Fighting 2.4Ghz interference in crowded space.

[mario_kmc]

Hello all,

We have a small ofice in the 3rd floor of a building and we are using 2 access points Aironet 1130 http://www.cisco.com/en/US/products/ps6087/index.html.

Since some time now the space has become really crowded with access points. 1 AP is on channel 5 and is almost unusable. The 2nd one on channel 11 has some APs to fight with and with smaller signall strenght.

I have been researching and read a bit about changing some access point parameters (RTS/CTS/timers) on a multi access point environment. I want to do that while waiting for 5GHz AP's (and because not all clients support 5GHz).

Anyone has experience with this? Can AP performance really be improved? How to do that in Cisco 1130?

Thanks very much

Mario

[fbarboza]

Hi , 

Remember that when working with wireless we should always do a site survey to be able 
to determine the current RF interference the site has, which channels are being used 
by other access points and how strong the signal is seen to those APs from where you 
are located. 

Then determine how many access points to get to cover the required area, where to 
locate each access point, how to distribute the 3 non overlapping channels 1-6-11 in 
the 20.4GHZ) between your access points and the 3rd party access points so that they 
do not create interference leaving the access points with the same channel the further 
apart, what power levels to use so that the overlap of the cell between your access 
points and 3rd party access points is not more than 15%, etc. 

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_q_and_a_item09186a00805e9a96.shtml

You can do tweaking as you mention on the access point settings to try to get 
the wireless clients to connect but which exact setting would work would be provided 
by the site survey results or configuring and testing until you get it to work. 

I would disable aironet extensions on the radio, set the data rates to best range 
and set the 3 non overlapping channel between your 3 access points to see how it goes. 

[mario_kmc]

Hi fbarboza,

Thanks for your time. I have been using InSSIDer for some time to survey Wi-Fi channels in our office. We have only 2 APs and our office is not big and the AP centrally located can cover all office averaging -65dBm RSSI at the edge farthest away from the AP.

I dont have any tools to survey non-WiFi interference.

Please check attached. One AP is using channel 4 (NetOneGuest) and the other is using channel 11 (NetOne) so they are already non-overlapping. As you can see there are many other signals on channels 1, 6 and 11. They are not managed by us.

I will have a look at the link you sent and try your suggestions.

[Shawn Purdy]

One thing I would try would be to reduce the power output of the AP. it shrinks the cell size to reduce interference. The cell size is smaller but it concentrates it's energy in smaller foot print as you have described. I did this at a warehouse once when I had one AP over powering another one.

Sent from Cisco Technical Support iPad App

[mario_kmc]

Hello Shawn,

Wouldn't reducing power degrade quality since I don't have control over the other AP's? Or are you suggesting I use more APs for the same area?

Thank you

Mario

[George Stefanick]

Mario,

Seems like you are very intersted in finding a solution for your problem. Wireless is a tricky technology. Let me take a few minutes and share with you a few things that I hope will help you better understand.

1. 2.4 GHz starts at 2.400 and ends at 2.4835 (2.400 - 2.4835)

2. IEEE allows 14 channels, depending on what country you live in. In the US we use 11 channels.

3. IEEE states, each channel will be center frequency and that each channel (on 2.4 ghz) requires 22 MHz (center channel) spacing. Also that each channel (1,2,3,4,etc) should have 5 Mhz spacing. So what does this all mean.

It means channel 1 will be center on 2.412 channel 2 will be (5 MHz away) 2.417 etc. You will note that channel 1 to channel 6 there is 25 MHz more than the 22 MHz that is required by the IEEE. Also channel 6 is 25 MHz away from 11. This is why we use 1 6 11.

4. If you have 2 aps on the same channel they should have a 20 dBm difference between them to lessen interference. By this I mean, if you have a ap #1 and you laptop reads -60. Your next ap (yours or your neighbors should be 20 away) like -80. If it is not you can have potential for interference.

5. Although you have APs on the sma channel doesnt mean they will interfer all the time. In fact, they will only interfer when they both tranmit at the same time. Example. If you are on a party line with 50 people. Say you and 1 other person talks. You being the ap and the other person client. Everything is good. Now suppose 2 more people talk while you talk, not there is interference.

6. Interferece will cuase retries. You will need to tool that will sniff your network and look at the retry rates.

7. By making your cell smaller you are limiting the over all cell coverage. By doing this you limit RF power. BUT i will say, in neigboring siutations lowerin the cell and adding more aps help. Also, adding aps and powering up your aps could help.

Not being there hard to really tell what your issue is. You should try to get your hands on some tools like Metageek.

I hope this makes sense.

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"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin

[mario_kmc]

Hi George no doubt I am,

At least attenuate the most until we can have expert survey and/or more AP's.

Your paragraphs were very helpfull especially 4. about the power difference between signals on the same channel.

7. Understood

Regarding 6. "retry rate". The 1130 has a GUI with radio detailed interface stats. Attached is the equivalent command using CLI and output. Can these stats either retries or any other stat? Is there a CLI command that can provide helpfull info?

looking for the "retry" keyword I found under transmit statistics: retries, "packets with one retry", "packets with more than one retry".

Below that there are statistics associated with each data rate.

I have also heard about "carrier busy test", is this any help? I did one (but its after work time) and got results below.

Frequency  Carrier Busy %

---------  --------------

   2412         10

   2417          7

   2422          5

   2427          3

   2432          9

   2437          4

   2442          5

   2447          5

   2452          2

   2457         13

   2462          6

   2467          5

   2472          2

Mario

[George Stefanick]

Actually, looking at the client or ap stats are more reliable then if you sniff the traffic with another tool to look at retires.What you are looking at is frames sent at different PHY rates and how many needed to be resent.  As for the CB test. I broke this down in a blog post here:  http://www.my80211.com/home/2010/7/2/wlc-carrier-busy-test.html

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin

[mario_kmc]

George,

Thanks for your reply and link. After a lot of reading I have found probable way of attenuating the problem.

Both AP's each one with its own SSID are "installed" on top of a rack. Just sitting there. That probably isnt any good.

solution 1. Move AP's one apart from the other and fix them in the ceiling. One in the current position  and the other some 7-10m apart. Change configuration so both AP's have the same 2 SSIDs (internal and guest) and play with power so they don't overlap too much.

solution 2. Do a loooot more reading!!! about those retries and the PHY layer of 802.11.

Thanks everyone for their contribution

[George Stefanick]

Yup, knowledge is power and very important when working with WiFi. Stop back if you have any other questions..

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
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‎"I'm in a serious relationship with my Wi-Fi. You could say we have a connection."