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A modem (a concatenation of modulator-demodulator) is

[a] device that allows two computers to communicate over telephone lines. It converts digital computer signals into analog format for transmission. A similar device at the other end converts the analog signal back into a digital format that the computer can understand.[1]
[a] device used to convert serial digital data from a transmitting terminal to a signal suitable for transmission over a telephone channel to reconvert the transmitted signal to serial digital data for the receiving terminal.[2]


The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals.

The most familiar example is a voiceband modem that turns the digital 1s and 0s of a personal computer into sounds that can be transmitted over the telephone lines and once received on the other end, converts those 1s and 0s back into a form used by a USB, serial, or network connection. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps". They can also be classified by baud, the number of distinct symbols transmitted per second; these numbers are directly connected, but not necessarily in linear fashion.

The highest speed modem used with a traditional telephone line, known as a 56K modem, offers a maximum data transmission rate of about 45,000 bits per second (bps). However, as the content on the World Wide Web has become more sophisticated, the limitations of relatively low data transmission rates (called "narrowband") such as 56K become apparent. For example, using a 56K modem connection to download a 10-minute video or a large software file can be a lengthy and frustrating exercise.


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