Non identified interference problem with AP121

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Hi,

I have a 75APs deployment and I ́ve experimenting a interference problem in just one AP. The interference it ́s located in the library, and the thing is that I have 3 APs and only one of them can see it. Nevertheless, it ́s resulting in connection problems, inestabilty and slow perfomance because there are located in channel 13 so my channel 11 cannot be used.

What can I do in order to discover what ́s the problem? I tried with rogue mitigation tool but it doesn ́t solve my problem,



Thanks in advance,
Raúl
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Raúl Tejeda Calero

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Posted 5 years ago

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Matthew Gast

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Hi Raúl,

If you're detecting interference but don't see another 802.11 device, it's likely that you have non-802.11 interference that's knocking out the 802.11 signal. Rogue mitigation will only help against 802.11 devices on your network, and this looks like it's a radio-layer interferer. Given that it affects only one of your devices, it's probably near there. Can you get a screen shot of the spectrogram on that device for us to look at?
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Raúl Tejeda Calero

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Hi Matthew,

Sure, I have it attached for you, The main problem is the location of the interference in channel 13 so I have problems with channel 11, moreover the interference is not continuos, so it comes some seconds and then dissapear, do you think I should modify the dynamic channels assignments??



Regards,
Raúl
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Crowdie, Champ

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Are you able to let the spectrum analysis continue for a few minutes, or at least until the swept spectrogram graph is completely populated, and then post a screen shot again?
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Raúl Tejeda Calero

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Sure,

Here it is.



Thanks in advance,
Raul
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Crowdie, Champ

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The interference you can see in channel 11 on the screenshot below is from a wireless security camera I detected at one of my sites (the activity around channel 1 is from the wireless network). You can see it has a very similar real time FFT and FTT duty cycle pattern as your interference source.

Another possible interference source are monitored alarms, such as fire alarms.



Do you have a spectrum analysis application for a laptop? (MetaGeek, AirMagnet, etc) If you have then you need to use this to locate the source.
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Raúl Tejeda Calero

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Hi Crowdie,

You seem to be right. We have found many non-WiFi problems but not only video bridge devices but also cordless phones and bluetooth devices...



The cameras are "geovision" we guess that will be the problem. I currently have no proper tools in order to meassure it. Do you know about a free tool that can be used to identify the problem while I get a AirMagnet or Ekahau license?

Regards,
Raúl
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Crowdie, Champ

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Unfortunately what is causing your issue is also going to make it difficult to resolve without specialist hardware.

When an 802.11 client attempts to gain medium access it looks at the modulated signal level (called "Clear Channel Assessment") and listens for any 802.11 frames. The 802.11 frames have a Duration/ID field that contains how long, in milliseconds, it will take to complete transmission. You will notice that I said "modulated signal level" rather than "signal level" as the 802.11 client can only detect 802.11 modulated signals and not non-802.11 modulated signal, "noise" or "interference".

In your case the interference is across multiple channels and any 802.11 client attempting to associate to an access point on any of those channels will be adversely affected by the interference, which they cannot detect. This is why when you purchase a portable spectrum analysis solution, such as AirMagnet or MetaGeek, part of the solution is always a spectrum analysis adapter (commonly a USB adapter).

To "protect" your 802.11 clients from interference the access points the 802.11 clients want to associate to must be able to detect and react to interference. If an access point on channel 1 is suddenly adversely affected by an interference source on channel 1 it must be able to move to another channel, if necessary. 802.11 clients in the same area as the access point will be forced to associate to the access point again on another channel, say channel 6, and thus avoid the interference on channel 1.
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Leo Bistmans

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This looks like a nice RF interference search tool:

http://www.aaronia.com/products/spect...
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Crowdie, Champ

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Raul,

I ran into a rather interesting 2.4 GHz environment this week and thought of your post: