How to measure SNR of a telephone Line? What is the device (without using router)?
Measuring the SNR of an ADSL signal is not a simple proposition. ADSL uses hundreds of subcarriers, each carrying a 16-QAM signal, as shown in the constellation diagram shown below, courtesy Wikimedia Commons.



The ADSL modem performs adaptive equalization to get the best possible constellation for each carrier, where "best possible" means that the individual points in the diagram have the least amount of "fuzziness" &mdash; the clusters of individual sample points are more tightly grouped. This accounts for the non-noise distortions of the channel (phone line), including its phase and frequency response.

The signal-to-noise ratio is derived from whatever fuzziness remains, which is due to external noise sources, crosstalk among the subcarriers and intersymbol interference. The modem does some statistical calculations that compare the actual signal sample values with their idealized values in order to come up with a number for SNR.

If the SNR on a particular subcarrier gets bad enough, the clusters will begin to overlap each other, leading to bit errors. When this happens, the modem can fall back to a simpler modulation scheme that has fewer points, or stop using that subcarrier altogether.

So the bottom line is, if you want to measure ADSL SNR, you need an ADSL modem chip. Since you already have one in your router, you might as well use it. Standalone SNR meters have modem chips inside them, too.