How do routers measure SNR?

[#1]

I have been having some REIN noise problems and this has lead me to monitor the SNR reported by the router, logging reading every 20 seconds.  Despite this the on/off an off/on transitions usually span several readings and I suspect this may be an artificial result because the reported SNR is averaged over quite a long time period (one minute, maybe).  This set me to wondering how the signal to noise ratio can actually be measured.  How can the router tell when there is no signal so a noise measurement can be made and how can the router arrange for this to happen, even when you are using the full download capacity of your line?

[#2]

Hi there,
Each router chipset has it's own way of interpreting the information but all essentially rely on digital signal processing. It's a very in depth subject but I've gained an understanding that it uses something called the "Nyquist–Shannon sampling theorem", that basically means algorithms determin the signal by reconstructing samples of it: http://en.wikipedia.org/wiki/Digital_signal_processing in short this seems to be very similar to how most of the current and last generation of smartphones determin and display signal strength.
Adam

[#3]

It's possible that the source of the interference takes a minute or so from being switched on to warm up or work its way up to working at full capacity, and the amount of interference it puts out increases during that time.