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From: Aaron Leonard on 1 Apr 2010 17:36


~ On Mar 26, 3:42�pm, Aaron Leonard <Aa...(a)Cisco.COM> wrote:
~ > Then look at co-channel interference, noise, etc. �Make sure that
~ > things look pretty solid.

~ Thanks everyone for the information and links... I have one more
~ question: How do I look at the co-channel interference and noise? Do I
~ need a spectrum analyzer?

Co-channel interference refers to multiple 802.11 cells (i.e. BSSIDs
AKA APs) overlapping in the same physical space. So as a first cut,
you can just use your site survey tool (whatever it may be) and see
what APs are present (at what signal levels) at each physical point
in your coverage area.

For example, let's say that you see the following APs:

On channel 1: AP1 @ -55dBm, AP2 @ -88, AP3 @ -89
On channel 6: AP4 @ -72dBm, AP5 @ -86
On channel 11: AP6 @ -53dBm, AP7 @ -84

Then I would say that you're fine wrt co-channel - a client in that location
should pick either AP1 or AP6, and any other APs on that channel are
30 dB away.

On the other hand, if you have:

Channel 1: AP1 @ -55dBm, AP2 @ -65, AP3 @ -70
Channel 6: AP4 @ -60dBm, AP5 @ -63
Channel 11: AP6 @ -58, AP7 @ -63

Then you have a co-channel problem - no matter whether the client picks
AP1, AP4 or AP6, there is another AP within 10 dB.

Now, with co-channel interference, you don't necessarily see any performance
problem, until you get under load. To measure the load effect, you really
would need a packet capture tool or similar at the time of load.

Noise, on the other hand, is non-802.11 energy. (Or what is *perceived*
as non-802.11 energy by the 802.11 radios - it could actually be energy
from some distant AP, or from an AP on an overlapping channel [like from
channel 3 when you're on channel 1.]) You would use a spectrum analyzer
(like Cisco Spectrum Expert) to measure noise.

Having said all of the above ... you can also just look at your
lightweight APs to measure the "load" (i.e. 802.11 interference,
from this/other APs in your RF group), "interference" (i.e. 802.11
interference from other devices *not* in your RF group) and "noise".

Example (from my lab):

(WLC4402) >show ap auto-rf 802.11b AP1142

Noise Information
Noise Profile................................ PASSED
Channel 1.................................... -93 dBm
Channel 2.................................... -90 dBm
Channel 3.................................... -81 dBm
Channel 4.................................... -94 dBm
Channel 5.................................... -59 dBm
Channel 6.................................... -92 dBm
Channel 7.................................... -89 dBm
Channel 8.................................... -90 dBm
Channel 9.................................... -87 dBm
Channel 10................................... -90 dBm
Channel 11................................... -94 dBm
Interference Information
Interference Profile......................... FAILED
Channel 1.................................... -70 dBm @ 2 % busy
Channel 2.................................... -128 dBm @ 0 % busy
Channel 3.................................... -46 dBm @ 12 % busy
Channel 4.................................... -39 dBm @ 2 % busy
Channel 5.................................... -128 dBm @ 0 % busy
Channel 6.................................... -128 dBm @ 0 % busy
Channel 7.................................... -128 dBm @ 0 % busy
Channel 8.................................... -128 dBm @ 0 % busy
Channel 9.................................... -56 dBm @ 1 % busy
Channel 10................................... -128 dBm @ 0 % busy
Channel 11................................... -44 dBm @ 23 % busy
Load Information
Load Profile................................. PASSED
Receive Utilization.......................... 0 %
Transmit Utilization......................... 1 %
Channel Utilization.......................... 29 %
Attached Clients............................. 1 clients
[ ... ]
Nearby APs
AP 00:12:44:b3:e0:00 slot 0.................. -8 dBm on 1 (10.0.47.3)
AP 00:19:07:c6:04:b0 slot 0.................. -22 dBm on 11 (10.0.47.11)
Radar Information
Channel Assignment Information
Current Channel Average Energy............... -60 dBm
Previous Channel Average Energy.............. -18 dBm
Channel Change Count......................... 96
Last Channel Change Time..................... Thu Apr 1 05:44:27 2010
Recommended Best Channel..................... 11

So this shows some real concerns:

- noise at -59 dBm on channel 5
- high interference on channels 1, 3, 4, 9 and 11
- potentially high load (29%) on your channel (11)
- APs excessively nearby (-8 and -22 dBm - they're all in the same rack actually)
- previous channel average energy -18dBm (that's super loud)
- 96 channel changes

If this were a real production network, rather than just in my messed-up
lab, I'd be worried ...

Hth,

Aaron
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