Acronym for High Data Rate Digital Subscriber Line
HDSL was developed in the late 1980s by BellCore? to provide a more cost-effective means of delivering a T1 data rate (1.544 Mbps). The standard T1 line uses alternate mark inversion (AMI) coding, which occupies a bandwidth of about 1.5 MHz. Because such high frequencies are involved, the attenuation characteristics limit the use of T1 to a distance of about 1km between repeaters. This, for many subscriber lines one or more repeaters are required, which adds to the installation and maintenance expense.
HDSL uses the 2B1Q coding scheme to provide a data rate of up to 2Mbps over two twisted pair lines with a bandwidth that extends only up to about 196 kHz. This enables a range of about 3.7km to be acheived.
-- Taken from "Data and Computer Communications" - William Stallings - 6th Ed, 
ISBN:0130863882
