Well, here are the two links you can check…
This one is a correspondence among 3GPP members . You can also check page 93 in Siemens Document titled GPRS/EGPRS Global Description.
The next link discusses about EDGE in general
http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16505610. Unfortunately this link is down most of the time, but you can try.