Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Asynchronous communication is the transmission of data between two devices that are not synchronized with one another via a clocking mechanism or other technique. Basically, the sender can transmit data at any time, and the receiver must be ready to accept information when it arrives. In contrast, synchronous transmission is a precisely timed stream of bits in which the start of a character is located by using a clocking mechanism.

In the mainframe/terminal environment, where asynchronous and synchronous transmissions were used abundantly, an asynchronous transmission is used to transmit characters from a terminal in which the user presses keys periodically. The receiving system knows to wait for the next keypress, even though that may take a relatively large amount of time. In contrast, synchronous transmissions are used as data links between large systems that transfer large amounts of information on a regular basis. The protocol is optimized to take advantage of slow links over public telephone systems, so extraneous bits are removed from the transmissions and clocks are used to separate characters.