With asynchronous transmission signal timing is not required; signals are sent in an agreed pattern of bits and if both ends are agreed on the pattern then communication can take place.
Bits are grouped together and consist of both data and control bits. If the signal is not synchronised the receiver will not be able to distinguish when the next group of bits will arrive. To overcome this the data is preceded by a start bit, usually binary 0, the byte is then sent and a stop bit or bits are added to the end. Each byte to be sent now incorporates extra control data. In addition to the control data small gaps are inserted between each chunk to distinguish each group.
In asynchronous transmission each bit remains timed in the usual way. Therefore, at bit level the transmission is still synchronous (timed). However, the asynchronous transmission is applied at byte level, once the receiver realises that there is a chunk of incoming data timing (synchronisation) takes place for the chunk of data.
Asynchronous transmission is relatively slow due to the increased number of bits and gaps. It is a cheap and effective form of serial transmission and is particularly suited for low speed connections such as keyboard and mouse.
One example of asynchronous transfer is Asynchronous Transfer Mode (ATM) switching. ATM allows voice, data and video to be transmitted in fixed length cells of 53 bytes.