What is the difference between ADSL, SDSL, IDSL?

ADSL (Asymmetric Digital Subscriber Line) is a transport that allows faster flow of information downstream than upstream. SDSL (Symmetric Digital Subscriber Line) supports one speed upstream and downstream regardless of information flow. IDSL (ISDN Digital Subscriber Line) is an ISDN technology that rides over existing twisted copper telephone lines to provide DSL service farther than ADSL or SDSL.