资料来源 : Free On-Line Dictionary of Computing

digital carrier
     
         A medium which can carry {digital}
        signals; broadly equivalent to the {physical layer} of the
        {OSI} seven layer model of networks.  Carriers can be
        described as {baseband} or {broadband}.  A baseband carrier
        can include direct current (DC), whereas broadband carriers
        are modulated by various methods into frequency bands which do
        not include DC.
     
        Sometimes a {modem} (modulator/demodulator) or {codec}
        (coder/decoder) combines several channels on one transmission
        path.  The combining of channels is called {multiplexing}, and
        their separation is called demultiplexing, independent of
        whether a modem or codec bank is used.  Modems can be
        associated with {frequency division multiplexing} (FDM) and
        codecs with {time division multiplexing} (TDM) though this
        grouping of concepts is somewhat arbitrary.
     
        If the medium of a carrier is copper telephone wire, the
        circuit may be called {T1}, {T3}, etc. as these designations
        originally described such.
     
        T1 carriers used a restored polar line coding scheme which
        allowed a baseband signal to be transported as broadband and
        restored to baseband at the receiver.  T1 is not used in this
        sense today, and indeed it is often confused with the {DS1}
        signal carried.
     
        (1996-03-31)