High Noise Margin
There are five main causes of a high noise margin. Only two are problems, the others are expected.
1 - The first easy cause is that your connection took place at a time between early dusk and late dawn. Background noise levels rise at dusk, peak overnight, and fall once full daylight returns.
In this case the noise margin will generally be 2-5dB above the normal range for your line and your connection speed down by 500-1500kbps.
The solution is to disconnect and reconnect in daylight hours. Some modem/routers you can do through the GUI, some using DMT or Routerstats, and some you have to power-cycle.
2 - The second easy cause to deal with often shows up as a very high number, typically 18dB or above, coupled with a very low connection/sync speed.
This frequently occurs after a one-off event like a thunderstorm in the area causing a burst or burst of (electrical) noise. This can cause a disconnection and then a reconnection while the huge noise is still present. As the target noise margin is re-applied at this point a very low sync results.
Once the noise goes away then noise margin rises by the amount of the noise at sync time.
So the first thing to try is always a disconnection and reconnection.
If that restores normal sync and noise levels then hopefully all is well. You should however think about what caused the noise if the answer isn’t obvious, and you may find up to three side-effects.
The most likely of those side-effects on a BT Wholesale based connection is that because of the low connection speed the IP Profile has dropped dramatically. There is also the possibility that Interleaving will have been turned on if it was previously off, and the target noise margin may have been raised.
3 - The expected cause is when a line is connecting at the maximum possible on the 8Mbps of BT-based ADSL Max. If the line is good enough in theory to connect faster then the noise margin rises at or immediately after the connection is made, to the level it would need to be to cause that connection speed if it were not limited.
4 - The awkward cause, only on BT-based systems, is when the DLM or engineer intervention raises the target margin. The margin immediately after connection will be near 9, 12 or 15dB, rather than the default 6dB.
If the increase is done by the system then it should self-correct after 10-14 days stability and moderate error rates. It is best not to disconnect or re-sync at all in the period but it seems the occasional one doesn’t matter. The self-correction causes a drop of 3dB in the target margin and an automatic re-sync. If the original rise was more than 3dB then the process needs to repeat as many times as required.
There is a high chance this automatic drop of target noise margin will occur overnight and in fact result in a lower connection speed. You are now in the situation covered at Cause 1 above. A daytime reconnection by you is needed and then let the process repeat if necessary as explained in the preceding paragraph.
If the increase is set by an engineer during investigation of a fault report then it is almost impossible to get it reduced again. Your ISP can request it but the usual response from BT is a flat No!
Two further points on automatic correction. First, it doesn’t always work. Second if it happens at night then the new connection speed may be lower than before, so a manual re-sync in the daytime is advised.
5 - Finally the unlucky cause. There is always the possibility that a target margin of 9,12 or 15dB is needed on your line anyway. See Unstable Lines on this page.