e.g. my billion estimates 72.9 on a 5.8db snrm, 73.4 on a 6.3db snrm so that 0.5db is 0.5mbit. Yet when the snrm goes to 6.4db the attainable jumps up to over 74. Then jumps again to over 75 with a 6.5db snrm.

Thats likely to do with the number of carrier bins available for actual use. Its always been like that even on adsl/adsl2+. The more tones that are actually available for use then the more bits can be loaded into the bins. Its why on adsl we say each 3dB of SNR can be worth anything between 400-1200 kbps and we use 800kbps as an average. On a very short line then its likely to be nearer 1200kbps, on a long line nearer to 400 kbps because it cant use all of the tones in the frequency band.

The same theory is why 3dB on vdsl is worth far more than on adsl... because vdsl has for more tones available for use.

Look at it this way.. (

*very rough calculation coming up**).. 2 lines both with exactly the same SNRm

if you have a line that can only use 100 tones then 100 x 60kbps = 6000 kbps

if you have a line that can use 255 tones then 255 x 60 kbps = 15300kbps

As line conditions deteriorate (through crosstalk or EMI/RFI or whatever) then the less bits can be loaded into the bins and the latter tones start becoming unavailable for use.

** QAM rate is 4,000 symbols per second, therefore each 3dB of SNR available in the sub channel over the base line will give approx 4kbps of sync speed, subject to a maximum of 60kbps (15 x 4kbps) per carrier.*----------

ETA

PS Just checked my stats... on my line 0.5 dB appears to be worth circa 2000 kbps.

Over the past 6 weeks Ive lost 1.5dB and 6000kbps of sync purely down to crosstalk.

Its more than on yours because I obviously have more tones available for use and am currently using all carriers. This is also the reason why crosstalk affects the short lines the most and we see the biggest losses from crosstalk. One of my big disturbers costs me 1dB each time he has his router on line.. that 1dB is worth 4000 kbps to me.