Practical UNIX and Internet Security, 3rd Edition

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 (modulation) at one end of the telephone line, and translate the tones back into the serial stream at the other end of the connection (demodulation). 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. Modems thus bypass firewalls, packet filters, and intrusion detection systems.

What’s more, even in this age of corporate LANs, cable modems, and DSL links, dialup modems are still the single most common way that people access the Internet. This trend is likely to continue through the first decade of the 21st century because dialup access is dramatically cheaper to offer than high-speed, always-on services.

Serial Interfaces

Information inside most computers moves in packets of 8, 16, 32, or 64 bits at a time, using 8, 16, 32, or 64 individual wires. ...