Figure 1
Revised: August 26, 2005
By Richard G. Baldwin
File: FwlProj080.htm
This laboratory project was prepared specifically for the benefit of my students who are enrolled in ITNW 1351, Fundamentals of Wireless LANs.
The project was designed under the assumption that students enrolled in the course have successfully completed the prerequisite course, ITNW 1325, Fundamentals of Networking Technologies.
The project design also assumes that the students are actively studying the material in the prescribed textbook for this course, which explains such complex topics as the IEEE 802.11g wireless specification.
Another browser window
I recommend that you open another copy of this document in a separate browser window so that you can easily view the discussion and the figures at the same time.
The purpose of this project is to teach you how to change the network Transmission Rate for the Linksys router, and to illustrate the results of making those changes.
The following equipment is required to complete this laboratory project:
The Linksys router supports a feature that your textbook author refers to as dynamic rate shifting. Linksys refers to it as the Auto-Fallback feature.
According to the textbook author,
"As a mobile device moves farther away from the AP, the transmission speed decreases. Known as dynamic rate shifting, this allows a mobile user to remain connected albeit at a slower speed."
The User Guide for the Linksys router describes it this way,
"The rate of data transmission should be set depending on the speed of your wireless network You can select from a range of transmission speeds, or you can select Auto to have the Router automatically use the fastest possible data rate and enable the Auto-Fallback feature. Auto-Fallback will negotiate the best possible connection speed between the Router and a wireless client. The default value is Auto."
How to change the Transmission Rate
You can change the Transmission Rate on the Wireless/Advanced Wireless Settings page in the administrator panel, as shown in Figure 1.
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Thirteen choices are available
When you pull down the choice box for Transmission Rate in Figure 1, you are allowed to select any one of the following thirteen choices:
What is the impact of the choices?
As mentioned earlier, you can select Auto to have the router automatically use the fastest possible data rate and enable the Auto-Fallback feature. Auto-Fallback will negotiate the best possible connection speed between the router and a wireless client.
Otherwise ...
When you select any Transmission Rate value in the above list other than Auto, the router is allowed to operate only at that rate regardless of other conditions such as the Signal Strength.
(As you can see, a Transmission Rate of 54 Mbps was selected for the router prior to capturing the screen shot shown in Figure 1.)
In this project, you will experiment with making changes in the Transmission Rate of a Linksys router and you will observe the impact of making those changes.
(Note that the parameter that is labeled Transmission Rate in the Linksys administrator panel shown in Figure 1 is labeled Speed in the Windows XP Wireless Network Connection Status dialog shown in Figure 2. Also note how the Signal Strength is indicated by green bars in Figure 2.)
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Behavior for Transmission Rate set to Auto
You will begin by experimentally determining the relationship between Transmission Rate and Signal Strength when the Transmission Rate is set to Auto.
You will do this by:
Behavior for 54 Mbps only
Then you will experimentally determine the behavior of the system when the Transmission Rate is set to 54 Mbps only and the laptop computer is once again moved to same location described above.
By now, you should know how to perform all of the following steps, so I will simply lay out the steps without providing detailed instructions on how to perform them.
Set up the wireless connections
Manually close all wireless network connections, making sure that all networks within range of your computer are put into Manual mode.
Open a wireless connection to the Linksys router.
Work with the administrator panel
Use the connection to the Linksys router to open the Linksys administrator panel.
Open the Wireless/Advanced Wireless Settings tab in the Linksys administrator panel.
Select Auto for Transmission Rate and click the Save Settings button.
Confirm signal strength and speed of the wireless connection
Open the Windows Wireless Network Connection Status dialog and confirm that you have five full bars of Signal Strength and a Speed of 54.0 Mbps.
Keep the dialog open and in a position on the screen where you can see the Signal Strength bars and the Speed as you perform the experiment
Confirm Internet connectivity and speed
Open a Google page in your web browser and confirm that you can successfully search for Baldwin Java. This search operation should produce lots of hits.
Move laptop away from the router
While monitoring the Windows Wireless Network Connection Status dialog shown in Figure 2, move your laptop computer away from the Linksys router to a distant location where the Signal Strength is reduced to only one or two full bars.
(If you don't have a laptop computer, you might be able to accomplish most of this by leaving your computer in one place and moving the router instead.)
Run some tests at the distant location
Once you arrive at that distant location:
Now, go back to the router
Return to the near proximity of the Linksys router where you have five full bars of Signal Strength.
Set the router to 54 Mbps speed
Use the connection to the Linksys router to open the administrator panel to the Wireless/Advanced Wireless Settings tab.
Select 54 Mbps for Transmission Rate and click the Save Settings button..
(What you have done is to specify that the router will be allowed to operate only at 54 Mbps regardless of any other conditions.)
Perform the same set of tests again
Take your laptop back to the same distant location as before and try to execute the same set of steps as before:
A summary of my results
When I arrived back at the distant location, the Signal Strength was once again showing only two full bars.
The connection appeared to be intact
The Wireless Network Connection dialog and the Wireless Network Connection Status dialog were both showing that the connection was intact.
The Wireless Network Connection Status dialog was also showing that the Speed was 54.0 Mbps.
Unable to connect to Google
However, when I attempted to execute a search using Google, the browser informed me that "google could not be found." It told me to "check the name and try again."
Unable to reconnect to the router
When I manually disconnected the wireless connection from the router and attempted to manually reconnect, the connection process never finished.
The dialogs weren't telling the truth
Clearly, even though the dialogs indicated that the connection was intact, I had lost communication with the router when I moved my laptop computer to the location where the Signal Strength was only two full bars.
The bottom line with Auto-Fallback (dynamic rate shifting)
When the router was allowed to use Auto-Fallback and adjust its speed to match the prevailing condition of low Signal Level, it maintained the connection but communicated more slowly (24 Mbps instead of 54 Mbps).
The bottom line without Auto-Fallback (dynamic rate shifting)
However, when Auto-Fallback was disabled and the router wasn't allowed to decrease its speed to compensate for the lower Signal Level, it simply quit working.
Somewhere between five full bars of signal level and two full bars of signal level, there is a crossover point below which the router is incapable of communicating at 54 Mbps. When that crossover point was reached, communications simply stopped. Unfortunately, this was not indicated by the information showing in the Windows dialogs.
Copyright 2005, Richard G. Baldwin. Reproduction in whole or in part in any form or medium without express written permission from Richard Baldwin is prohibited.
Richard has participated in numerous consulting projects and he frequently provides onsite training at the high-tech companies located in and around Austin, Texas. He is the author of Baldwin's Programming Tutorials, which have gained a worldwide following among experienced and aspiring programmers. He has also published articles in JavaPro magazine.
In addition to his programming expertise, Richard has many years of practical experience in Digital Signal Processing (DSP). His first job after he earned his Bachelor's degree was doing DSP in the Seismic Research Department of Texas Instruments. (TI is still a world leader in DSP.) In the following years, he applied his programming and DSP expertise to other interesting areas including sonar and underwater acoustics.
Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems.
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Figure 2