Two APs on the same channel... How far to keep them away?

[igorfi]

Hi Folks. I'm doing a project where an access point will be in every single room (there is a reason for this). Now the question is, how far away to keep two of the same channels APs from each other to avoid co-channel interference, and hence entirely eliminate competition between them? I would like to know this in dBi. Obviously, if one station will "sense" another station signal at 60 dBi, then they will be competing. So how much dBi from the other AP will not trigger CSMA/CA? 85dBi? 90dBi? Do you have a reliable source on this? I have a good books, but they don't mention this.

Thank you.

 

[t53186]

You can download a phone app (wifi analyzer) which can be used to monitor signal strength and channel usage. You can manually select the channel in the AP configuration.

 

[igorfi]

Hi!

I know, but I need to know the amount of dBi at which one station will neglect another on the same channel.

 

[DeauteratedDog]

It will vary somewhat between manufacturers, but somewhere around -85 dBm.

My source is Cisco's "Wireless LAN Design Guide for High Density Client Environments in Higher Education".

 

[igorfi]

Hi DeauteratedDog.

Thanks. I see that CWNA mentions the same -85dBm. But it says about the background noise. If it exceeds -85dBm, then it can degrade the transmission. But the problem is that two APs on the same channel will not consider each other as noise. They either compete or not. But I think you are right here, probably the manufacturer sets the CSMA/CA thresholds. So there is no right answer, I guess.

 

[DeauteratedDog]

If the AP detects another signal stronger than -85 then it will not attempt to transmit over it - APs take turns. If the other signal is below -85 then the AP will transmit, but the competing signal will reduce the SNR (Signal to Noise Ratio) and thus the maximum data rates - you might only get QAM-16 instead of QAM-64 (for example).

 

[igorfi]

You absolutely correct! You wrote that faster than I got some data sheets for Cisco. It is called a Rx sensitivity:

https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-3800-series-access-points/datasheet-c78-736498.html

At -101dBm it still can operate on 1Mbps.

Thank you!