Modems
Dial-up or analog modems, which enable computers to communicate with each other over telephone lines, derive their name from the process they perform. A modem sending data modulates digital computer data into analog data suitable for transmission over telephone lines to the receiving modem, which demodulates the analog data back into computer form. Modems share two characteristics with serial ports:
Both use serial communication to send and receive information.
Both often require adjustment of transmission speed and other options.
In fact, most external modems require a serial port to connect them to the computer; some external modems use the USB port instead.
Modems come in five types: add-on card, external, PC Card, motherboard-integrated, ...
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