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.
In asynchronous serial communication in 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.
In this case, the asynchronously transferred blocks are called data packets, for example ATM cells.
The opposite is circuit switched communication, which provides constant bit rate, for example ISDN and SONET/SDH.
Examples include email[3] and bulletin-board systems, where participants send or post messages at different times than they read them.