Particularly on ADSLx, less so on FTTC, downstream noise margin is very likely to fall by anything up to 3dB
overnight. The interference hitting the copper between you and the DSLAM/MSAN is greater at night.
A simple fact. It's to do with reflections off one of the atmospheric belts when the sun is down. Which is why it is always recommended to connect well inside daylight hours for highest speed, and at night for greatest stability.
Think about it. For decades the default sync-time noise margin was 6dB. SpeedTouch modems would lose connection if it went below 3dM. Netgear would hold it as low as 0dB or even sometimes -1dB.
Why did BT Wholesale Sky and TalkTalk all set it at 6dB? Because it was deemed to be the lowest safe level. Nowadays with improved modem technology at both ends, BT Wholesale allow DLM to set connection time SNRM at 3dB. Openreach are trialling 3-5dB as you know for VDSL2.
It's a safety margin. It is expected to fall. It is rarely a fault, and if it is there will be much more serious and noticeable symptoms that a slight overnight decrease. Remember this page
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
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