modem

Definition of modem in The Network Encyclopedia.

What is Modem (in computer networking)?

Generally, any type of data communications equipment (DCE) that enables digital data transmission over the analog Public Switched Telephone Network (PSTN). The term “modem” (which actually stands for “modulator/demodulator”) is usually reserved for analog modems, which interface, through a serial transmission connection such as the RS-232 interface, with data terminal equipment (DTE) such as computers.

The modem converts the digital signal coming from the computer into an analog signal that can be carried over a Plain Old Telephone Service (POTS) line. The term “digital modem” is sometimes used for ISDN terminal adapters, but this is something of a misnomer because no signal modulation actually takes place.

Modems were developed in the 1960s by Bell Labs, which developed a series of standards called the Bell Standards. These standards defined modem technologies of up to a 9600-bps transmission speed. The Bell Standards have been superseded by the V series standards from the International Telecommunication Union (ITU), which defines standards of up to V.90 (which supports 56-Kbps downloads and 33.6-Kbps uploads).

Modems generally have two interfaces:

  • An RS-232 serial transmission interface for connecting to the DTE, usually the computer
  • An RJ-11 telephone interface for connecting to the 4-wire PSTN telephone outlet in the local loop connection

Modem types include the following:

  • Internal modems, which are installed as interface cards inside the computer and might use some of the machine’s CPU processing power for functions such as encoding and data compression.
  • External modems, which are generally more expensive and connect to the serial port on the computer using a DB9 or DB25 connector. External modems are useful when several users need to share a modem.
  • PCMCIA modems, which are credit-card-sized modems for laptop computers used by mobile workers.
  • Voice/data/fax modems, which can be used for file transfer, sending and receiving faxes, and voice mail using associated software.