A modulator-demodulator, modem for short, is a device that converts digital computer signals into analog form and vice versa. This simply means that it translates computer information into sound that can be transmitted over telephone lines. For computer to computer communications, each computer must have a modem. Both external and internal modems are available for microcomputers. Modems are also distinguished by transmission speed and other capabilities such as error correction or data compression.
Parity controls how error checking is performed.
In full duplex, the most common, the host computers echoes characters to the remote computer. In half-duplex, the remote computer echoes characters to itself. One common emulations is the DEC VT-100 terminal. With appropriate software, a PC equipped with communications hardware can act as a terminal on a mainframe or minicomputer. Students at Millsaps, for example, can dial into the Millsaps system and access any of the mainframes with a proper account.