A device used to convert a computer's digital signal to an analogue signal for communication is called a:

A modem converts a computer's digital signals into analogue form for transmission and back again on receipt.

The name modem comes from modulator-demodulator, describing this two-way conversion.

A bridge simply joins two network segments and forwards frames between them.

A switch directs data frames to the specific device they are addressed to within a network.

A network interface card connects a single computer to a network but does not do digital-to-analogue conversion.

Modems let digital computers communicate over analogue lines like telephone wires.