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Modelling and Simulation of Systems

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Title: Modelling and Simulation of Systems


1
Modelling and Simulationof Systems
2
Useful References
  • Introduction to Computer Simulation
  • A System Dynamics Modelling Approach
  • Roberts et al, Call No 003.3 INT
  • Operations Research - An Introduction
  • Hamdy Taha, 6E, Prentice-Hall, Call No CA003 TAH
  • Network II.5 User Manual

3
Model
  • A standard or example for imitation or comparison
  • Cheaper to construct a wrong model than a
    wrong system
  • Enables a variety of ideas to be tried
  • Simplifies complexity
  • Reduces the scale of the system
  • Provides a plan or map of the system
  • Deconstructs a system into its components

4
Modelling and Simulationof Systems
  • System - a collection of interacting components
    (elements) that function together for some
    purpose
  • the circulatory system
  • a computer network
  • Simulation - to simulate is to imitate something
  • A simulation involves a model that mimics
    important components (elements) of the system
    under study

5
Modelling and Simulation
  • A simulation may involve a
  • physical model
  • a mental conception
  • a mathematical model
  • a computer model
  • a combination of the above
  • Simulation has been practiced for centuries in
    games, art and engineering, in these activities
    participants
  • derive abstract models from concrete situations,
  • they then interpret the models to provide
    insights into the original situations

6
Modelling and Simulation
  • Given a model cannot include all the elements
    and all the interactions between the elements
    that make up a system the model is
    simpler than the phenomenon it represents
  • model aircraft versus real aircraft --gt yet
    engineers can use this physical model in a wind
    tunnel to predict the flight characteristics of
    the real aircraft
  • Newtons law of gravitational attraction is a
    mathematical model of the force of gravity --gt
    yet by interpreting its symbols we can learn
    about a physical phenomenon

7
Modelling and Simulation
  • As a simplification of reality,
    a model
    will not produce reliable information
    on every aspect of the system
    being modelled

8
Modelling and Simulation
  • It is often useful to be able to
  • predict the affect on a Systems Performance, of
    a change to the system or change in the systems
    workload
  • determine the minimum network resources required
    to meet an applications requirements
  • Ask Questions like -
  • How is Response Time (R/T) affected
    ?
  • How is Transaction Throughput
    affected ?
  • Where is the bottleneck ?
  • Will one 64kbps link be sufficient
    to handle data traffic ?

9
Modelling and Simulation
  • Common Approaches
  • 1) Do an After-the-Fact Study
  • 2) Make a Simple Projection
  • 3) Develop a Mathematical Model of the system
  • 4) Develop a Computer Model of the system

10
1) Do an After-the-Fact Study
  • Basically you make the changes then Wait-and-See
    what happens
  • This tends to lead to
  • unhappy clients, ie average R/T goes from 3secs
    to 30secs
  • unwise purchases being made
  • In effect, it is no option at all
  • It is also a way of ensuring you have a short but
    (in)glorious career

11
2) Make a Simple Projection
  • Make a simple projection based on current
    knowledge of the Systems behaviour
  • For example we could assume a Straight line
    (y2x1) relationship between R/T (y) and System
    load (x) based on the behaviour of the current
    system up to a transaction load of 500 trans/sec
  • Refer fig Projected versus Actual Response time
  • However what if there is an Exponential (y2x21)
    relationship ?
  • R/T collapses between 0.8 and 0.9
    system load

12
3) Develop a Mathematical Model
  • A Mathematical Model can be expressed as a Set
    of Equations that can be solved to to yield a set
    of estimated results that allow us to predict
    the affect of System Change on
  • Response Time
  • Transaction Throughput
  • Number of communications links required
  • etc
  • For Distributed Computing Systems which involve,
    shared facilities - networks, databases,
    printers, routers - Mathematical Models based on
    Queuing Theory provide a reasonable fit to reality

13
4) Develop a Computer Model
  • Given a simulation language - Simscript or
    simulation package - Network II.5, the analyst
    can model reality in great detail
  • Overheads - Time to build Model
  • - Time to run simulation
  • - Time to analyse results
  • Due to the development of (cheap) powerful CPUs
    simulation is increasingly used in training,
    system design
  • Flight simulators
  • Architectural design, walk through the proposed
    building
  • Virtual Reality environments

14
Modelling - Telephone Industry
  • The Telephone industry developed a model of voice
    traffic patterns in order to design and provision
    a profitable circuit switching network
  • Engineers can
  • Over Provision design a non-blocking switching
    system that guarantees all users can get a dial
    tone at anytime. Cost high -gt user
    fees high -gt few users -gt low profit
  • Under Provision under design, too many users
    will be denied service and again low profit
  • Modelling and Simulation aim to reduce the risk
    of Over or Under provision, hence resulting in a
    network (design) that will provide services most
    of the time

15
Modelling and Simulation
  • Risk of Over Provisioning
  • Money spent needlessly on hardware and services
    that remain under utilized
  • Risk of Under Provisioning
  • Users immediately suffer a fixed period of lost
    productivity while additional time and money is
    spent to upgrade the network
  • Refer fig 1.1 Degree of Network Provisioning

16
Model Building - Phases
  • Problem Definition
  • Involves recognizing and defining a problem to
    study that is amenable to analysis in systems
    terms
  • contains variables that vary over time
  • causes of the variability can be described
  • causes can be contained in a closed system
  • System Conceptualization
  • Involves committing to paper the important
    influences believed to be operating within a
    system

17
Model Building - Phases
  • Model Representation
  • Model represented in the form of computer code
  • Model Behaviour
  • Computer simulation can be used to determine how
    all of the variables (elements) within the system
    behave over time

18
Model Building - Phases
  • Model Evaluation
  • Tests are performed to evaluate the models
    quality and validity eg
  • checking for logical consistency
  • matching model output against observed data
    collected over time
  • formal statistical tests for variables used
    within the simulation
  • Policy Analysis and Model Use
  • The model is used to test alternative system
    designs that might be implemented as the real
    system

19
Modelling and Simulationof Systems
  • Lunar Lander Model
  • Initial state
  • Internal and External variables
  • Deterministic
  • Nondeterministic
  • Two Types of Model
  • Discrete Event Simulation
  • Continuous Simulation

20
Modelling and SimulationLunar Lander - A Simple
Game
  • The aims are
  • to land on the moon
  • achieve a low rate of descent
  • leave enough fuel for takeoff
  • The game is a simulation that manipulates
    variables representing
  • altitude
  • rate of descent
  • amount of fuel remaining

21
Modelling and SimulationLunar Lander - A Simple
Game
  • The simulation begins (the Initial State) by
    displaying
  • initial rate of descent
  • initial altitude
  • amount of fuel available to burn
  • Prompted for next amount of fuel to burn, then
    calculate
  • fuel remaining after the burn
  • the new altitude
  • the new rate of descent

22
Modelling and SimulationLunar Lander
  • The variables can be classed as
  • internal - rate of descent, altitude, amount of
    fuel remaining
  • external - amount of fuel used in next burn
  • These variables control the outcome of the
    simulation
  • The variables are related via equations
    once the amount of fuel
    to be burned is assigned,
    the value of the other variables can be
    calculated

23
Modelling and SimulationLunar Lander
  • The model is Deterministic
  • to the extent that the same sequence of fuel
    quantities entered always results in the same
    outcome
  • Once a winning sequence is discovered ,
    the equations imply, it
    will always produce a soft landing
  • The model is Nondeterministic
  • to the extent that we cannot predict the sequence
    of fuel amounts a player might enter

24
Modelling and SimulationLunar Lander
  • How can we make the game (model) more
    nondeterministic ?
  • Start each new game with a random altitude and
    rate of descent

25
Modelling and SimulationTwo Types of Model
  • Discrete Model
  • deals with a system whose behaviour changes only
    at given instants (in time)
  • eg when a customer enters or leaves the system
    (bank)
  • Continuous Model
  • deals with a system whose behaviour changes
    continuously (in time)
  • where variables (attributes) of system undergo
    smooth continuous changes - eg electric current
  • usually represented in terms of differential or
    algebraic equations

26
Modelling and Simulation Discrete Event
  • Another characteristic of the Lunar Lander model
    is its discrete outcome
  • When the fuel is entered for the next burn the
    lander instantly assumes a new altitude and rate
    of descent
  • The simulation computes the future state of the
    modelled system at discrete time intervals,

    often determined by events in the simulation
    eg the amount of
    fuel entered for the next burn

27
Modelling and Simulation Continuous
  • Newtons Mathematical Model of gravitational
    attraction
  • is based on infinitesimal divisions of time and
    space and produces a continuous range of results
  • generally described by deterministic
    differential or algebraic equations that describe
    the interactions among the different attributes
    of the system

28
Simulation for Network Planning and Management
  • Network managers are likely to have at least two
    major duties
  • planning new installations or network upgrades
  • the day-to-day management of the network itself

29
Planning New Installations orNetwork Upgrades
  • When large systems are being designed,
    the task may be quite complex
    and it is
    becoming more common to see planners resorting to
    the use of modelling and simulation
  • Simulation will aid in
  • Concept testing
  • Comparison of alternatives
  • Documentation of proposals
  • Planned system must support the business function

30
Concept Testing
  • Models of networks can be developed
    and using a simulation
    design tool tested (Proof of Concept)
    to see which model is the most likely to be
    suitable as the basis of
    building a workable network

31
Comparison of alternatives
  • A good simulation design tool should permit rapid
    creation of a simulated environment
    and also
    simple modification of the network type for
    comparison of differing systems prior to final
    selection
  • This should be able to be done easily and
    efficiently
  • This gives the designer the opportunity to
    test different systems
    and network alternatives and come up
    with a workable design

32
Documentation of proposals
  • The simulation design tool should also support
    high-level reporting as an aid

    in producing documented proposals
    for evaluation by
    management

33
Supporting the Business Function
  • The results of the simulation should allow the
    building of a workable system
  • that satisfies the functional requirements
    defined in the application specifications
  • that carries the required load with an adequate
    response time
  • that provides an efficient interface with the
    people who will be using it
  • that is easily expandable
  • that is not unreasonable in terms of its
    implementation cost and ongoing support costs

34
Network II.5
  • Is a good example of a simulation design tool
  • It permits the rapid development and
    analysis of prototype networks

35
Network II.5 - From User Manual
  • Is a design tool which takes a computer system
    description you specify and provides measures of
  • hardware utilization
  • software execution
  • message delivery
  • response times
  • contention
  • Is used both to evaluate the ability of a
    proposed system (configuration) to meet the
    required workload and to evaluate competing
    designs

36
Network II.5 - From User Manual
  • Is designed to model a wide variety of computer
    architectures, from a single processor to a
    complex system of processors and storage devices
    connected in a network
  • It allows the portions of a computer system of
    special interest to be modelled at a detailed
    level while the rest of the system is modelled at
    a coarser level
  • It is a tool not a computer language with an
    interactive graphical interface than can be used
    after a short period of training

37
Day-to-Day Management
  • In modern networks where switching is common,
    the level of network monitoring
    (of traffic
    generated by each node in the network)
    that is easy in shared-media (non-switched)
    networks is now not possible
  • The volume of traffic generated by Monitoring
    agents can be detrimental to network performance
    (refer lecture 11)
  • The monitors cannot cover the whole network
    (refer lecture 11)

38
Day-to-Day Management
  • Simulating the functioning of the network is one
    way of being able to monitor the level of service
    that the network provides
  • This can be done by testing the network
    in a simulated environment
    using
    real traffic
    that is generated by
    real nodes on the network
  • The traffic details can be sampled for a period
    of time for each node (eg using a Monitoring
    agent like RMON)
    and then imported into the simulation
    environment

39
Day-to-Day Management
  • RMON a Monitoring Agent
  • collects statistics (packet counts, error rates
    etc) with management station retrieving data
    after some time duration
  • Refer lecture 11

40
Day-to-Day Management
  • The traffic that is recorded can be used by the
    simulation design tool to generate more traffic
  • The simulated traffic created is based upon the
    samples of real traffic recorded
  • Network II.5 uses a tool called TRAFLINK III that
    can import recorded data from a number of popular
    monitoring tools

41
TrafLink III - From User Manual
  • Provides the link between Network II.5 and other
    software packages
  • It allows you to import network topology
    information and traffic traces into Network II.5
    and use the developed model as a foundation for
    what-if scenarios
  • It enables you to represent your existing network
    topology as a Network II.5 model

42
Introduction to Network Performance
  • COT3151

43
Network Performance
  • Networks are built to support business
    applications
  • word processing
  • computer-aided drawing
  • electronic mail
  • imaging etc
  • Critical Success Factor - each business
    application must be performed in real time
  • Real Time - definition depends on the application
    and can vary from msecs to hours

44
  • What Real Time Responses are Expected to satisfy
    the following Business Applications ?
  • Word Processing
  • Computer aided drawing
  • Electronic mail

45
Real Time Response Expected
  • Word Processing
  • user loads document, makes changes, writes back
    to file server
  • read/write operations should be a few seconds at
    most
  • Computer aided drawing
  • drawing activities are iterative
  • users expect subsecond response time when loading
    a library symbol or deleting a component from a
    drawing
  • Electronic mail
  • may take minutes or hours without users becoming
    upset with network performance

46
  • How is User Productivity affected if Response
    Time is poor ?

47
Real Time Response Expected
  • User Productivity
  • Declines if applications do not respond quickly
    enough, as User
  • frustration sets in
  • attitudes deteriorate
  • work output slows down
  • confidence in the network becomes low
  • Network Performance must be sufficient to support
    the users/applications response time
    expectations

48
Common Performance Metrics
  • User Response Time
  • determined by adding up all the delays that
    application packets endure before a response is
    returned to user
  • delays - client side think time,
  • network delays in forwarding packets,
  • server side processing time etc
  • Two Definitions
  • Network Utilization
  • current use of available bandwidth, 10, 20 etc
  • ability to provide additional bandwidth on demand
  • ability to provide dedicated bandwidth to an
    application

49
Network Performance
  • good performance must be available day and night
  • response time must be provided when needed most
  • end of fiscal quarter
  • end of month
  • 9am - 11am each morning to handle Peak
    Interactive Traffic Load etc
  • good performance must be maintained when
  • additional users (remote or local) added to
    network
  • new department moves onto network
  • new application is deployed enterprise wide
  • additional network printers installed etc

50
Additional Users
  • If additional users (remote or local) are to be
    added to network
  • need to anticipate performance impact in advance
  • if analysis indicates
  • extra traffic can be carried -gt proceed
  • response time will degrade -gt delay until
    selection and implementation of appropriate
    network upgrades

51
Network Performance
  • The ability to predict network performance allows
    us to
  • Keep user response time low
  • Increase user productivity
  • Supply adequate network bandwidth
  • Provide for future growth
  • Ensure successful deployment of new applications
  • Validate response time goals of new network
    designs
  • Troubleshoot for bottlenecks
  • Choose among several competing network
    applications
  • Choose the best alternative network topology

52
  • What are the Costs of a Poorly Performing Network
    ?

53
Costs of a Poorly Performing Network
  • Putting a dollar figure on network down time is
    an exercise in estimation
  • A Network is considered to be down when users can
    no longer work
  • If the network is down
  • A salesperson writing a quotation for a customer
    risks losing business if the quotation cannot be
    delivered on time
  • A point-of-sale clerk who cannot authorize a
    customers credit card will lose a sale and
    possibly a customer
  • Electronic mail between a contractor and a
    supplier is delayed and a critical order date is
    missed

54
Costs of a Poorly Performing Network
  • For many companies the enterprise network (their
    computer systems and networks) has become Mission
    Critical that is, failure or poor performance
    means the company is out of business

55
Costs of a Poorly Performing Network
  • Large Australian company with offices across all
    states and New Zealand
  • Has central corporate office and numerous branch
    offices and manufacturing sites interconnected by
    a WAN
  • Branch offices
  • provide sales and support services to customers
  • transmit sales orders to the corporate office
  • communicate with manufacturing sites for product
    support
  • Corporate office
  • communicates with appropriate manufacturing sites
    to schedule product shipments to customers

56
Costs of a Poorly Performing Network
  • The enterprise network is Mission Critical to
    such a company. Performance problems can cost
    dollars in lost sales.
  • Other mission critical systems ?

57
Costs of a Poorly Performing Network
  • Productivity - Loss Estimate
  • Full-time administrative assistant 60 per hr
    includes on-costs
  • 10 assistants
  • Access to File server is slow in the afternoons
    -gt 10 of an assistants time is unproductive
  • Estimate
  • 48weeks x 5days/week x 8hrs/day 1920 hrs
  • 0.10 x 1920 x 60 x 10 115,200 lost annually

58
Costs of a Poorly Performing Network
  • Productivity - Loss Estimate
  • Software Company - pending product release
  • 40 developers and testers
  • 7 development systems acting as a repository for
    source code and each equipped with software
    development tools
  • Periodical build is done, all source code and
    data files are compiled and linked to produce
    final product
  • Turnaround time to be competitive these builds
    should take less than 12 hrs so that testing can
    resume in the morning
  • If builds start to take longer, the next release
    will slip because developers cannot test until
    late next morning

59
Costs of a Poorly Performing Network
  • Productivity - Loss Estimate
  • Assume a new release will generate a 1 million
    in revenue over 6 mths
  • A slip of 1 mth represents a direct loss of
    167,000
  • A slip in release date may also mean a competitor
    takes market share
  • The loss of productivity due to a late delivery,
    missed deadline, or an inoperative application is
    just as real, though not easily estimated in
    dollar terms

60
Costs of a Poorly Performing Network
  • Lost sales opportunities
  • Low customer and user satisfaction
  • Slipped schedules
  • Low user morale
  • Out of business
  • Loss of market share

61
Performance Studies
  • Allow us to answer some Real World network
    performance questions as exemplified by following
    examples
  • How does User Response Time change ?

62
How does User Response Time Change ?
  • Situation Resource Utilization is low and there
    are few active users. Examples
  • when there are only a few digital images accessed
    per hour -gt Users do not have to
    wait
  • when there is little new mail to process -gt the
    Email system responds quickly
  • when there are only a few small reports printed
    per hour -gt No waiting for
    print jobs
  • In this situation Response time
  • will remain constant
  • is independent of the number of users, can add
    more users without and increase

63
How does User Response Time Change ?
  • Situation Resource Utilization High and Many
    active users
  • In this situation Response time
  • Increases as new users are added
  • will not remain constant
  • Long queues and lengthy wait times, users
    discover another way to measure response time -
    their position in the print queue
  • Refer fig 1-2
  • Queuing theory and simulation methods can provide
    estimates of how response time behaves versus the
    number of users

64
How does User Response Time Change ?
  • Situation Resource Utilization High and Many
    active users
  • Given this situation Response Time increases with
    increases in
  • the number of active users
  • user workload intensity
  • the volume of data moved
  • queue depth
  • level of network utilisation

65
Should a Server be Upgraded or a Second Server
installed ?
  • Response time includes delay introduced by the
    server
  • As workload on the server increases, its
    utilisation approaches 100 and user response
    time degrades
  • Should we upgrade server by
  • increasing number of disks
  • installing faster disks
  • replacing LAN adaptor with 32 bit version
  • adding more RAM
  • upgrading CPU
  • Where is the bottleneck ?

66
Should a Server be Upgraded or a Second Server
installed ?
  • Another option is to purchase a second server and
    redistribute the user workload between them
  • Queuing Theory can be used to answer this
    question.

67
Should a Server be Upgraded or a Second Server
installed ?
  • Queuing Theory - it is always better to have one
    fast server than two half speed servers
  • When the single fast server is run at
  • low utilisation, its response time will be almost
    half of the half-speed servers
  • high utilisation, its response time will be about
    equal to a half-speed servers
  • What other issue may come into consideration ?

68
How many PCs can be supported on the a LAN before
Response Time deteriorates ?
  • Analysis
  • PC generates about 1 utilisation
  • Keep LAN utilisation below 40 as response time
    becomes erratic as LAN utilisation exceeds about
    40
  • Users say response time is more important than
    utilisation
  • If we know up to 50 of users are active at any
    time, how many PCs can we attach to the LAN ?
  • A LAN analyser or an SNMP management station can
    be used to get a more accurate measure of the
    traffic generated by the PCs
  • What Media Access Control technique is being used
    ?

69
How many PCs can be supported on the a LAN before
Response Time deteriorates ?
  • Max Utilisation Users Active Nos of PCs
    Traffic per PC
  • Max Utilisation 40/100
  • Users Active 50/100
  • Traffic per PC 1/100

70
Can we add a NEW application to the Enterprise
Network ?
  • The ability of the network to support additional
    traffic comes under scrutiny when a new
    application is to be deployed,
  • new application and database servers
  • additional user workstations
  • LAN and WAN traffic will increase.
  • Analysis
  • benchmark new application on an isolated LAN and
    measure LAN traffic generated by each transaction
  • determine location of application and database
    servers
  • determine number and type of transactions that
    will occur at each location

71
Can we add a NEW application to the Enterprise
Network ?
  • Analysis
  • determine network topology
  • measure existing work traffic flows
  • Input topology and traffic details into a
    simulation tool
  • perform simulation to provide response time
    figures for each user site and utilisation
    figures for WANs and LANs
  • Result - a reproducible documented analysis that
    answers how the new application can be deployed

72
How do we decide WAN Link Speeds ?
  • Available Bandwidth
  • varies with offered traffic load
  • goes to near-zero at saturation, response time
    becomes more unpredictable at this point, ie the
    variation of response time around its mean
    increases dramatically
  • Network designers can provide and control
    bandwidth to achieve a certain level of response
    time.
  • eg they can fine-tune a routers priority
    queues to favour
  • interactive over bulk traffic

73
How do we decide WAN Link Speeds ?
  • Need to consider
  • network topology
  • location of clients and servers
  • work habits of the users
  • response time requirements
  • throughput requirements
  • transaction traffic loads offered by clients and
    servers
  • Using techniques such as mean-value analysis,
    worst-case analysis, queuing theory, or
    simulation, link speeds can be adjusted to meet
    the requirements
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