chapter1_sqqp3033

8/3/2019 Chapter1_SQQP3033

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Introduction to SimulationChapter 1


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Objectives

i. Define simulation, system and modelii. List types of simulation modelsiii. List types of constructive simulationiv. Introduce Discrete Event Simulation (DES)

conceptsv. List suitable problems for DESvi. Overview of a simulation studyvii. Identify different tools for building simulation

modelsviii.List advantages and disadvantages of simulationix. List paths to failure and success in conducting a

simulation project


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Educational perspective: Simulation is a digitallearning material which allows learners toperform hands-on activities (e.g., mouseclicking, text entering, etc.) in order to receive

additional tasks or information. Engineering perspective: Simulation refers to

the process of designing a model of a realsystem for the purpose of understanding the

behaviour of the system and/or evaluatingvarious strategies on its operations based onspecified objectives of a study.

Simulation


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System & Model

System a group or collection of interrelatedelements that cooperate to accomplish somestated objective; e.g., bank, airport, etc.

Modeling the process of producing a model

Model a representation of a system s operations

or processes similar to but simpler than the system it

represents enables the analyst to predict the effect of

changes to the system


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live simulations (or role playing) - real peopleoperate simulated operations of a real systemusing real equipment (e.g., training exercise of afire drill).

virtual simulations - real people operatesimulated equipment in a simulated environment(e.g., flight and vehicle simulators).

constructive simulations - real people operatecomputerised models by performing interactionsand get feedbacks.

Types of Simulation Models


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Type Learning support Example

Soft skill Simulation Exposing learners with workexperiences before they actually deal

with real situations in order to improvelearners communication and decisionskills

Software applicationsimulations

Situation-based simulations;e.g., Business andManagement training,customer and sales training,doctor-patient interactionsand customer services

Procedural Simulationsor Virtual products

Understanding the physicalcharacteristics of real life equipmentand learning to use complicatedequipment

Mechanical devicesimulations; e.g.,medical/manufacturing /homeelectronic equipment

Discrete Event

Simulations (DES)

Understanding the operation of a

system which is represented by achronological sequence of events

Queuing systems

Manufacturing systems Logistic models

System Dynamics (SD) Understanding the behaviour ofcomplex systems over time whichinvolves internal feedback loops, stocks(entities which accumulate or depleteover time) and flow (the rate change of

the stock)

Policy analysis and design Population systems Ecological systems Economic systems

Types of Constructive Simulations


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Limit our study to Discrete Event Simulation(a.k.a. DES)

DES operations of a system is represented by achronological sequence of events (i.e., discretepoints of times where state variables changevalues)

State variables variables that affect systemperformance; e.g., resource utilization, totalproduction, number in a queue, etc.

Discrete Event Simulation (DES)


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A model intended for a DES study is amathematical model developed using DES tools(languages or software).

Mathematical model classifications include:

deterministic input and output variables arefixed values

stochastic at least one of the input or outputvariables is probabilistic

static time is not taken into account dynamic time-varying interactions among

variables are taken into account Typically, DES models are stochastic and

dynamic

DES (contd.)


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Entity any objects that enter a system, move

through a series of processes, and then leavethe system; e.g., customers, patients, parts,

etc. can have individual characteristics

(attributes)- a common characteristic of all

entities but with a specific value that candistinguish the entity; e.g., name, priority, duedate, account number, etc.

DES Concepts


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DES Concepts (contd.)

Resources any objects that process entities;e.g., workers, machines, etc.

Queue a space to wait a resource.

DES Model a computer program thatrepresents the logic of a system as entities withattributes arrive, join queues to be allocated and

processed with a number of required resources,and then exit the system.


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Conclusion

AlteredSystem

Real World Simulation Study

DES Study Schematic

SimulationAnalysis

SimulationExperiment

Simulation

Model

SystemUnderStudy


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DES Projects

DES modeling involves two fields: Art system analysis and modeling Science the programming and statistical

components


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DES Project (contd.)

Most are conducted by team

Need two types of people:1) people who know and understand the

considered system; e.g., designers, systems,

manufacturing or process engineers2) people who know how to formulate and model

the system as well as program the model(simulation specialists)

If we do not have (2) hire people with the necessary skills contract the modeling to outside consultants


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train some of our own people data collection,statistical skills and programming skills

some combination of the above

DES Project (contd.)


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Suitable Problems For DES

Generally, when there is a need to model andanalyze randomness in a system. Morespecifically: It is impossible or extremely expensive to

modify certain processes in the real world; e.g.,large manufacturing systems

Problems in which mathematical model can be

formulated but analytic solutions are eitherimpossible or too complicated (e.g., complexsystems like large scale queuing models)


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It is impossible or extremely expensive tovalidate the mathematical model describingthe system; e.g., due to insufficient data

Suitable areas:

Computer and communication systems Manufacturing Transportation Service systems

Suitable Problems For DES (contd.)


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A Simulation Study

Steps that should be present in any simulation

study:1) Problem DefinitionClearly defining the goals of

the study The purpose why are we studying the problem What questions do we hope to answer?

2) System DefinitionDetermining the boundariesand restrictions to be used in defining the system(or process) and investigating how the systemworks Divide the system into logical subsystems. Define the entities and resources Define the basic flow patterns of entities through the

stations using flow diagrams


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3) Conceptual Model FormulationDeveloping apreliminary model either graphically (e.g. blockdiagram or process flow chart) or in pseudo-codeto define the components, descriptive variables,

and interactions (logic) that constitute the system4) Collect and process real system dataIdentifying and collecting the input data neededby the model. Example:

Time between arrival (Inter-arrival Time) Processing time at each station Route time (travel time) between stations

-very time consuming and difficult (1/3 of the total

time used in the study)

A Simulation Study (contd.)


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Potential sources of input data Historical records Observational data Similar systems Operator estimates Vendor's claims Designer estimates Theoretical considerations

5) Model Translation Formulating the model in an

appropriate simulation language



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6) Verification and ValidationVerification make sure the computer programperforms as expected and intended.Validation the process of reaching an

acceptable level of confidence that the inferencesdrawn are correct and applicable to the real-worldsystem being represented. Does the model adequately represent the real-world

system. Is the model generated behavioral data characteristic of

the real-world system's behavioral data? Does the simulation model user have confidence in the

model's results?



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7) Design and Run Experiment A simulationexperiment is a test or a series of tests in whichmeaningful changes are made to the inputvariables of a simulation model so that we may

observe and identify the reasons for changes inthe performance measures

8) Analysis and Interpretation Drawing inferencesfrom the data generated by the simulation runs


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9) Implementation and DocumentationReportingthe results, putting the results to use, recordingthe findings, and documenting the model and itsuse. Choosing an appropriate vocabulary (no

technical jargon). Length and format of both written and verbal

reports (short and concise). Must address the issues that the sponsor or

user consider important.



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DES Tools

A simulation model can be built using: General-purpose programming languages General-purpose simulation language more

flexible

Special purpose simulation packages areeasier to learn

Some of the advantages of using a simulationpackage Reduction of the programming task Provision of conceptual guidance Increased flexibility when changing the model Fewer programming errors Automated gathering of statistics


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Types Of Simulation Packages Examples

Programming Languages FORTRAN, C++, Visual BasicSimulation Languages/packages AweSIM , Extend , GPSS/H , Micro Saint,

SIMSCRIPT , SLX, MODSIM III, SIMPLE++

High Level Simulators Arena, SIMUL8, Flexim

Application-Oriented Simulators Manufacturing:

AutoMod, ProModel, QUEST,WITNESS

Communication/computer:

COMNET III, NETWORK II.5, OPNET Modeler

Business:BP$IM, Extend+BPR, ProcessModel,ServiceModel, SIMPROCESS

Health Care:

MedModel

DES Tools (contd.)


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We can test new designs, layouts, etc. withoutcommitting resources to their implementation

It can be used to explore new staffing policies,operating procedures, decision rules,organizational structures, information flows, etc.without disrupting the ongoing operations

Simulation allows us to identify bottlenecks in

information, material and product flows and testoptions for increasing the flow rate

It allows us to test hypothesis about how or why

certain phenomena occur in the system

Advantages of Simulation


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Simulation allows us to control time. Thus we canoperate the system for several months or years ofexperience in a matter of seconds allowing us toquickly look at long time horizons or we can slow

down phenomena for study It allows us to gain insights into how a modeled

system actually works and understanding of which

variables are most important to performance Simulation's great strength is its ability to let us

experiment with new and unfamiliar situations andto answer "what if" questions

Advantages of Simulation (contd.)


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Simulation modeling is an art that requiresspecialized training and therefore skill levels ofpractitioners vary widely. The utility of the studydepends upon the quality of the model and the

skill of the modeler

Gathering highly reliable input data can be timeconsuming and the resulting data is sometimes

highly questionable. Simulation cannotcompensate for inadequate data or poormanagement decisions

Disadvantages of Simulation


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Simulation models are input-output models, i.e.they yield the probable output of a system for agiven input. They are therefore "run" rather than

solved. They do not yield an optimal solution,rather they serve as a tool for analysis of thebehavior of a system under conditions specifiedby the experimenter

Disadvantages of Simulation (contd.)


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Failure to define a clear and achievable goal.

Inadequate planning and underestimating theresources needed

Inadequate user participation

Writing code too soon before the system is reallyunderstood

Inappropriate level of included detail (usually toomuch)

Wrong mix of team skills

Paths to Failure


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Paths to Failure (contd.)

Lack of trust, confidence and backing bymanagement


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Have clearly defined and achievable goals Be sure we have adequate resources available to

successfully complete the project on time

Have management's support and have it knownto those who must cooperate with us in supplyinginformation and data

Assure that we have all the necessary skillsrequired available for the duration of the project

Be sure that there are adequate communicationchannels to the sponsor and end users

Paths to Success


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Paths to Success (contd.)

Have a clear understanding with the sponsor andend users as to the scope and goals of theproject as well as schedules

Have good documentation of all planning andmodeling efforts

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