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.
