Planning And Analysing Wlan Using Opnet Computer Science Essay
This is the second, more advanced laboratory exercise dealing with network simulation and more specifically, with wireless networks. The exercise is based on an OPNETWORKS tutorial (OPNETWWORKS is the official OPNET conference).
Students are expected to have completed successfully the first OPNET simulation (assignment 2.1). For those of you lacking experience, there is an introductory assignment that you can follow, in the R: drive (assignment 2a – net simulation – tutorial.pdf)
. There is also an introductory slide show to be viewed before starting this assignment. It will give you some basic understanding of fundamental WLAN procedures and the way that OPNET deals with them. It will also describe in more detail the problems posed in this assignment.
The assignment is divided in two parts: (both are pre-created for your convenience)
Lab 1 deals with the “hidden station” problem, a situation where two WLAN terminals can both “see” a third one but not each other. As a result, they try to initiate transmission at the same time and their packets collide and are destroyed. The situation is resolved by the introduction of two procedures: Request To Send / Clear To Send (RTS/CTS).
Lab 2 investigates the scenario of adding more Access Points (AP) on a given office space, in order to distribute the traffic more evenly, avoid major bottlenecks and improve throughput.
Learning Outcomes
After completing this lab exercise you should be able to:
Set up a wireless network using OPNET.
Select specific attributes of the WLAN to fine-tune your WLAN
Understand the impact of the RTS/CTS mechanism as a measure to prevent the hidden node problem
Become familiar with WLAN model attributes needed to configure BSSs
Be able to select an appropriate WLAN topology according to the application traffic
2. Starting OPNET
To start OPNET under Windows XP/2000 perform the following steps:
Go to Start button and locate under “SED Software” the group called OPNET modeller 14.5 (or similar) then click the item called OPNET modeller.
Wait until OPNET starts up and a valid license is obtained from the network. Then click OK.
Create a directory and copy the files that come with this assignment (WLAN directory) in your scratch drive (either D: or your personal disk space). Within OPNET, select File / Model files / Add model directory. Point to your newly created directory and choose make this the default directory.
3. Lab 1: Hidden Node Scenario
Methodology
Run a simulation on a wireless LAN (WLAN) scenario, which suffers from “hidden node” problem
Observe the effect of the problem using the collected statistics
Enable the RTS/CTS feature to overcome this issue
Verify that the network performance improves as a result of enabling the RTS/CTS frame exchange
Start OPNET Modeler
Open project “1332_WLAN”.
Select File / Open.
Select Project.
Choose “1332_WLAN”.
Click OK.
Observe traffic settings
Right-click on Node B.
Select Edit Attributes.
Observe the inter-arrival time parameters
Expand Traffic Generation Parameters.
Observe the settings.
Observe the packet size parameters
Expand Packet Generation Arguments.
Observe the settings.
Click Cancel.
Run simulation
Click on the Configure/Run Discrete Event Simulation (DES) button.
Notice that the simulation Duration is 1000 seconds.
Click Run.
When the simulation completes, click the Close button.
View node movement and results
Click on the Hide/Show Graph Panels button.
Select DES / Panel Operations / Panel Templates / Load with Latest Results.
Observe the results shown.
Move the result panels around, and make them smaller if necessary, so that you can see both the results and the nodes in the network.
Select View / Show Time Controller.
Time Controller is a new feature that can be used to see how network data, like location of a mobile node that is moving along its trajectory or a result panel that shows a time-varying statistic, changes over time.
Click on Configure… button.
Set the value of Slider end time to 1000s and click on OK.
Note that this was also the duration of our simulation run.
On Time Controller window click on >> to advance the animation with 1-minute steps.
Observe that, with each step, the green vertical bars on results panels moving to the right and the mobile node is traversing its trajectory (e.g., moving between minutes 5 and 12).
Results analysis
At 350 seconds, Node A becomes a hidden node to Node B (and vice versa) for approximately 300 seconds due to the defined trajectory pattern.
During this period, since Node A and Node B can’t detect each other’s transmissions, the collision probability for their transmissions increases.
Increased collisions probability leads to higher number of collisions and retransmissions.
Wireless LAN delay increases due to packet loss and retransmissions.
Expected throughput is achieved only through a high number of retransmissions.
Close Time Controller window and click the Hide/Show Graph Panels button to hide the graphs.
Applying RTS/CTS mechanism
Switch scenarios
Select Scenarios / Switch to Scenario.
Select “hidden_node_rts_cts” scenario.
Select all WLAN stations
Right-click on any station.
Choose Select Similar Nodes.
Edit node attributes
Right-click on any of the selected stations.
Select Edit Attributes.
Click Apply changes to Selected Objects.
Expand the attribute Wireless LAN / Wireless LAN Parameters.
Click on the value field of RTS Threshold (bytes).
Select Edit…
Enter 1024.
Recall that the minimum size of the generated packets is 1500 bytes.
Close both attribute editing windows by clicking on OK.
Type Ctrl+S to save the project.
Run simulation
Click on the Configure/Run Discrete Event Simulation (DES) button.
Notice that the simulation Duration is 1000 seconds.
Click Run.
When the simulation completes click on Close button.
View results
Click the Hide/Show Graph Panels button.
Select DES / Panel Operations / Panel Templates / Load with Latest Results.
The panels will be updated with the latest results.
Like we did in the first part of the lab, feel free to study the results also by using Time Controller.
Results analysis
At 350 seconds, Node A becomes a hidden node to Node B (and vice versa) for approximately 300 seconds.
During this period, use of RTS/CTS frame exchange reduces the number of collisions (and therefore retransmissions) significantly.
CTS message sent by the Receiver node informs the sender nodes about the upcoming data transmission attempt of the other node and its reservation of the channel.
There are still more transmissions compared to when Node A and Node B can detect each other’s transmissions, because they sense the ongoing activity on the medium still only when the Receiver responses to the other sender’s RTS with a CTS.
Due to less number of retransmissions, the wireless LAN delay drops drastically for the period when the nodes are hidden to each other.
The delay when Node A and B can hear each other is now higher because of the overhead caused by the RTS/CTS exchanges.
Click the Hide/Show Graph Panels button to hide the graphs.
Type Ctrl+S to save the project.
4. Lab 2: Infrastructure ESS (Extended Service Set)
Methodology
Create an independent BSS wireless LAN network that spans multiple floors on a building
Configure appropriate attributes and then run a simulation
Study the statistics to observe that the network performance is not satisfactory
Add more access points to build an ESS network
Limit the number of access points as 3 to prevent channel overlapping, hence no interference
Run another simulation to find out whether the additional access points improve the network performance
Instructions
Switch to the Infastructure_BSS scenario
Select Scenario / Switch to scenario.
Select “Infastructure_BSS” .
Completing the network
Open the object palette
Click on the Open Object Palette button.
Add 3 wireless workstations to each level
Click on the wlan_wkstn (fix) node inside the object palette.
Place nine workstations to the project workspace as shown on the following figure.
When done, right-click in the workspace to end object creation.
Close the object palette window
Configuring the workstations
Select all WLAN stations
Right-click on any station.
Choose Select Similar Nodes.
Configure traffic
Right-click on any of the selected stations.
Select Edit Attributes.
Check the box Apply changes to selected objects.
Expand the attribute Applications / Application: Supported Profiles.
Add one row by setting row value to 1.
Expand this new row.
Set Profile Name to Wlan_engineer.
Set WLAN data rate to 1 Mbps
Expand the attribute Wireless LAN / Wireless LAN Parameters.
Set Data Rate to 1 Mbps.
Click OK to save attribute value changes.
Click on the workspace to deselect the nodes.
Choosing the statistics for collection
Observe selected statistics
Right-click in an empty area of the workspace and select Choose Individual DES Statistics.
Expand Global Statistics.
Observe statistics chosen
FTP
HTTP
Remote Login
Wireless LAN
Click Cancel.
Running simulation and viewing results
Run simulation
Save the project by typing Ctrl+S.
Click on the Configure/Run Discrete Event Simulation (DES) button.
Notice that the simulation Duration is 10 minutes.
Click on Run button.
When the simulation completes click on Close button.
View results
Click the Hide/Show Graph Panels button.
Select DES / Panel Operations / Panel Templates / Load with the Latest Results.
The panels will be updated with the latest results.
Results may be different due to node positioning! To get the matching results, use the reference project.
Results analysis
High application response time
Application data received is lower than the application load
Wireless LAN access is very high and remains high
The network is saturated; WLAN MACs drop packets because their buffers are congested and full
Click the Hide/Show Graph Panels button to hide the graphs.
Type Ctrl+S to save the project.
Scaling capacity by aggregating bandwidth
Adding new access points
Duplicate scenario
Select Scenarios / Duplicate Scenario.
Type “Infrastructure_ESS” .
Click OK.
Create new access points for level 3 and 4
Select the access point node at level 2 (AP_Level_2) by clicking on it.
Press Ctrl+C or select Edit / Copy.
Press Ctrl+V or select Edit / Paste.
Click on an area on level 3.
Click on the Switch node to connect the link from the access point.
Repeat above steps to create the network as shown below.
Configuring each access point
Edit the attributes of the access point on level 2 (AP_level_2)
Right-click on the access point node AP_level_2.
Select Edit Attributes.
Expand Wireless LAN / Wireless LAN Parameters.
Set the value of the BSS Identifier to the level number (2 for level 2, 3 for level 3,…).
Click OK.
Click in the workspace to deselect the node.
Edit the attributes of the access points on level 3 and 4
Repeat the previous steps for access points on levels 3 and 4.
Configuring the workstations
Configure all WLAN workstations on level 2
Click on each node while holding down Shift key.
Do not select the access points or the switch!
Right-click on any of the selected stations.
Select Edit Attributes.
Check the Apply changes to Selected Objects box.
Expand Wireless LAN / Wireless LAN Parameters.
Set the value of the BSS Identifier to the level number (2 for level 2, 3 for level 3, …).
Click OK.
Click in the workspace to deselect the nodes.
Configure the workstations on level 3 and 4
Repeat the previous steps for workstations on levels 3 and 4.
Running simulation and viewing results
Run simulation
Type Ctrl+S to save the project.
Click on the Configure/Run Discrete Event Simulation (DES) button.
Notice that the simulation Duration is 10 minutes.
Click Run.
When the simulation completes click on Close button.
Import an analysis configuration file to compare results
Menu: Scenarios / Scenario Components / Import… / Analysis Configuration.
Select “1332_infrastructure_ess”.
Choose Yes to destroy the existing panels.
Select DES / Panel Operations / Panel Templates / Load with Latest Results.
The panels will be updated with the latest results.
Results may be different due to the position of nodes!
Results analysis
Deployment of additional access points increased the WLAN capacity
Sharing the clients among the access points reduced the contention for each shared medium
No WLAN packet drops are observed anymore
Application throughput increased significantly
Lower WLAN delay due to less contention lowered also application end-to-end delay despite higher throughput
Increasing the number of collision domains is an useful alternative when the data rate cannot be increased
Click the Hide/Show Graph Panels button to hide the graphs.
Type Ctrl+S to save the project.
5. Report
Your report should include:
A description of your methods.
An example of an OPNET report file.
All derived throughput-load diagrams.
A short interpretation for each diagram.
A comparative analysis of theory and simulation.
Conclusions.
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