If I timed the sync of my modem to a little after dawn which would be at 3dB , it would rise a little during the day and drop down to 0dB most nights whilst retaining sync. Some nights it would drop down to a negative SNR and -1.0dB was the lowest I actually saw and I have screen shots at -0.7dB.
I actually achieved over 100 days without loss of sync with it dropping to 0dB most nights. A good stable line and a decent modem is what is needed.
M H C
taurus excreta cerebrum vincit