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My
Publications -
Tutorials
Roaming
to the Wrong AP
By
Eric Geier
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Originally Published by Wi-Fi
Planet on May 17, 2007 -
When you
connect to a wireless network, you are connecting to a single access
point (AP) -- though the radio card (A.K.A. client card, network
interface card, or NIC) in your laptop will roam to other APs with the
same SSID (or network name) , which allows mobile users to move about
a facility and continue to use the network. However, not all client
radios use the exact same reasoning and techniques to deal with
roaming , and radio frequency (RF) interference and other
environment conditions can cause problems.
If your
wireless network is outfitted with multiple APs, and users experience
poor signal strength and performance despite proper coverage in the
area, the user’s radio card may not be connecting to the closest or
most ideal access point for that specific location.
Here are a few
symptoms of a client connecting to the wrong AP:
Verifying
the Problem
Since these
symptoms are similar to other network issues, you should try to verify
that the radio client is connecting to the wrong AP, and not just
sitting too far away from the right one.
One way to
verify this problem is to find where the nearest APs are located in
relation to the location of the problem client, using a facility map
of the AP locations. Check to see which AP the client is connected to
when problems occur. This can be done by checking the association
lists on the APs, or by using a Wi-Fi analyzer. Then check if
there’s a closer or more ideal AP for the client, keeping in mind
the environment of the facility.
For example,
say you checked your AP association lists and saw that the client is
connected to AP 2, which is located on the next floor down. However,
the facility map indicates that AP 1 is slightly closer to the client
and doesn’t have any obstructions in the way. Therefore, you can
conclude that the radio is likely connecting to the wrong AP.
Quick Fixes
Here are a few
things to try in order to temporarily fix the problem:
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Restart
the radio card. Simply disabling and re-enabling the radio
card may work. During initial scanning, the client may choose the
correct AP.
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For
mobile clients, move around. Moving the client around may make
the radio card realize there’s a better AP in range.
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Use MAC
address filtering. You could block the client from connecting
to “incorrect” AP(s) by only allowing clients with specific
MAC addresses.
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Change
AP transmit power. You could slightly decrease the transmit
power on “incorrect” AP(s) or slightly increase the transmit
power on “correct” AP(s).
Fixing the
Underlying Problem
Although you
may be able to find a temporary fix for this problem, you may want to
do some in-depth analysis of the network and RF conditions. For
example, check to see if occasional RF interference causes the
problem, and analyze AP placement. You should also keep in mind that
the underlying cause of the problem may be the client card’s
scanning methods and techniques; therefore, you could try replacing
the card, or see if there’s a driver update that may fix the issue.
You may also want to contact the manufacturer to report the issues
you’ve found.
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