14.1 Modems: Theory of Operation

Modems are devices that let computers transmit information over ordinary telephone lines. The word explains how the device works: modem is an acronym for "modulator/demodulator." Modems translate a stream of information into a series of tones (modulating) at one end of the telephone line, and translate the tones back into the serial stream at the other end of the connection (demodulating). Most modems are bidirectional  - every modem contains both a modulator and a demodulator, so a data transfer can take place in both directions simultaneously.

Modems have a flexibility that is unparalleled by other communications technologies. Because modems work with standard telephone lines, and use the public telephone network to route their conversations, any computer that is equipped with a modem and a telephone line can communicate with any other computer that has a modem and a telephone line, anywhere in the world. Thus, even in this age of the Internet and local area networks, modems are still the single most common way that people access computers remotely. This trend is likely to continue into at least the near future, especially with the continuing popularity of specialized dial bulletin board systems ( BBSS ).

Early computer modems commonly operated at 110 or 300 baud, transmitting information at a rate of 10 or 30 characters per second, respectively. Today, in 1996, few computer modems are sold that are not capable of 14,400 bits per second (bps), and modems that zip along at 28,800 bps are increasingly popular. Some modems that send data synchronously, with a precision clock, are capable of rates in excess of 100,000 bps. Special modems on digital ISDN lines are also capable of speeds in excess of 100,000 bps. With data compression included, and new technology constantly being offered, we expect to see common modems with increasingly higher speeds (and smaller physical sizes) as time goes on.