LANTRONIX has a good description of 56K modems.
Intel's website has a good description of the Bell, MNP, and CCITT modem standards.
Cable-Modems.Org offers a fairly detailed web site just on cable modems.
For a very comprehensive frame relay web page displaying over 100 links to frame relay articles, see Alliance Datacom's page.
A tutorial on ATM can be found at the rad.com website and at International Engineering Consortium.
Basic information on ATM can be located at the ATM Forum's web site.
More information on DSL can be found at 2wire's learning center.