Let's hope openreach get it right this time.... 
TBH I dont forsee too much of a problem. It is only proof of concept. Shorter lines could benefit.
Its already in use on 21CN (which doesnt have G.INP) without any issue. If the line experiences issues and generates more errors, then in theory it should back off in a day or two. 3dB has been in use by other ISP DLM systems for >10yrs.
6dB is needed for ADSL1, but advancements in dsl algorithms since ADSL2(+) has lowered this to 3dB for stable lines - which means that if a line doesn't have more than a 1dB daily swing and drop below 2dB, then hopefully they should be fine. ***
The fly in the ointment with NGA DLM is its over zealous use of capping. :/ ... and the ECI inability to do vectoring and upstream g.inp.
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ETA
Whilst I am unhappy about lack of g.inp (& vectoring) on the ECI's - and how things do keep getting delaying for retransmission... perhaps because this keeps getting delayed then why should it stop them pre-trialling something which could benefit some ECI users.
In an ideal world it would be nice if ECI users could get vectoring and g.inp, and as such those users are getting an inferior service, so why not at least see if the XdB could at least benefit some lines.
***
reg member Kondrado who is on adsl 2+ quite often pushes his line to below 2dB without any undue consequences.
Ive even tried it myself when on BE* a few times but cba to keep tweaking after every reboot, but I do know it ran ok several times at 2dB or less after a crosstalker came online. The caveat with doing this on NGA and pushing a line to its max, is that damn cap.
