(high data rate DSL) This variety created in the late 1980s delivers symmetric service at speeds up to 2.3 Mbps in both directions. Available at 1.5 or 2.3 Mbps, this symmetric fixed rate application does not provide standard telephone service over the same line and is already standardized through ETSI and ITU (International Telecommunications Union). Seen as an economical replacement for T1 or E1, it uses one, two or three twisted copper pairs.
HDSL2: (2nd generation HDSL) This variant delivers 1.5 Mbps service each way, supporting voice, data, and video using either ATM (asynchronous transfer mode), private-line service or frame relay over a single copper pair. This ATIS standard (T1.418) for this symmetric service gives a fixed 1.5 Mbps rate both up and downstream. HDSL2 does not provide standard voice telephone service on the same wire pair. HSDL2 differs from HDSL in that HDSL2 uses one pair of wires to convey 1.5 Mbps whereas ANSI HDSL uses two wire pairs.
HDSL4: HDSL4 is virtually the same as HDSL2 except it achieves about 30% greater distance than HDSL or HDSL2 by using two pairs of wire (thus, 4 conductors), whereas HDSL2 uses one pair of wires.

This definition is in context to DSL. See more contextual defintions for HDSL.