## Synchronous/asynchronous communications

### Synchronous transmission #### A synchronous system is one in which the transmitter and receiver are operating continuously at the same number of symbols per second and are maintained, by suitable correction, in the desired phase relationship. Synchronous operation requires an accurate timing signal in the receive modem, which can be derived either from a separate transmitted reference or from symbol transitions within the data signal itself. A separate reference requires additional power or bandwidth, while a data-derived reference requires frequent symbol transitions to occur in the received waveform, that is, long strings of 1s or 0s are not acceptable. ### Asynchronous transmission #### An asynchronous system is one in which the symbol rate can vary marginally with time and no rigid timing constraint is applied. Asynchronous operation is usually characterized by the use of start and stop bits to signify the beginning and end of a character that is to be sent, for example. the RS232 protocol.