The name modem is the abbreviation of the term Modulator and Demodulator. As the name implies, both functions are included in a modem. A modem is used to convert digital signals into analog signals capable of being transmitted over telephone lines. At the receiving end of the system, modem is used to demodulate the analog signals and reconstruct the equivalent digital output. Modems are placed at both ends of the communication circuit as shown in Fig .
Fax (or) Facsimile
Fax (or) Facsimile, is an electronic system for transmitting graphical information by wire or radio. It is used to send printed material by scanning and converting it into electronic signals. These signals modulate a carrier to be transmitted over the telephone lines. Since modulation is involved, fax transmission can also take place by radio.
The components of a fax system are illustrated in Fig . Although facsimile is used to transmit pictures, it is not TV because it does not transmit sound messages or live scenes and motion. However, it does use scanning techniques that are in same way generally similar to those used in TV. A scanning process is used to break up a printed document into many horizontal scan lines which can be transmitted and reproduced serially.