Sources
The information from this page comes from many sources.  Having worked as a Tier 1 technical support agent for two years under SBC (now AT&T) I learned a lot about the DSL network.  Some of this information comes from training.  A large amount of this information comes from the excellent help of many techs at http://www.broadbandreports.com/.  Most of these techs are field technicians for SBC, but many work for other companies as well.  Some of this information I learned from asking questions, helping out others, and watching the field techs help people out.  Below is a list of links showing where much of this information comes from.
http://www.dslreports.com/faq/11721
http://www.geocities.com/cccmt@sbcglobal.net/dslflash
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