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Modem
 
is a devices that converts the analog signal to digital signal and back. It is an abbreviation for “modulator / demodulator”. Modems are mainly used for transferring digital data via analog data circuit. This circuit might be represented by a telephone line, coaxial cable, wireless transfer, etc.


Modem types

  • modems for dial-up connection to standard analog telephone network
  • terminal adaptors – TA, sometimes incorrectly called "ISDN modems" for connecting  to digital telephone network
  • modems for hired point-to-point connection – a two- or four-wire line
  • modems for broadband cable connection - CableDSL
  • modems for broadband telephone connection - ADSL
  • GSM, UMTS modem/card for data connection via GSM, UMTS network. See GPRS, EGPRS, HSDPA.
  • various radio modems (full-duplex or half-duplex)

For controlling most of the standard modems a set of commands is used called AT (the so-called Hayes-compatible). Special modems may use their own communication protocol.

Modem


Telephone modem

Telephone modem for dialup connection converts the digital signal into the band used for normal calls (standard telephone band ranging from 0.3 to 3.4 kHz). Various types of modulation are used, especially multiple-state quadrature amplitude modulation (with e.g. 16 states, i.e. 16-QAM). Modern modems use protocols for automatic detection and correction of errors (see ECC), automatic quality watching, etc. Present telephone modems can reach the speed of 56 kb/s which is the physical maximum on the analog telephone line. It is one of the most commonly used ways to connect a home computer to the internet.

Faxmodem

Most of the telephone modems ISDN modems and GSM/GPRS/UMTS modems allow for transferring faxes of the group 3.

Modem for hired connection

These modems are usually fully compatible with telephone modems while providing the possibility of interconnection using a non dial-up connection over a hired analog line. Two-wire line is used just like in case of classic telephone connection or a four-wire one when one pair of wires is used for each direction of the traffic. Therefore you can watch and set the parameters of the transfer for each direction separately. Besides, the four-wire line can be used together with a suitable full-duplex radio station for creating a radio modem.

First telephone modems were created in the 1950s within the American military project SAGE. The maximum speed of the first modems was 300 bit/s, later it was gradually increased to 1200 bit/s, 2400 bit/s and 9600 bit/s.

Present-day telephone modems work according to the ITU-T V.90 standard that defines the maximum speed of 56 kb/s for download and 33,6 kb/s for upload.