Using channels other than 1, 6 and 11

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Mar 9th, 2010
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I had a strange happening just recently and I can not explain what happened.  I was hoping that someone may beable to shead some light on what may have been happening.

I was running a WLC 2125 with 24 AP's (6 1252's and 18 1142's)  The location was within a convention center and this system was supply wireless internet access for attendees.  At one point new associations to the wireless system started to fail.  Two laptops side by side acted differently.  Both showed a strong signal however one could connect and work fine while the other failed to even show up as attempting to connect to the wireless controller.  In one location where there was a high number of wireless devices I had 5 access points spead around the room.  The area covered roughly an area of 200 x 100 feet.  I verified that TPC and DCA where working and I was seeing proper channel assignments as well as power being set correctly.  This area many people could not connect.  The other area was smaller.  This was maybe 70x40 feet and had 5 AP's spread around the room.  This area worked perfectly with no complaints.

Now comes the strange part.  When the 2 of the 5 AP's where shutdown and any rogue AP's where placed on say channels 2,7, and 10 then the issue disappeared.  The rogue AP's where operated my booths and where allowed to operate for the event.  What I can not figure out is why decreasing the number of AP's and the use of non-standard channels fixed the issue with clients not being able connect.

Any informatiomn on where to look to explain why this solved the issue would be great.



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Leo Laohoo Tue, 03/09/2010 - 13:20
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Go to Wirelesss > 802.11a > RRM > General and under Clients (1 to 75) what is the value?  By default, the AP will accept 12 clients.

Kayle Miller Tue, 03/09/2010 - 13:24
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     I am not sure I can truly explain what happened or what you saw, but there are a number of things that could have been contributing to the issues. For example the issue could have been a result of channel congestion/utilization; this is a larger issue in really dense environments where you deploy 5, 10, 15 access points in a small area due to the potential user count. In most cases when this is done all data rates are enabled (1,2,5.5,6,9,11,12,18,24,36,48,54) what that means is that all the beacons are sent at the lowest data rate (1mbps). This scenario creates alot of traffic on the channel which in the case of a really dense deployment there are many AP's sharing the same channel and they are all performing the same functions. George S. had a great posting to another issue that discusses this in great detail somewhere here in the mobility community.

     That alone can cause clients to have issues because if all of the channel is used by beacons there is no time, bandwidth, or cycles to service the client data traffic. Now lets add the other side of the communication to the equation; you have all these clients out there polling and sending beacons, along with association requests, authentication requests, Acks, etc... all this traffic contributes to the overall channel utilization as well further compounding the possible issues; because if 2 clients talk to the AP at the same time one of then is going to get pushed aside for a time, but it will continue to look for other access points, causing additional traffic on other channels.

     This scenario is lessened by moving the other vendor's AP's to non-standard channels because although they are over-lapping your channel, they still lessen the overall all traffic load. Also by shutting down two of the access points you also drastically cut down on the amount of channel utlization the clietns were experiencing.

     The final HUGE variable here is that depending on the client driver they can have an effect on how the client functions in a highly dense environment.

Hope this helps explain at least some of your question..  If you feel this information was vaulable feel free to rate it.



Robert Phillips Tue, 03/09/2010 - 15:27
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Kayle I think you have going me on a track that may help explain it in greater detail.  I was always under the assumtion that if you put a AP on a overlapping channel that it would cause the clients to believe the frequency is clear to sent however when they do send the interfence would prevent the data from both this client as well as the client on the other overlapping channel which will then cause both to repeat and thereby increase the congestion.

I was under the assumtion that even if two clients are both on the same channel yet on different SSID's then they would hear that the frencency is busy and wait thereby decrease the colllisions.  How is this not the case?

I do know I had turned all speeds at and below 5.5 off but maybe it would have helped to turn off even more but I didn't want to prevent someone with an old 802.11B client from being able to connect.

Now is it possible that even though by shutting down the two extra AP's that we where still very close to capacity.  By shutting down the Ap's it opened up the airspace be removing the beacons but as soon asthe clients used up that space the same issues would have started to appear again?


Scott Fella Tue, 03/09/2010 - 20:31
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Might as well throw in my 2cents.... First off I would of disabled RRM and disabled all data rates below 11mbs.  This would keep AP's from switching channels.... dropping devices and transmit the beacons at the lowest 11mbs.  Being in an open area you probably would still see the AP pretty far.  Anyway's.... disabling the 2 AP's might of helped sort of stabilizing RRM, but it was probably a coincidence that the rouge AP's helped.  You probably had to look at the noise level on the different channels which could of been high and caused issues with clients associating.  How many SSID's did you use by the way.

Robert Phillips Thu, 03/11/2010 - 07:13
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I was broadcasting only a single SSID, however there where about 10 rogue networks running thier own SSID's.  All but one rogue network was running a single AP.

One of the other things I saw that I could not explain is that two people where standing side by side.  One was working via Wireless and the other not.  I could see active clients on all of the AP's and it appears that once they had assciated everything was fine.  They had great performance and could easily roam from location to location.

Scott Fella Thu, 03/11/2010 - 07:59
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That is a tough senerio.... again... a lot of noise will cause issues with clients associating.  Nothing you can do with other wireless in the area besides trying to adjust channels or use 802.11a since most laptop devices support that now a days.

dennischolmes Fri, 03/12/2010 - 14:57
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Now for my 2 cents worth as well. The effect of adjacent channel interference

in an already congested rf environment would be devatstating. You had several APs already beaconing 1,2,and 5.5 beacons. Add to that your rogue APs on adjacent channels, most assuredly broadcasting low data rate beacons as well, and you have a recipe for disaster. You would have had better success by lowering the power to 12.5mw -25mw and disabling 1,2,and 5.5mb data rates and attempting to get the rogues to change channels that are appropriate. Also make sure that like Leo said, the client number per AP was bumped up to 20 for additional capacity. Note that doing this would really wipe out any ability to provide voice or latency based applications but I don't suspect you would be offering any of those at this show anyway. Having Spectrum Expert would have told you the definitive answer on spectrum saturation.


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