Source : Free On-Line Dictionary of Computing

Duplex High Speed Data
     
        (DHSD) A term which describes a {full-duplex} channel that can
        carry 64 kilobits per second.
     
        This is the kind of service provided by an Inmarsat-B type
        portable earth station or a {leased line} (not {ISDN}).
     
        (1995-02-02)