Network and infrastructure
Analogue line
An analogue line carries sound as a continuous electrical signal whose shape directly reproduces that of the sound wave.
It is the medium fax machines were designed for. A fax modem does not emit voice but calibrated tones, and an analogue line reproduces them as they are, interpreting nothing. That neutrality is exactly what a packet network loses, where the signal is compressed according to what a human ear perceives — a criterion unrelated to a modem.