How do modems achieve the 56kbps

[Lin1]

How can modems achieve 56kbps when the theoretical limit it approx 30kbps.Are they using some special software techniques or what kind of modulation are they using.

 

[jimbopalmer]

this is the official document on how they do it:

http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-V.92-200011-I!!PDF-E&type=items

I tried to remain child-like, all I acheived was childish.