from
The Free On-line Dictionary of Computing (8 July 2008)
digital carrier
<hardware, communications> 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)