My understanding is that the noise level in a WLAN is a result of the environment where wireless equipment is in use.
I have a site where the noise is around -98 dB when connected to an AP with the speed fixed on 11Mb. But when I connect to another AP in the same location, on a different channel with the speed not fixed (it can go up to 54Mb) the noise goes up to -72 dB.
The noise level may vary over time, which may explain this... however a more likely explanation (especially if you can repeat this scenario reliably) is that there is simply more noise on the channel that the second AP is running on.
The art here is to make sure as far as possible (whilst still using the non-overlapping channels 1,6 and 11) that each AP is running on the channel with the lowest noise level, whilst still not using the same channel as an adjacent AP.
The noise level shouldn't be impacted too much by the speed of the transmissions from the AP.
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We considered this possibility while but there are 2 installed APs on the site. One on channel 1 and another channel 11. Both seem to be affected by the increase in noise when transmitting at 54 Mb.
I have attached a copy of the levels that were measured using the Cisco ADU. The installed AP is on channel 11 and we set the test AP to channel 1. I will be returning to the site and try different channels and see if fixing the speed to 11 Mb makes any difference. Thanks.