A modem (modulator-demodulator) is a mechanism that changes an analog carrier signal to predetermine digital information, as well as also demodulates such a carrier signal to interpret the transmitted information. The aim is to create a signal that can be transmitted simply as well as decoded to repeat the unusual digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.

The most well-known illustration is a voiceband modem that turn out the digital 1s as well as 0s of a individual PC into sounds that can be broadcasted in excess of the telephone lines of basic previous telephone services (POTS), with once received on the further side, changes those 1s and 0s back into a form used by a USB, Ethernet, serial, or else network connection.

Modems are usually categorized by the quantity of data they can send in a specified time, frequently calculated in bits for each second (bit/s, or else bps). They can also be categorizing by Baud, the number of times the modem changes its signal state for each second. Such as, the ITU V.21 standard being used by audio frequency-shift keying, aka tones, to carry 300 bit/s by means of 300 baud, while the unique ITU V.22 standard enabled 1,200 bit/s with 600 baud by means of phase-shift keying.

Quicker modems are being specially used by Internet clients all day, especially cable modems as well as ADSL modems. In telecommunications, wide-band radio modems send out repeating frames of data at extremely elevated data rates in excess of microwave radio links. Narrow-band radio modem is used for low data rate up to 19.2k mostly for confidential radio networks. Various microwave modems broadcast more than a hundred million bits for each second. Optical modems broadcast data above optical fibers. Most international data links now make use of optical modems transmitting over underwater optical fibers. Optical modems regularly have data rates in surplus of a billion (1x109) bits for each second. One kilobit for each second (kbit/s, kb/s, or kbps) as used in this article means 1,000 bits per second as well as not 1,024 bits per second. For instance, a 56k modem can transfer data at up to 56,000 bit/s (7 kB/s) over the phone line.