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 The Bell Standards have been superseded by the V series standards from the International Telecommunication Union (ITU), which defines standards of up to V.90 (which supports 56-Kbps downloads and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.