Asynchronous communication
Asynchronous communication is a continuous stream of data that uses a start bit to flag the beginning of a byte of information, and ends with a stop bit and sometimes a parity bit, used for error checking. This type of communication does not use a timing mechanism to control the flow of data or to interpret the start or end of discreet pieces of data, rather it uses its own encapsulation overhead to ensure that data is received error free, in the order transmitted. Asynchronous communication can take place at speeds up to 28,800 bits per second over normal phone lines. Higher rates (up to 38,400 bits per second) can be achieved in directly connected systems, or when data compression is used.
Synchronous communication
Synchronous communication relies on the presence of a clocking system at both ends of the transmission, and these clocks must be synchronized at the beginning of the session so that the timing of the transmission, not start- and stop-bit encapsulation, defines where data begins and ends.
When binary data is transmitted, a unique 8-bit pattern is used to define the start of the data stream. To preserve the timing during the session, a special bit-transition pattern is embedded in the digital signal that assists in maintaining the timing between the sender and receiver. Synchronous communication can achieve much higher rates than asynchronous.