Asynchronous communication is a form of data transmission between devices that does not require the clocks of the receiving computer and the sending computer to be closely synchronised. Data communication based on asynchronous transmission is transmitted irregularly and there is no real-time engagement in communication. Rather than use an external clock, a start and stop bit is used to indicate the start of a message and the end of a message respectively. For instance, when transmitting an ASCII character (7 bits of data), 3 other bits are included:

·        The start bit, which alerts the receiver that data is being sent

·        The parity bit ( may be used for error checking)

·        The  stop bit, which indicates the end of the message

This means a total of 10 bits are sent. The start and the stop bits are the control characters.

Start bit

7 bit ASCII

Characters

Parity

Stop bit

 

 

When calculating transmission efficiency, all the 10 bits mentioned above are used (this includes the 2 control characters and the 8 characters of useful data).

Efficiency = 8/10 *100% = 80%.

Asynchronous communication is very useful for generating communication that occurs at irregular intervals. Examples of Asynchronous communication include emailing which does not require the receiver to be logged on at the time the message is being sent. Other examples include discussion boards, forums, Streaming video and audio (used for communication of information or teaching). Asynchronous communication is also used as a  mode of data transmission between computers and other devices such as printers, fax machines and modems.

See also:

Synchronous communication