Shannon’s result was derived from information-theory arguments and applies
to any channel subject to thermal noise. Counterexamples should be treated in the
same category as perpetual motion machines. For ADSL to exceed 13 Mbps, it
must either improve the SNR (for example by inserting digital repeaters in the
lines closer to the customers) or use more bandwidth, as is done with the evolution
to ASDL2+.