The term modem is a contraction of modulator/demodulator. A modem interfaces a computer to a telephone line, digital subscriber line (DSL), cable system, fiber-optic network, wireless network, or radio transceiver, allowing you to communicate with other computer users and to “surf the Internet.”
Modems work at various speeds, usually measured in kilobits per second (kbps) or megabits per second (Mbps). Sometimes you’ll hear about speed units called the baud and kilobaud. (A kilobaud is 1000 baud.) Baud and bits per second are almost the same units, but they are not identical. People sometimes use the term baud when they really mean bps. Modem speeds, particularly in cable, fiber-optic, and wireless networks, keep increasing as computer communications technology advances. Modems are rated according to the highest data speed they can handle. A typical telephone modem works at about 56 kbps. Modems for more advanced connections operate much faster.
A computer works with binary digital signals, which are rapidly fluctuating direct currents. For digital data to be conveyed over a telephone or radio circuit, the data must be converted to analog form. In a telephone modem or radio-transceiver modem, this is done by changing the digit 1 into an audio tone, and the digit 0 into another tone with a different pitch. The result is an extremely fast back-and-forth alternation between the two tones.
In modulation, digital data from the computer is changed into analog data for transmission over the telephone line or radio medium. The modulator is therefore a digital-to-analog converter (D/A converter or DAC). Demodulation changes the analog signals from the telephone line or radio medium back to digital signals that a computer can understand. The demodulator is thus an analogto- digital converter (A/D converter or ADC). The highest practical speed for this type of modem is approximately 56,000 bits per second (bps), or 56 kilobits per second (kbps).
Amateur radio operators use a variety of digital modes at considerably slower speeds than 56 kbps for Internet-like communications. The principal advantage of amateur radio lies in the fact that it can work when all else fails. Such communications are not fast, but no infrastructure is needed. This makes it possible for amateur radio operators to provide emergency communications into and out of areas stricken by natural disasters that destroy the conventional communications infrastructure.
Internal versus External Modems
An internal modem is a printed-circuit board, also called a card. Virtually all computers, both desktop and notebook, are sold with internal modems that can interface with the telephone line at speeds of up to 56 kbps.
An external modem is a self-contained unit. It has a cord that runs to either the computer’s serial data port (also called the communications port) or one of the USB ports, and another cord that runs to the telephone line, cable system, or satellite dish. If you want to use a DSL, cable, or satellite Internet connection, you will usually have to buy or rent an external modem from the service provider.