ADSL (Asymmetric Digital Subscriber Line) employs two different transmission speeds, with the downstream speed (from the provider to the user) usually being much higher than the upstream speed (from the user to the corporate host). ADSL can achieve downstream data rates up to 8 Mbps and upstream rates to 1 Mbps.
IDSL (ISDN Digital Subscriber Line) is a form of DSL that uses ISDN provisioning and testing, and can coexist with current analog and ISDN services. IDSL is usually limited to 144 Kbps upstream and downstream, but can sometimes provide further reach than other DSL solutions because it does not have the same distance limitations.
VDSL (Very High Speed Digital Subscriber Line) promises even higher speeds than ADSL, although over much shorter distances. Standardization on speeds and technological specifications are currently in progress.
RADSL (Rate Adaptive Digital Subscriber Line) adjusts the data transmission rate to match the quality of the phone line. RADSL users get the very best performance their telephone line is conditioned to provide, providing transmission rates of up to 7 Mbps downstream and 1 Mbps upstream.
HDSL/SDSL (High Data Rate Digital Subscriber Line/Symmetric Digital Subscriber Line) utilize two standard phone lines for 1.5 Mbps transmission speeds and offer the capability to combine three phone lines for 2.0 Mbps speeds. HDSL and SDSL are intended as lower cost replacements for dedicated and fractional T-1 lines 2 .