Science Fair Project Encyclopedia A modem (a portmanteau word
constructed from modulator and demodulator) is a device that
modulates an analog carrier signal (sound), to encode digital
information, and that also demodulates such a carrier signal to
decode the transmitted information. Put simply, it turns the
digital '1s and 0s' into sounds that can be transmitted over the
Plain Old Telephone System (POTS), and once received on the
other side, converts those sounds back into 1s and 0s. The goal
is to produce a signal that can be transmitted easily and
decoded to reproduce the original digital data. Primarily used
to communicate via telephone lines, modems can be used over any
means of transmitting analog signals, from driven diodes to
radio. Modems were first introduced as a part of the SAGE air-
defense system in the 1950s, connecting terminals located at
various airbases, radar sites and command-and-control centers to
the SAGE director centers scattered around the US and Canada.
SAGE ran on dedicated communic