Asynchronous communication

In telecommunications, asynchronous communication is transmission of data, generally without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream. Any timing required to recover data from the communication symbols is encoded within the symbols. A notable exception is the RS-232 port, and some derivatives, which are asynchronous, but still have an external clock signal available, although not commonly used. The most significant aspect of asynchronous communications is that data is not transmitted at regular intervals, thus making possible variable bit rate, and that the transmitter and receiver clock generators do not have to be exactly synchronized all the time.

Physical layer

In asynchronous serial communication the physical protocol layer, the data blocks are code words of a certain word length, for example octets (bytes) or ASCII characters, delimited by start bits and stop bits. A variable length space can be inserted between the code words. No bit synchronization signal is required. This is sometimes called character oriented communication. Examples are the RS-232C serial standard, and MNP2 and V.2 modems and older.