Co-Channel Interference- The old problem!!

[Flavio Miranda]

Hello everybody, This is kind a very old subject but I´d like to discuss here some details surrounding CCO that may be not that clear to everybody, at least for me is not.
We all know that to users to have a good experience using their device, the network must provide a good SNR (among many others thing, of course).   But, what I´d like to highlight here is at which point CCO can interfere in SNR level. I mean, put many APs on the same floor and on the same channel, may impact on the user experience due to the medium contention but not necessarily due low SNR. When exactly CCO starts to interfere on SNR ? I believe when the AP is far from each other and is not able to decode the other´s AP signal, then,considering this signal as a noise floor and lowering SNR. But, this value is a bit confuse. I am trying to figure out, how far it must be. In terms of frequency, I heard some value about 20 dBm.But, it is not clear at which frequency value I should start. I mean, if the cell boundary is -67 dBm, then value greater than -87 dBm should not interfere anymore and value between -68 dBm to -86 dBm should be seen as noise. But the bondary may vary depending  on the deployment.

I am attaching a picture to help. 

 Somebody is able to put some light here ?

[George Stefanick]

hello,

the rule is 20 dB. In other words wherever your client is and the channel they are on the next signal on the same channel needs to be 20 dB away to limit CCC cochannel contention. 

make sense?

[Flavio Miranda]

Yes, make sense but did not explain what I´m trying to understand. Probably my terrible English don´t let me to be more clear, but, what I am trying to get here is from which value I need to consider this 20 dB. From the value determined as a Cell boundary ? I don't think so, because this value depends on the network and I can change it according to my goal. Must exist a value from which the AP starts to consider each other´s signal as noise and a value which AP ignores each other´s signal completely.

[George Stefanick]

From ap to to ap perspective it would be the same as a client. Make sure each ap radio on the same channel doesnt hear the other under 20. 

Make sense ? 

[mohanak]

At the edge of each voice cell, the received signal strength indication (RSSI) measurement should be -67 dBm if you are using a Cisco Unified Wireless IP Phone 7921G. It is recommended that you have RSSI above 35 at the edge of the cell, which is equivalent to -67dBm for optimum preformance on the phone.

Each cell in the network should overlap with the adjacent cells in order to facilitate uninterrupted handoff as a client moves between cells and to provide a minimum service even in case of access point failure. For a typical voice deployment, Cisco recommends a 15 to 20 percent overlap of a given access point's cell from each of the adjoining cells.

http://www.cisco.com/c/en/us/solutions/collateral/wireless/4400-series-wireless-lan-controllers/net_implementation_white_paper0900aecd804f1a46.html