This is all silly and not helping the OP. I made an minor comment about non-compliance and I stand by that.
You are wrong, as given an approximate 24Mps downstream sync on Annex A there is a fair chance that there would be no change to it on Annex M. To me, the implication is the person producing that page understood little about the Annex M issue, as they have reduced the maximum downstream by 6Mbps.
At around 16Mbps downstream Annex A there is indeed a loss of 2.5Mbps downstream sync if moving to Annex M, and virtually no increase in upstream sync. As the Annex A d/s sync increases the loss by moving to Annex M decreases, and the upstream gain increases.
From around 20Mbps and upwards d/s on Annex A, a move to Annex M normally has little effect on the downstream, and the upstream rises towards 2.5Mbps as the line length decreases.
Based on empirical evidence from multiple DMT bit-loading graphs years ago on Be - information that is no longer available due to tbb archiving. It's to do with the way the different frequencies attenuate, and the way the higher frequencies come into use with possibly full 14-bit availability per bin as line length decreases. Thus allowing the modems to compensate for the low frequencies handed over to the upstream.
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