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A modem (modulator-demodulator) is a Device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. 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, from driven diodes to radio.

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 of POTS, and once received on the other side, converts those 1s and 0s back into a form used by a USB, Ethernet, 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 (bit/s, or bps). They can also be classified by Baud, the number of times the modem changes its signal state per second.

Faster modems are used by Internet users every day, notably cable modems and ASDL Modems. In telecommunication, wide-band radio modems transmit repeating frames of data at very high data rates over microwaves radio links. Narrow-band radio modem is used for low data rate up to 19.2k mainly for private radio networks. Some microwave modems transmit more than a hundred million bits per second.Optical modem transmit data over optical fibers. Most intercontinental data links now use optical modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x10⁹) bits per second. One kilobit per second (kbit/s, kb/s, or kbps) as used in this article means 1000 bites per second and not 1024 bites per second. For example, a 56k modem can transfer data at up to 56,000 bit/s (7 kB/s) over the phone line.

In the summer of 1960, the name Data-Phone was introduced to replace the earlier term digital subset. The 202 Data-Phone was a half duplex asynchronous service that was marketed extensively in late 1960. In 1962, the 201A and 201B Data-Phones were introduced. They were synchronous modems using two-bit-per-baud phase shift keyingfull duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels running on their own set of two wires each. (PSK). The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the 201B provided

The famous Bell 103A dataset standard was also introduced by Bell Labs in 1962. It provided full-duplex service at 300 baud over normal phone lines. Frequency shift keyingHz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the KSR33, the ASR33, and the IBM 2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems. was used with the call originator transmitting at 1,070 or 1,270

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