Source : Free On-Line Dictionary of Computing

High bit-rate Digital Subscriber Line
     
         (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)