Lab 2

Switches versus Hubs

Overview Ethernet hubs work at the physical layer, simply repeating any frames they receive on one port onto all other ports. For this reason, they are also called multiport repeaters. Switches, on the other hand, forward frames only to the port that is connected to the frame’s destination. All the nodes connected to a hub are considered to be part of the same broadcast domain. In other words, any frame transmitted by one of the nodes will be received by all the rest. This configuration can limit throughput, as all nodes must share the capacity of the LAN. Since switches forward frames only on a single line, they create multiple broadcast domains, leading to great gains in throughput. The throughput of a switch is restricted, however, by its processing speed, the speed at which it can forward frames onto the correct outgoing link. Objective To examine the change in throughput in a local area network when upgrading from a hub to a switch. Build the Simulation Model Start up OPNET IT Guru Academic Edition. Select the File tab => New... Choose Project and click on OK. Change the Project Name to xx_Switch_vs_Hub (where xx are your initials). Set the Scenario Name to Hub and click on OK. In the Initial Topology window, select Create Empty Scenario and click on Next. In the Choose Network Scale window, select Office and click on Next. In the Specify Size window, accept the default values and click on Next. In the Select Technologies window, include the ethernet and ethernet_advanced model families, and click on Next. In the Review window, click on OK.

First, we will build a LAN in which the workstations are connected together with an Ethernet hub. An easy way to create a network with a large number of nodes in OPNET is with the Rapid Configuration tool.

OPNET Lab Manual Select the Topology tab => Rapid Configuration. Set the Configuration to Star, and click on OK. Set the Center Node Model to ethernet16_hub. Set the Periphery Node Model to ethernet_station. Set the Link Model to 10BaseT. Set the Number to 12, and click on OK to create the LAN.

Right click on the hub and select Set Name. Set the Name to Hub. Click on OK to close the window. Now we need to set up the traffic patterns for the Ethernet stations. Right click on any of the stations and choose Select Similar Nodes. Next, right click on one of the stations and choose Edit Attributes. Put a check in the checkbox next to Apply Changes to Selected Objects. Expand the Traffic Generation Parameters and Packet Generation Arguments attributes. Set the ON State Time to constant(1000), and the OFF State Time to constant(0). This will ensure that the stations are always sending. Set the Interarrival Time (seconds) to exponential(0.005) and the Packet Size (bytes) to constant(1000). Click on OK to apply the changes and close the window. Each station will now generate traffic at an average rate of one 1000-byte packet every 5 milliseconds. You can calculate the average traffic that each node will generate from the interarrival time and the packet size. For instance, 1000 bytes/packet * 8 bits/byte * 1 packet/0.005 sec = 1.6 Mbps

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Switches versus Hubs

We are done creating the hub-based LAN. Your model should look like the one presented next.

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OPNET Lab Manual Configure the Simulation Select the Simulation tab => Choose Individual Statistics… Expand the Global Statistics item and the Ethernet item, and select the Delay (sec) statistic. Expand the Traffic Sink item and select the Traffic Received (bits/sec) statistic. Expand the Traffic Source item and select the Traffic Sent (bits/sec) statistic. Expand the Node Statistics item and the Ethernet item, and select the Collision Count statistic. Click on OK to close window.

Select Simulation => Configure Discrete Event Simulation… Under the Common tab, set the Duration to 2, and the unit to minute(s). Click on OK to close the window.

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Switches versus Hubs

Duplicate the Scenario Let us now build another scenario which uses an Ethernet switch as the center of the LAN instead of the hub. This will allow us to compare the performance of the two designs. Choose Scenarios => Duplicate Scenario, and name the new scenario Switch. Click on OK to create the scenario. Right click on the hub and choose Edit Attributes. Left click on the model attribute and choose ethernet16_switch_adv from the pull-down menu. Click on OK to make the change.

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Right click on the switch and choose Set Name. Set the Name to Switch and click on OK to close the window.

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Switches versus Hubs

Run the Simulation Select the Scenarios tab => Manage Scenarios… Edit the Results field in both rows and set the values to or . Click on OK to run both scenarios (one after the other). When the simulation has completed, click on Close to close the window.

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Inspect and Analyze Results Select the Results tab => Compare Results… Select and expand the Global Statistics item, and the Ethernet item and select the Delay (sec) statistic. View the statistic in As Is mode. Click on Show for a more detailed graph. This statistic shows the delivery delay for Ethernet frames. You can see that the delay for the switch scenario is small and constant, while the delay for the hub scenario is growing without bound. This is to be expected as we are loading the hub far past its capacity. Remember that we said each node was generating 1.6 Mbps of traffic. With 12 nodes in the system, that comes to 12 * 1.6 Mbps = 19.2 Mbps of offered load. The hub is operating at only 10 Mbps (since we specified 10BaseT links in the configuration). Click on the close window icon and Delete the panel. Click on the statistic again to disable the preview.

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Switches versus Hubs

Expand the Traffic Sink item, and select the Traffic Received (bytes/sec) statistic. View the statistic in As Is mode. Click on Show for a more detailed graph. This statistic shows the sum total of traffic received by all stations in the LAN. You can see that the traffic received in the hub case tops out near 10 Mbps, the rated speed of the hub. The switch, on the other hand, easily delivers nearly twice that much traffic. Click on the close window icon and Delete the panel. Click on the statistic again to disable the preview.

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Expand the Traffic Source item, and select the Traffic Sent (bytes/sec) statistic. View the statistic in As Is mode. This statistic shows the sum total of traffic sent by all stations in the LAN. You can see that the same amount of traffic was generated in both scenarios. The switch, however, can actually carry this much traffic. Click on the statistic again to disable the preview. You can always click on Show for more detail. Click on Close to close the Compare Results window.

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Switches versus Hubs

Lastly, we will look at a statistic which only applies to the hub scenario. Select the Scenarios tab => Switch to Scenario and select the Hub scenario. Select the Results tab => View Results… Expand the Object Statistics, Office Network, Hub, and Ethernet items. Select the Collision Count statistic. View the statistic in As Is mode. This statistic shows the number of collisions that occurred at the hub during the simulation. You can see that close to 2000 collisions per second occurred. This is due to the heavy overloading of the hub. Click on Close to close the View Results window.

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Save your model and close all windows. Questions 1. The switching capacity of a switch determines the amount of load it can process. Duplicate the Switch scenario and name the new scenario Switching_Speed. Edit the switch and expand the Bridge Parameters attribute. Set the Packet Service Rate (packets/sec) attribute to 2000. Now the switch can only handle 2000 frames per second. Calculate the number of packets generated (total) using the Packet Generation parameters of the Ethernet stations. Rerun the simulation and examine the Ethernet delay, Traffic Received, and Traffic Sent statistics. Explain your results. 2. Continue with the Switching_Speed scenario you created in Question 1. Determine how large the Packet Service Rate must be in order for the switch to handle all the offered load. In other words, how large must the rate be in order for the Traffic Received to equal the Traffic Sent? Explain your answer. 3. Duplicate the original Switch scenario and name the scenario Smooth_Traffic. Right click on one of the Ethernet stations and choose Select Similar Objects. Right click again on one of the Ethernet stations and choose Edit Attributes. Edit the Packet Generation Parameters and set the interarrival time to constant(0.005). The average rate of traffic generation will be the same, but the frames will be generated regularly, rather than on the basis of an exponential (bursty) distribution. Rerun the simulation and compare your results to the Switch scenario results.

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