Asynchronous Transmission is a mode of serial transmission for modems and other devices in which the data is transmitted as a continuous stream of bytes separated by start and stop bauds.
Asynchronous transmission is essentially character-based with additional bauds between each character (8-bit frame) to enable synchronization at the receiever. Asynchronous communication is not synchronized by a timer mechanism or clock, and asynchronous devices are not bound to send or receive information at an exact transmission rate. Instead, the sender and receiver negotiate transmission speeds based on physical layer limitations and the need to maintain a reliable flow of information. Asynchronous transmission is mainly suitable for low-speed transmission.
An optional Parity bit for error checking can be located immediately before the stop baud in each character frame. With parity correction, an 8-bit character requires 3 bits of control information (start, stop, and parity bits), which means adds to an overhead of 3/8 or 38 percent.
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