All the info comes from DSLStats 4.53.1 on a HG612
Right. Have you given us attainable figures, instead of the actual sync speeds?
Do you have a "connection stats" tab within the "telnet data" section?
If so, copying the data here would help.
Perhaps that's not the case with VDSL?
I don't recall a correlation in plain ADSL; my technical background tells me there shouldn't be a correlation either - but I could be wrong. Because the upstream band sits only at the lowest frequencies, and isn't overlapped, it rarely gets affected by distance, downstream speeds, or crosstalk from other users.
There was rumoured to be a link in ADSL2+, which is why Plusnet did not automatically increase upstream speeds after switchover to 21CN. However, that link turns out to not have been as significant as they thought. Like ADSL, I can't think of a real reason
For ADSL2+ Annex M, you do see a correlation: Some frequencies that are (for ADSL2+) normally used for downstream are instead used for upstream. Downstream speed ought to decrease, while upstream should increase, when using annex M
In VDSL, there are multiple upstream and downstream bands that don't overlap, so you see the same effect as ADSL (ie no correlation). I don't know of an equivalent of Annex M here.
However, VDSL has different bandplans that an operator can choose, which use different boundaries between the upstream & downstream bands. Choosing a different bandplan will allow for different speeds up & down... we saw such a change in 2011 when BT swapped from the 40/10 setup (using 7MHz and the 997 bandplan) in preparation for the introduction of 80/20 (using 17MHz and the 998 bandplan). Some people could see a re-balance happen between their up- and down-stream, even before 80/20 packages became available.
Because some of the VDSL upstream frequency bands are higher than in ADSL, they do suffer from the effects of distance - which is why the upstream can degrade in VDSL.