hdsl

from The Free On-line Dictionary of Computing (8 July 2008)

High bit-rate Digital Subscriber Line
HDSL

   <communications, protocol> (HDSL) A form of {Digital
   Subscriber Line}, providing {T1} or {E1} connections over two
   or three {twisted-pair} copper lines, respectively.  Unlike
   most other forms of DSL HDSL is not a typical consumer
   service, it's used mostly to replace traditional T1/E1
   connections, such as connecting {PBXes} to {telco} offices.
   The advantage of HDSL over the {Alternate Mark Inversion} line
   coding scheme traditionally used on T1/E1 lines is that it
   requires about an order of magnitude lower bandwidth to carry
   the same traffic.

   (1998-05-18)

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

HDSL
       High data / bit rate Digital Subscriber Line (BELLCORE, AT&T,
DSL, BIT)

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

HDSL
       High-level Data Specification Language