AIUI the "Impacted" (B) range is allowing for imperfect home wiring. In the days of "engineer install only" that line didn't exist, and when it came in the A range was the same as the previous only range.
If you are an early adopter on your cabinet you are likely to be somewhere around the top of the range. As the cabinet becomes more and more populated it is probable the real speed and the estimate will decrease, due to new crosstalk. If you are not an early adopter, you will just slot into the range somewhere, depending on the take-up there.
Unfortunately the crosstalk isn't simply dependent on the number on the cabinet. It is very dependent on the proximity of in-use pairs. You could see many join and have no effect, then one more really hits you. Alternatively, that one is the first after you and you get hit immediately but then stay stable. Or, it's your joining that hits that one and you start off affected! Plus any permutation you care to think about wrt such factors.
What is guaranteed is that unless what you get is clearly a fault, (and that can take a long time to prove), you get what you get and Openreach don't give a damn. As a result, your ISP can do (many censored words) very little.
A final factor is Interleaving
. If that kicks in the normal (lowest) hit is about 10Mbps with a latency increase of 8ms. Unlike the old ADSLx DLM interleaving however, this one does revert to Fast Path if it proves over a couple of weeks or so to have been an isolated burst of errors/noise.
My broadband basic info/help site - www.robertos.me.uk
| Domains,site and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 56.6/14.1Mbps @ 600m. - BQM
"Angels can fly because they can take themselves lightly."
- G K Chesterton.