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Computing (FOLDOC) dictionary
digital carrier
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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.