I have fiber optic line, I just recently upgraded my connection from 2 mb to 4 mb and I noted a major decrease of Upstream SNR.
Before upgrading to 4 mb my line condition was very good like SNR always stays at 30+, line attenuation was like 5-8
But now look at SNR UP
I tried to change modulation from adsl2 to g.dmt but both give same stats. Also before upgrade I never face disconnection issue usual connection uptime was like 5 to 6 days, but now within 9 hours it disconnected 12 times.