GENERAL PRINCIPLES, SIMULATION SOFTWARE

Unit 5

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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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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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• 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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• 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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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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• 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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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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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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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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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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LIST PROCESSING

• Lists: Basic properties and operations• Using Arrays for list processing• Using dynamic Allocations and Linked Lists

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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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• 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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• 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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SINGLE- SERVER QUEUE SIMULATION IN JAVA

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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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• 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

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END OF UNIT 5Thank you