I did not know that yesterday!

A modem (a portmanteau* constructed from modulate and demodulate) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio. Experiments have even been performed in the use of modems over the medium of two cans connected by a string.

The most familiar example of a modem turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those sounds back into 1s and 0s. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps".

The 300 bit/s modems used frequency-shift keying to send data. In this system the stream of 1's and 0's in computer data it translated into sounds which can be easily sent on the phone lines. In the Bell 103 system the originating modem sends 0's by playing a 1070 Hz tone, and 1's at 1270 Hz, with the answering modem putting its 0's on 2025 Hz and 1's on 2225 Hz. These frequencies were chosen carefully, they are in the range that suffer minimum distortion on the phone system, and also are not harmonics of each other. For the 103F leased line version, internal strapping selected originate or answer operation. For dial models, the selection was determined by which modem originated the call. Modulation was so slow and simple that some people were able to learn how to whistle short bits of data into the phone with some accuracy.
Lovely - now you know :)


* portmanteau is a term in linguistics that refers to a word or morpheme that fuses two or more grammatical functions.)