The speeds can and do vary forever as the VDSL MUST react to bursts of noise and things like crosstalk over the life of the line, just like with ADSL and ADSL2+
In fact DLM actually seems to work on BT's FTTC/VDSL implementation.
Or, at least, it works in both directions (unlike ADSL DLM which only ever seemed to increase margins and reduce speeds).
Ever since getting FTTC (around 18 months ago) I've had full speed (40Mbps, then 80 when that was rolled out), with few - if any - unforced resyncs.
Over the Christmas period my sync dropped by around 20Mbps, with the IP profile following it, and stayed there (regardless of daytime reboots, etc.) yet almost 5 days (to the hour) from my last forced reboot, my speed is back up to where it's always been and the IP profile has immediately jumped back up.
My guess is loads of tacky external Christmas lights between here and the cab.
Whatever it was, I'm happy to report BT's DLM doesn't seem anywhere near as broken as it was on ADSL
vDSL2 FTTC Infinity with BT
DL Sync 80Mbps
UL Sync 20Mbps
Edited by adebov (Mon 07-Jan-13 17:17:24)