Network Simulation Experiments Manual: A Systems Approach, 4th Edition Manual

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NETWORK
SIMULATION
EXPERIMENTS
MANUAL

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Prepared by Emad Aboelela, Ph.D.
University of Massachusetts Dartmouth

NETWORK
SIMULATION
EXPERIMENTS
MANUAL
Second Edition

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07 08 09 10 11
5 4 3 2 1

CONTENTS

Preface

vii

Laboratory 0

Introduction

Laboratory 1

Ethernet

Laboratory 2

Token Ring

Laboratory 3

Switched LANs

Laboratory 4

Network Design

Laboratory 5

ATM

Laboratory 6

RIP: Routing Information Protocol

Laboratory 7

OSPF: Open Shortest Path First

Laboratory 8

Border Gateway Protocol (BGP)

Laboratory 9

TCP: Transmission Control Protocol

106

Laboratory 10

Queuing Disciplines

118

Laboratory 11

RSVP: Resource Reservation Protocol

130

Laboratory 12

Firewalls and VPN

147

Laboratory 13

Applications

158

Laboratory 14

Wireless Local Area Network

173

Laboratory 15

Mobile Wireless Network

185

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Preface
Welcome to the Network Simulation Experiments Manual, 2nd Edition. As networking systems have
become more complex and expensive, hands-on experiments based on networking simulation have become
essential for teaching the key computer networking topics to students and professionals. The simulation
approach is highly useful because it provides a virtual environment for a variety of desirable features such
as modeling a network based on specified criteria and analyzing its performance under different scenarios.
This manual has 15 laboratory experiments that cover a variety of networking designs and protocols. The
experiments in this manual do not require programming skills as a prerequisite. They are generic and can
be easily expanded to utilize new technologies and networking standards. With the free easy-to-install
software, the OPNET IT Guru Academic Edition, networking students and professionals can implement the
experiments from the convenience of their homes or workplaces. The manual is suitable for a singlesemester course on computer networking at the undergraduate or beginning graduate level. Professors can
pick the experiments that are appropriate to their class.
The new materials in the 2nd edition of the manual are:
A new Prelab Activities section in every laboratory assignment. This section includes
recommended readings from the textbook as well as references to animations from the Net-SEAL
project (www.net-seal.net)1. These activities are intended to reinforce the student’s understanding
of the concepts related to the topics covered in the laboratory.
Two new laboratory experiments to cover wireless networking protocols.
One new laboratory experiment to cover the Border Gateway Protocol (BGP).
A new component to address the ICMP protocol has been added to the RIP lab.
A new component to address the effect of encryption has been added to the VPN lab.
OPNET IT Guru Academic Edition provides a virtual environment for modeling, analyzing, and predicting
the performance of IT infrastructures, including applications, servers, and networking technologies. Based
on OPNET's award-winning IT Guru product, Academic Edition is designed to complement specific lab
exercises that teach fundamental networking concepts. The commercial version of IT Guru has broader
capabilities designed for the enterprise IT environment, documentation, and professional support. OPNET
software is used by thousands of commercial and government organizations worldwide, and by over 500
universities. For more information, visit www.opnet.com. OPNET and IT Guru are trademarks of OPNET
Technologies, Inc.
I would like to extend my appreciation to Professor Larry Peterson and Dr. Bruce Davie for giving me the
opportunity to associate the laboratory experiments of this manual with their valuable book. I want to thank
the folks at Morgan Kaufmann who have helped to bring this project to life. Many thanks to the reviewers,
from academia and OPNET, who read through the various drafts of the experiments and provided me with
extremely valuable feedback. My gratitude to the Net-SEAL project team, with special appreciation to Neal
Charbonneau for the superb job he did in the project. I want to thank my family for their consideration and
enthusiastic assurance throughout the development of this project. Last, but not least, I want to thank you
for choosing the manual. I welcome your emails to report bug or to suggest improvements.

Emad Aboelela, Ph.D.
eaboelela@umassd.edu
University of Massachusetts Dartmouth
July 2007

The materials in the www.net-seal.net website are based upon work supported by the National Science Foundation under
Grant No. DUE-0536388. Any opinions, findings and conclusions or recommendations expressed in this website are those
of the Net-SEAL project team and do not necessarily reflect the views of the National Science Foundation (NSF).

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Laboratory

Introduction
Basics of OPNET IT Guru Academic Edition
Objective
This lab teaches you the basics of using OPNET IT Guru Academic Edition. OPNET IT
Guru Academic Edition enables students to better understand the core concepts of
networking and equips them to effectively troubleshoot and manage real-world network
infrastructures.

Overview
OPNET’s IT Guru provides a Virtual Network Environment that models the behavior of
your entire network, including its routers, switches, protocols, servers, and individual
applications. By working in the Virtual Network Environment, IT managers, network and
system planners, and operations staff are empowered to diagnose difficult problems more
effectively, validate changes before they are implemented, and plan for future scenarios
including growth and failure.
OPNET's Application Characterization Environment (ACE) module for IT Guru enables
enterprises to identify the root cause of end-to-end application performance problems and
to solve them cost-effectively by understanding the impact of changes.
In this lab, you will learn the basics of the OPNET IT Guru Academic Edition software. You
will learn how to setup and run OPNET IT Guru Academic Edition. You will become
familiar with some of its capabilities by running some tutorials.
The labs in this manual are implemented with OPNET IT Guru Academic Edition release
9.1.A (Build 1997). If you want to download the software, please visit the following site to
register with OPNET technology:
www.opnet.com/university_program/itguru_academic_edition/
Recommended System Configuration, Platforms, and Software:
-

1.5 GHz processor or better
512 MB – 2 GB RAM
1 GB disk space
Display: 1024 x 768 or higher resolution, 256 or more colors
Adobe Acrobat reader.
The English language version of the following operating systems are supported:
• Microsoft Windows NT (Service Pack 3, 5, or 6a)

Network Simulation Experiments Manual

•
•

Windows 2000 Professional (Service Pack 1, 2, and 4 are supported but not
required)
Windows XP (Service Pack 1 is required; Service Pack 2 is supported but not
required.)

Prelab Activities
Read Chapter 1 from "Computer Networks: A Systems Approach", 4th Edition.

Go to www.net-seal.net/animations.php and play the following animation:
- No Network.

Procedure
Start OPNET IT Guru Academic Edition
To start OPNET IT Guru Academic Edition:
1. Click on Start Programs OPNET IT Guru Academic Edition x.x
OPNET IT Guru Academic Edition, where x.x is the software version (e.g.,
9.1).
2. Read the Restricted Use Agreement and if you agree, click I have read this
SOFTWARE AGREEMENT and I understand and accept the terms and
conditions described herein.
Now you should see the starting window of OPNET IT Guru Academic Edition as
shown:

Introduction

Check the OPNET Preferences
The OPNET Preferences let you display and edit environment attributes, which control
program operations. In this lab you will check three of these attributes.
1. After starting OPNET, from the Edit menu, choose Preferences.
2. The list of environment attributes is sorted alphabetically according to name. You
can locate attributes faster by typing any part of the attribute’s name in the Find
field.
3. Check the value of the license_server attribute. It has the name of the License
Server’s host. If IT Guru is getting its license from the local host (i.e., the
computer on which the software was installed), the value of license_server
should be localhost as shown in the following figure.
4. Set the license_server_standalone attribute to TRUE. This attribute specifies
whether the program acts as its own license server.
5. A model directory is a directory that contains OPNET model files. If the directory
is listed in the mod_dirs environment attribute, then OPNET programs will use
the models in that directory. Check the value of the mod_dirs attribute. The first
directory in the list is where your own models will be saved. In the future you
might need to access that directory to back up, copy, or move your models. IT
Guru saves numerous files for every single project you create.
6. Click OK to close the dialog box.

Network Simulation Experiments Manual

Run the Introduction Tutorial
Now you will run the introductory tutorial that teaches you the basics of using OPNET IT
Guru.
1. From the Help menu, select Tutorial.
2. Go over the Introduction lesson from the list of Basic Lessons.

Run the Small Internetworks Tutorial
In this tutorial you will learn how to use OPNET IT Guru features to build and analyze
network models.
1. From the Help menu, select Tutorial.
2. Carry out the Small Internetworks tutorial from the list of Basic Lessons.

Exercises
1) In the project you created for the Small Internetworks tutorial, add a new

scenario as a duplicate of the first_floor scenario. Name the new scenario
expansion2. In the expansion2 scenario expand the network the same way as
in the expansion scenario but with 30 nodes in the second floor instead of 15
nodes. Run the simulation and compare the load and delay graphs of this new
scenario with the corresponding graphs of the first_floor and expansion
scenarios.

Lab Report
The laboratory report of this lab (and also all the following labs in this manual) should
include the following items/sections:
-

A cover page with your name, course information, lab number and title, and
date of submission.

A summary of the addressed topic and objectives of the lab.

Implementation: a brief description of the process you followed in conducting
the implementation of the lab scenarios.

Results obtained throughout the lab implementation, the analysis of these
results, and a comparison of these results with your expectations.

Answers to the given exercises at the end of the lab. If an answer
incorporates new graphs, analysis of these graphs should be included here.

A conclusion that includes what you learned, difficulties you faced, and any
suggested extensions/improvements to the lab.

Laboratory

Ethernet
A Direct Link Network with Media Access Control
Objective
This lab is designed to demonstrate the operation of the Ethernet network. The simulation
in this lab will help you examine the performance of the Ethernet network under different
scenarios.

Overview
The Ethernet is a working example of the more general Carrier Sense, Multiple Access
with Collision Detect (CSMA/CD) local area network technology. The Ethernet is a
multiple-access network, meaning that a set of nodes sends and receives frames over a
shared link. The “carrier sense” in CSMA/CD means that all the nodes can distinguish
between an idle and a busy link. The “collision detect” means that a node listens as it
transmits and can therefore detect when a frame it is transmitting has interfered (collided)
with a frame transmitted by another node. The Ethernet is said to be a 1-persistent
protocol because an adaptor with a frame to send transmits with probability 1 whenever a
busy line goes idle.
In this lab you will set up an Ethernet with thirty nodes connected via a coaxial link in a bus
topology. The coaxial link is operating at a data rate of 10 Mbps. You will study how the
throughput of the network is affected by the network load as well as the size of the
packets.

Prelab Activities
Read section 2.6 from "Computer Networks: A Systems Approach", 4th Edition.

Go to www.net-seal.net/animations.php and play the following animation:
-Hub.

Network Simulation Experiments Manual

Procedure
Create a New Project
To create a new project for the Ethernet network:
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project Click OK Name the project _Ethernet, and
the scenario Coax Click OK.
Local area networks
(LANs) are designed to
span distances of up to a
few thousand meters.

3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Choose Office from the Network Scale list
Click Next Assign 200 to X Span and keep Y Span as 100 Click Next
twice Click OK.
4. Close the Object Palette dialog box.

Create the Network
To create our coaxial Ethernet network:
1. To create the network configuration, select Topology Rapid Configuration.
From the drop-down menu choose Bus and click OK.
2. Click the Select Models button in the Rapid Configuration dialog box. From the
Model List drop-down menu choose ethcoax and click OK.
3. In the Rapid Configuration dialog box, set the following eight values and click OK.
The eth_tap is an
Ethernet bus tap that
connects a node with the
bus.
The eth_coax is an
Ethernet bus that can
connect nodes with bus
receivers and
transmitters via taps.

Ethernet

4. To configure the coaxial bus, right-click on the horizontal link Select Advanced
Edit Attributes from the menu:
a. Click on the value of the model attribute Select Edit from the dropdown menu Choose the eth_coax_adv model.
b. Assign the value 0.05 to the delay attribute (propagation delay in sec/m).
c. Assign 5 to the thickness attribute.
d. Click OK.

A higher delay is used
here as an alternative to
generating higher traffic
which would require
much longer simulation
time.

Thickness specifies the
thickness of the line used
to “draw” the bus link.

5. Now you have created the network. It should look like the illustration below.
6. Make sure to save your project.

Network Simulation Experiments Manual

Configure the Network Nodes
To configure the traffic generated by the nodes:
1. Right-click on any of the 30 nodes Select Similar Nodes. Now all nodes in the
network are selected.
2. Right-click on any of the 30 nodes Edit Attributes.
3. Check the Apply Changes to Selected Objects check box. This is important to
avoid reconfiguring each node individually.
4. Expand the Traffic Generation Parameters hierarchy:
The argument of the
exponential distribution
is the mean of the
interval between
successive events. In the
exponential distribution
the probability of
occurrence of the next
event by a given time is
not at all dependent
upon the time of
occurrence of the last
event or the elapsed time
since that event.

a. Change the value of the ON State Time to exponential(100) Change
the value of the OFF State Time to exponential(0). (Note: Packets are
generated only in the "ON" state.)
5. Expand the Packet Generation Arguments hierarchy:
a. Change the value of the Packet Size attribute to constant(1024).
b. Right-click on the Interarrival Time attribute and choose Promote
Attribute to Higher Level. This allows us to assign multiple values to
the Interarrival Time attribute and hence to test the network
performance under different loads.

The interarrival time is
the time between
successive packet
generations in the "ON"
state.

6. Click OK to return back to the Project Editor.
7. Make sure to save your project.

Ethernet

Configure the Simulation
To examine the network performance under different loads, you need to run the simulation
several times by changing the load into the network. There is an easy way to do that.
Recall that we promoted the Interarrival Time attribute for package generation. Here we
will assign different values to that attribute:
Make sure that the
1. Click on the Configure/Run Simulation button:
Common tab is chosen Assign 15 seconds to the Duration.

2. Click on the Object Attributes tab.
3. Click on the Add button. The Add Attribute dialog box should appear filled with
the promoted attributes of all nodes in the network (if you do not see the attributes
in the list, close the whole project and reopen it). You need to add the Interarrival
Time attribute for all nodes. To do that:
a. Click on the first attribute in the list (Office Network.node_0.Traffic
Generation ….) Click the
Wildcard button Click on
node_0 and choose the
asterisk (*) from the dropdown menu Click OK.
b. A new attribute is now
generated containing the
asterisk (the second one in
the list), and you need to
add it by clicking on the
corresponding cell under the
Add? column.
c. The Add Attribute dialog box
should look like the shown
one Click OK.

Network Simulation Experiments Manual

4. Now you should see the Office Network.*.Traffic Generation Parameter … in
the list of simulation object attributes. Click on that attribute to select it Click the
Values button of the dialog box.
5. Add the following nine values. (Note: To add the first value, double-click on the
first cell in the Value column Type “exponential (2)” into the textbox and hit
enter. Repeat this for all nine values.)

6. Click OK. Now look at the upper-right corner of the Simulation Configuration
dialog box and make sure that the Number of runs in set is 9.

7. For each simulation of the nine runs, we need the simulator to save a “scalar”
value that represents the “average” load in the network and to save another
scalar value that represents the average throughput of the network. To save

Ethernet

these scalars we need to configure the simulator to save them in a file. Click on
the Advanced tab in the Configure Simulation dialog box.
8. Assign _Ethernet_Coax to the Scalar file text field.

9. Click OK and then save your project.

Choose the Statistics
To choose the statistics to be collected during the simulation:
1. Right-click anywhere in the project workspace (but not on one of the nodes or
links) and select Choose Individual Statistics from the pop-up menu Expand
the Global Statistics hierarchy.
a. Expand the Traffic Sink hierarchy Click the check box next to Traffic
Received (packets/sec) (make sure you select the statistic with units of
packets/sec),
b. Expand the Traffic Source hierarchy Click the check box next to
Traffic Sent (packets/sec).
c.

Click OK.

2. Now to collect the average of the above statistics as a scalar value by the end of
each simulation run:
A probe represents a
request by the user to
collect a particular piece
of data about a
simulation.

a. Select Choose Statistics (Advanced) from the Simulation menu.
b. The Traffic Sent and Traffic Received probes should appear under the
Global Statistic Probes.

Network Simulation Experiments Manual

Right-click on Traffic Received probe Edit Attributes. Set the scalar
data attribute to enabled Set the scalar type attribute to time
average Compare to the following figure and click OK.

d. Repeat the previous step with the Traffic Sent probe.
e. Select save from the File menu in the Probe Model window and then
close that window.
f.

Now you are back to the Project Editor. Make sure to save your project.

Run the Simulation
To run the simulation:
Make sure that 15
1. Click on the Configure/Run Simulation button:
second(s) (not hours) is assigned to the Duration Click Run. Depending on
the speed of your processor, this may take several minutes to complete.
2. Now the simulator is completing nine runs, one for each traffic generation
interarrival time (representing the load into the network). Notice that each
successive run takes longer to complete because the traffic intensity is increasing.
3. After the nine simulation runs complete, click Close.
4. Save your project.
When you rerun the simulation, OPNET IT Guru will “append” the new results to the
results already in the scalar file. To avoid that, delete the scalar file before you start a new
run. (Note: Deleting the scalar file after a run will result in losing the collected results from
that run.)
•

Go to the File menu Select Model Files Delete Model Files
Select ( .os): Output Scalars Select the scalar file to be deleted; in this lab it

Ethernet

is _Ethernet_Coax Confirm the deletion by clicking OK
Click Close.

View the Results
To view and analyze the results:
1. Select View Results (Advanced) from the Results menu. Now the Analysis
Configuration tool is open.
2. Recall that we saved the average results in a scalar file. To load this file, select
Load Output Scalar File from the File menu Select _Ethernet-Coax from the pop-up menu.
3. Select Create Scalar Panel from the Panels menu Assign Traffic
Source.Traffic Sent (packets/sec).average to Horizontal Assign Traffic
Sink.Traffic Received (packets/sec).average to Vertical Click OK.

4. The resulting graph should resemble the one below:

Network Simulation Experiments Manual

Further Readings
−

OPNET Ethernet Model Description: From the Protocols menu, select Ethernet
Model Usage Guide.

Exercises
1) Explain the graph we received in the simulation that shows the relationship

between the received (throughput) and sent (load) packets. Why does the
throughput drop when the load is either very low or very high?
2) Create three duplicates of the simulation scenario implemented in this lab. Name

these scenarios Coax_Q2a, Coax_Q2b, and Coax_Q2c. Set the Interarrival
Time attribute of the Packet Generation Arguments for all nodes (make sure to
check Apply Changes to Selected Objects while editing the attribute) in the
new scenarios as follows:
-

Coax_Q2a scenario: exponential(0.1)
Coax_Q2b scenario: exponential(0.05)
Coax_Q2c scenario: exponential(0.025)

In all the above new scenarios, open the Configure Simulation dialog box and
from the Object Attributes delete the multiple-value attribute (the only attribute
shown in the list).
Choose the following statistic for node 0: Ethcoax →Collision Count. Make sure
that the following global statistic is chosen: Global Statistics→Traffic
Sink→Traffic Received (packet/sec). (Refer to the Choose the Statistics section
in the lab.)
Run the simulation for all three new scenarios. Get two graphs: one to compare
node 0’s collision counts in these three scenarios and the other graph to compare
the received traffic from the three scenarios. Explain the graphs and comment on
the results. (Note: To compare results you need to select Compare Results from
the Results menu after the simulation run is done.)
3) To study the effect of the number of stations on Ethernet segment performance,

create a duplicate of the Coax_Q2c scenario, which you created in Exercise 2.
Name the new scenario Coax_Q3. In the new scenario, remove the oddnumbered nodes, a total of 15 nodes (node 1, node 3, …, and node 29). Run the
simulation for the new scenario. Create a graph that compares node 0’s collision
counts in scenarios Coax_Q2c and Coax_Q3. Explain the graph and comment
on the results.

Ethernet

4) In the simulation a packet size of 1024 bytes is used (Note: Each Ethernet packet

can contain up to 1500 bytes of data). To study the effect of the packet size on
the throughput of the created Ethernet network, create a duplicate of the
Coax_Q2c scenario, which you created in Exercise 2. Name the new scenario
Coax_Q4. In the new scenario use a packet size of 512 bytes (for all nodes). For
both Coax_Q2c and Coax_Q4 scenarios, choose the following global statistic:
Global Statistics→Traffic Sink→Traffic Received (bits/sec). Rerun the
simulation of Coax_Q2c and Coax_Q4 scenarios. Create a graph that compares
the throughput as packets/sec and another graph that compares the throughput
as bits/sec in Coax_Q2c and Coax_Q4 scenarios. Explain the graphs and
comment on the results.

Lab Report
Prepare a report that follows the guidelines explained in Lab 0. The report should include
the answers to the above exercises as well as the graphs you generated from the
simulation scenarios. Discuss the results you obtained and compare these results with
your expectations. Mention any anomalies or unexplained behaviors.

Laboratory

Token Ring
A Shared-Media Network with Media Access Control
Objective
This lab is designed to demonstrate the implementation of a token ring network. The
simulation in this lab will help you examine the performance of the token ring network
under different scenarios.

Overview
A token ring network consists of a set of nodes connected in a ring. The ring is a single
shared medium. The token ring technology involves a distributed algorithm that controls
when each node is allowed to transmit. All nodes see all frames. The destination node,
which is identified in the frame header, saves a copy of the frame as the frame flows past
the node. With a ring topology, any link or node failure would render the whole network
useless. This problem can be solved by using a star topology where nodes are connected
to a token ring hub. The hub acts as a relay, known as a multistation access unit (MSAU).
MSAUs are almost always used because of the need for robustness and ease of node
addition and removal.
The “token,” which is just a special sequence of bits, circulates around the ring; each node
receives and then forwards the token. When a node that has a frame to transmit sees the
token, it takes the token off the ring and instead inserts its frame into the ring. When the
frame makes its way back around to the sender, this node strips its frame off the ring and
reinserts the token. The token holding time (THT) is the time a given node is allowed to
hold the token. From its definition, the THT has an effect on the utilization and fairness of
the network, where utilization is the measure of the bandwidth used versus that available
on the given ring.
In this lab, you will set up a token ring network with 14 nodes connected in a star topology.
The links you will use operate at a data rate of 4 Mbps. You will study how the utilization
and delay of the network are affected by the network load as well as the THT.

Prelab Activities
Read section 2.7 from "Computer Networks: A Systems Approach", 4th Edition.

Token Ring

Procedure
Create a New Project
To create a new project for the token ring network:
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _Token, and
the scenario Balanced Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Choose Office for the Network scale
Click Next three times Click OK.
4. Close the Object Palette and then save your project.

Create the Network
To create our token ring network:
1. Select Topology Rapid Configuration. From the drop-down menu choose
Star and click OK.
2. Click the Select Models button in the Rapid Configuration dialog box. From the
Model List drop-down menu choose token_ring and click OK.
3. In the Rapid Configuration dialog box, set the following six values and click OK.

The tr32_hub node
model is a token ring
hub supporting up to 32
connections at 4 or 16
Mbps. The hub forwards
an arriving packet to the
next output port. There
is no queuing of packets
in the hub itself as the
processing time is
considered to be zero.

The TR4 link connects
two token ring devices
to form a ring at 4
Mbps.

Network Simulation Experiments Manual

4. You have now created the network, and it should look like the following:

5. Make sure to save your project.

Configure the Network Nodes
Here you will configure the THT of the nodes as well as the traffic generated by them. To
configure the THT of the nodes, you need to use the tr_station_adv model for the nodes
instead of the current one, tr_station.
1. Right-click on any of the 14 nodes Select Similar Nodes. Now all nodes in the
network are selected.
2. Right-click on any of the 14 nodes Edit Attributes.
a. Check the Apply Changes to Selected Objects check box. This is
important to avoid reconfiguring each node individually.
The following figure shows the attributes we will change in steps 3 to 6:

Token Ring

The THT (token
holding time) specifies
the maximum amount
of time a token ring
MAC (media access
control) may use the
token before releasing it.

The interarrival time is
the time between
successive packet
generations in the "ON"
state.

3. Click on the model value: tr_station and select Edit from the drop-down menu.
Now select tr_station_adv from the extended drop-down menu.
4. To test the network under different THT values, you need to “promote” the THT
parameter. This allows us to assign multiple values to the THT attribute.
a. Expand the Token Ring Parameters hierarchy.
b. Right-click on the THT Duration attribute Choose Promote Attribute
to Higher Level.
5. Expand the Traffic Generation Parameters hierarchy Assign
exponential(100) to the ON State Time attribute Assign exponential(0) to the
OFF State Time attribute. (Note: Packets are generated only in the "ON" state.)
6. Expand the Packet Generation Arguments
exponential(0.025) to the Interarrival Time attribute.
7. Click OK to return back to the Project Editor.
8. Make sure to save your project.

hierarchy

Assign

Network Simulation Experiments Manual

Configure the Simulation
To examine the network performance under different THTs, you need to run the simulation
several times by changing THT with every run of the simulation. There is an easy way to
do that. Recall that we promoted the THT Duration attribute. Here we will assign different
values to that attribute:
1. Click on the Configure/Run Simulation button:
2. Make sure that the Common tab is chosen Assign 5 minutes to the Duration.

3. Click on the Object Attributes tab Click the Add button.
4. As shown in the following Add Attribute dialog box, you need to add the THT
Duration attribute for all nodes. To do that:
a. Add the unresolved attribute: Office Network.*.Token Ring
Parameters[0].THT Duration by clicking on the corresponding cell under the
Add? column Click OK.

Token Ring

5. Now you should see the Office Network.*.Token Ring Parameters[0].THT
Duration in the list of simulation object attributes (widen the “Attribute” column to
see the full name of the attribute). Click on that attribute Click the Values
button, as shown below.

6. Add the following six values. (Note: To add the first value, double-click on the first
cell in the Value column Type “0.01” into the textbox and hit enter. Repeat this
for all six values.)

Network Simulation Experiments Manual

7. Click OK. Now look at the upper-right corner of the Simulation Configuration
dialog box and make sure that the Number of runs in set is 6.

8. For each of the six simulation runs we need the simulator to save “scalar” values
that represent the “average” values of the collected statistics. To save these
scalars we need to configure the simulator to save them in a file. Click on the
Advanced tab in the Configure Simulation dialog box.
9. Assign _Token_Balanced to the Scalar file text field.

10. Click OK and then save your project.

Token Ring

Choose the Statistics
To choose the statistics to be collected during the simulation:
1. Right-click anywhere in the project workspace (but not on a node or link) and
select Choose Individual Statistics from the pop-up menu.
a. Expand the Global Statistics hierarchy:
−
−
The utilization is a
measure of the
bandwidth used versus
that available on the
given ring.

Expand the Traffic Sink hierarchy Click the check box next
to Traffic Received (packets/sec).
Expand the Traffic Source hierarchy Click the check box
next to Traffic Sent (packets/sec).

b. Expand the Node Statistics hierarchy:
−
c.

Expand the Token Ring hierarchy Click the check box next
to Utilization.

Click OK.

2. Now we want to collect the average of the above statistics as a scalar value by
the end of each simulation run.
a. Select Choose Statistics (Advanced) from the Simulation menu.
A probe represents a
request by the user to
collect a particular piece
of data about a
simulation.

b. The Traffic Sent and Traffic Received probes should appear under the
Global Statistic Probes. The Utilization probe should appear under the
Node Statistics Probes.
c.

Right-click on Traffic Received probe Edit Attributes. Set the scalar
data attribute to enabled Set the scalar type attribute to time
average Compare to the following figure and click OK.

d. Repeat the previous step with the Traffic Sent and Utilization probes.

Network Simulation Experiments Manual

3. Since we need to analyze the effect of THT on the network performance, THT
must be added as an “input” statistic to be recorded by the simulation. To do that:
a. Select Create Attribute Probe from the Objects menu. Now a new
attribute is created under the Attribute Probes hierarchy as shown.
b. Right-click on the new attribute probe and select Choose Attributed
Object from the pop-up menu Expand the Office Network hierarchy
Click on node_0 (actually you can pick any other node) Click OK.

Right-click again on the new attribute probe and select Edit Attributes
from the pop-up menu Assign the Token Ring Parameter[0].THT
Duration value to the “attribute” Attribute, as shown in the figure
Click OK.

4. Select save from the File menu in the Probe Model window and then Close the
window.
5. Now you are back to the Project Editor. Make sure to save your project.

Token Ring

Duplicate the Scenario
The token ring network scenario we just implemented is balanced: the distribution of the
generated traffic in all nodes is the same. To compare performance, you will create an
“unbalanced” scenario as follows:
1. Select Duplicate Scenario from the Scenarios menu and give it the name
Unbalanced Click OK.
2. Select node_0 and node_7 by shift-clicking on both nodes Right-click on one
of these two selected nodes and select Edit Attributes Expand the Traffic
Generation Parameters hierarchy Expand the Packet Generation
Arguments hierarchy Change the value of the Interarrival Time attribute to
exponential(0.005) as shown. Make sure to check the Apply Changes to
Selected Objects box before you click OK.

3. Select all nodes except node_0 and node_7 Right-click on one of the
selected nodes and select Edit Attributes Change the value of the
Interarrival Time attribute to exponential(0.075) as in the previous step. Make
sure to check the Apply Changes to Selected Objects box before you click
OK.
4. Click anywhere in the workspace to unselect objects.
5. Click on the Configure/Run Simulation button:
Click on the Advanced
tab in the Configure Simulation dialog box
Assign _Token_Unbalanced to the Scalar file text field.
6. Click OK and then save your project.

Network Simulation Experiments Manual

Run the Simulation
To run the simulation for both scenarios simultaneously:
1. Go to the Scenarios menu Select Manage Scenarios.
2. Change the values under the Results column to (or )
for both scenarios. Compare to the following figure.

3. Click OK to run the simulations. Depending on the speed of your processor, this
may take several minutes to complete.
4. After the simulation completes the 12 runs, 6 for each scenario, click Close.
5. Save your project.
When you rerun the simulation, OPNET IT Guru will “append” the new results to the
results already in the scalar file. To avoid that, delete the scalar file before you start a new
run.
•

Go to the File menu Select Model Files Delete Model Files From the
list, choose other model types Select ( .os): Output Scalars Select the
scalar file to be deleted; in this lab they are
_Token_Balanced and
_Token_Unbalanced Click Close.

Token Ring

Scalar File from the File menu Select _Token_Balanced from
the pop-up menu.
3. Select Create Scalar Panel from the Panels menu Select the scalar panel
data as shown in the following dialog box: THT for Horizontal and Utilization for
Vertical. (Note: If any of the data is missing, make sure that you carried out steps
2.c and 2.d in the Choose the Statistics section.)

4. Click OK.
5. To change the title of the graph, right-click on the graph area and choose Edit
Graph Properties Change the Custom Title to Balanced Utilization as
shown.

6. Click OK. The resulting graph should resemble the one shown below. Do not
close the graph and continue with the following step.

Network Simulation Experiments Manual

7. To compare with the Unbalanced scenario, load its scalar file, select Load
Output Scalar File from the File menu
Select _Token_Unbalanced from the pop-up menu.
8. Select Create Scalar Panel from the Panels menu Select the scalar panel
data as in step 3.
9. Click OK Change the graph title to Unbalanced as in step 5 Click OK. The
resulting graph should resemble the one shown below. Do not close this graph or
the previous one and continue with the following step.

Token Ring

10. To combine the above two graphs on a single graph, select Create Vector Panel
from the Panels menu Click on the Display Panel Graphs tab Select both
Balanced and Unbalanced statistics Choose Overlaid Statistics from the
drop-down menu in the right-bottom area of the dialog box as shown.

11. Click Show and the resulting graph should resemble the one shown below.

Network Simulation Experiments Manual

12. Repeat the same process to check the effect of the THT on Traffic Received for
both scenarios. Assign the appropriate titles to the graphs.
13. The resulting graph, which combines the Traffic Received statistic for both the
Balanced and Unbalanced scenarios, should resemble the following one:

Token Ring

Further Readings
−

OPNET Token Ring Model Description: From the Protocols menu, select Token
Ring Model Usage Guide.

Exercises
1) Why does the utilization increase with higher THT values?
2) Create a duplicate scenario of the Balanced scenario. Name the new scenario

Q2_HalfLoad. In the Q2_HalfLoad scenario, decrease the load into the network
(i.e., load from all nodes in the network) by half and repeat the simulation.
Compare the utilization and traffic received in the Q2_HalfLoad scenario with
those of the Balanced scenario.
Hints:
-

Decreasing the load from a node by half can be done by doubling the
“Interarrival Time” of the node’s Packet Generation Arguments.
Do not forget to assign a separate “scalar file” for the new scenario.

3) Create a duplicate scenario of the Balanced scenario. Name the new scenario

Q3_OneNode. In the Q3_OneNode scenario, reconfigure the network so that
node_0 generates a traffic load that is equivalent to the traffic load generated by
all nodes in the Balanced scenario combined. The rest of the nodes, node_1 to
node_13, generate no traffic. Compare the utilization and traffic received in
Q3_OneNode scenario with those of the Balanced scenario.
Hints:
-

One way to configure a node so that it does not generate traffic is to set
its Start Time (it is one of the Traffic Generation Parameters) to the
special value Never.
Do not forget to assign a separate “scalar file” for the new scenario.

Laboratory

Switched LANs
A Set of Local Area Networks Interconnected by Switches
Objective
This lab is designed to demonstrate the implementation of switched local area networks.
The simulation in this lab will help you examine the performance of different
implementations of local area networks connected by switches and hubs.

Overview
There is a limit to how many hosts can be attached to a single network and to the size of a
geographic area that a single network can serve. Computer networks use switches to
enable the communication between one host and another, even when no direct
connection exists between those hosts. A switch is a device with several inputs and
outputs leading to and from the hosts that the switch interconnects. The core job of a
switch is to take packets that arrive on an input and forward (or switch) them to the right
output so that they will reach their appropriate destination.
A key problem that a switch must deal with is the finite bandwidth of its outputs. If packets
destined for a certain output arrive at a switch and their arrival rate exceeds the capacity of
that output, then we have a problem of contention. In this case, the switch will queue, or
buffer, packets until the contention subsides. If the contention lasts too long, however, the
switch will run out of buffer space and be forced to discard packets. When packets are
discarded too frequently, the switch is said to be congested.
In this lab you will set up switched LANs using two different switching devices: hubs and
switches. A hub forwards the packet that arrives on any of its inputs on all the outputs
regardless of the destination of the packet. On the other hand, a switch forwards incoming
packets to one or more outputs depending on the destination(s) of the packets. You will
study how the throughput and collision of packets in a switched network are affected by
the configuration of the network and the types of switching devices that are used.

Switched LANs

Prelab Activities
Read section 3.1 from "Computer Networks: A Systems Approach", 4th Edition.

Go to www.net-seal.net/animations.php and play the following animations:
- Switch.
- Switched Network With No Server.
- Switched Network With Server.

Procedure
Create a New Project
1.

Start the OPNET IT Guru Academic Edition Choose New from the File
menu.

2. Select Project and click OK Name the project
_SwitchedLAN, and the scenario OnlyHub Click OK.
3.

In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Choose Office from the Network Scale list
Click Next three times Click OK.

4. Close the Object Palette dialog box.

Create the Network
To create our switched LAN:
1. Select Topology Rapid Configuration. From the drop-down menu choose
Star and click OK.
2. Click the Select Models button in the Rapid Configuration dialog box. From the
Model List drop-down menu choose ethernet and click OK.
The prefix ethernet16_
indicates that the device
supports up to 16
Ethernet connections.

3. In the Rapid Configuration dialog box, set the following five values: Center Node
Model = ethernet16_hub, Periphery Node Model = ethernet_station, Link
Model = 10BaseT, Number=16, Y=50, and Radius = 42 Click OK.

Network Simulation Experiments Manual

The 10BaseT link
represents an Ethernet
connection operating at
10 Mbps.

4. Right-click on node_16, which is the hub Edit Attributes Change the name
attribute to Hub1 and click OK.
5. Now that you have created the network, it should look like the following one.
6. Make sure to save your project.

Configure the Network Nodes
Here you will configure the traffic generated by the stations.
1. Right-click on any of the 16 stations (node_0 to node_15) Select Similar
Nodes. Now all stations in the network are selected.
2. Right-click on any of the 16 stations Edit Attributes.
a. Check the Apply Changes to Selected Objects check box. This is
important to avoid reconfiguring each node individually.

Switched LANs

3. Expand the hierarchies of the Traffic Generation Parameters attribute and the
Packet Generation Arguments attribute Set the following four values:
4. Click OK to close the attribute editing window(s).
5. Save your project.

Choose Statistics
To choose the statistics to be
collected during the simulation:
The Ethernet Delay
represents the end to
end delay of all packets
received by all the
stations.

1. Right-click anywhere in the
project workspace and
select Choose Individual
Statistics from the pop-up
menu.

Traffic Received (in
packets/sec) by the
traffic sinks across all
nodes.

2. In the Choose Results
dialog box, choose the
following four statistics:

Traffic Sent (in
packets/sec) by the
traffic sources across all
nodes.

Collision Count is the
total number of
collisions encountered
by the hub during packet
transmissions.

3. Click OK.

Network Simulation Experiments Manual

Configure the Simulation
Here we need to configure the duration of the simulation:

1. Click on the Configure/Run Simulation button:
2. Set the duration to be 2.0 minutes.
3. Click OK.

Duplicate the Scenario
The network we just created utilizes only one hub to connect the 16 stations. We need to
create another network that utilizes a switch and see how this will affect the performance
of the network. To do that we will create a duplicate of the current network:
1. Select Duplicate Scenario from the Scenarios menu and give it the name
HubAndSwitch Click OK.
. Make sure that Ethernet is
2. Open the Object Palette by clicking on
selected in the pull-down menu on the object palette.
3. We need to place the shown hub and switch in the new scenario.

4. To add the Hub, click its icon in the object palette Move your mouse to the
workspace Click to drop the hub at a location you select. Right-click to indicate
you are done deploying hub objects.
5. Similarly, add the Switch and then close the Object Palette.
6.

Right-click on the new hub Edit Attributes Change the name attribute to
Hub2 and click OK.

Right-click on the switch Edit Attributes Change the name attribute to
Switch and click OK.

8. Reconfigure the network of the HubAndSwitch scenario so that it looks like the
following one.

Switched LANs

Hints:
a. To remove a link, select it and choose Cut from the Edit menu (or simply hit
the Delete key). You can select multiple links and delete all of them at once.
b. To add a new link, use the 10BaseT link available in the Object Palette.

9. Save your project.

Run the Simulation
To run the simulation for both scenarios simultaneously:
1. Select Manage Scenarios from the Scenarios menu.
2. Change the values under the Results column to (or )
for both scenarios. Compare to the following figure.

3. Click OK to run the two simulations. Depending on the speed of your processor,
this may take several minutes to complete.

Network Simulation Experiments Manual

4. After the two simulation runs complete, one for each scenario, click Close.
5. Save your project.

View the Results
To view and analyze the results:
1. Select Compare Results from the Results menu.
time_average is the
average value over time
of the values generated
during the collection
window. This average is
performed assuming a
“sample-and-hold”
behavior of the data set
(i.e., each value is
weighted by the amount
of time separating it
from the following
update and the sum of
all the weighted values is
divided by the width of
the collection window).
For example, suppose
you have a 1-second
bucket in which 10
values have been
generated. The first 7
values were generated
between 0 and 0.3
seconds, the 8th value at
0.4 seconds, the 9th
value at 0.6 seconds ,
and the 10th at 0.99
seconds. Because the last
3 values have higher
durations, they are
weighted more heavily in
calculating the time
average.

2. Change the drop-down menu in the lower-right part of the Compare Results
dialog box from As Is to time_average, as shown.

3. Select the Traffic Sent
(packets/sec) statistic and click
Show. The resulting graph should
resemble the one below. As you
can see, the traffic sent in both
scenarios is almost identical.

Switched LANs

Select the Traffic Received (packets/sec) statistic and click Show. The resulting graph should
resemble the one below. As you see, the traffic received with the second scenario, HubAndSwitch, is
higher than that of the OnlyHub scenario.

4. Select the Delay (sec) statistic and click Show. The resulting graph should
resemble the one below. (Note: Result may vary slightly due to different node
placement.)

Network Simulation Experiments Manual

5. Select the Collision Count statistic for Hub1 and click Show.
6. On the resulting graph right-click anywhere on the graph area Choose Add
Statistic Expand the hierarchies as shown below Select the Collision
Count statistic for Hub2 Change As Is to time_average Click Add.

7. The resulting graph should resemble the one below.

8. Save your project.

Switched LANs

Further Readings
−

OPNET Building Networks: From the Protocols menu, select Methodologies
Building Network Topologies.

Exercises
1) Explain why adding a switch makes the network perform better in terms of

throughput and delay.
2) We analyzed the collision counts of the hubs. Can you analyze the collision count

of the “Switch”? Explain your answer.
3) Create two new scenarios. The first one is the same as the OnlyHub scenario

but replace the hub with a switch. The second new scenario is the same as the
HubAndSwitch scenario but replace both hubs with two switches, remove the
old switch, and connect the two switches you just added together with a 10BaseT
link. Compare the performance of the four scenarios in terms of delay,
throughput,
and
collision
count.
Analyze
the
results.
Note: To replace a hub with a switch, right-click on the hub and assign
ethernet16_switch to its model attribute.

Laboratory

Network Design
Planning a Network with Different Users, Hosts, and Services
Objective
The objective of this lab is to demonstrate the basics of designing a network, taking into
consideration the users, services, and locations of the hosts.

Overview
Optimizing the design of a network is a major issue. Simulations are usually used to
analyze the conceptual design of the network. The initial conceptual design is usually
refined several times until a final decision is made to implement the design. The objective
is to have a design that maximizes the network performance, taking into consideration the
cost constraints and the required services to be offered to different types of users. After
the network has been implemented, network optimization should be performed periodically
throughout the lifetime of the network to ensure maximum performance of the network and
to monitor the utilization of the network resources.
In this lab you will design a network for a company that has four departments: Research,
Engineering, E-Commerce, and Sales. You will utilize a LAN model that allows you to
simulate multiple clients and servers in one simulation object. This model dramatically
reduces both the amount of configuration work you need to perform and the amount of
memory needed to execute the simulation. You will be able to define a profile that
specifies the pattern of applications employed by the users of each department in the
company. By the end of this lab, you will be able to study how different design decisions
can affect the performance of the network.

Prelab Activities
Read section 3.2 from "Computer Networks: A Systems Approach", 4th Edition.

Go to www.net-seal.net/animations.php and play the following animations:
- Adding Switches.

Network Design

Procedure
Create a New Project
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _NetDesign,
and the scenario SimpleNetwork Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Choose Campus from the Network Scale
list Click Next Choose Miles from the Size drop-down menu and assign 1
for both X Span and Y Span Click Next twice Click OK.

Create and Configure the Network
Initialize the Network:
Application Config is
used to specify
applications that will be
used to configure users
profiles.

1. The Object Palette dialog box should be now on the top of your project space. If it
. Make sure that the internet_toolbox is
is not there, open it by clicking
selected from the pull-down menu on the object palette.
2. Add to the project workspace the following objects from the palette: Application
Config, Profile Config, and a subnet.

Profile Config describes
the activity patterns of a
user or group of users in
terms of the applications
used over a period of
time. You must define
the applications using
the Application Config
object before using this
object.

a. To add an object from a palette, click its icon in the object palette Move your
mouse to the workspace Left-click to place the object. Right-click when
finished. The workspace should contain the following three objects:

3. Close the Object Palette dialog box and save your project.

Network Simulation Experiments Manual

Configure the Services:
1. Right-click on the Application Config node Edit Attributes Change the
name attribute to Applications Change the Application Definitions attribute
to Default Click OK.
2. Right-click on the Profile Config node Edit Attributes Change the name
attribute to Profiles Change the Profile Configuration attribute to Sample
Profiles Click OK.
Sample Profiles provides patterns of applications employed by users such as
engineers, researchers, salespeople, and multimedia users.

Configure a Subnet:
1. Right-click on the subnet node Edit Attributes Change the name attribute
to Engineering and click OK.
2. Double-click on the Engineering node. You get an empty workspace, indicating
that the subnet contains no objects.
3. Open the object palette

and make sure it is still set to internet_toolbox.

4. Add the following items to the subnet workspace: 10BaseT LAN, ethernet16
Switch, and a 10BaseT link to connect the LAN with the Switch Close the
palette.
5. Right-click on the 10BaseT LAN node Edit Attributes Change the name
attribute to LAN Observe that the Number of Workstations attribute has a
value of 10. Click in the Value column for the Application: Supported Profiles
attribute, and select Edit. You should get a table in which you should do the
following:
a. Set the number of rows to 1.
b. Set the Profile Name to Engineer. Note: Engineer is one of the
“sample” profiles provided within the Profile Config object.
c. Click OK twice.
The object we just created is equivalent to a 10-workstation star topology LAN.
The traffic generated from the users of this LAN resembles that generated by
“engineers.”
6. Rename the ethernet16 Switch to Switch.
7. The subnet should look like the shown one.
8. Save your project.

Network Design

Configure All Departments:
1. Now you have completed the configuration of the Engineering department
subnet. To go back to the main project space, click the Go to the higher level
button.
The subnets of the other departments in the company should be similar to the
engineering one except for the supported profiles.
2. Make three copies of the Engineering subnet we just created: Click on the
Engineering node From the Edit menu, select Copy From the Edit menu,
select Paste three times, placing the subnet in the workspace after each, to
create the new subnets.
3. Rename (right-click on the subnet and select Set Name) and arrange the subnets
as shown below:

4. Double-click the Research node Edit the attributes of its LAN Edit the
value of the Application: Supported Profiles attribute Change the value of
the Profile Name from Engineer to Researcher Click OK twice Go to the
higher level by clicking the

button.

5. Repeat step 4 with the Sales node and assign to its Profile Name the profile
Sales Person.
6. Repeat step 4 with the E-Commerce node and assign to its Profile Name the
profile E-commerce Customer.
7. Save your project.

Network Simulation Experiments Manual

Configure the Servers:
Now we need to implement a subnet that contains the servers. The servers have to
support the applications defined in the profiles we deployed. You can double-check those
applications by editing the attributes of our Profile node. Inspect each row under the
Applications hierarchy, which in turn, is under the Profile Configuration hierarchy. You
will see that we need servers that support the following applications: Web browsing, Email,
Telnet, File Transfer, Database, and File Print.
and add a new subnet Rename the new
1. Open the Object Palette
subnet to Servers Double-click the Servers node to enter its workspace.
2. From the Object Palette, add three ethernet_servers, one ethernet16_switch,
and three 10BaseT links to connect the servers with the switch.
3. Close the Object Palette.
4. Rename the servers and the switch as follows:

5. Right-click on each one of the above servers and Edit the value of the
Application: Supported Services attribute.
i. For the Web Server add four rows to support the following services: Web
Browsing (Light HTTP1.1), Web Browsing (Heavy HTTP1.1), Email
(Light), and Telnet Session (Light).
ii. For the File Server add two rows to support the following services: File
Transfer (Light) and File Print (Light).
iii. For the Database Server add one row to support the following service:
Database Access (Light).
6. Go back to the project space by clicking the Go to the higher level
7. Save your project.

button.

Network Design

Connect the Subnets:
Now all subnets are ready to be connected together.
1. Open the Object Palette
and add four 100BaseT links to connect the
subnets of the departments to the Servers subnet.
As you create each link, make sure that it is configured to connect the
“switches” in both subnets to each other. Do this by choosing them from the
drop-down menus as follows:

2. Close the Object Palette.
3. Now your network should resemble the following one:

4. Save your project.

Network Simulation Experiments Manual

Choose the Statistics
To test the performance of our network we will collect one of the many available statistics
as follows:
1. Right-click anywhere in the project workspace and select Choose Individual
Statistics from the pop-up menu.
2. In the Choose Results dialog box, choose the following statistic:

Page Response Time
is the required time to
retrieve the entire page.

3. Click OK.

Configure the Simulation
Here we need to configure the duration of the simulation:

1. Click on the Configure/Run Simulation
2. Set the duration to be 30.0 minutes.
3. Press OK.

button.

Network Design

Duplicate the Scenario
In the network we just created we assumed that there is no background traffic already in
the links. In real networks, the links usually have some existing background traffic. We will
create a duplicate of the SimpleNetwork scenario but with background utilization in the
100BaseT links.
Link utilization is the
percentage of the used
link bandwidth.

1. Select Duplicate Scenario from the Scenarios menu and give it the name
BusyNetwork Click OK.
2. Select all the 100BaseT links simultaneously (click on all of them while holding
the Shift key) Right-click on anyone of them Edit Attributes Check the
Apply Changes to Selected Objects check box.
3. Expand the hierarchy of the Background Utilization attribute Expand the row
0 hierarchy Assign 99 to the background utilization (%) as shown below.

4. Click OK.
5. Save your project.

Network Simulation Experiments Manual

3. Click OK to run the two simulations. Depending on the speed of your processor,
this may take several seconds to complete.
4. After the two simulation runs complete (one for each scenario), click Close.
5. Save your project.

Network Design

View the Results
To view and analyze the results:
1. Select Compare Results from the Results menu.
2. Change the drop-down menu in the lower-right part of the Compare Results
dialog box from As Is to time_average as shown.

3. Select the Page Response Time (seconds) statistic and click Show. The
resulting graph should resemble the one below. (Note: Results may vary slightly
due to different node placement.)

Network Simulation Experiments Manual

Further Readings
−

OPNET Configuring Applications and Profiles: From the Protocols menu, select
Applications Model Usage Guide Configuring Profiles and
Applications.

Exercises
1) Analyze the result we obtained regarding the HTTP page response time. Collect

four other statistics, of your choice, and rerun the simulation of the Simple and the
Busy network scenarios. Get the graphs that compare the collected statistics.
Comment on these results.
2) In the BusyNetwork scenario, study the utilization% of the CPUs in the servers

(Right-click on each server and select Choose Individual Statistics CPU
Utilization).
3) Create a new scenario as a duplicate of the BusyNetwork scenario. Name the

new scenario Q3_OneServer. Replace the three servers with only one server
that supports all required services. Study the utilization% of that server’s CPU.
Compare this utilization with the three CPU utilizations you obtained in the
previous exercise.
4) Create a new scenario as a duplicate of the BusyNetwork scenario. Name the

new scenario Q4_FasterNetwork. In the Q4_FasterNetwork scenario, replace
all 100BaseT links in the network with 10Gbps Ethernet links and replace all
10BaseT links with 100BaseT links. Study how increasing the bandwidth of the
links affects the performance of the network in the new scenario (e.g., compare
the HTTP page response time in the new scenario with that of the
BusyNetwork).

Laboratory

ATM
A Connection-Oriented, Cell-Switching Technology
Objective
The objective of this lab is to examine the effect of ATM adaptation layers and service
classes on the performance of the network.

Overview
Asynchronous Transfer Mode (ATM) is a connection-oriented, packet-switched
technology. The packets that are switched in an ATM network are of a fixed length, 53
bytes, and are called cells. The cell size has a particular effect on carrying voice traffic
effectively. The ATM Adaptation Layer (AAL) sits between ATM and the variable-length
packet protocols that might use ATM, such as IP. The AAL header contains the
information needed by the destination to reassemble the individual cells back into the
original message. Because ATM was designed to support all sorts of services, including
voice, video, and data, it was felt that different services would have different AAL needs.
AAL1 and AAL2 were designed to support applications, like voice, that require guaranteed
bit rates. AAL3/4 and AAL5 provide support for packet data running over ATM.
ATM provides QoS capabilities through its five service classes: CBR, VBR-rt, VBR-nrt,
ABR, and UBR. With CBR (constant bit rate), sources transmit stream traffic at a fixed
rate. CBR is well-suited for voice traffic that usually requires circuit switching. Therefore,
CBR is very important to telephone companies. UBR, unspecified bit rate, is ATM’s besteffort service. There is one small difference between UBR and the best-effort model.
Because ATM always requires a signaling phase before data is sent, UBR allows the
source to specify a maximum rate at which it will send. Switches may make use of this
information to decide whether to admit or reject the new VC (virtual circuit).
In this lab you will set up an ATM network that carries three applications: Voice, Email, and
FTP. You will study how the choice of the adaptation layer as well as the service classes
can affect the performance of the applications.

Prelab Activities
Read section 3.3 from "Computer Networks: A Systems Approach", 4th Edition.

Network Simulation Experiments Manual

Procedure
Create a New Project
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _ATM, and the
scenario CBR_UBR Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Select Choose From Maps from the
Network Scale list Click Next Choose USA from the maps Click Next
From the Select Technologies list, include the atm_advanced Model Family as
shown in the following figure Click Next Click OK.

Create and Configure the Network
Initialize the Network:
1. The Object Palette dialog box should now be on the top of your project
. Make sure that
workspace. If it is not there, open it by clicking
atm_advanced is selected from the pull-down menu on the object palette.
2. Add to the project work space the following objects from the palette: Application
Config, Profile Config, two atm8_crossconn_adv switches, and a subnet.
a. To add an object from a palette, click its icon in the object palette Move your
mouse to the workspace and click to place the object Right-click to get out of
“object creation mode.”
3. Close the Object Palette dialog box and rename (right-click on the node Set
Name) the objects you added as shown and then save your project:

ATM

Configure the Applications:
1. Right-click on the Applications node Edit Attributes Expand the
Application Definitions attribute and set rows to 3 Name the rows: FTP,
EMAIL, and VOICE.

PCM stands for Pulse
Code Modulation. It is a
procedure used to
digitize speech before
transmitting it over the
network.

i. Go to the FTP row Expand the Description hierarchy Assign High
Load to FTP.
ii. Go to the EMAIL row Expand the Description hierarchy Assign High
Load to Email.
iii. Go to the VOICE row Expand the Description hierarchy Assign PCM
Quality Speech to Voice.

2. Click OK and then save your project.

Network Simulation Experiments Manual

Configure the Profiles:
1. Right-click on the Profiles node Edit Attributes Expand the Profile
Configuration attribute and set rows to 3.
i. Name and set the attributes of row 0 as shown:

ii. Name and set the attributes of row 1 as shown:

ATM

iii. Name and set the attributes of row 2 as shown. (Note: To set the Duration to
exponential(60), you will need to assign “Not Used” to the “Special Value”)
Close the Object Palette dialog box.

Configure the NorthEast Subnet:
1. Double-click on the NorthEast subnet node. You get an empty workspace,
indicating that the subnet contains no objects.
and make sure that atm_advanced is selected from
2. Open the object palette
the pull-down menu on the object palette..
3. Add the following items to the subnet workspace: one atm8_crossconn_adv
switch, one atm_uni_server_adv, four atm_uni_client_adv, and connect them
with bidirectional atm_adv links Close the palette Rename the objects as
shown.

Network Simulation Experiments Manual

4. Change the data rate attribute for all links to DS1.
Hint: To edit the
attributes of multiple
nodes in a single
operation, select all
nodes simultaneously
using shift and left-click;
then Edit Attributes of
one of the nodes, and
select Apply Changes
to Selected Objects.

5. For both NE_Voice1 and NE_Voice2, set the following attributes:
i. Set ATM Application Parameters to CBR only.
ii. Expand the ATM Parameters hierarchy Set Queue Configuration to
CBR only.
iii. Expand the Application: Supported Profiles hierarchy Set rows to 1
Expand the row 0 hierarchy Set Profile Name to VOICE_P.
iv. Application: Supported Services Edit its value Set rows to 1 Set
Name of the added row to VOICE Click OK.
v. Expand the Application: Transport Protocol hierarchy Voice Transport
= AAL2.

Client Address is the
Transport Adaptation
Layer (TPAL) address of
the node. This value
must be unique for each
node.
The TPAL model suite
presents a basic, uniform
interface between
applications and
transport layer models.
All interactions with a
remote application
through TPAL are
organized into sessions.
A session is a single
conversation between
two applications through
a transport protocol.

6. For NE_Voice1, select Edit Attributes Edit the value of the Client Address
attribute and write down NE_Voice1.
7. For NE_Voice2, select Edit Attributes Edit the value of the Client Address
attribute and write down NE_Voice2.
8. Configure the NE_DataServer as follows:
i. Application: Supported Services Edit its value Set rows to 2 Set
Name of the added rows to: EMAIL and FTP Click OK.
ii. Expand the Application: Transport Protocol Specification hierarchy
Voice Transport = AAL2.
iii. Edit the value of the Server Address attribute and write down
NE_DataServer.
9. For both NE_Data1 and NE_Data2, set the following attributes:

The queue
configuration specifies
a one-to-one mapping
between output port
queues and the QoS that
they support. A specific
queue may be
configured to support a
specific QoS.

i. Expand the ATM Parameters hierarchy Set Queue Configuration to
UBR.
ii. Expand the Application: Supported Profiles hierarchy Set rows to 2
Set Profile Name to FTP_P (for row 0) and to EMAIL_P (for row 1).
10. For NE_Data1, select Edit Attributes Edit the value of the Client Address
attribute and write down NE_Data1.
11. For NE_Data2, select Edit Attributes Edit the value of the Client Address
attribute and write down NE_Data2.
12. Save your project.

ATM

Add Remaining Subnets:
1. Now you completed the configuration of the NorthEast subnet. To go back to the
project space, click the Go to the higher level

button.

The subnets of the other regions should be similar to the NorthEast one except for the
names and client addresses.
2. Make three copies of the subnet we just created.
3. Rename (right-click on the node Set Name) the subnets and connect them to
the switches with bidirectional atm_adv links as shown. (Note: You will be asked
to pick the node inside the subnet to be connected to the link. Make sure to
choose the “switch” inside each subnet to be connected.)

4. Change the data rate for all links to DS1.
5. Select and double-click each of the new subnets (total four subnets) and change
the names, client address, and server address of the nodes inside these
subnets as appropriate (e.g., replace NE with SW for the SouthWest subnet).

Hint: To do step 6, you
can right-click on any
voice station and choose
Edit Similar Nodes.
This brings up a table in
which each node
occupies one row and
attributes are shown in
the columns.
Follow the same
procedure with similar
steps in this lab.

Network Simulation Experiments Manual

6. For all voice stations in all subnets (total of eight stations), edit the value of the
Application: Destination Preferences attribute as follows:
i. Set rows to 1 Set Symbolic Name to Voice Destination Click on (…)
under the Actual Name column Set rows to 6 For each row choose a
voice station that is not in the current subnet. The following figure shows the
actual names for one of the voice stations in the NorthEast subnet:

7. For all data stations in all subnets (total of eight stations), configure the
Application: Destination Preferences attribute as follows:
i. Set rows to 2 Set Symbolic Name to FTP Server for the one row and
Email Server for the other row For each symbolic name (i.e., FTP Server
and Email Server), click on (…) under the Actual Name column Set rows
to 3 For each row choose a data server that is not in the current subnet.
The following figure shows the actual names for one of the data stations in
the NorthEast subnet:

ATM
Hint: To do step 8 in a
single operation, you can
use the right-click menu
on any switch to Select
Similar Nodes; then
Edit Attributes, and
check Apply Changes
to Selected Objects.
This feature does work,
even across objects in
different subnets.

8. For all switches in the network (total of six switches), configure the
Max_Avail_BW of the CBR queue to be 100%, as shown below, and the
Min_Guaran_BW to be 20%.

Max_Avail_BW is the
maximum bandwidth
allocated to this queue.
Calls will be admitted
into this queue only if
they are within the
maximum available
bandwidth requirement.

9. Save your project.

Network Simulation Experiments Manual

3. Click OK.

ATM

Configure the Simulation
Here we need to configure the duration of the simulation:

1. Click on the Configure/Run Simulation button:

2. Set the duration to be 10.0 minutes.
3. Click OK. We will be running the simulation later.

Duplicate the Scenario
In the network we just created, we used the CBR service class for the Voice application
and the UBR service class for the FTP and Email applications. To analyze the effect of
such different classes of services, we will create another scenario that is similar to the
CBR_UBR scenario we just created but it uses only one class of service, UBR, for all
applications. In addition, to test the effect of the ATM adaptation layer, in the new scenario
we will use AAL5 for the Voice application rather than AAL2.
1. Select Duplicate Scenario from the Scenarios menu and give it the name
UBR_UBR Click OK.
2. For all voice stations in all subnets, reconfigure them as follows. (Check the note
below for a faster way to carry out this step.)
i. Set ATM Application Parameters to UBR only.
ii. ATM Parameters Set Queue Configuration to UBR.
iii. Application: Transport Protocol Set Voice Transport to AAL5.
3. Save your project.
Note: One easy way to carry out step 2 above is through the network browser as
follows:
-

Select Show Network Browser from the View menu.

Select Nodes from the drop-down menu, and check the Only Selected check
box as shown in the following figure.

Write voice in the find field and click Enter.

In the network browser you should see a list of all voice stations selected.

Right-click on any of the voice stations in the list, select Edit Attributes, and
check Apply Changes to Selected Objects.

Carry out the configuration changes in step 2 above.

To hide the network browser, deselect Show Network Browser from the View
menu.

Network Simulation Experiments Manual

3. Click OK to run the two simulations. Depending on the speed of your processor,
this may take several minutes to complete.
4. After the two simulation runs complete, one for each scenario, click Close.
5. Save your project.

ATM

View the Results
To view and analyze the results:
1. Select Compare Results from the Results menu.
2. Change the drop-down menu in the right-lower part of the Compare Results
dialog box from As Is to time_average as shown.

3. Select the voice Packet Delay Variation statistic and click Show. The resulting
graph should resemble the one below. (Note: Result may vary slightly due to
different node placement.)

Network Simulation Experiments Manual

Further Readings
−

OPNET ATM Model Description: From the Protocols menu, select ATM
Model Usage Guide.

Exercises
1) Analyze the result we obtained regarding the voice Packet Delay Variation time.

Obtain the graphs that compare the Voice packet end-to-end delay, the Email
download response time, and the FTP download response time for both
scenarios. Comment on the results.
2) Create another scenario as a duplicate of the CBR_UBR scenario. Name the

new scenario Q2_CBR_ABR. In the new scenario you should use the ABR class
of service for data, i.e., the FTP and Email applications in the data stations.
Compare the performance of the CBR_ABR scenario with that of the CBR_UBR
scenario.
Hints:
- To set ABR class of service to a node, assign ABR Only to its ATM
Application Parameters attribute and ABR only (Per VC Queue) to its
Queue Configuration (one of the ATM Parameters).
- For all switches in the network (total of 6 switches), configure the
Max_Avail_BW of the ABR queue to be 100% and the Min_Guaran_BW to
be 20%.
3) Edit the FTP application defined in the Applications node so that its File Size is

twice the current size (i.e., make it 100000 bytes instead of 50000 bytes). Edit the
EMAIL application defined in the Applications node so that its File Size is five
times the current size (i.e., make it 10000 bytes instead of 2000 bytes). Study
how this affects the voice application performance in both the CBR_UBR and
UBR_UBR scenarios. (Hint: to answer this exercise, you might need to create
duplicates of the CBR_UBR and UBR_UBR scenarios. Name the new scenarios
Q3_CBR_UBR and Q3_UBR_UBR respectively)

Laboratory

RIP: Routing Information Protocol
A Routing Protocol Based on the Distance-Vector Algorithm
Objective
The objective of this lab is to configure and analyze the performance of the Routing
Information Protocol (RIP) model.

Overview
A router in the network needs to be able to look at a packet’s destination address and then
determines which one of the output ports is the best choice to get the packet to that
address. The router makes this decision by consulting a forwarding table. The
fundamental problem of routing is: How do routers acquire the information in their
forwarding tables?
Routing algorithms are required to build the routing tables and hence forwarding tables.
The basic problem of routing is to find the lowest-cost path between any two nodes, where
the cost of a path equals the sum of the costs of all the edges that make up the path.
Routing is achieved in most practical networks by running routing protocols among the
nodes. The protocols provide a distributed, dynamic way to solve the problem of finding
the lowest-cost path in the presence of link and node failures and changing edge costs.
One of the main classes of routing algorithms is the distance-vector algorithm. Each node
constructs a vector containing the distances (costs) to all other nodes and distributes that
vector to its immediate neighbors. RIP is the canonical example of a routing protocol built
on the distance-vector algorithm. Routers running RIP send their advertisements regularly
(e.g., every 30 seconds). A router also sends an update message whenever a triggered
update from another router causes it to change its routing table.
The Internet Control Message Protocol (ICMP) can be utilized to analyze the performance
of the created routes. It can be used to model traffic between routers without the need of
running applications in an end node.
In this lab you will set up a network that utilizes RIP as its routing protocol. You will analyze
the routing tables generated in the routers, and you will observe how RIP is affected by
link failures. You will also utilize the ICMP to create echo reply messages (i.e., ping) to
analyze the created routes.

Network Simulation Experiments Manual

Prelab Activities
Read sections 4.1.5, 4.1.7, and 4.2.2 from "Computer Networks: A Systems
Approach", 4th Edition.

Go to www.net-seal.net/animations.php and play the following animations:
- The Address Resolution Protocol (ARP).
- ARP with Multiple Networks.
- Routing.

Procedure
Create a New Project
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _RIP, and the
scenario NO_Failure Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Select Campus from the Network Scale
list Click Next three times Click OK.

Create and Configure the Network
Initialize the Network:
1. The Object Palette dialog box should now be on top of your project workspace. If
The ethernet4_slip8_
gtwy node model
represents an IP-based
gateway supporting four
Ethernet hub interfaces
and eight serial line
interfaces. IP packets
arriving on any interface
are routed to the
appropriate output
interface based on their
destination IP address.
The Routing
Information Protocol
(RIP) or the Open
Shortest Path First
(OSPF) protocol may be
used to dynamically and
automatically create the
gateway's routing tables
and select routes in an
adaptive manner.

. Make sure that the internet_toolbox is
it is not there, open it by clicking
selected from the pull-down menu on the object palette.
2. Add to the project workspace the following objects from the palette: one
ethernet4_slip8_gtwy router and two 100BaseT_LAN objects.
a. To add an object from a palette, click its icon in the object palette Move your
mouse to the workspace Click to place the object Right-click to stop
creating objects of that type.
3. Use bidirectional 100BaseT links to connect the objects you just added as in the
following figure. Also, rename the objects as shown (right-click on the node Set
Name).
4. Close the Object Palette dialog box.
5. Save your project.

RIP: Routing Information Protocol

Configure the Router:
1. Right-click on Router1 Edit Attributes Expand the IP Routing
Parameters hierarchy and set the following:
i. Routing Table Export = Once at End of Simulation. This asks the router to
export its routing table at the end of the simulation to the simulation log.
2. Click OK and then save your project.

Add the Remaining LANs:
1. Highlight or select simultaneously (using shift and left-click) all five objects that
you currently have in the project workspace (one router, two LANs, and two links).
You can click-and-drag a box around the objects to do this.
2. Press Ctrl+C to copy the selected objects and then press Ctrl+V to paste them.

The PPP_DS3 link has
a data rate of 44.736
Mbps.

3. Repeat step 2 three times to generate three new copies of the objects and
arrange them in a way similar to the following figure. Rename all objects as
shown.
4. Connect routers, as shown, using PPP_DS3 links.

Network Simulation Experiments Manual

Choose the Statistics
RIP traffic is the total
amount of RIP update
traffic (in bits)
sent/received per
second by all the nodes
using RIP as the routing
protocol in the IP
interfaces in the node.
Total Number of
Updates is the number
of times the routing table
at this node gets updated
(e.g., due to a new route
addition, an existing
route deletion, and/or a
next hop update).

To test the performance of the RIP protocol, we will collect the following statistics:
1. Right-click anywhere in the project workspace and select Choose Individual
Statistics from the pop-up menu.
2. In the Choose Results dialog box, check the following statistics:
a. Global Statistics RIP Traffic Sent (bits/sec).
b. Global Statistics RIP Traffic Received (bits/sec).
c. Nodes Statistics Route Table Total Number of Updates.
3. Click OK and then save your project.

Configure the Simulation
Here we need to configure some of the simulation parameters:
1.

Click on

and the Configure Simulation window should appear.

2. Set the duration to be 10.0 minutes.
Auto Addressed means
that all IP interfaces are
assigned IP addresses
automatically during
simulation. The class of
address (e.g., A, B, or C)
is determined based on
the number of hosts in
the designed network.
Subnet masks assigned
to these interfaces are
the default subnet masks
for that class.
Export causes the autoassigned IP interface to
be exported to a file
(name of the file is
ip_addresses.gdf and
gets saved in the primary
model directory).

3. Click on the Global Attributes tab and change the following attributes:
a. IP Dynamic Routing Protocol = RIP. This sets the RIP protocol to be
the routing protocol of all routers in the network.
b. IP Interface Addressing Mode = Auto Addressed/Export.
c. RIP Sim Efficiency = Disabled. If this attribute is enabled, RIP will stop
after the "RIP Stop Time." But we need the RIP to keep updating the
routing table in case there is any change in the network (as we will see
in the second scenario).
4. Click OK and then save the project.

RIP: Routing Information Protocol

The Ping Scenario
In this scenario we will utilize the ping model to print the list of traversed nodes while the
ICMP request message is sent to the destination and the ICMP response is received from
the destination. Traversed routes are logged in the simulation log file..
1. Select Duplicate Scenario from the Scenarios menu and name it ICMP_Ping
Click OK.
2. Select both Router1 and Router3 simultaneously (click on both of them while
holding the Shift key) Select the Protocols menu IP Demands
Configure Ping Traffic on Selected Nodes.
3. Change the Pattern attribute to Record Route as shown Click OK.

Notice that a Ping Parameter node will be added to the project space. In addition the ping
demand is created between Rotuer1 and Router3 as a dotted line.

The Failure Scenario
The routers in the network we created will build their routing tables with no need to update
these tables further because we didn’t simulate any node or link failures. In this scenario
we will simulate failures so that we can compare the behavior of the routers in both cases.
1. Select Duplicate Scenario from the Scenarios menu and name it Failure
Click OK.
. Select the Utilities palette from the drop2. Open Object Palette by clicking
down menu Add a Failure Recovery object to your workspace and name it
Failure as shown Close the Object Palette dialog box.

Network Simulation Experiments Manual

3. Right-click on the Failure object Edit Attributes Expand the Link
Failure/Recovery Specification hierarchy Set rows to 1 Set the attributes
of the added row, row 0, as follows:

This will “fail” the link between Router1 and Router2 200 seconds into the
simulation.
5. Click OK and then save the project.

RIP: Routing Information Protocol

3. Click OK to run the three simulations. Depending on the speed of your
processor, this may take several seconds to complete.
4. After the three simulation runs complete, one for each scenario, click Close
Save your project.

View the Results
Compare the Number of Updates:
1. Select Compare Results from the Results menu.
2. Change the drop-down menu in the right-lower part of the Compare Results
dialog box to Stacked Statistics and Select Scenarios as shown.

Network Simulation Experiments Manual

3. Select the Total Number of Updates statistic for Router1 and click Show
Select the NO_Failure and Failure scenarios in the Select Scenarios dialog box.
4. You should get two graphs, one for each scenario. Right-click on each graph and
select Draw Style Bar.
5. The resulting
graphs should
resemble the
following (you
can zoom in on
the graphs by
clicking-anddragging a box
over the region
of interest):

RIP: Routing Information Protocol

Obtain the IP Addresses of the Interface:
Before checking the contents of the routing tables, we need to determine the IP address
information for all interfaces in the current network. Recall that these IP addresses are
assigned automatically during simulation, and we set the global attribute IP Interface
Addressing Mode to export this information to a file.
1. From the File menu choose Model Files Refresh Model Directories. This
causes OPNET IT Guru to search the model directories and update its list of files.
2. From the File menu choose Open From the drop-down menu choose Generic
Data File Select the _RIP-NO_Failure-ip_addresses file (the
other file created from the Failure scenario should contain the same information)
Click OK.

3. The following is a part of the gdf file content. It shows the IP addresses assigned
to the interfaces of Router1 in our network. For example the interface of Router1
that is connected to Net11 has the IP address 192.0.0.1 (Note: Your result
may vary due to different nodes placement.) The Subnet Mask associated with
that interface indicates that the address of the subnetwork, to which the interface
is connected, is 192.0.0.0 (i.e., the logical AND of the interface IP address and
the subnet mask).

4. Print out the layout of the network you implemented in this lab. On this layout,
from the information included in the gdf file, write down the IP addresses
associated with Router1 as well as the addresses assigned to each subnetwork
as shown in the following two figures (Note: Your IP addresses may vary due to
different nodes placement.)

Network Simulation Experiments Manual

Getting the Ping Report:
1. To check the content of the ping report for Router1:
i. Go to the ICMP_Ping scenario Go to the Results menu Open
Simulation Log Click on the field PING REPORT for “Campus Network
Router1”. The report should resemble the following one:

RIP: Routing Information Protocol

Compare the Routing Tables Content:
1. To check the content of the routing tables in Router1 for the Failure and
NO_Failure scenarios:
i. Go to the Results menu Open Simulation Log Expand the hierarchy
on the left as shown below Click on the field COMMON ROUTE TABLE.

2. Carry out the previous step for the Failure and NO_Failure scenarios. The
following are partial contents of Router1’s routing table for both scenarios (Note:
Your results may vary due to different nodes placement):

Routing table of Router1 (NO_Failure scenario):

Loopback interface
allows a client and a
server on the same host
to communicate with
each other using
TCP/IP.

Network Simulation Experiments Manual

Routing table of Router1 (Failure scenario):

RIP: Routing Information Protocol

Further Readings
−

RIP: IETF RFC number 2453 (www.ietf.org/rfc.html).

−

ICMP: IETF RFC number 792 (www.ietf.org/rfc.html).

Exercises
1) Obtain and analyze the graphs that compare the sent RIP traffic for the Failure

and NO_Failure scenarios. Make sure to change the draw style for the graphs to
Bar.
2) Describe and explain the effect of the failure of the link connecting Router1 to

Router2 on the routing tables of Router1.
3) Create another scenario as a duplicate of the Failure scenario. Name the new

scenario Q3_Recover. In this new scenario have the link connecting Router1 to
Router2 recover after 400 seconds (make sure to keep the failure that occurs at
the 200th second). Generate and analyze the graph that shows the effect of this
recovery on the Total Number of Updates in the routing table of Router1. Check
the contents of Router1‘s routing table. Compare this table with the
corresponding routing tables generated in the NO_Failure and Failure scenarios.
4) Change the Ping packet size to 5000 bytes (Hint: Edit the attributes of the Ping

Parameters node). Run the simulation to generate a new Ping report. What is the
effect of the new size on the ICMP packet response time?

Laboratory

OSPF: Open Shortest Path First
A Routing Protocol Based on the Link-State Algorithm
Objective
The objective of this lab is to configure and analyze the performance of the Open Shortest
Path First (OSPF) routing protocol.

Overview
In Lab 6 we discussed RIP, which is the canonical example of a routing protocol built on
the distance-vector algorithm. Each node constructs a vector containing the distances
(costs) to all other nodes and distributes that vector to its immediate neighbors. Link-state
routing is the second major class of intra-domain routing protocol. The basic idea behind
link-state protocols is very simple: Every node knows how to reach its directly connected
neighbors, and if we make sure that the totality of this knowledge is disseminated to every
node, then every node will have enough knowledge of the network to build a complete
map of the network.
Once a given node has a complete map for the topology of the network, it is able to decide
the best route to each destination. Calculating those routes is based on a well-known
algorithm from graph theory—Dijkstra’s shortest-path algorithm.
OSPF introduces another layer of hierarchy into routing by allowing a domain to be
partitioned into areas. This means that a router within a domain does not necessarily need
to know how to reach every network within that domain—it may be sufficient for it to know
how to get to the right area. Thus, there is a reduction in the amount of information that
must be transmitted to and stored in each node. In addition, OSPF allows multiple routes
to the same destination to be assigned the same cost and will cause traffic to be
distributed evenly over those routers.
In this lab, you will set up a network that utilizes OSPF as its routing protocol. You will
analyze the routing tables generated in the routers and will observe how the resulting
routes are affected by assigning areas and enabling load balancing.

OSPF: Open Shortest Path First

Prelab Activities
Read sections 4.2.3 and 4.2.4 from "Computer Networks: A Systems Approach",
4th Edition.

Go to www.net-seal.net/animations.php and play the following animation:
- Routing.

Procedure
Create a New Project
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _OSPF, and the
scenario No_Areas Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Select Campus from the Network Scale
list Click Next three times Click OK.

Create and Configure the Network
Initialize the Network:
The slip8_gtwy node
model represents an IPbased gateway
supporting up to eight
serial line interfaces at a
selectable data rate. The
RIP or OSPF protocols
may be used to
automatically and
dynamically create the
gateway's routing tables
and select routes in an
adaptive manner.
The PPP_DS3 link has
a data rate of 44.736
Mbps.

1. The Object Palette dialog box should now be on top of your project workspace. If
it is not there, open it by clicking
menu on the object palette.

. Select the routers item from the pull-down

a. Add to the project workspace eight routers of type slip8_gtwy. To add an
object from a palette, click its icon in the object palette Move your mouse to
the workspace and click to place the object You can keep on left-clicking to
create additional objects. Right-click when you are finished placing the last
object.
2. Switch the palette configuration so it contains the internet_toolbox. Use
bidirectional
PPP_DS3 links to
connect the
routers. Rename
the routers as
shown below.
3. Close the Object
Palette and then
save your project.

Network Simulation Experiments Manual

Configure the Link Costs:
1. We need to assign link costs to match the following graph:
5

2. Like many popular commercial routers, OPNET router models support a
parameter called a reference bandwidth to calculate the actual cost, as follows:
Cost = (Reference bandwidth) / (Link bandwidth)
where the default value of the reference bandwidth is 1,000,000 Kbps.
3. For example, to assign a cost of 5 to a link, assign a bandwidth of 200,000 Kbps
to that link. Note that this is not the actual bandwidth of the link in the sense of
transmission speed, but merely a parameter used to configure link costs.
4. To assign the costs to the links of our network, do the following:
i. Select all links in your network that correspond to the links with a cost of 5 in
the above graph by shift-clicking on them.
ii. Select the Protocols menu IP Routing Configure Interface Metric
Information.
iii. Assign 200000 to the Bandwidth (Kbps) field Check the Interfaces
across selected links radio button, as shown Click OK.

5. Repeat step 4 for all links with a cost of 10 but assign 100,000 Kbps to the
Bandwidth (Kbps) field.
6. Repeat step 4 for all links with a cost of 20 but assign 50,000 Kbps to the
Bandwidth (Kbps) field.
7. Save your project.

OSPF: Open Shortest Path First

Configure the Traffic Demands:
1. Select both RouterA and RouterC by shift-clicking on them.
i. Select the Protocols menu IP Demands Create Traffic Demands
Check the From RouterA radio button as shown Keep the color as
blue Click Create. Now you should see a blue-dotted line representing the
traffic demand between RouterA and RouterC.

2. Select both RouterB and RouterH by shift-clicking on them.
i. Select the Protocols menu IP Demands Create Traffic Demands
Check the From RouterB radio button Change the color to red Click
OK Click Create.
Now you can see the lines representing the traffic demands as shown.

3. To hide these lines: Select the View menu Select Demand Objects Select
Hide All.
4. Save your project.

Network Simulation Experiments Manual

Configure the Routing Protocol and Addresses:
1. Select the Protocols menu IP Routing Configure Routing Protocols.
2. Check the OSPF check box Uncheck the RIP check box Uncheck the
Visualize Routing Domains check box, as shown:

3. Click OK.
Auto-Assign IP
Addresses assigns a
unique IP address to
connected IP interfaces
whose IP address is
currently set to autoassigned. It does not
change the value of
manually set IP
addresses.

4. Select RouterA and RouterB only Select the Protocols menu IP
Routing Select Export Routing Table for Selected Routers Click OK on
the Status Confirm dialog box.
5. Select the Protocols menu IP Addressing Select Auto-Assign IP
Addresses.
6. Save your project.

Configure the Simulation
Here we need to configure some of the simulation parameters:

Click on

and the Configure Simulation window should appear.

2. Set the duration to be 10.0 minutes.
3. Click OK and then save your project.

OSPF: Open Shortest Path First

Duplicate the Scenario
In the network we just created, all routers belong to one level of hierarchy (i.e., one area).
Also, we didn’t enforce load balancing for any routes. Two new scenarios will be created.
The first new scenario will define two new areas in addition to the backbone area. The
second one will be configured to balance the load for the traffic demands between
RouterB and RouterH.

The Areas Scenario:
1. Select Duplicate Scenario from the Scenarios menu and give it the name
Areas Click OK.
2. Area 0.0.0.1:
i. Select the three links that connect RouterA, RouterB, and RouterC by shiftclicking on them Select the Protocols menu OSPF Configure Areas
Assign the value 0.0.0.1 to the Area Identifier, as shown Click OK.

Loopback interface
allows a client and a
server on the same host
to communicate with
each other using
TCP/IP.

ii. Right-click on RouterC Edit Attributes Expand the OSPF Parameters
hierarchy Expand the Loopback Interfaces hierarchy Expand the row0
hierarchy Assign 0.0.0.1 to the value of the Area ID attribute Click OK.

3. Area 0.0.0.2:
i. Click somewhere in the project workspace to disable the selected links and then
repeat step 2-i for the three links that connect RouterF, RouterG, and RouterH
but assign the value 0.0.0.2 to their Area Identifier.

Network Simulation Experiments Manual

4. To visualize the areas we just created, select the Protocols menu OSPF
Visualize Areas Click OK. The network should look like the following one with
different colors assigned to each area (you may get different colors though).
Note:
-

The area you did not configure is the backbone area and its Area
Identifier = 0.0.0.0.

The figure shows the links with a thickness of 3.

The Balanced Scenario:
1. Under the Scenarios menu, Switch to Scenario Select No_Areas.
OPNET provides two
types of IP load
balancing:
With Destination
Based, load balancing is
done on a perdestination basis. The
route chosen from the
source router to the
destination network is
the same for all packets.
With Packet Based,
load balancing is done
on a per-packet basis.
The route chosen from
the source router to the
destination network is
redetermined for every
individual packet.

2. Select Duplicate Scenario from the Scenarios menu, and give it the name
Balanced Click OK.
3. In the new scenario, select both RouterB and RouterH by shift-clicking on them.
4. Select the Protocols menu IP Routing Configure Load Balancing
Options Make sure that the option is Packet-Based and the radio button
Selected Routers is selected as shown Click OK.

5. Save your project.

OSPF: Open Shortest Path First

Run the Simulation
To run the simulation for the three scenarios simultaneously:
1. Go to the Scenarios menu Select Manage Scenarios.
2. Click on the row of each scenario and click the Collect Results button. This
should change the values under the Results column to as shown.

View the Results
The No_Areas Scenario:
1. Go back to the No_Areas scenario.
2. To display the route for the traffic demand between RouterA and RouterC:
Select the Protocols menu IP Demands Display Routes for
Configured Demands Expand the hierarchies as shown and select RouterA
Æ RouterC Go to the Display column and pick Yes Click Close.

Network Simulation Experiments Manual

3. The resulting route will appear on the network as shown:

4. Repeat step 2 to show the route for the traffic demand between RouterB and
RouterH. The route is as shown below. (Note: Depending on the order in which
you created the network topology, the other “equal-cost” path can be used, that is,
the RouterB-RouterA-RouterD-RouterF-RouterH path).

The Areas Scenario:
1. Go to scenario Areas.
2. Display the route for the traffic demand between RouterA and RouterC. The
route is as shown:

3. Save your project.

OSPF: Open Shortest Path First

The Balanced Scenario:
1. Go to scenario Balanced.
2. Display the route for the traffic demand between RouterB and RouterH. The
route is as shown:

3. Save your project.

Further Readings
−

OPNET OSPF Model Description: From the Protocols menu, select OSPF
Model Usage Guide.

−

OSPF: IETF RFC number 2328 (www.ietf.org/rfc.html).

Exercises
1) Explain why, for the same pair of routers, the Areas and Balanced scenarios

result in different routes than those observed in the No_Areas scenario.
2) Using the simulation log, examine the generated routing table in RouterA for

each one of the three scenarios. Explain the values assigned to the Metric
column of each route.
Hints:
-

Refer to the View Results section in Lab 6 for information about
examining the routing tables. You will need to set the global attribute IP
Interface Addressing Mode to the value Auto Addressed/Export and
rerun the simulation.
To determine the IP address information for all interfaces, you need to
open the Generic Data File that contains the IP addresses and
associated with the scenarios.

Network Simulation Experiments Manual
3) OPNET allows you to examine the link-state database that is used by each router

to build the directed graph of the network. Examine this database for RouterA in
the No_Areas scenario. Show how RouterA utilizes this database to create a
map for the topology of the network and draw this map (This is the map that will
be used later by the router to create its routing table.)

Note: A stub network
only carries local traffic
(i.e., packets to and from
local hosts). Even if it
has paths to more than
one other network, it
does not carry traffic for
other networks (RFC
1983).

Hints:
- To export the link-state database of a router, Edit the attributes of the
router and set the Link State Database Export parameter (one of the
OSPF Parameters, under Processes) to Once at End of Simulation.
- You will need to set the global attribute IP Interface Addressing Mode
to the value Auto Addressed/Export. This will allow you to check the
automatically assigned IP addresses to the interfaces of the network.
(Refer to the notes of exercise 2 above.)
- After rerunning the simulation, you can check the link-state database by
opening the simulation log (from the Results menu). The link-state
database is available in Classes OSPF LSDB_Export.
4) Create another scenario as a duplicate of the No_Areas scenario. Name the new

scenario Q4_No_Areas_Failure. In this new scenario simulate a failure of the
link connecting RouterD and RotuerE. Have this failure start after 100 seconds.
Rerun the simulation. Show how that link failure affects the content of the linkstate database and routing table of RouterA. (You will need to disable the global
attribute OSPF Sim Efficiency. This will allow OSPF to updat the routing table if
there is any change in the network.)
5) For both No_Areas and Q4_No_Areas_Failure scenario, collect the Traffic

Sent (bits/sec) statistic (one of the Global Statistics under OSPF). Rerun the
simulation for these two scenarios and obtain the graph that compares the
OSPF’s Traffic Sent (bits/sec) in both scenarios. Comment on the obtained
graph.

Laboratory

Border Gateway Protocol (BGP)
An Interdomain Routing Protocol
Objective
The objective of this lab is to simulate and study the basic features of an interdomain
routing protocol called Border Gateway Protocol (BGP).

Overview

The Internet is organized as a set of routing domains. Each routing domain is called an
autonomous system (AS). Each AS is controlled by a single administrative entity (e.g., an
AS of a single service provider). Each AS has a unique 16-bit identification number. This
number is assigned by a central authority. An AS employs its own intradomain routing
protocol (e.g., RIP or OSPF). Different ASs establish routes among each other through
interdomain routing protocols. The border gateway protocol (BGP) is one of the major
interdomain routing protocols.
The main goal of BGP is to find any path to the destination that is loop-free. This is
different from the common goal of intradomain routing protocols, which is to find an
optimal route to the destination based on a specific link metric. The routers that connect
different ASs are called border gateways. The task of the border gateways is to forward
packets between ASs. Each AS has also at least one BGP speaker. BGP speakers
exchange reachability information among ASs.
BGP advertises the complete path to the destination AS as an enumerated list. This way
routing loops can be avoided. A BGP speaker can also apply some policies such as
balancing the load over the neighboring ASs. If a BGP speaker has a choice of several
different routes to a destination, it will advertise the best one according to its own local
policies. BGP is defined to run on top of TCP and hence BGP speakers do not need to
worry about acknowledging received information or retransmission of sent information.
In this lab you will set up a network with three different ASs. RIP will be used as the
intradomain routing protocol and BGP as the interdomain one. You will analyze the routing
tables generated in the routers as well as the effect of applying a simple policy.

Network Simulation Experiments Manual

Prelab Activities
Read section 4.3 from "Computer Networks: A Systems Approach", 4th Edition.

Go to www.net-seal.net/animations.php and play the following animation:
- IP Subnets.

Procedure
Create a New Project
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _BGP, and the
scenario No_BGP Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Select Campus from the Network Scale
list Click Next three times Click OK.

. Make sure that the internet_toolbox is
it is not there, open it by clicking
selected from the pull-down menu on the object palette.
2. Add to the project workspace the following objects from the palette: six
ethernet4_slip8_gtwy routers and two 100BaseT_LAN objects.
a. To add an object from a palette, click its icon in the object palette Move your
mouse to the workspace Click to place the object Right-click to stop
creating objects of that type.
3. Use bidirectional PPP_DS3 links to connect the routers you just added as in the
following figure. Also, rename the network objects as shown (right-click on the
node Set Name).
4. Use a bidirectional 100BaseT link to connect LAN_West to Router1 and another
100BaseT link to connect LAN_East to Router6 as in the following figure.
5. Close the Object Palette dialog box.
6. Save your project.

Border Gateway Protocol (BGP)

Routers Configuration:

Redistribute w/
Default. allows a router
to have a route to a
destination that belongs
to another Autonomous
System

1. Highlight or select simultaneously (using shift and left-click) all six routers
Right-click on any router Edit Attributes Check the Apply Changes to
Selected Objects check box.
2. Expand the BGP Parameters hierarchy and set the following:
i. Redistribution Æ Routing Protocols Æ RIP Æ Redistribute w/ Default.
3. Expand the IP Routing Parameters hierarchy and set the following:
i. Routing Table Export = Once at End of Simulation. This asks the router to
export its routing table at the end of the simulation to the simulation log.
4. Expand the RIP Parameters hierarchy and set the following:
i. Redistribution Æ Routing Protocols Æ Directly Connected Æ
Redistribute w/ Default.
5. Click OK and then save your project.

Application Configuration:
1. Right-click on LAN_West Edit Attributes Assign All to Application:
Supported Services Assign West_Server to the LAN Server Name attribute
as shown Click OK.
Notice that two objects for Applications and Profiles will be added automatically
to the project.

Network Simulation Experiments Manual

2. Right-click on LAN_East Edit Attributes:
i. Expand the Application: Supported Profiles hierarchy Set rows to 1
Expand the row 0 hierarchy Set Profile Name to E-commerce
Customer.
ii. Edit the Application: Destination Preferences attribute as follows:
Set rows to 1 Set Symbolic Name to HTTP Server Edit Actual Name
Set rows to 1 In the new row, assign West_ Server to the Name column.
3. Click OK three times and then save your project..

Configure the Simulation
Here we need to configure some of the simulation parameters:
1.

Click on

and the Configure Simulation window should appear.

2. Set the duration to be 10.0 minutes.
3. Click on the Global Attributes tab and make sure that the following attributes
are assigned as follows:

Border Gateway Protocol (BGP)

Auto Addressed means
that all IP interfaces are
assigned IP addresses
automatically during
simulation. The class of
address (e.g., A, B, or C)
is determined based on
the number of hosts in
the designed network.
Subnet masks assigned
to these interfaces are
the default subnet masks
for that class.

a. IP Interface Addressing Mode = Auto Addressed/Export.
b. IP Routing Table Export/Import = Export.
c.

RIP Sim Efficiency = Disabled. If this attribute is enabled, RIP will stop
after the "RIP Stop Time." But we need the RIP to keep updating the
routing table in case there is any change in the network.

4. Click OK and then save the project.

Export causes the autoassigned IP interface to
be exported to a file
(name of the file is
ip_addresses.gdf and
gets saved in the primary
model directory).

Choose the Statistics
1. Right-click on LAN_East and select Choose Individual Statistics from the popup menu From the Client HTTP hierarchy choose the Traffic Received
(bytes/sec) statistic Click OK.
2. Right-click on the link that connects Router2 to Router3 and select Choose
Individual Statistics from the pop-up menu From the point-to-point
hierarchy choose the “Throughput (bits/sec) -->” statistic Click OK.
Note: If the name of the link is “Router3 <-> Router2” then you will need to choose
the “Throughput (bits/sec) <--” statistic instead.
3. Right-click on the link that connects Router2 to Router4 and select Choose
Individual Statistics from the pop-up menu From the point-to-point
hierarchy choose the “Throughput (bits/sec) -->” statistic Click OK.
Note: If the name of the link is “Router4 <-> Router2” then you will need to choose
the “Throughput (bits/sec) <--” statistic instead.
4. Save your project.

Network Simulation Experiments Manual

Router Interfaces and IP Addresses
Before setting up the routers to use BGP, we need to get the information of the routers’
interfaces along with the IP addresses associated to these interfaces. Recall that these IP
addresses are assigned automatically during simulation, and we set the global attribute IP
Interface Addressing Mode to export this information to a file.

1. First we need to run the simulation. Click on
window should appear Click on Run.

and the Configure Simulation

2. After the simulation run completes click Close.
3. From the File menu choose Model Files Refresh Model Directories. This
causes OPNET IT Guru to search the model directories and update its list of files.
4. From the File menu choose Open From the drop-down menu choose Generic
Data File Select the <_BGP-No_BGP -ip_addresses file
Click OK.
The file that contains all the information about router interfaces and their IP addresses will
open. Table 1 shows the interface number and IP addresses between the six outers in our
projects. For example Router1 is connected to Router2 through interface (IF) 11, which is
assigned 192.0.1.1 as its IP address. A router is connected to itself by a Loopback
interface as shown. Create a similar table for your project but note that your result may
vary due to different nodes placement.

Routers
1
2
3
4
5
6

1
IF: 12
IP: 192.0.2.1
IF: 10
IP: 192.0.1.2

2
IF: 11
IP: 192.0.1.1
IF: 12
IP: 192.0.8.1
IF: 10
IP: 192.0.4.1
IF: 10
IP: 192.0.7.2

IF: 11
IP: 192.0.4.2
IF: 12
IP: 192.0.6.1
IF: 11
IP: 192.0.5.2
IF: 10
IP: 192.0.3.2

IF: 4
IP: 192.0.7.1
IF: 11
IP: 192.0.5.1
IF: 12
IP: 192.0.10.1
IF: 11
IP: 192.0.9.2

IF: 4
IP: 192.0.3.1
IF: 4
IP: 192.0.9.1
IF: 12
IP: 192.0.12.1
IF: 10
IP: 192.0.11.2

IF: 4
IP: 192.0.11.1
IF: 12
IP: 192.0.14.1

Table 1: Interfaces that connect the routers and their assigned IP addresses

Border Gateway Protocol (BGP)

Creating the BGP Scenario
In the network we just created, all routers belong to the same autonomous system. We will
divide the network into three autonomous systems and utilize BGP to route packets
among these systems.
1. Select Duplicate Scenario from the Scenarios menu and name it BGP_Simple
Click OK.
2. Highlight or select simultaneously (using shift and left-click) Router1 and Router2
Right-click on Router1 Edit Attributes Check the Apply Changes to
Selected Objects check box.
3. Expand the IP Routing Parameters hierarchy and set the Autonomous System
Number to 12 Click OK.
4. Repeat steps 2 and 3 above for routers Router3 and Router4. Assign their
Autonomous System Number to 34.
5. Repeat steps 2 and 3 above for routers Router5 and Router6. Assign their
Autonomous System Number to 56.

The following figure shows the created autonomous systems. The figure shows also the
interfaces that connect routers across different autonomous systems. There interfaces are
taken from Table 1 above (note: the interface numbers in your project may vary). The next
step is to disable the RIP protocol on the shown interfaces.

AS 12

AS 34

AS 56

11
10
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Network Simulation Experiments Manual

6. Right-click on Router2 Edit Attributes Expand the IP Routing
Parameters hierarchy Expand the Interface Information hierarchy Expand
row 4 hierarchy Click on the values of the Routing Protocol(s) attribute
Disable RIP as shown click OK twice.

7. Repeat step 6 above for all other interfaces that connect routers across
autonomous systems (i.e., All the eight inter-domain interfaces shown above).
8. Save your project.

Configuring the BGP Neighbor Information
If you try to run the simulation of the BGP_Simple scenario, you will receive hundreds of
errors! This is because there is no routing protocol running between the inter-domain
routers. Therefore, no routing tables are created to deliver packets among autonomous
systems. The solution is to utilize BGP by defining the neighbors of inter-domain routers.
Table 2 shows the neighbors of the routers that will run BGP. Neighbors are defined by
their interface IP address and the AS number. For each router in Table 2 carry out the
following step:
1. Right-click on the router Edit Attributes Expand the BGP Parameters
hierarchy Expand the Neighbor Information hierarchy Assign to the rows
attribute the value 1 for Router1 and Router6. For all other routers, assign the
value 3 to the rows attribute Utilize Table 2 to assign the corresponding values
to the IP Address, Remote AS, and Update Source attributes for each of the
added rows.

Border Gateway Protocol (BGP)

Note: the values to be assigned to the IP Address attribute have to match the
values you will collect in your Table 1.

2. Save your project.

Routers

Router1

Router2

Router3

Router4

Router5

Router6

BGP Parameters Neighbor Information
row 0
row 1
row 2
IP Address: 192.0.8.1
Remote AS: 12
Update Source: Loopback
IP Address: 192.0.2.1
IP Address: 192.0.7.2
IP Address: 192.0.4.1
Remote AS: 12
Remote AS: 34
Remote AS: 34
Update Source: Not Used Update Source: Not Used Update Source: Loopback
IP Address: 192.0.10.1
IP Address: 192.0.3.2
IP Address: 192.0.4.2
Remote AS: 34
Remote AS: 56
Remote AS: 12
Update Source: Not Used Update Source: Not Used Update Source: Loopback
IP Address: 192.0.6.1
IP Address: 192.0.9.2
IP Address: 192.0.7.1
Remote AS: 34
Remote AS: 56
Remote AS: 12
Update Source: Not Used Update Source: Not Used Update Source: Loopback
IP Address: 192.0.14.1
IP Address: 192.0.9.1
IP Address: 192.0.3.1
Remote AS: 56
Remote AS: 34
Remote AS: 34
Update Source: Not Used Update Source: Not Used Update Source: Loopback
IP Address: 192.0.12.1
Remote AS: 56
Update Source: Loopback
Table 2: Neighbors’ info for inter-domain routers

Creating the BGP with Policy Scenario
BGP allows for routing policies that can be enforced using route maps. We will utilize this
feature to configure Router2 to redirect its load on the two egress links of its autonomous
system.
1. First make sure that your project is in the BGP_Simple scenario. Select
Duplicate Scenario from the Scenarios menu and name it BGP_Policy Click
OK.
2. Right-click on Router2 Edit Attributes Expand the IP Routing
Parameters hierarchy Expand the Route Map Configuration hierarchy
Set the attributes as shown in the following figure.
The purpose of the created route map is to reduce the degree of preference of
the “route to AS 56” to the value 10 (Note: the normal value is "99", which is
calculated as 100 - number of AS that should be crossed to reach the
destination).

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Network Simulation Experiments Manual

The next step is to assign the above route map to the link connecting Router2 to Router3.
This way traffic from Router2 to AS 56 will be preferred to go through Router4 instead.
3. Right-click on Router2 Edit Attributes Expand the BGP Parameters
hierarchy Expand the Neighbor Information hierarchy Expand the row that
has the IP address of Router3 interface (it is row 0 in my project) Expand the
Routing Policies hierarchy Set its attribute as shown in the following figure.
4. Click OK and save your project.

Border Gateway Protocol (BGP)

101

Run the Simulation
To run the simulation for the three scenarios simultaneously:
1. Go to the Scenarios menu Select Manage Scenarios.
2. Change the values under the Results column to (or )
for the three scenarios. Compare to the following figure.

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Network Simulation Experiments Manual

3. Click OK to run the three simulations. Depending on the speed of your
processor, this may take several minutes to complete.
4. After the three simulation runs complete, one for each scenario, click Close
Save your project.

View the Results
Compare the Routing Tables Content:
1. To check the content of the routing tables in Router2 for both scenarios:
i. Go to the Results menu Open Simulation Log Expand the hierarchy
on the left as shown below Click on the field COMMON ROUTE TABLE in
the row corresponds to Rouer2.

2. Carry out the previous step for scenario No_BGP and scenario BGP_Simple.
The following are partial contents of Router2’s routing table for both scenarios
(Note: Your results may vary due to different nodes placement):

Routing table of Router2 for the No_BGP scenario:

Border Gateway Protocol (BGP)

103

Routing table of Router2 for the BGP_Simple scenario:

Compare the load in the network:
1. Select Compare Results from the Results menu.
2. Change the drop-down menu in the right-lower part of the Compare Results
dialog box from As Is to time_average as shown.

3. Select and show the graphs of the following statistics: Traffic Received in
LAN_East, throughput in the Router2-Router3 link, and throughput in the
Router2-Router4 link. The resulting graphs should resemble the graphs below.

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Network Simulation Experiments Manual

Border Gateway Protocol (BGP)

105

Further Readings
−

A Border Gateway
(www.ietf.org/rfc.html).

−

Application of the Border Gateway Protocol in the Internet: IETF RFC number
1772 (www.ietf.org/rfc.html).

−

BGP-4 Protocol Analysis: IETF RFC number 1774 (www.ietf.org/rfc.html).

Protocol

(BGP-4):

IETF

RFC

number

1771

Exercises
1) Obtain and analyze the routing table for Router5 in the project before and after

applying BGP.
2) Analyze the graphs that show the throughput in both Router2-Router3 link and

Router2-Router4 link. Explain the effect of applying the routing policy on these
throughputs.
3) Create another scenario as a duplicate of the BGP_Simple scenario. Name the

new scenario BGP_OSPF_RIP. In this new scenario change the intradomain
routing protocol in AS 56 to be OSPF instead of RIP. Run the new scenario and
check the contents of Router5‘s routing table. Analyze the content of this table.

Laboratory

TCP: Transmission Control Protocol
A Reliable, Connection-Oriented, Byte-Stream Service
Objective
This lab is designed to demonstrate the congestion control algorithms implemented by the
Transmission Control Protocol (TCP). The lab provides a number of scenarios to simulate
these algorithms. You will compare the performance of the algorithms through the analysis
of the simulation results.

Overview
The Internet’s TCP guarantees the reliable, in-order delivery of a stream of bytes. It
includes a flow-control mechanism for the byte streams that allows the receiver to limit
how much data the sender can transmit at a given time. In addition, TCP implements a
highly tuned congestion-control mechanism. The idea of this mechanism is to throttle how
fast TCP sends data to keep the sender from overloading the network.
The idea of TCP congestion control is for each source to determine how much capacity is
available in the network, so that it knows how many packets it can safely have in transit. It
maintains a state variable for each connection, called the congestion window, which is
used by the source to limit how much data the source is allowed to have in transit at a
given time. TCP uses a mechanism, called additive increase/multiplicative decrease, that
decreases the congestion window when the level of congestion goes up and increases the
congestion window when the level of congestion goes down. TCP interprets timeouts as a
sign of congestion. Each time a timeout occurs, the source sets the congestion window to
half of its previous value. This halving corresponds to the multiplicative decrease part of
the mechanism. The congestion window is not allowed to fall below the size of a single
packet (the TCP maximum segment size, or MSS). Every time the source successfully
sends a congestion window worth of packets, it adds the equivalent of one packet to the
congestion window; this is the additive increase part of the mechanism.
TCP uses a mechanism called slow start to increase the congestion window “rapidly” from
a cold start in TCP connections. It increases the congestion window exponentially, rather
than linearly. Finally, TCP utilizes a mechanism called fast retransmit and fast recovery.
Fast retransmit is a heuristic that sometimes triggers the retransmission of a dropped
packet sooner than the regular timeout mechanism
In this lab you will set up a network that utilizes TCP as its end-to-end transmission
protocol and analyze the size of the congestion window with different mechanisms.

TCP: Transmission Control Protocol

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Prelab Activities
Read sections 5.1 and 5.2 from "Computer Networks: A Systems Approach", 4th
Edition.

Go to www.net-seal.net/animations.php and play the following animations:
- TCP Connections.
- TCP Multiplexing.
- TCP Buffering and Sequencing.
- User Datagram Protocol (UDP).

Procedure
Create a New Project
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _TCP, and the
scenario No_Drop Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Select Choose From Maps from the
Network Scale list Click Next Choose USA from the Map List Click Next
twice Click OK.

Create and Configure the Network
Initialize the Network:
The ip32_cloud node
model represents an IP
cloud supporting up to
32 serial line interfaces at
a selectable data rate
through which IP traffic
can be modeled. IP
packets arriving on any
cloud interface are
routed to the appropriate
output interface based
on their destination IP
address. The RIP or
OSPF protocol may be
used to automatically
and dynamically create
the cloud's routing tables
and select routes in an
adaptive manner. This
cloud requires a fixed
amount of time to route
each packet, as
determined by the
Packet Latency
attribute of the node.

1. The Object Palette dialog box should now be on the top of your project space. If it
. Make sure that the internet_toolbox item is
is not there, open it by clicking
selected from the pull-down menu on the object palette.
2. Add to the project workspace the following objects from the palette: Application
Config, Profile Config, an ip32_Cloud, and two subnets.
a. To add an object from a palette, click its icon in the object palette Move your
mouse to the workspace Click to drop the object in the desired location
Right-click to finish creating objects of that type.
3. Close the palette.
4. Rename the objects you added as shown and then save your project:

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Configure the Applications:
1. Right-click on the Applications node Edit Attributes Expand the
Application Definitions attribute and set rows to 1 Expand the new row
Name the row FTP_Application.
i. Expand the Description hierarchy Edit the FTP row as shown (you will
need to set the Special Value to Not Used while editing the shown
attributes):

2. Click OK twice and then save your project.

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Configure the Profiles:
1. Right-click on the Profiles node Edit Attributes Expand the Profile
Configuration attribute and set rows to 1.
i. Name and set the attributes of row 0 as shown Click OK.

Configure the West Subnet:
1. Double-click on the West subnet node. You get an empty workspace, indicating
that the subnet contains no objects.
and make sure that the internet_toolbox item is
2. Open the object palette
selected from the pull-down menu.
The ethernet4_slip8_
gtwy node model
represents an IP-based
gateway supporting four
Ethernet hub interfaces
and eight serial line
interfaces.

3. Add the following items to the subnet workspace: one ethernet_server, one
ethernet4_slip8_gtwy router, and connect them with a bidirectional 100_BaseT
link Close the palette Rename the objects as shown.

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4. Right-click on the Server_West node Edit Attributes:
i. Edit Application: Supported Services Set rows to 1 Set Name to
FTP_Application Click OK.
ii. Edit the value of the Server Address attribute and write down Server_West.
iii. Expand the TCP Parameters hierarchy Set both Fast Retransmit and
Fast Recovery to Disabled.
5. Click OK and then save your project.
Now, you have completed the configuration of the West subnet. To go back to the top
level of the project, click the Go to next higher level

button.

Configure the East Subnet:
1. Double-click on the East subnet node. You get an empty workspace, indicating
that the subnet contains no objects.
and make sure that the internet_toolbox item is
2. Open the object palette
selected from the pull-down menu.
3. Add the following items to the subnet workspace: one ethernet_wkstn, one
ethernet4_slip8_gtwy router, and connect them with a bidirectional 100_BaseT
link Close the palette Rename the objects as shown.

4. Right-click on the Client_East node Edit Attributes:
i. Expand the Application: Supported Profiles hierarchy Set rows to 1
Expand the row 0 hierarchy Set Profile Name to FTP_Profile.
ii. Assign Client_ East to the Client Address attributes.
iii. Edit the Application: Destination Preferences attribute as follows:
Set rows to 1 Set Symbolic Name to FTP Server Edit Actual Name
Set rows to 1 In the new row, assign Server_West to the Name column.
5. Click OK three times and then save your project.
6. You have now completed the configuration of the East subnet. To go back to the
project space, click the Go to next higher level

button.

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Connect the Subnets to the IP Cloud:
1. Open the object palette

2. Using two PPP_DS3 bidirectional links connect the East subnet to the IP Cloud
and the West subnet to the IP Cloud.
3. A pop-up dialog box will appear asking you what to connect the subnet to the IP
Cloud with. Make sure to select the “routers.”
4. Close the palette.

Choose the Statistics
OPNET provides the
following capture
modes:

1. Right-click on Server_West in the West subnet and select Choose Individual
Statistics from the pop-up menu.

All values—collects
every data point from a
statistic.

2. In the Choose Results dialog box, choose the following statistic:

Sample—collects the
data according to a userspecified time interval or
sample count. For
example, if the time
interval is 10, data is
sampled and recorded
every 10th second. If the
sample count is 10, every
10th data point is
recorded. All other data
points are discarded.
Bucket—collects all of
the points over the time
interval or sample count
into a “data bucket” and
generates a result from
each bucket. This is the
default mode.

TCP Connection Congestion Window Size (bytes) and Sent Segment
Sequence Number.
3. Right-click on the Congestion Window
Size (bytes) statistic Choose Change
Collection Mode In the dialog box check
Advanced From the drop-down menu,
assign all values to Capture mode as
shown Click OK.

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4. Right-click on the Sent Segment Sequence Number statistic Choose
Change Collection Mode In the dialog box check Advanced From the
drop-down menu, assign all values to Capture mode.
5. Click OK twice and then save your project.
6. Click the Go to next higher level

button.

Configure the Simulation
Here we need to configure the duration of the simulation:

1. Click on

and the Configure Simulation window should appear.

2. Set the duration to be 10.0 minutes.
3. Click OK and then save your project.

Duplicate the Scenario
In the network we just created we assumed a perfect network with no discarded packets.
Also, we disabled the fast retransmit and fast recovery techniques in TCP. To analyze the
effects of discarded packets and those congestion-control techniques, we will create two
additional scenarios.
With fast retransmit,
TCP performs a
retransmission of what
appears to be the
missing segment,
without waiting for a
retransmission timer to
expire.

1. Select Duplicate Scenario from the Scenarios menu and give it the name
Drop_NoFast Click OK.
2. In the new scenario, right-click on the IP Cloud Edit Attributes Assign
0.05% to the Packet Discard Ratio attribute.
3. Click OK and then save your project.

After fast retransmit
sends what appears to be
the missing segment,
congestion avoidance
but not slow start is
performed. This is the
fast recovery algorithm.

4. While you are still in the Drop_NoFast scenario, select Duplicate Scenario from
the Scenarios menu and give it the name Drop_Fast.

The fast retransmit and
fast recovery algorithms
are usually implemented
together (RFC 2001).

6. Click OK and then save your project.

5. In the Drop_Fast scenario, right-click on Server_ West, which is inside the West
subnet Edit Attributes Expand the TCP Parameters hierarchy Enable
the Fast Retransmit attribute Assign Reno to the Fast Recovery attribute.

TCP: Transmission Control Protocol

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View the Results
To view and analyze the results:
To switch to a scenario,
choose Switch to
Scenario from the
Scenarios menu or just
press Ctrl+.

1. Switch to the Drop_NoFast scenario (the second one) and choose View Results
from the Results menu.
2. Fully expand the Object Statistics hierarchy and select the following two results:
Congestion Window Size (bytes) and Sent Segment Sequence Number.

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3. Click Show. The resulting graphs should resemble the ones below.

TCP: Transmission Control Protocol

115

4. To zoom in on the details in the graph, click and drag your mouse to draw a
rectangle, as shown above.
5. The graph should be redrawn to resemble the following one:

6. Notice the Segment Sequence Number is almost flat with every drop in the
congestion window.
7. Close the View Results dialog box and select Compare Results from the Result
menu.
8. Fully expand the Object Statistics hierarchy as shown and select the following
result: Sent Segment Sequence Number.

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9. Click Show. After zooming in, the resulting graph should resemble the one below.

TCP: Transmission Control Protocol

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Further Readings
−

OPNET RSVP Model Description: From the Protocols menu, select RSVP
Model Usage Guide.

Exercises
1) Analyze the graphs we obtained in this lab. Show the effect of RSVP on the Voice

application and explain the obtained numbers of Path and Resv states.
2) How does the data rate of the link connecting the East and West routers affect

the performance (e.g., Packet End-to-End Delay) of the Voice and Video
Conference applications? To answer this question, create a new scenario as a
duplicate of the QoS_RSVP scenario. Name the new scenario Q2_HighRate. In
the Q2_HighRate scenario replace the current PPP_DS1 link (data rate 1.544
Mbps) with a PPP_DS3 link (data rate 44.736 Mbps).

Laboratory

Firewalls and VPN
Network Security and Virtual Private Networks
Objective
The objective of this lab is to study the role of firewalls and Virtual Private Networks
(VPNs) in providing security to shared public networks such as the Internet.

Overview
Computer networks are typically a shared resource used by many applications for many
different purposes. Sometimes the data transmitted between application processes is
confidential, and the application users would prefer that others not be able to read it.
A firewall is a specially programmed router that sits between a site and the rest of the
network. It is a router in the sense that it is connected to two or more physical networks
and it forwards packets from one network to another, but it also filters the packets that flow
through it. A firewall allows the system administrator to implement a security policy in one
centralized place. Filter-based firewalls are the simplest and most widely deployed type of
firewall. They are configured with a table of addresses that characterize the packets they
will and will not forward.
A VPN is an example of providing a controlled connectivity over a public network such as
the Internet. VPNs utilize a concept called an IP tunnel—a virtual point-to-point link
between a pair of nodes that are actually separated by an arbitrary number of networks.
The virtual link is created within the router at the entrance to the tunnel by providing it with
the IP address of the router at the far end of the tunnel. Whenever the router at the
entrance of the tunnel wants to send a packet over this virtual link, it encapsulates the
packet inside an IP datagram. The destination address in the IP header is the address of
the router at the far end of the tunnel, while the source address is that of the encapsulating
router.
In this lab you will set up a network where servers are accessed over the Internet by
customers who have different privileges. You will study how firewalls and VPNs can
provide security to the information in the servers while maintaining access for customers
with the appropriate privilege.

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Prelab Activities
Read section 4.5.3 from "Computer Networks: A Systems Approach", 4th Edition.

Procedure
Create a New Project
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _VPN, and the
scenario NoFirewall Click OK.
3. Click Quit on the Startup Wizard.
4. To remove the world background map, select the View menu Background
Set Border Map Select NONE from the drop-down menu Click OK.

Create and Configure the Network
Initialize the Network:
1. Open the Object Palette dialog box by clicking
. Make sure that the
internet_toolbox item is selected from the pull-down menu on the object palette.
The ppp_server and
ppp_wkstn support one
underlying SLIP (Serial
Line Internet Protocol)
connection at a
selectable data rate.

PPP_DS1 connects two
nodes running PPP. Its
data rate is 1.544 Mbps.

2. Add the following objects, from the palette, to the project workspace (see figure
below for placement): Application Config, Profile Config, an ip32_cloud, one
ppp_server, three ethernet4_slip8_gtwy routers, and two ppp_wkstn hosts.
a. To add an object from a palette, click its icon in the object palette Move your
mouse to the workspace and click where you want to place the object Rightclick to indicate you are done creating objects of this type.
3. Rename the objects you added and connect them using PPP_DS1 links, as
shown below:

4. Save your project.

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Configure the Nodes:
Several example
application
configurations are
available under the
Default setting. For
example, "Web
Browsing (Heavy
HTTP1.1)" indicates a
Web browsing
application performing
heavy browsing using
HTTP 1.1 protocol..

1. Right-click on the Applications node Edit Attributes Assign Default to the
Application Definitions attribute Click OK.
2. Right-click on the Profiles node Edit Attributes Assign Sample Profiles to
the Profile Configuration attribute Click OK.
3. Right-click on the Server node Edit Attributes Assign All to the
Application: Supported Services attribute Click OK.
4. Right-click on the Sales A node Select Similar Nodes (make sure that both
Sales A and Sales B are selected).
i. Right-click on the Sales A node Edit Attributes Check the Apply
Changes to Selected Objects check-box.
ii. Expand the Application: Supported Profiles attribute Set rows to 1
Expand the row 0 hierarchy Profile Name = Sales Person (this is one of
the “sample profiles” we configured in the Profiles node).
iii. Click OK.
5. Save your project.

Choose the Statistics
DQ Query Response
Time is measured from
the time when the
database query
application
sends a request to the
server to the time it
receives a response
packet.

HTTP Page Response
Time specifies the time
required to retrieve the
entire page with all the
contained inline objects.

1. Right-click anywhere in the project workspace and select Choose Individual
Statistics from the pop-up menu.
2. In the Choose Results dialog, check the following statistics:
i. Global Statistics DB Query Response Time (sec).
ii. Global Statistics HTTP Page Response Time (seconds).
3. Click OK.
4. Right-click on the Sales A node and select Choose Individual Statistics from
the pop-up menu.
5. In the Choose Results dialog, check the following statistics:
i. Client DB Traffic Received (bytes/sec).
ii. Client Http Traffic Received (bytes/sec).
6. Click OK.
7. Right-click on the Sales B node and select Choose Individual Statistics from
the pop-up menu.
8. In the Choose Results dialog, check the following statistics:
i. Client DB Traffic Received (bytes/sec).
ii. Client Http Traffic Received (bytes/sec).
9. Click OK and then save your project.

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The Firewall Scenario
In the network we just created, the Sales Person profile allows both sales sites to access
applications such as Database Access, Email, and Web Browsing from the server (check
the Profile Configuration of the Profiles node). Assume that we need to protect the
database in the server from external access, including the salespeople. One way to do
that is to replace Router C with a firewall as follows:
1. Select Duplicate Scenario from the Scenarios menu and name it Firewall
Click OK.
2. In the new scenario, right-click on Router C Edit Attributes.
3. Assign ethernet2_slip8_firewall to the model attribute.
4. Expand the hierarchy of the Proxy Server Information attribute Expand the
row 1, which is for the Database application, hierarchy Assign No to the Proxy
Server Deployed attribute as shown:

Proxy Server
Information is a table
defining the
configuration of the
proxy servers on the
firewall. Each row
indicates whether a
proxy server exists for a
certain application and
the amount of additional
delay that will be
introduced to each
forwarded packet of that
application by the proxy
server.

5. Click OK and then save your project.

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Our Firewall configuration does not allow database-related traffic to pass through the
firewall (it filters such packets out). This way, the databases in the server are
protected from external access. Your Firewall scenario should look like the following
figure.

The Firewall_VPN Scenario
In the Firewall scenario, we protected the databases in the server from “any” external
access using a firewall router. Assume that we want to allow the people in the Sales A site
to have access to the databases in the server. Since the firewall filters all database-related
traffic regardless of the source of the traffic, we need to consider the VPN solution. A
virtual tunnel can be used by Sales A to send database requests to the server. The
firewall will not filter the traffic created by Sales A because the IP packets in the tunnel will
be encapsulated inside an IP datagram.
The ethernet4_slip8_
gtwy node model
represents an IP-based
gateway supporting four
Ethernet hub interfaces
and eight serial line
interfaces. IP packets
arriving on any interface
are routed to the
appropriate output
interface based on their
destination IP address.
The Routing
Information Protocol
(RIP) or the Open
Shortest Path First
(OSPF) protocol may be
used to dynamically and
automatically create the
gateway's routing tables
and select routes in an
adaptive manner.

1. While you are in the Firewall scenario, select Duplicate Scenario from the
Scenarios menu and give it the name Firewall_VPN Click OK.
2. Remove the link between Router C and the Server.
. Make sure that the
3. Open the Object Palette dialog box by clicking
internet_toolbox is selected from the pull-down menu on the object palette.
i. Add to the project workspace one ethernet4_slip8_gtwy and one IP VPN
Config (see the figure below for placement).
ii. From the Object Palette, use two PPP_DS1 links to connect the new router
to Router C (the firewall) and to the Server, as shown below.
iii. Close the Object Palette dialog box.
4. Rename the IP VPN Config object to VPN.

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5. Rename the new router to Router D as shown:

Configure the VPN:
1. Right-click on the VPN node Edit Attributes.
i. Expand the VPN Configuration hierarchy Set rows to 1 Expand row 0
hierarchy Edit the value of Tunnel Source Name and enter Router A
Edit the value of Tunnel Destination Name and enter Router D.
ii. Expand the Remote Client List hierarchy Set rows to 1 Expand row 0
hierarchy Edit the value of Client Node Name and enter Sales A.
iii. Click OK and then save your project.

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Simulating Encryption:
A virtual tunnel between Sales A and the Server does not guarantee security for the
contents of the transferred database packets. If the contents of these packets are
confidential, encryption of these packets will be needed. In OPNET AE, the effect of
packets encryption can be simulated by the available compression function. Two of the
available compression schemes are the Per-Interface Compression and the Per-Virtual
Circuit Compression as shown. Once you edit the Compression Information attribute of an
interface, OPNET adds the IP Config node to the project.

Per-Interface Compression compresses the entire packet (including the headers). This
means the packet is decompressed and compressed at each hop on the route. Per-Virtual
Circuit Compression compresses the packet payload only. Therefore, compression and
decompression take place only at the end nodes. One of the exercises by the end of this
lab will ask you to create a new scenario to utilize the compression function.

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View the Results
To view and analyze the results:
1. Select Compare Results from the Results menu.
2. Expand the Sales A hierarchy Expand the Client DB hierarchy Select the
Traffic Received statistic.
3. Change the drop-down menu in the middle-lower part of the Compare Results
dialog box from As Is to time_average as shown.

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4. Press Show and the resulting graph should look similar to the following figure.
Your graph may not match exactly due to node placement.

5. Create a graph similar to the previous one, but for Sales B:

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6. Create two graphs similar to the previous ones to depict the Traffic Received by
the Client Http for Sales A and Sales B.

Note: Results may vary slightly due to different node placement.

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Further Readings
−

The Impact of Internet Link Capacity on Application Performance: From the
Protocols menu, select Methodologies Capacity Planning.

−

Virtual Private Networks: IETF RFC number 2685 (www.ietf.org/rfc.html).

Exercises
1) From the obtained graphs, explain the effect of the firewall, as well as the

configured VPN, on the database traffic requested by Sales A and Sales B.
2) Compare the graphs that show the received HTTP traffic with those that show the

received database traffic.
3) Generate and analyze the graph(s) that show the effect of the firewall, as well as

the configured VPN, on the response time (delay) of the HTTP pages and
database queries.
4) In the Firewall_VPN scenario we configured the VPN node so that no traffic from

Sales A is blocked by the firewall. Create a duplicate of the Firewall_VPN
scenario and name the new scenario Q4_DB_Web. In the Q4_DB_Web
scenario we want to configure the network so that:
a. The databases in the server can be accessed only by the people in the
Sales A site.
b. The web sites in the server can be accessed only by the people in the
Sales B site.
Include in your report the diagram of the new network configuration including any
changes you made to the attributes of the existing or added nodes. Generate the
graphs of the DB traffic received and the HTTP traffic received for both Sales A
and Sales B to show that the new network meets the above requirements.
5) Create a duplicate of the Firewall_VPN scenario and name the new scenario

Q5_Compression. In the new scenario simulate packet encryption between
Sales A and the Server by allowing Per-Virtual Circuit Compression in both
nodes. As encryption takes more time than compression, edit the attributes of the
Per-Virtual Circuit Compression row (row 3) in the IP Config node. Assign
3E-006 and 1E-006 to Compression Delay and Decompression Delay
respectively. Study the effect of compression on the DB Query response time
between Sales A and the Server.

Laboratory

Applications
Network Application Performance Analysis
Objective
The objective of this lab is to analyze the performance of an Internet application protocol
and its relation to the underlying network protocols. In addition, this lab reviews some of
the topics covered in previous labs.

Overview
Network applications are part network protocol (in the sense that they exchange
messages with their peers on other machines) and part traditional application program (in
the sense that they interact with the users).
OPNET’s Application Characterization Environment (ACE) provides powerful visualization
and diagnosis capabilities that aid in network application analysis. ACE provides specific
information about the root cause of application problems. ACE can also be used to predict
application behavior under different scenarios. ACE takes as input a real trace file
captured using any protocol analyzer, or using OPNET’s capture agents (not included in
the Academic Edition).
In this lab you will analyze the performance of an FTP application. You will analyze the
probable bottlenecks for the application scenario under investigation. You will also study
the sensitivity of the application to different network conditions such as bandwidth and
packet loss. The trace was captured on a real network, which is shown in the figure below,
and already imported into ACE. The FTP application runs on that network; the client
connects to the server over a 768 Kbps Frame Relay circuit with 36 ms of latency. The
FTP application downloads a 1 MB file in 37 seconds. Normally the download time for a
file this size should be about 11 seconds.

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Prelab Activities
Read sections 9.1.1 and 9.1.3 from "Computer Networks: A Systems Approach",
4th Edition.

Go to www.net-seal.net/animations.php and play the following animations:
- Internet Access.
- Email Protocols.

Procedure
Open the Application Characterization Environment
1. Start OPNET IT Guru Academic Edition Choose Open from the File menu.
2. Select Application Characterization from the pull-down menu.
3. Select FTP_with_loss from the list Click OK.

Visualize the Application
After opening the file, ACE shows the Data Exchange Chart (DEC), depicting the flow of
application traffic between tiers. The DEC you see on your screen may or may not show
the FTP Server tier as the top tier. If it does not, drag the tier label from the bottom to the
top, so your screen matches the one shown in the following diagram.

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The Data Exchange
Chart can display the
following:
- The Application
Chart, which shows the
flow of application
traffic between tiers.
- The Network Chart,
which shows the flow of
network traffic between
tiers, including the
effects of network
protocols on application
traffic. Network
protocols split packets
into segments, add
headers, and often
include mechanisms to
ensure reliable data
transfer. These network
protocol effects can
influence application
behavior.

Network Simulation Experiments Manual

1. Select Network Chart Only from the drop-down menu as shown below.
2. Differentiate the messages flowing in different directions by selecting View
Split Groups.

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To get a better understanding of this traffic, you will zoom in on the transaction. To
understand how the Application Chart and Network Chart views differ, you will view both
simultaneously.
3. Select Application and Network Charts from the drop-down menu in the middle
of the dialog box.
4. To disable the split groups view, select View Split Groups.
5. Select View Set Visible Time Range Set Start Time to 25.2 and End
Time to 25.5 Click OK.
6. The Application Message Chart shows a single message flowing from the FTP
Server to the Client. To show the size, rest the cursor on the message to show
the tooltip. Client Payload is shown as 8192.

The Network Chart shows that this application message causes many packets to flow
over the network. These packets are a mix of large (blue and green) packets from the FTP
Server to the Client and small (red) packets from the Client to the FTP Server. As the red
color indicates, these packets contain 0 bytes of application data. They are the
acknowledgments sent by TCP.

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Analyze with AppDoctor
AppDoctor’s Summary of Delays provides insight into the root cause of the overall
application delay.
1. From the AppDoctor menu, select Summary of Delays Check the Show
Values checkbox.
Notice that the largest contributing factor to the application response time is
protocol/congestion. Only about 30 percent of the file download time is caused by the
limited bandwidth of the Frame Relay circuit (768 Kbps). Notice also that application delay
(processing inside the node) by both the Client and FTP Server is a very minor
contributing factor to the application response time.

2. Close the Summary of Delays dialog box.
The Diagnosis function of AppDoctor should give further insight into the cause of the
protocol/congestion delay.
3. From the AppDoctor menu, select Diagnosis.
The diagnosis shows four bottlenecks: transmission delay, protocol/congestion delay,
retransmissions, and out of sequence packets. One factor that contributes to
protocol/congestion delay is retransmissions. So it is no surprise that here, in the more
detailed diagnosis, you see retransmissions listed as a bottleneck. The out-ofsequence packets, also listed as a bottleneck, are a side effect of the retransmissions.
Correcting that issue will probably also cure the out of sequence packets problem.

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4. Close the Diagnosis Window.
AppDoctor also provides summary statistics for the application transaction.
5. From the
AppDoctor
menu, select
Statistics.
Notice that 52
retransmissions
occurred during
a file transfer
composed of
1281 packets,
yielding a
retransmission
rate of 4%.
6. Close the
Statistics
window.

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Examine the Statistics
To view the actual network throughput, use the Graph Statistics feature.
1. From the Data Exchange Chart, select Graph Statistics from the Graph menu
(or click the button:

2. Select the two Network Throughput statistics that measure Kbits/sec.

3. Click Show.

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4. Return to the Graph Statistics window Unselect the throughput statistics and
select only the two Retransmissions statistics Change the Bucket Width to
100 ms Click Show.
ACE divides the entire
task duration into
individual buckets of
time and calculates a
mean or total value for
each interval. The default
bucket width is 1000
msec; you can change
this value in the Bucket
Width (msec) field of the
ACE statistic browser.

Ideal TCP Window Size
In TCP, rather having a fixed-size sliding window, the receiver advertises a window
size to the sender. This is done using the AdvertisedWindow field in the TCP
header. The sender is then limited to having no more than a value of
AdvertisedWindow bytes of unacknowledged data at any given time. The receiver
selects a suitable value for AdvertisedWindow based on the amount of memory
allocated to the connection for the purpose of buffering data. This procedure is called
flow control, and its idea is to keep the sender from overrunning the receiver’s buffer.

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In addition, TCP maintains a new state variable for each connection, called
CongestionWindow, which is used by the source to limit how much data it is allowed
to have in transit at a given time. The congestion window is congestion control’s
counterpart to flow control’s advertised window. It is dynamically sized by TCP in
response to the congestion status of the connection.
TCP will send data only if the amount of sent-but-not-yet-acknowledged data is less
than the minimum of the congestion window and the advertised window. ACE
autocalculates the optimum window size based on the bandwidth-delay product as
follows:
The bandwidth-delay
product of a connection
gives the “volume” of
the connection—the
number of bits it holds.
It corresponds to how
many bits the sender
must transmit before the
first bit arrives at the
receiver.

1. Return to the Graph Statistics window Select the TCP In-Flight Data (bytes)
FTP_Server to Client statistic Assign 1000 to the Bucket Width (msec).

2. Click Show. From the graph, the ideal window size calculated by ACE is about 7
KB.

3. You can now close all opened graphs (delete the panel when you are asked to do
that) and close the Graph Statistics window.

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Impact of Network Bandwidth
ACE QuickPredict enables you to study the sensitivity of an application to network
conditions such as bandwidth and latency.
1. Click on the QuickPredict button:

2. In the QuickPredict Control dialog box assign 512Kbps to the Min Bandwidth
field and 10Mbps to the Max Bandwidth field Click the Update Graph button.

3. The resulting graph should resemble the following one:

4. Close the graph and the QuickPredict Control dialog box.

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Deploy an Application
OPNET IT Guru can be used to perform predictive studies of applications that are
characterized in ACE. ACE uses the trace files to create application fingerprints that
characterize the data exchange between tiers. From these fingerprints, a simulation
can show how the application will behave under different conditions. For example, the
ACE topology wizard can be used to build a network model from the ACE file of this
lab, FTP_with_loss, to answer the following question: What will the performance of
the FTP application be when deployed to 100 simultaneous users over an IP
network?
Follow the following steps to answer the above question:
1. From the IT Guru main window, select File New Select Project from the
pull-down menu Click OK.
2. Name the project _FTP, and the scenario ManyUsers Click
OK.
3. In the Startup Wizard, select Import from ACE Click Next.

4. The Configure ACE Application dialog box appears.
i. Set the Name field to FTP Application.
ii. Set the Repeat application field to 2. This field controls how many times a
user executes the application per hour.
iii. Leave the limit at the default value, Infinite.
iv. Click on Add Task In the Contained Tasks table, click on the word
Specify... Select FTP_with_loss from the pull-down menu.
v. Click Next.

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5. The Create ACE Topology dialog box appears. Set Number of Clients to 100
and set Packet Latency to 40. Leave all other settings at the default values.
6. Click Create.

7. Select File Save Click OK to save the project.

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The ACE Wizard creates a topology similar to the one shown. The Tasks,
Applications, and Profiles objects have all been configured according to the trace
files and the entries you made in the ACE Wizard. You can customize them further.

Run the Simulation and View the Results:
Now that the topology is created, you can run the simulation.
1. Click on the Configure / Run Simulation action button

2. Use the default values. Click Run. Depending on the speed of your processor,
this may take several minutes to complete.
3. Close the dialog box when the simulation completes Save your project.
4. Select View Results from the Results menu Expand the Custom
Application hierarchy Select the Application Response Time (sec) statistic.

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5. Click Show. The resulting graph should resemble the following:

Further Readings
−

File Transfer Protocol (FTP): IETF RFC number 959 (www.ietf.org/rfc.html).

Exercises
1) Explain why the Client to FTP Server messages are mainly 0-byte messages?
2) Utilizing the AppDoctor’s Summary of Delays, what is the effect of each of the

following upgrades on the FTP download time?
a. Server upgrade
b. Bandwidth upgrade
c. Protocol(s) upgrade
3) How do retransmissions contribute to protocol/congestion delay? and explain why

“out of sequence” packets are constitute a side effect of retransmissions.
4) Which protocol is responsible for the retransmission, IP, TCP, or FTP? Explain.

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5) In the Network Throughput graph, the throughput from the FTP Server to the

Client has an average value of about 300 Kbps and has a spike to about 500
Kbps. But the Frame Relay circuit has an available bandwidth of 768 Kbps.
Explain why the throughput is not close to the available bandwidth.
6) Explain how the TCP in-flight data is used as an indicator of the TCP window size

and how the bandwidth-delay product of the connection is used as an indicator of
the ideal window size.
7) Comment on the graph we received that shows the relation between the network

bandwidth and the FTP application response time. Why does the response time
look unaffected by increasing the bandwidth beyond a specific point?
8) In the Deploy an Application section, a network model with multiple users was

created based on the ACE file, FTP_with_loss. Duplicate the created scenario to
create a new one with the name Q8_ManyUsers_ExistingTraffic. In this new
scenario add an “existing” traffic of 80% load in the network. Examine how the
existing traffic affects the FTP application's response time. Note: A simple way to
simulate the existing traffic is to apply background utilization traffic of 80% to the
link between the Remote Router and the IP Cloud.

Laboratory

Wireless Local Area Network
Medium Access Control for Wirelessly Connected Stations
Objective
This lab addresses the MAC (Medium Access Control) sublayer of the IEEE 802.11
standard for WLAN (wireless local area network). Different options of this standard are
studied in this lab. The performance of these options is analyzed under different scenarios.

Overview
The IEEE 802.11 standard provides wireless connectivity to computerized stations that
require rapid deployment such as portable computers. The Medium Access Control
(MAC) sublayer in the standard includes two fundamental access methods: distributed
coordination function (DCF) and the point coordination function (PCF). DCF utilizes the
carrier sense multiple access with collision avoidance (CSMA/CA) approach. DCF is
implemented in all stations in the wireless local area network (WLAN). PCF is based on
polling to determine the station that can transmit next. Stations in an infrastructure network
optionally implement the PCF access method.
In addition to the physical CSMA/CA, DCF and PCF utilize virtual carrier-sense
mechanism to determine the state of the medium. This virtual mechanism is implemented
by means of the network allocation vector (NAV). The NAV provides each station with a
prediction of future traffic on the medium. Each station uses NAV as an indicator of time
periods during which transmission will not be initiated even if the station senses that the
wireless medium is not busy. NAV gets the information about future traffic from
management frames and the header of regular frames being exchanged in the network.
With DCF, every station senses the medium before transmitting. The transmitting station
defers as long as the medium is busy. After deferral and while the medium is idle, the
transmitting station has to wait for a random backoff interval. After the backoff interval and
if the medium is still idle, the station initiates data transmission or optionally exchanges
RTS (request to send) and CTS (clear to send) frames with the receiving station. The
effect of RTS and CTS frames will be studied in the following lab.
With PCF, the access point (AP) in the network acts as a point coordinator (PC). The PC
uses polling to determine which station can initiate data transmission. It is optional for the
stations in the network to participate in PCF and hence respond to poll received from the
PC. Such stations are called CF-Pollable stations. The PCF requires control to be gained
of the medium by the PC. To gain such control, the PC utilizes the Beacon management
frames to set the network allocation vector (NAV) in the network stations. As the

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mechanism used to set NAV is based on the DCF, all stations comply with the PC request
to set their NAV whether or not they are CF-Pollable. This way the PC can control frame
transmissions in the network by generating contention free periods (CFP). The PC and the
CF-Pollable stations do not use RTS/CTS in the CFP.
The standard allows for fragmentation of the MAC data units into smaller frames.
Fragmentation is favorable in case the wireless channel is not reliable enough to transmit
longer frames. Only frames with a length greater than a fragmentation threshold will be
fragmented. Each fragment will be sent independently and will be separately
acknowledged. During a contention period, all fragments of a single frame will be sent as
burst with a single invocation of the DCF medium access procedure. In case of PCF and
during a contention free period, fragments are sent individually following the rules of the
point coordinator (PC).

Prelab Activities
Read section 2.8.2 from "Computer Networks: A Systems Approach", 4th Edition.

Procedure
Create a New Project
To create a new project for the Ethernet network:
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project Click OK Name the project _WirelessLAN,
and the scenario DCF Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Choose Office from the Network Scale list
and check Use Metric Units Click Next twice Click OK.

Create and Configure the Network
To create our wireless network:
1. The Object Palette dialog box should be now on the top of your project space. If it
. Make sure that the wireless_lan is selected
is not there, open it by clicking
from the pull-down menu on the object palette.
2. Add to the project workspace the following objects from the palette: 9
wlan_station_adv (fix).
a. To add an object from a palette, click its icon in the object palette
Move your mouse to the workspace Left-click to place the object.
Right-click when finished.

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3. Close the Object Palette dialog box Arrange the stations in the workspace as
shown in the following figure Save your project.

Configure the wireless nodes:
1. Repeat the following for each of the nine nodes:
Right-click on the node Edit Attributes Assign to the Wireless LAN MAC
Address attribute a value equals to the node number (e.g., address 1 is assigned
to node_1) Assign to the Destination Address attribute the corresponding
value shown in the following table Click OK.
¾

Node
name
node_0
node_1
node_2
node_3
node_4
node_5
node_6
node_7
node_8

The following figure shows the values assigned to the Destination
Address and Wireless LAN MAC Address attributes for node_1.

Destination
Address
Random
5
8
6
7
1
3
4
2

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Traffic Generation Parameters:
1. Select all the nodes in the network simultaneously except node_0 (click on all of
them while holding the Shift key) Right-click on any of the selected nodes (i.e.,
node_1 to node_8) Edit Attributes Check the Apply Changes to
Selected Objects check box.
2. Expand the hierarchies of the Traffic Generation Parameters and the Packet
Generation Arguments attributes Edit the attributes to match the values
shown in the following figure Click OK.

3. Select all the nodes in the network simultaneously including node_0 Rightclick on any of the selected nodes Edit Attributes Check the Apply
Changes to Selected Objects check box.

Buffer Size specifies the
maximum size of the
higher layer data buffer
in bits. Once the buffer
limit is reached, the data
packets arrived from
higher layer will be
discarded until some
packets are removed
from the buffer so that
the buffer has some free
space to store these new
packets.

4. Expand the hierarchy of the Wireless LAN Parameters attribute Assign the
value 4608000 to the Buffer Size (bits) attribute Click OK.

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177

5. Right-click on node_0 Edit Attributes Expand the Wireless LAN
Parameters hierarchy and set the Access Point Functionality to Enabled
Click OK.

Choose the Statistics
To test the performance of the network in our DCF scenario, we will collect some of the
available statistics as follows:
1. Right-click anywhere in the project workspace and select Choose Individual
Statistics from the pop-up menu.
2. In the Choose Results dialog box, Expand the Global Statistics and Node
Statistics hierarchies choose the following five statistics:

3. Click OK and then save your project.

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Configure the Simulation
Here we will configure the simulation parameters:
1.

Click on

and the Configure Simulation window should appear.

2. Set the duration to be 10.0 minutes.
3. Click OK and then save your project.

Duplicate the Scenario
In the network we just created, we did not utilize many of the features explained in the
overview section. By default the distributed coordination function (DCF) method is used for
the medium access control (MAC) sublayer. We will create three more scenarios to utilize
the features available from the IEEE 802.11 standard. In the DCF_Frag scenario we will
allow fragmentation of the MAC data units into smaller frames and test its effect on the
network performance. The DCF_PCF scenario utilizes the point coordination function
(PCF) method for the medium access control (MAC) sublayer along with the DCF method.
Finally, in the DCF_PCF_Frag scenario we will allow fragmentation of the MAC data and
check its effect along with PCF.

The DCF_Frag Scenario:
1. Select Duplicate Scenario from the Scenarios menu and give it the name
DCF_Frag Click OK.

Fragmentation
Threshold specifies the
fragmentation threshold
in bytes. Any data packet
received from higher
layer with a size greater
than this threshold will
be divided into
fragments, which will be
transmitted separately
over the radio interface.
Regardless of the value
of this attribute, if the
size of a higher layer
packet is larger than the
maximum MSDU size
allowed by the IEEE
802.11 WLAN standard,
which is 2304 bytes, then
such a packet will not be
transmitted by the MAC,
and it will be
immediately discarded
when received.

1. Select all the nodes in the DCF_ Frag scenario simultaneously (click on all of
them while holding the Shift key) Right-click on anyone of them Edit
Attributes Check the Apply Changes to Selected Objects check box.
2. Expand the hierarchy of the Wireless LAN Parameters attribute Assign the
value 256 to the Fragmentation Threshold (bytes) attribute Click OK.

3. Right-click on node_0 Edit Attributes Expand the Wireless LAN
Parameters hierarchy and set the Access Point Functionality to Enabled
Click OK.

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179

The DCF_PCF Scenario:
To switch to a scenario,
choose Switch to
Scenario from the
Scenarios menu or just
press Ctrl+.

1. Switch to the DCF scenario, select Duplicate Scenario from the Scenarios
menu and give it the name DCF_PCF Click OK Save your project.
2. Select node_0, node_1, node_3, node_5, and node_7 in the DCF_PCF scenario
simultaneously (click on these nodes while holding the Shift key) Right-click
on anyone of the selected nodes Edit Attributes.
3. Check Apply Changes to Selected Objects Expand the hierarchy of the
Wireless LAN Parameters attribute Expand the hierarchy of the PCF
Parameters attribute Enable the PCF Functionality attribute Click OK.

4. Right-click on node_0 Edit Attributes Expand the Wireless LAN
Parameters hierarchy and set the Access Point Functionality to Enabled.
5. Click OK and save your project.

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The DCF_PCF_Frag Scenario:
1. Switch to the DCF_Frag scenario, select Duplicate Scenario from the
Scenarios menu and give it the name DCF_PCF_Frag Click OK Save
your project.
2. Select node_0, node_1, node_3, node_5, and node_7 in the DCF_PCF_Frag
scenario simultaneously (click on these nodes of them while holding the Shift
key) Right-click on anyone of the selected nodes Edit Attributes.
3. Check Apply Changes to Selected Objects Expand the hierarchy of the
Wireless LAN Parameters attribute Expand the hierarchy of the PCF
Parameters attribute Enable the PCF Functionality attribute Click OK.
4. Right-click on node_0 Edit Attributes Expand the Wireless LAN
Parameters hierarchy and set the Access Point Functionality to Enabled.
5. Click OK and save your project.

Run the Simulation
To run the simulation for the four scenarios simultaneously:
1. Go to the Scenarios menu Select Manage Scenarios.
2. Click on the row of each scenario and click the Co llect Results button. This
should change the values under the Results column to as shown.

3. Click OK to run the four simulations. Depending on the speed of your processor,
this may take several seconds to complete.
4. After the simulation of the four scenarios complete, click Close and then save
your project.

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181

View the Results
To view and analyze the results (Note: Actual results will vary slightly based on the actual
node positioning in the project):
1. Select Compare Results from the Result menu.

time_average is the
average value over time
of the values generated
during the collection
window. This average is
performed assuming a
“sample-and-hold”
behavior of the data set
(i.e., each value is
weighted by the amount
of time separating it
from the following
update and the sum of
all the weighted values is
divided by the width of
the collection window).

Delay represents the
end to end delay of all
the packets received by
the wireless LAN MACs
of all WLAN nodes in
the network and
forwarded to the higher
layer.
This delay includes
medium access delay at
the source MAC,
reception of all the
fragments individually,
and transfer of the
frames via AP, if access
point functionality is
enabled.

2. Change the drop-down menu in the lower-right part of the Compare Results
dialog box from As Is to time_average Select the Delay (sec) statistic from
the Wireless LAN hierarchy as shown.

3. Click Show to show the
result in a new panel. The
resulting graph should
resemble the shown one.

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4. Go to the Compare Results dialog box Follow the same procedure to show the
graphs of the following statistics from the Wireless LAN hierarchy: Load
(bits/sec) and Throughput (bits/sec). The resulting graphs should resemble the
following ones.

Load represents the
total load (in bits/sec)
submitted to wireless
LAN layers by all other
higher layers in all
WLAN nodes of the
network.
This statistic does not
include the bits of the
higher layer packets that
are dropped by WLAN
MACs upon arrival and
not considered for
transmission due to, for
example, insufficient
space left in the higher
layer packet buffer of the
MAC.

Throughput represents
the total number of bits
(in bits/sec) forwarded
from wireless LAN
layers to higher layers in
all WLAN nodes of the
network.

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183

5. Go to the Compare Results dialog box Expand the Object Statistics hierarchy
Expand the Office Network hierarchy Expand the hierarchy of two nodes.
One node should have PCF enabled in the DCF_PCF scenario (e.g., node_3)
and the other node should have PCF disabled (e.g., node_2) Show the result
of the Delay (sec) statistic for the chosen nodes. The resulting graphs should
resemble the following ones.

6. Repeat step 5 above but for the Retransmission Attempts (packets) statistic.
The resulting graphs should resemble the following ones.

7. Close all graphs and the Compare Results dialog box Save your project.

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Further Readings
−

ANSI/IEEE Standard 802.11, 1999 Edition: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) Specifications.

Exercises
1) Based on the definition of the statistic Load, explain why with PCF enabled the

load is lower than if DCF is used without PCF.
2) Analyze the graphs that compare the Delay and Throughput of the four

scenarios. What are the effect of utilizing PCF and fragmentation on these two
statistics?
3) From the last four graphs, explain how the performance of a node without PCF is

affected by having PCF enabled in other nodes in the network.
4) Create two new scenarios as duplicates of the DCF_PCF scenario. Name the first

new scenario DCF_allPCF and the second new scenario DCF_twoPCF. In
DCF_allPCF, enable the PCF attribute in all 8 nodes: node_1 through node_8
(Note: do not include node_0 in any of your attribute editing). In DCF_twoPCF,
disable the PCF attribute in node_3 and node_5 (this will leave only node_1 and
node_7 with PCF enabled). Generate the graphs for the Delay, Load, and
Throughput statistics and explain how the number of PCF nodes might affect the
performance of the wireless network.
5) For all scenarios, select the Media Access Delay statistic from the Global

Statistics Æ Wireless LAN hierarchy. Re-run the simulation for all scenarios.
Generate the graph that compares the Media Access Delay statistic of all
scenarios. Analyze the graph explaining the effect of PCF, fragmentation, and
number of PCF nodes on media access delay.

Laboratory

Mobile Wireless Network
A Wireless Local Area Network with Mobile Stations
Objective
This lab simulates mobility in wireless local area network. The effect of mobility on the
TCP performance is studied. In addition, the lab studies how the request to send (RTS)
and clear to send (CTS) frames are utilized in avoiding the hidden node problem usually
induced by mobility in WLAN.

Overview

One of the requirements of the IEEE 802.11 standard is to handle mobile stations in
wireless local area networks. Mobile stations are defined as the stations that access the
LAN while in motion. IEEE 802.11 handles station mobility within the MAC sublayer and
hence such mobility is hidden from the higher layers in the network. However the
disconnection and reconnection events induced by mobility in WLAN significantly affect
the performance of higher layer protocols such as TCP. For example TCP interprets
disconnection due to mobility as congestion and hence it multiplicatively decreases its
congestion window size. After reconnection, TCP takes unnecessary longer time to
recover the congestion window to a size that matches the available bandwidth.
IEEE 802.11 utilizes the request to send (RTS) and clear to send (CTS) frames in various
circumstances to further minimize collisions. RTS and CTS are especially useful in solving
the hidden node problem in WLANs that have mobile stations. Exchanging the RTS and
CTS between the sender and the receiver informs nearby stations that a transmission is
about to begin. Duration information in RTS/CTS frames are used to set the network
allocation vector (NAV) in all stations that are within the reception range of the RTS/CTS
frames. This way the problem of a hidden sender can be solved as any station that sees
the CTS frame knows that it is close to the receiver, and therefore cannot transmit for the
period of time indicated in the NAV. If transmitted data frames are short, sending
RTS/CTS frames is not recommended as it adds overhead inefficiency. Therefore, a
threshold is defined to use RTS/CTS only on frames longer than a specified length.
In this lab we will simulate a wireless LAN with mobile workstations and server. The
workstations will run an FTP application to upload files to the server. We will study the
effect of node mobility on the performance of the TCP connection for the FTP session. We
will study also the role of the request to send (RTS) and clear to send (CTS) frames in
avoiding the hidden node problem usually induced by mobility in wireless LAN.

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Prelab Activities
Read sections 2.8.2 and 4.2.5 from "Computer Networks: A Systems Approach",
4th Edition.

Procedure
Create a New Project
1. Start OPNET IT Guru Academic Edition Choose New from the File menu.
2. Select Project and click OK Name the project _MobileWLAN,
and the scenario Mobile_noRTSCTS Click OK.
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
Scenario is selected Click Next Select Campus from the Network Scale list
Click Next Make sure that Kilometer is the unit chosen for the Size and then
assign 2 and 1 to the X Span and Y Span respectively Click Next twice
Click OK.

Create and Configure the Network
Initialize the Network:
1. The Object Palette dialog box should now be on the top of your project space. If it
. Make sure that the wireless_lan is selected
is not there, open it by clicking
from the pull-down menu on the object palette.
2. Add to the project workspace the following objects from the palette: Application
Config, Profile Config, two wlan_wkstn (mob), and one wlan_server (mob).
a. To add an object from a palette, click its icon in the object palette Move your
mouse to the workspace Click to drop the object in the desired location
Right-click to finish creating objects of that type.
3. Close the palette.
4. Arrange and rename the objects you added as shown:

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5. Position the workstations and the server according to the x and y positions shown
in the following table:
a. To position an object: Right-click on the object Advanced Edit Attributes
Edit the x position and y position attributes.
Node
ClientA
FTP_Server
ClientB

x position
1.25
1.5
1.75

Configure the Applications:

0% for the “Command
Mix (Get/Total)”
attribute means all the
FTP sessions will be
only “Send” from the
clients to the server.

1. Right-click on the Applications
node Edit Attributes
Expand the Application
Definitions attribute and set
rows to 1 Expand the new
row Name the row
FTP_Application.
i. Expand the Description
hierarchy Edit the FTP
row as shown (you will need
to set the Special Value to
Not Used while editing the
shown attributes):

2. Click OK twice and then save your
project.

y position
0.5
0.5
0.5

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Configure the Profiles:
1. Right-click on the Profiles node Edit Attributes Expand the Profile
Configuration attribute and set rows to 1.
i. Name and set the attributes of row 0 as shown Click OK.

Configure the Applications in the Server and Clients:
1. Right-click on the FTP_Server node Edit Attributes:
i. Edit the Server Address attribute Assign the value FTP_Server to it.
ii. Edit Application: Supported Services Set rows to 1 Set Name to
FTP_Application Click OK twice.

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2. Select both ClientA and ClientB in the network simultaneously (click on both of
them while holding the Shift key) Right-click on one of them Edit Attributes
Check the Apply Changes to Selected Objects check box:
i. Expand the Application: Supported Profiles hierarchy Set rows to 1
Set Profile Name to FTP_Profile as shown Click OK.

ii. Edit the Application: Destination Preferences attribute as follows:
Set rows to 1 Set Symbolic Name to FTP Server Edit Actual Name
Set rows to 1 In the new row, assign FTP_Server to the Name column.
3. Click OK three times and then save your project.

Configure the Trajectory:
The trajectory attribute
specifies the name of an
ASCII trajectory file that
specifies the times and
locations that a mobile
node will pass through
as the simulation
progresses..

1. Right-click on ClientA Edit Attributes Assign trajectory_1 to the
trajectory attribute Click OK.
2. A green trajectory will appear on the project workspace. Right-click on that
trajectory and select Edit Trajectory.
3. In the Edit Trajectory Information dialog box, name the trajectory _left_trajectory Click OK.
4. From the Edit menu, choose Preferences. Check the value of the mod_dirs
attribute. A list of directories will appear. The first directory in the list is where a
trajectory file with the name _left_trajectory.trj is saved. Edit that file
using any text editor (e.g., Notepad). Replace all the contents of the file with the
info shown in the following figure then save and close the text editor.

Version: 2
Position_Unit: Kilometers
Altitude_Unit: Meters
Coordinate_Method: relative
Altitude_Method: absolute
locale: English_United States.1252
Coordinate_Count: 6
# X Position
Y Position
0
,0
-0.75
,0
-0.75
,0.02
-1
,0.02
-1
,0.04
0
,0.04

Altitude
,0
,0
,0
,0
,0
,0

Traverse Time
,0h0m0.00s
,0h0m20.97s
,0h0m2.24s
,0h0m6.99s
,0h0m2.24s
,0h0m27.96s

Wait Time
,0h2m0.00s
,0h1m0.00s
,0h0m0.00s
,0h0m0.00s
,0h0m30.00s
,0h0m0.00s

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5. Right-click on ClientA Edit Attributes Assign
_left_trajectory to the trajectory attribute Click OK.
6. The new trajectory should look exactly like the following one. Right-click on the
trajectory and select Edit Trajectory.

7. In the Edit Trajectory Information dialog box, verify that the trajectory info matches
the values shown in the following figure:
Note that the trajectory makes ClientA start moving after 2 minutes from the
beginning of the simulation. ClientA waits at X Pos 0.5 for 1 minute and at X Pos
0.25 for 20 seconds.

8. Click OK and then save your project.

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Configure the Simulation
Here we will configure the simulation parameters:
The Seed attribute is an
integer that is used by
the simulation’s random
number generator. Its
default value is 128.

Click on

and the Configure Simulation window should appear.

2. Assign 10.0 minutes to the Duration attribute.
3. Assign 256 to the Seed attribute.
4. Click OK and then save your project.

Choose the Statistics
To test the performance of our
mobile wireless network we will
collect some of the available statistics
as follows:
1. Right-click anywhere in the project
workspace and select Choose
Individual Statistics from the
pop-up menu.
2. In the Choose Results dialog box,
Expand the Node Statistics
hierarchy choose the shown
three statistics.
3. Right-click on the Congestion
Window Size (bytes) statistic
Choose Change Collection Mode
In the dialog box check
Advanced From the drop-down
menu, assign all values to
Capture mode as shown Click
OK.

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4. Right-click on the Traffic Received (bytes) statistic Choose Change Collection
Mode In the dialog box check Advanced From the drop-down menu, assign all
values to Capture mode.
5. Click OK twice and then save your project.

Duplicate the Scenario
We will create one more scenarios to utilize the request to send (RTS) and clear to
send (CTS) frames to study their effect on minimizing collisions.
1. Select Duplicate Scenario from the Scenarios menu and give it the name
Mobile_RTSCTS Click OK.
2. Select ClientA, FTP_server, and ClientB simultaneously (click on all of them
while holding the Shift key) Right-click on anyone of them Edit Attributes
Check the Apply Changes to Selected Objects check box.
3. Expand the hierarchy of the Wireless LAN Parameters attribute Assign the
value 256 to the Rts Threshold (bytes) attribute

4. Click OK and then save your project.

Run the Simulation
To run the simulation for both scenarios simultaneously:
1. Go to the Scenarios menu Select Manage Scenarios.

Mobile Wireless Network

193

2. Click on the row of each scenario and click the Collect Results button. This
should change the values under the Results column to as shown.

3. Click OK to run both simulations. Depending on the speed of your processor, this
may take several seconds to complete.
4. After the simulation of both scenarios complete, click Close and then save your
project.

View the Results
To view and analyze the results (Note: Actual results will vary slightly based on the actual
node positioning in the project):
1. Select Compare Results from the Result menu.
2. Select the Congestion Window Size (bytes) statistic for ClientA from the TCP
Connection hierarchy as shown.

3. Click Show to show the result in a new panel.

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4. Repeat the above steps for the following statistics:
a. Congestion Windows of ClientB;
b. Load for CLientA; and
c. Load for ClientB.
The resulting graphs should resemble the following graphs.

5. Go back to the Compare Results dialog box Expand the TCP Connection
hierarchy for the FTP server Select the Traffic Received (bytes) – Conn 1
statistic Select sample_sum to replace As Is as shown in the following figure
Click Show.
6. Repeat the above step for the Traffic Received (bytes) – Conn 2 statistic.
7. The resulting graphs should resemble the following graphs.

Mobile Wireless Network

195

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Further Readings
−

ANSI/IEEE Standard 802.11, 1999 Edition: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) Specifications.

−

Transmission Control Protocol: IETF RFC number 793 (www.ietf.org/rfc.html).

Exercises
1) Explain how Load and Congestion Window Size are affected by the mobility of

ClientA.
2) Explain how enabling RTS/CTS helps in avoiding the hidden node problem and

hence explain the effect of RTS/CTS frames on the network performance.
3) The graphs show that the server terminates the FTP session with CleintA earlier if

RTS/CTS is enabled. On the other hand, the server terminates the FTP session
with CleintB later if RTS/CTS is enabled. Explain why.
4) Create a new scenario as a duplicate of the Mobile_noRTSCTS scenario. Name

the new scenario twoMobiles_noRTSCTS. Create a second new scenario as a
duplicate of the Mobile_RTSCTS scenario. Name the second new scenario
twoMobiles_RTSCTS. In both new scenarios, edit the attribute of the
FTP_Server and assign _left_trajectory to its trajectory
attribute. Run the simulation for all scenarios and create the graphs for the Load
(bits/sec), Congestion Window Size (bytes), and Traffic Received (bytes)
statistic results as we did in this lab. Analyze the graphs explaining the effect of
the server mobility on the network performance.

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