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)