Riverbed Modeler Academic Edition

Laboratory

1

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 14 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.

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Procedure

Create a New Project

To create a new project for the Ethernet network:

1. Start Riverbed Modeler Academic Edition ⇒ Choose New from the File menu.

2. Select Project ⇒ Click OK ⇒ Name the project <your initials>_Ethernet, and the scenario Coax_2 ⇒ Make sure that the Use Startup Wizard is checked ⇒ 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 Finish.

4. Close the Object Tree 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 Next.

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.

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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.

4. To configure the coaxial bus, right-click on the horizontal link ⇒ Select Edit Attributes (Advanced) from the menu:

a. Click on the value of the model attribute ⇒ Select Edit from the drop-

down 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.

5. Now you have created the network. It should look like the illustration below.

6. Make sure to save your project.

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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.

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.

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

state.

4. Expand the Traffic Generation Parameters hierarchy:

a. Change the value of the ON State Time to exponential(100) ⇒ Change

the value of the OFF State Time to exponential(0.00001). (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) ⇒ Change the value of the Interarrival Time attribute to exponential(2).

6. Click OK to return back to the Project Editor.

7. Make sure to save your project.

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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 DES 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. Select Choose Statistics (Advanced) from the DES 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.

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 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.

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Run the Simulation

To run the simulation:

1. Click on the Configure/Run Simulation button: ⇒ Assign 30 second(s) (not

hours) to the Duration ⇒ Click Run. Depending on the speed of your processor, this may take several minutes to complete.

2. After the simulation run is complete, click Close.

3. Save your project.

4. Select Duplicate Scenario from the Scenarios menu.

5. Name the new scenario Coax_1.

6. Right-click on any of the 30 nodes ⇒ Select Similar Nodes. Now all nodes in the network are selected.

7. Right-click on any of the 30 nodes ⇒ Edit Attributes. 8. Check the Apply Changes to Selected Objects check box. This is important to

avoid reconfiguring each node individually. 9. Expand the Traffic Generation Parameters hierarchy ⇒ Expand the Packet

Generation Arguments hierarchy ⇒ Change the value of the Interarrival Time attribute to exponential(1).

10. Go to back to step 1 in this page and repeat the instructions 8 times for the

Interarrival Times of exponential(0.5), exponential(0.25), exponential(0.1), exponential(0.05), exponential(0.035), exponential(0.03), exponential(0.025) and exponential(0.02) using the scenarios names: Coax_05, Coax_025, Coax_01, Coax_005, Coax_0035, Coax_003, Coax_025 and Coax_002. Notice that each time the simulator is completing a run for different traffic generation interarrival time (representing the load into the network) and that each successive run takes longer to complete because the traffic intensity is increasing.

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View the Results

To view and analyze the results:

1. Click on the View Results button: Now the Results Browser is open.

2. Select the DES Parametric Studies tab.

3. From the Results for drop-down menu, select Current Project.

4. Uncheck and check again the results for your project in order to check all the

results.

5. Uncheck Coax_0025

6. Uncheck Include vectors.

7. Expand the Scalar Statistics ⇒ Expand the Traffic Sink and Traffic Source.

8. Right click on Traffic Received and select Set as Y-Series

9. Right click on Traffic Sent and select Set as X-Series

10. The resulting graph should resemble the one below:

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Questions

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) Use three duplicates of the simulation scenario implemented in this lab named

Coax_01, Coax_005, and Coax_0025. Make sure that the Interarrival Time

attribute of the Packet Generation Arguments for all nodes in the scenarios are

as follows:

- Coax_01 scenario: exponential(0.1)

- Coax_005 scenario: exponential(0.05)

- Coax_0025 scenario: exponential(0.025)

Choose the following statistic for node 0: Node Statistics →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 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

Results in the DES menu after the simulation runs is done.)

3) To study the effect of the number of stations on Ethernet segment performance,

create a duplicate of the Coax_0025 scenario. Name the new scenario

Coax_Q3. In the new scenario, remove the odd- numbered 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_0025 and Coax_Q3. Explain the graph and comment on the results.

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_0025 scenario. Name the new scenario Coax_Q4. In the new scenario

use a packet size of 512 bytes (for all nodes). For both Coax_0025 and

Coax_Q4 scenarios, choose the following global statistic:

Global Statistics→Traffic Sink→Traffic Received (bits/sec). Rerun the

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simulation of Coax_0025 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_0025 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 questions 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.