Franc is properly summarizing basic science as it conforms
to FCC regulation. Concept defined in one of the world's
greatest papers, written by Claude Shannon, reprinted in the
Bell Labs Technical Journal in July and Oct of 1948, then
reprinted again in Scientific American July 1949. IOW
Shannon's Law defines modem speed. Underlying concept should
be common knowledge to any computer power user. Concepts that
Franc Zabkar has properly summarized.

Power increases to permit 56K would create too much
crosstalk. Power is limited by FCC regulation. Shannon's Law
therefore says maximum data rates would be 53K.

Franc Zabkar wrote:
AFAIK, the FCC mandates restrictions on the maximum signal levels,
not the data rate. It's just that data rates in excess of 53333bps
usually require signal levels greater than those allowed. At least
that's the explanation that is often given at comp.dcom.modems.

- Franc Zabkar