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.
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