A method of data transmission in which data is transmitted in small fixed-size groups, typically corresponding to a character and containing between five and eight bits, and in which the timing of bits within the group is not directly determined by some form of clock. The standard practice is to precede each group of data by a start bit, whose duration indicates the expected duration of each subsequent bit within the group, and to follow the group by a stop bit whose duration is at least one and a half times that of the start bit. The presence of the start and stop bits necessarily reduces the rate of sending genuine data bits. Asynchronous transmission is normally used only for relatively slow data rates, up to say 2400 bps. See also synchronous transmission.