Why does my AC68U router show 4 different SNR margin figures?

[Eattelf]

The DSL log page of my router (Asus DSL-AC68U) shows a SNR Margin value for 'Downstream' under a heading of 'Line Stats' plus three more values for SNR Margin under a heading of 'VDSL Band Status', one for D1, one for D2 and one for D3 (and corresponding upstream ones)

I understand what SNR is and the difference between the ratio and the margin. I also understand that D1,2,3 are referring to the different bands in use but please can someone explain where the 'downstream' value comes from?

It doesn't seem to always be the largest or the average of D1,2,3

Which figure gives me the better idea of my overall line quality?

Which figure gives me the better idea of my overall line quality?

 

[Tech9]

Which figure gives me the better idea of my overall line quality?

The modem is doing it's thing, leave the numbers alone. Do you get DL/UL Internet speed close to what you pay for?

 

[Eattelf]

Actually No. When its up I'm getting less than 1/4 of the dl speed I had a month ago and for the last few weeks it has only been up at all for a couple of hours at a time, then down for 5 hrs. The SNR margin is flapping around like crazy between 0.5 dB and 6.5 dB.

The reason for my question is that I have written an application to monitor the SNR margin over a long period of time and save the results to a file that I can send to my ISP to illustrate the issue. Since my code is taking the dB value from the internal web page of the router. I particularly want to know which of the four SNR margin figures gives the best idea of my line quality (as I said in my post) and how the single download figure relates to the three individual band numbers, as it doesn't always seem to be the max or the average.

@ Tech9 If you know the answer please can you post it, rather than simply dismissing my question.