The Free On-line Dictionary of Computing (30 December 2018):

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)