Riverbed Modeler Academic Edition

Laboratory

3

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 it 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.

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Procedure

Create a New Project

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

menu.

2. Select Project and click OK ⇒ Name the project

<your initials>_SwitchedLAN, and the scenario OnlyHub ⇒ Make sure that the Use Startup Wizard is checked ⇒ 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 Finish.

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 Next.

The prefix ethernet16_ indicates that the device supports up to 16 Ethernet connections.

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

2. Click the Select Models button in the Rapid Configuration dialog box. From the

Model List drop-down menu choose ethernet and click OK.

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, X=50, and Radius = 42 ⇒ Click OK.

4. Right-click on node_16, which is the hub ⇒ Set Name ⇒ 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.

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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.

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). Save your project.

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Choose Statistics

To choose the statistics to be collected during the simulation:

1. Right-click anywhere in the project workspace and select Choose Individual

DES Statistics from the pop-up menu.

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

The Ethernet Delay represents the end to end delay of all packets received by all the stations.

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

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.

Configure and Run 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 Run.

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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.

2. Open the Object Palette by clicking on . Make sure that Ethernet is selected

in the pull-down menu on the object palette.

3. We need to place a hub and a switch in the new scenario. (They are circled in the

following figure.)

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.

6. Close the Object Palette.

7. Right-click on the new hub ⇒ Set Name ⇒ Change the name attribute to Hub2 and click OK.

8. Right-click on the switch ⇒ Set Name ⇒ Change the name attribute to Switch and click OK.

9. Reconfigure the network of the HubAndSwitch scenario so that it looks like the

following one:

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10. Remove All links by selecting them and hitting the Delete key.

11. Add a new links, using the 10BaseT link available in the Object Palette.

12. Save your project.

Configure and Run 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 Run.

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View the Results

To view and analyze the results:

1. Select Compare Results from Results in the DES 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. From the Results for drop-down menu, select Current Project.

4. Check OnlyHub and HubAndSwitch.

5. Expand Global Statistics ⇒ expand Ethernet ⇒ expand Traffic Sink ⇒ expand

Traffic Source.

6. Expand Object Statistics ⇒ expand Office Network ⇒ expand Hub1 ⇒ expand

Ethernet.

7. 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.

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8. 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.

9. 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.)

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10. Select the Collision Count statistic for Hub1 and click Show.

7. 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.

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8. The resulting graph should resemble the one below.

9. Save your project.

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Questions

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.

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.