PowerPoint Presentation

Computer Simulation ofNetworksComputer Simulation ofNetworks

ECE/CSC 777: Telecommunications Network Design

Fall, 2013, Rudra Dutta

Objective - Performance StudyObjective - Performance Study

Direct Measurement

–Reactive

Disrupts the user/system, affects the behavior/characteristics

Can be done only on completed running systems

Predictive

–Analytical Methods

Useful if the model is available and is computationally efficient

–Most systems are complex and require highly complex mathematicalmodels

Usually only steady state can be modeled

–Simulation

Give more detail than analytical modeling

–Very useful when large design space to be explored

–Can study dynamic behavior

Not exact and involves some amount of approximation

–Can explore only part of operational space

Halfway approach - some implementation

Simulation and ModelSimulation and Model

Image courtesy of GSU, “Hyperphysics” site, Rovio

SimulationSimulation

Programmatic representation of system entities

–We know how bits and pieces of the system work algorithmically

Possibly for some bits we know mathematically

–Implement these algorithms and let them interact

Two main types of simulation

–Discrete event

discrete points in time, using random number generators

–Continuous time

State changes occur continuously across time

Packages

–E.g. OPNET (commercial), NS-2 (open source), OMNeT++

–Levels of detail

E.g. packet level, flow level

Discrete Event SimulationDiscrete Event Simulation

Useful for speeding up when details are abstracted

Decide what happens first

–Many things happen after this that are not modeled in thesimulation

–E.g. in GBN the next thing after receiving frame is sending ACK

Decide what events are triggered by this event that aremodeled by simulation

–Arrange in order of time of occurrence

Jump to next event, do the same

–Until you run out of events, or have run it for “long enough”

Discrete Event ExampleDiscrete Event Example

Sender transmits threemessages at times 0s, 10s,20s

The channel takes 2s topropagate

Receiver takes a randomtime between 1s and 9s toprocess

Sends back either a “moreinfo” request or an “OK”,alternately

Upon “more info”, sendersends one more messageimmediately

0s: S sends P1

10s: S sends P2

20s: S sends P3

2s: R rcvs P1

Discrete Event ExampleDiscrete Event Example

Sender transmits threemessages at times 0s, 10s,20s

The channel takes 2s topropagate

Receiver takes a randomtime between 1s and 9s toprocess

Sends back either a “moreinfo” request or an “OK”,alternately

Upon “more info”, sendersends one more messageimmediately

0s: S sends P1

10s: S sends P2

20s: S sends P3

13s: R rcvs P1+

11s: S sends P1+

11s: S rcvs MI

9s: R sends MI

2s: R rcvs P1

12s: R rcvs P2

Event ProcessorsEvent Processors

Each event generates new events

–Event generation algorithm is an algorithmic model ofthe part of the system being simulated

–“Callback” handlers or processors

Processing algorithms represent specificentities, e.g. GBN receiver

–These can obviously grow quite complex

Usually the bulk of the effort of simulation

–Also the problem specific part

–The Discrete Event framework is an engine and canbe commoditized

–Hence the concept of “entity models” that plug intosuch an engine

Introduction to OPNETIntroduction to OPNET

A variegated software package

–Modeler, IT Guru, SP Guru, …

Modeler: a comprehensive development environmentdriven from GUI

Features

–Object-oriented modeling

–Discrete event simulator

–Integrated data analysis tool

–More scalable and efficient simulation engine

–Hundreds of protocol and vendor device models

–Flexibility to develop detailed custom models

Extensive online documentation and tutorials - quickoverview here

Specification

Data Collectionand Simulation

Analysis

Using OPNETUsing OPNET

The cycle is repeated until we achieve the correctobjective

Typical ApplicationsTypical Applications

Network (LAN/WAN) performance modeling

Network planning

R & D in communications architectures andprotocols

Resource sizing

…

Model SpecificationModel Specification

Objective - developing a representation of thesystem

Mirrors the hierarchical structure of realnetworks/systems/…

Primarily three levels of abstraction

–Network Model (highest level)

Entire network, e.g., the entire Internet

–Node Model

Individual devices, e.g., computers, routers, servers, …

–Process Model (lowest level)

Basic level of functionality

Typically individual protocols

Creating Models in OPNET (1-a)Creating Models in OPNET (1-a)

Project Editor  Network Model

–Main work place for creating a network simulation

–Where you can :

Create a network model using models from the StandardModel Library (the highest level of abstraction)

Choose statistics to collect

Execute a simulation

View results

–Two options

Object Palette (use built-in models)

Build your own nodes

Creating Models in OPNET (1-b)Creating Models in OPNET (1-b)

Corresponds to thenetwork architecture /topology

Creating Models in OPNET (2-a)Creating Models in OPNET (2-a)

Node Editor  Node Model

2nd level of abstraction

Used to define the behavior of each network object(node/system)

Important constituents of node model are

–Modules

They model some internal aspect of node behavior

E.g. simple traffic source, processor, …

–Packet streams (flow of data)

Connect modules

Two options

–Use the library

–Build your own process modules

Creating Models in OPNET (2-b)Creating Models in OPNET (2-b)

Corresponds to theinternal structure ofdevices in the network

Creating Models in OPNET (3-a)Creating Models in OPNET (3-a)

Process Editor  Process Model

The lowest level of abstraction

Represented by Finite State Machines (FSMs)

–State (icons)

–Transitions (lines)

Operations performed in each state or for atransition are described in embedded C/C++code blocks

Creating Models in OPNET (3-b)Creating Models in OPNET (3-b)

Many Other EditorsMany Other Editors

E.g. link model editor, path editor, …

Probe editor

–Specify the statistics to be collected during simulation,e.g. delay, throughput, utilization, …

–Two types of statistics

Global

Local

Two ways to collect statistics

–Probe editor

–Straight from the project editor

Simulation & AnalysisSimulation & Analysis

Simulation execution

–using the “configure simulation” tool, or “advancedconfigure simulation” tool

–Can specify various attributes, simulation time andother details

Analysis

–Directly from project editor

–No. of ways to analyze the results: get time-average,peak values, etc.

–Graphical analysis