I'm currently decorating my study where the VDSL router is installed and have had to, temporarily move it to the hall where the master socket is installed. The master socket is equipped with a VDSL MK2 faceplate. From that I run a (filtered) phone extension and a single pair (unfiltered) VDSL feed to the study using CAT5 cabling. The length of this cable is less than 10 metres. I had been used to getting a reliable DS sync speed of about 58-59mbps, despite being 650 metres from the cabinet (as measured by me using GPS).
I figured when plugging in the router into the hall master VDSL unfiltered socket I'd suffer some loss of speed as the study extension forms what's called a "bridged tap" - that is an unterminated pair as a T off the main path. What I had not expected was that it would lose me fully 25mbps downstream and almost 3mbps upstream (out of 10mbps). That is simply astonishing and if anything demonstrates how sensitive VDSL is to the quality of domestic extension wiring it is this. I figure if I put a balanced termination on the extension it might help, but I simply disconnected the unfiltered extension pair and got back my original speed (in fact, as you'd expect, a fraction more as I'd shortened the cable run to the cabinet by about 10m).
These are the stats from the master without the extension (similar in the study extension).
6. Data rate: 10470 / 59987
7. Maximum data rate: 10496 / 60012
8. Noise margin: 6.2 / 6.0
9. Line attenuation: 30.1 / 21.3
10. Signal attenuation: 29.9 / 20.5
This is what happens when operating from the master with that <10m of CAT5 pair extension to the study.
6. Data rate: 7558 / 33144
7. Maximum data rate: 7562 / 33922
8. Noise margin: 5.7 / 6.4
9. Line attenuation: 32.9 / 21.4
10. Signal attenuation: 21.0 / 18.4
That's almost a 9dB increase in downstream signal attenuation from less than 10m of CAT5 pair leading to a 45% drop in DS speed. OK, I know some of the theory of signal reflected off of unterminated transmission lines, but even so...