The modem (portmanteau, for modulator-demodulator), is a device used to communicate with remote users via a telephone line. It allows for example to exchange (send/receive) files, faxes, to connect to the Internet, to exchange e-mails, to telephone or to receive television. television.

It can also be defined as an acronym.

Technically, the device is used to convert digital data from the computer into analog data that can be transmitted by a conventional telephone line and vice versa. Today in the world of voice over IP, this analog data can also be transmitted in an encoded voice communication without any type of compression.

It is an electronic device, in independent box or in card to be inserted in a computer, which makes it possible to circulate (receive and send) digital data on a channel analog. It performs the modulation: coding of digital data, synthesis of an analog signal which is generally a modulated carrier frequency. The demodulation operation performs the reverse operation and allows the receiver to obtain the digital information.

We speak of modem to designate the devices intended to communicate digital machines (computers, embedded systems) through an analog network (switched telephone network, electrical network, radio networks …).

All these modem categories are often used to access the Internet (or to send or receive faxes, to connect to Minitel services…), or even to make television digital voice.

Since the end of the 1990s, many telecommunications standards have appeared and, therefore, as many new types of modems: ISDN (or ISDN), ADSL, GSM, GPRS, Wi-Fi, Wimax…

The main characteristic of a modem is its transmission speed. This is expressed in bits per second (bit/s or bps) or in kilobits per second (kbit/s or kbps). When it connects the modem makes a recognizable noise.

There have been modems working at 150, 300, 600, 1200 bps, 4.8, 9.6, 14.4, 28.8, and 33.6 kbps. For several years, the 56Â kbit/s standard has been the standard. At this speed, we come close to the theoretical limits of information flow for a telephone line using a single carrier frequency. For higher speeds, systems using multiple carriers have been developed, such as ADSL. These techniques obviously require the use of specific modems.

Modems were first used in the SAGE air defense system in the late 1950s. The purpose was to connect terminals located at airbases, radar sites and command and control centers at SAGE centrals scattered across the US. and in Canada. SAGE used a system of dedicated lines but the equipment at their ends was similar to modern modems.

IBM was SAGE’s main supplier of computers and modems. A few years later, a meeting between the CEO of American Airlines and an IBM regional manager led to the creation of a mini-SAGE used as an automatic ticketing system, for which the terminals placed in agencies selling tickets, were linked to a central computer responsible for managing availability and the calendar. The system, known as “Sabre” was a distant relative of the modern Saber system.

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which device sends signals from a computer over a network