Also known as start-stop transmission. Data transmission that is not synchronized between two or more computers across a circuit. The transmitting device sends data intermittently, rather than in a steady stream or at regular intervals. Such transmission is characterized as character-framed, as each character is preceded by a start bit that alerts the receiving computer of its arrival and succeeded by one or two stop bits that signal the end of the character. As illustrated in Figure A-7, an optional parity bit may be included for error control. Multiple characters commonly are organized into blocks, with an additional error control mechanism, such as a cyclic redundancy check (CRC), for improved error performance. Kermit, XMODEM, and ZMODEM are examples of asynchronous protocols. See also asynchronous, CRC, error control, frame, Kermit, parity bit, synchronous, synchronous transmission, XMODEM, and ZMODEM.
The transmission of data in which each character is a self-contained unit with its own start and stop bits. Intervals between characters may be uneven. It is the common method of transmission between a computer and an analog modem, although the modem may switch to synchronous transmission to communicate with the other modem. Also called "start/stop transmission." Contrast with synchronous transmission.