Below is an extract of a brief we received about uneccessary 'Lift & Shifts'. Hope it adds to your understanding of VDSL ?
NGA DLM profiles CCT’s perform differently to other forms of ADSL, and this means that the full sync speed will not be achievable at the PCP.
For example, if a CCT is syncing at 29MB at the End User’s premises, and there were no issues on the D-side pair, you should only expect to achieve sync of something around 29MB at the PCP.
A ‘Lift and Shift’ would initially increase the sync speed at the PCP, but DLM would eventually return the speed back to 29MB, as this is the maximum that the D-side can support.
That brief confirms to be BT are drilling rubbish into their staff meaning when people like me get fobbed off the person telling me that truly believe it themselves.
What are you speaking of is probably line banding which the new DLM does instead of adjusting target noise margins (I assume so it cant be overriden by people tweaking the noise margin CPE side).
BT appear to have a policy on FTTC where there is pretty much no SLA/fault threshold, and that their DLM system can throttle without limits until things stabilise. Combined with providing CPE's that hide information from the end user the result is the ability to lie to the end user. On adsl there was eg. the FTR.
Also interesting is that line speed estimates clearly look hugely under estimated, they appear to be pretty much worse case scenarios meaning if someone has a line that matches their estimated speed whilst they may feel satisfied it probably means they either have a fault or high levels of crosstalk around the top of the percentile's expected but BT can pass it off as fine.
My personal case as an example.
Day 1 I had 80/20 sync. Estimated 65.9/20 On HG modem, ECI dslam.
2 weeks later (if I remember right as was months ago now) I unlocked modem and the stats showed attainable of 110/36.
I noticed error rate was high considering how high my noise margin was and what others were reporting with short lines. I was averaging around 300 crc errors a day and I did have occasional error bursts even when I had those stats.
24 hours after I unlocked modem attainable fell sharply to 90/36. so 2 weeks after install.
Week after that attainable fell again to 73/24. With this sync speed and lost all my snr margin on the downstream I was now averaging 1200 crc errors a day, and the bursts now were serious when they occured causing me to be interleaved 3 times.
I probably would have got a better result if I at this point pushed the fault. Openreach have a advisory that if the line drops 25% in a short time its a fault. My problem was they look at 25% drop in visible speed. Since everything above 80 is supposedbly invisible my drop was from 80 to 73 not from 110 to 73. So I didnt report the fault.
Then later on 1-2 months later so about a month or so ago. I did report the fault as things started getting even worse, I had instability whilst interleaved, sync speeds falling below 60mbit, outages lasting multiple hours. Initially when I escalated to the BT chairmans office the first guy @i had spoke to was very sympathetic and arranged the engineer. But apaprently he was going on holday so someone else would be handling my case. The new guy was nowhere near as sympathetic and when the engineer proved to be a fail, (basically he did basic jdsu tests, refused to do a pair swap and said because he couldnt find anything on his tests was nothing to be done.) he said if no more outages he will consider fault closed, my sync speed is above my estimate so thats that. (slightly above 66 sync 65.9 estimate). I still at this point did not tell BT I had access to my stats instead I said I knew the line could do 110 as my install engineer showed me. I was basically been fed information similiar to what bald eagle posted that its normal for a line to slow down after install as its by design. the engineer visiting even claimed my initial 80 sync was impossible and thats on FTTP only. Welcome to BT fault resolution *sighs*.
Then I decided to let BT know I am a samknows tester, suddenly the guy decides I merit a new engineer so one is booked again. He comes today, changes the cable from my pole and I think great someone who is motivated. I also told this engineer I had the modem unlocked and I am not in the mood for fake information.
So my stats prior to the cable swap were 66/20 with 68/27 attainable. My line is fast path still since the last engineer but on a banded profile capped to 74 down. After the cable swap even with higher attenuation my attainable jumps to 97/39 yeah!! but with sync of 74mbit due to the banding. fixed? Sadly not. BT in their wisdom decide even tho he has done the work to put in a new cable of the pole I had to be switched back to the dropwire that I was sharing with my neighbour in the above flat. SO I am back on my old lower sync. I argued with a guy over the phone (I think his manager) for 20 minutes but the decision stands, back to shared dropwire. The guy who booked the engineer from the chairmans office said he will get back to me, I said dont waste your time, get me quote for dedicated dropwire. He said will ring me back tommorow.
Incidently after finding in BT's t&c's that they already breached my contract by refusing to fix previously I have already been offered a get out fo my entire contract without penalty and given 3 months free broadband, they have pretty much admitted the fault by their actions but refuse to fix it.