DMT on VDSL2 (Proposed rates calculation)

Hi
Have just signed up to Think broadaband forum.
This is my first posting. Am hoping someone can help me with some calculations

I understand that VDSL2 used DMT coding . Have been trying to find information on how the promised data rates are achieved.
for example profile 17a
17MHz with 4095 tones
each tone has 15 bits of data
and symbol rate is 4000 (4khz)
4000 * 4095 * 15 = 246Mbps
this is way more than what is listed. I understand this is upstream plus downstream

Does anyone have any information on this or links to good websites