Crosstalk can also cause crc errors even without an impact on speed, so it may be there is lines in any given area (maybe my area) that are having stability issues so then power is cutback to reduce crosstalk to manage the errors, maybe not I dont know.
To be fair I havent read documentation anywhere that states CP's use powermasking to manage downstream crosstalk (only upstream) and on downstream to protect adsl services, I just assumed if its used on the upstream it logical its used on the downstream as well. Then I made further assumptions based on what some end users have reported and what I am seeing on my own line. But in the VDSL spec's power can be cutback on individual tones if desired.
An end user eg. is more likely to notice a 15mbit speed drop off to 5mbit than someone on 80mbit drop off to 70mbit. So a CP making choices on who to hinder for the sake of fairness and all that maybe chooses to help the 15mbit user over the 80mbit user. Indeed I havent reported this as a fault, but if I had a 15mbit sync dropoff to 5mbit I would almost certianly be crying about it to tech support on a regular basis.
In terms of crc errors. on the 5.8 power my upstream had about 50% of the downstream crc error rate. On 6.1 its now about 20% with the extra margin, on 6.6 it was extremely low.
To me it looks fairly clearcut my upstream power is been fiddled with, but far less clearcut on the downstream. The downstream could easily be just down to huge crosstalk.
BT Infinity 2 Since Dec 2012 - Estimate 65.9/20 - Attainable peak 110/36 - Current Sync 71/20