The HDLC module implements one of the protocols in the Data Link
    layer. The HDLC (High-level Data Link Control) protocol is intended
    to cover a wide range of applications; for example, one-way, two-way
    alternate or two way simultaneous data communication between
    data stations which are usually buffered, including operations on
    different types of data circuits; for example multipoint/point-
    to-point, duplex/half-duplex, and switched/non-switched. This
    implementation uses HDLC to offer reliable communication at the
    Data Link layer for point-to-point synchronous data lines over a
    wide area network link. The HDLC module typically runs as a Data
    Link module under the CLNS Network protocol.

                               NOTE

    For Tru64 UNIX, the WAN Device Drivers are provided as an
    installable subset within the product HP Wide Area Network Support
    for Tru64 UNIX.  You must install this subset before you can refer
    to HDLC module entities in an NCL command.

    For OpenVMS, you must install the WAN Device Drivers from the
    HP X.25 for OpenVMS product before you can refer to the HDLC
    module entities in an NCL command.