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A medium which can carry digitalsignals; broadly equivalent to the physical layer of theOSI seven layer model of networks. Carriers can bedescribed as baseband or broadband. A baseband carriercan include direct current (DC), whereas broadband carriersare modulated by various methods into frequency bands which donot include DC.Sometimes a modem (modulator/demodulator) or codec(coder/decoder) combines several channels on one transmissionpath. The combining of channels is called multiplexing, andtheir separation is called demultiplexing, independent ofwhether a modem or codec bank is used. Modems can beassociated with frequency division multiplexing (FDM) andcodecs with time division multiplexing (TDM) though thisgrouping of concepts is somewhat arbitrary.If the medium of a carrier is copper telephone wire, thecircuit may be called T1, T3, etc. as these designationsoriginally described such.T1 carriers used a restored polar line coding scheme whichallowed a baseband signal to be transported as broadband andrestored to baseband at the receiver. T1 is not used in thissense today, and indeed it is often confused with the DS1signal carried.