asynchronous transmission

Definition of asynchronous transmission in The Network Encyclopedia.

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What is Asynchronous Transmission?

A mode of serial transmission for modems and other telecommunication devices in which the data is transmitted as a continuous stream of bytes separated by start and stop bits.

This is in contrast to synchronous transmission in which some timer or clocking mechanism is used to ensure a steady flow of data between the devices.

How it works

In asynchronous communication, only about 80 percent of the transmitted bits actually contain data, while the other 20 percent contain signaling information in the form of start and stop bits. Each data frame starts with a start bit and ends with a stop bit, with data bits in between. When the receiving station receives a start bit, it knows that pure data will follow. When a stop bit is received, it knows the data frame has ended and waits for the next one.

Asynchronous transmission is essentially character-based with additional bits between characters to enable synchronization and error correction. An optional parity bit for error checking can be located immediately before the stop bit in each frame. With parity correction, an 8-bit character requires 3 bits of control information (start, stop, and parity bits), which means an actual overhead of 3/8 or 38 percent.

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 hardware limitations and the need to maintain a reliable flow of information. Asynchronous transmission is mainly suitable for low-speed transmission, but speeds can be increased by using data compression.