<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
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