HDSL is a rather obscure telco technology that may not be familiar to those coming from a mostly Internet background. It's a transport technology that allows T1 lines to be provisioned without repeaters every 6000 feet and works by transmitting the DS1 frame payload in a physically different way. An HDSL system consists of two metallic loops like a traditional T1 system. However, whereas in a traditional T1 system each loop carries data in only one direction, in an HDSL system each loop carries one half of the DS1 payload in both directions. HDSL thus lowers the Nyquist frequency of the signal at the expense of more complex electronics to handle data being transmitted in both directions on the same pair. HDSL is little known among those who work mostly at the Internet level because it's normally completely transparent to the user — HDSL terminals on both sides provide a standard DS1 interface, and the customer side HDSL terminal unit is part of the active demark. Other versions of HDSL exist in Europe where 2 or 3 loops are used to carry an E1 instead of a T1.

Unlike SDSL, HDSL is a real standard. It is defined by a formal specification, and compatibility and compliance are determined by adherence to this specification rather than implementation details. An engineer desiring to build an HDSL terminal unit is free to use any vendor's chip or make a discrete implementation.

HDSL uses the 2B1Q line code, which was originally designed for the North American ISDN BRI U interface; HDSL has adapted it for use with much higher data rates. While the 2B1Q line code itself is the same in ISDN and HDSL, the line activation protocol is different. The different activation sequences are the reason why you hear fast clicks (formally called TL tones) when you connect a lineman's handset to an ISDN or IDSL line, whereas on an SDSL line (derived from HDSL standards) you hear white noise instead, which is actually the audible low frequency tail of the HTU-C S0 signal.