Network - Communication, Asynchronous
Asynchronous communications are those in which a transmission may take place at a variable rate, and in which byte boundaries are indicated by a combination of start and stop bits. Transmission elements are distinguished by these special bits. This is in contrast to synchronous communication, in which transmission elements are identified by reference to a clock or other timing mechanism. Examples of asynchronous processes include voice or data transmissions (commonly using modems), terminal-host communications, and file transfer. Modems, terminals, pointer devices, and printers are all devices that use asynchronous communications. In asynchronous communication, the occurrence of the special start bit indicates that a byte is about to be transmitted. The duration of the start bit indicates the length of a bit interval (duration of a single signal value), which represents the speed at which that byte is going to be transmitted. In a sense, asynchronous transmissions synchronize for each byte. With respect to the communication, both sender and receiver need to agree on the number of start and stop bits, and also on whether a parity bit will be used. This information is necessary to identify the transmission elements. If a parity bit is used, knowing what kind of parity is operating will help interpret the transmission contents. Asynchronous transmissions are less effi- cient than synchronous (time-based) ones. For example, the start and stop bit around each byte represent 25 percent overhead for an asynchronous byte. Because of this lesser efficiency, asynchronous communications cannot attain the bandwidths possible with synchronous transmissions. On the other hand, asynchronous transmissions are much more flexible, forgiving, and easier to correct than the faster moving synchronous transmissions. SEE ALSO Communication, Synchronous