Introduction to Simulation
Internet Computing Laboratory @ KUT
Youn-Hee Han
What is Simulation?
General Procedure of “Paper Work”
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Related Works
 Reading Papers and Books
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New Idea
Develop a new proposal
Performance analysis
 Experiments with real system
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Implementation are time-consuming works
 Experiments with a physical model of the real system
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Emulation
 Experiments with a mathematical model of the real system
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Simulation
 Less assumption
Mathematical Analysis
 Much assumption
Data Structure
What is Simulation?
Performance Analysis
Measure/estimate performance
Improve operation
System
Experiment
with the
actual system
Real System
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Experiment with
a physical model
of the system
Test-bed
Emulation
Experiment with a
mathematical model
of the system
Simulation
Mathematical
Analysis
Data Structure
What is Mathematical Analysis?
Mathematical Analysis
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An abstract and simplified representation of a system
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Specifies
 Important components
 Assumptions/approximations about how the system works
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Not an exact re-creation of the original system!
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If model is simple enough, study it with…
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Queueing Theory
Linear Programming
Differential Equations...
But, the simple model is really enough?
If model is complex, simulation is the only way!!!
Data Structure
What is Simulation?
Simulation

The Oxford English Dictionary describes “simulation” as
 Simulation is the technique of imitating the behavior of some
situation or system (economic, mechanical, etc.) by means of an
analogous model, situation, or apparatus, either to gain
information more conveniently or to train personnel.
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In Wikipedia…
 Simulation is an imitation of some real thing, state of affairs, or
process. The act of simulating something generally entails
representing certain key characteristics or behaviors of a selected
physical or abstract system.
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Another way…
 simulation is the technique of a building a model of a real or
proposed system so that the behavior of the system under specific
conditions may be studied.
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One of the key powers of simulation is the ability to model the
behavior of a system as time progresses.
Data Structure
Then… what is system?
System
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In Wikipedia
 System is a set of entities, real or abstract, comprising a whole
where each component interacts with or is related to at least one
other component.
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In the American Heritage Dictionary of the English Language…
 System is a group of interacting, interrelated, or interdependent
elements forming a complex whole.
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Another way…
 System is a set of interacting components or entities operating
together to achieve a common goal or objective.
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Any object which has no relation with any other element of the
system is not part of that system but rather of the system
environment.
Data Structure
And… what is model?
Model

In the American Heritage Dictionary of the English Language…
 Model is a schematic description of a system, theory, or
phenomenon that accounts for its known or inferred properties and
may be used for further study of its characteristics.
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Another way…
 Model is a set of assumptions/approximations about how the
system works
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We can study the model instead of the real system
Can try wide-ranging ideas with the model
 Make your mistakes on the computer where they don’t count,
rather than for real where they do count
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Data Structure
And… what is model?
Model Conceptualization
Real World System
Select correct level of details (assumptions)
Assumed system
Abstract essential features
- Events, activities, entities, attributes,
resources, and their relationships
Conceptual model
Logical model
Simulation model
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- Performance measures
- Data requirements
Shows the logical relationships among the
elements of the model
Flowchart model
Data Structure
And… what is model?
Getting Answers From Models
ACTUAL SYSTEM
Operating Policies
• Single queue, parallel servers
• FIFO
(X)
(Y)
• Waiting Times
Input Parameters
• # of servers
Output Parameters
MODEL
• Inter-arrival Time Distribution
• System Size
•…
• Service Time Distributions
Y = f (X)
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Data Structure
Level of Details
Too little detail result in lost of information and goals
cannot be accomplished
Too much detail requires:
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more time and effort
longer simulation runs
more likely to contain errors
Accuracy of the model
Scope & level of details
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Cost of model
Scope & level of details
Data Structure
How to Simulate?
By hand
Spreadsheets
Programming in General Purpose Languages
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Java
Simulation Languages
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SIMAN
Simulation Packages
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NS2, OPNet, OMNet++, J-Sim, Arena…
Data Structure
Data Collection and Analysis
Collect data for input analysis and validation
Analysis of the data
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Determine the random variables
Fit distribution functions
Draw graphs representing the analysis results
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Data Structure
Documentation & Refactoring
Program Documentation
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Future modifications
Creates confidence
Progress Reports
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Frequent reports (e.g. monthly) are suggested
Alternative scenarios
Performance measures or criteria used
Results of experiments
Recommendations
Produce “Papers”
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Data Structure
Pros. and Cons. of Simulation
Advantages
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When mathematical analysis methods are not available,
simulation may be the only investigation tool
When mathematical analysis methods are available, but are so
complex that simulation may provide a simpler solution
Allows comparisons of alternative designs or alternative
operating policies
Allows time compression or expansion
Data Structure
Pros. and Cons. of Simulation
Disadvantages
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For a (stochastic) model, simulation just estimates the output
while an analytical solution, if available, produces the exact
output
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Often expensive and time consuming to develop
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An invalid model may result with confidence in wrong results.
Data Structure
What is Discrete Event Simulation?
Discrete Event Simulation
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the operation of a system is represented as a chronological
sequence of events.
Each event occurs at an instant in time and marks a change of
state in the system
model world as events
 maintain queue of events, ordered by time
 maintain [virtual time]
 repeat:
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extract event at head
process it
if processing generates another event, add it to queue
each event takes predefined amount of virtual time, arbitrary
amount of real time
 having a slow CPU makes simulation run slower (in real time), but
doesn’t change result
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Data Structure
What is Discrete Event Simulation?
Discrete Event Example
S
D
Event queue
TIME
EVENT
A.
1s
S decides to send pkt to D
S starts sending pkt
B.
1.005s
S finishes transmitting pkt to D
C.
1.01s
D begins to recv pkt
D.
1.015s
D recvs entire pkt and delivers to
application
A.
B.
C.
D.
delay
value
transmission
0.005s
propagation
0.01s
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S
D
Data Structure