SNR

SNR, short for Signal-to-Noise ratio is the amount usable signals your line can see opposed to the amount of noise it can hear. It is measured in decibels (dB), with a high SNR representing a clearer signal. Low SNR is generally a bad thing, as the signal should stand out a lot from any background noise.

There is an important difference between SNR and SNR Margin, with reference to DSL -

The SNR is what we see on the below graph - each tone has from about 10dB of SNR at the highest frequencies to over 50dB of SNR at the lowest frequencies. The SNR does NOT change on any given line (except for a small drop during dark hour). This cannot be altered by the ISP.

The SNR Margin is what we refer to when we talk about the "Margin" displayed in a router status page. The SNR Margin is the difference between the Available SNR per Tone and the Bit Loading Per Tone. Say for example, a line has a 48dB SNR at 200KHz. If the SNR Margin was set to 6.0dB then 42dB of the tones (SNR) at 200KHz would be used (loaded with bits), and 6.0dB of tones (SNR) at 200KHz would go unused, reserved for fluctuations in SNR at various times of day or bitswapping if other loaded tones become unavailable later at night due to interference.

The SNR Margin is in no way the TOTAL SNR available on the line, but the DIFFERENCE between what is available, and what is being USED.

Good ISPs can set your Margin to a level that is best for your line, a lower Margin setting would give a higher sync speed whereas a higher Margin would give a slower speed. The amount of Margin can be seen as the amount of "wiggle room" the connection has. The more is has, the more stable it becomes, as it will be able to deal with bigger fluctuations in noise without disconnecting. Normally, the Margin is set in 3dB increments. Normally 6, 9, 12 and 15dB. Some ISPs will offer 3dB, but there are only a few lines that will be stable on this setting.

It is known however for a downstream Margin to be set to 18dB if however the line suffers constant errors (more than ten) in one hour. This will result in increased loss of sync and a 2 week wait before it is reduced in 3dB increments.

Here is a DMT tool graph, the yellow one shows the Total SNR available, with the Blue graph showing how much of the available SNR is being loaded with bits. It is the difference between the Yellow and the Blue graph that the router reports as the SNR Margin to you and I.