• There are 40 standard blocks in GPSS.
• Blocks represents events, delays and other actions that
affect transaction flow
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24. • First version by IBM in 1961.
• GPSS/H is the widely used version . Unlike the original
IBM implementation GPSS/H includes built-in files and
screen I/O , use of an arithmetic expression as a block of
operand , an interactive debugger, faster execution ,
expanded control statements , ordinary variables and
arrays.
• Single-server queue simulation in GPSS/H 24

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26. END OF UNIT 5
Thank you
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