Asynchronous transmission refers to the case when the sender commences transmission of bytes at any instant of time. Asynchronous transmission uses start and stop bits to signify the beginning bit. ASCII character would actually be transmitted using 10 bits. For example, "0100 0001" would become "1 0100 0001 0". The extra one (or zero, depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended.
Synchronous transmission uses no start and stop bits, but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signal(s) built into each component. A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in the protocol for sending/receiving data, so some bytes could become corrupted (by losing bits).
A unique sequence of fixed number of bits , called flag is prefixed to each data block , called frame . The receiver first detects the flag bits and then identifies the boundaries of frame.
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