Network and infrastructure
Modem
A modem, or modulator-demodulator, turns digital data into a signal that can travel over a phone line, then converts it back at the other end.
An analogue line can only carry waves within the voice band. The modem encodes bits by varying the frequency, phase or amplitude of a carrier, following standards that set the speed and method. Two modems can only communicate if they share at least one of those standards, which they check at the start of the call.
Why it matters for fax
Every fax machine contains a modem, and the whole transmission depends on it: negotiation happens at low speed, the image at the highest speed the line supports. When a network distorts the signal, the document is not at fault; the two modems' ability to understand each other is.