The type of method used depends on the source of the timing information. If the timing in the receiver is generated by the receiver, separately from the transmitter, the transmission is termed asynchronous . If the timing is generated, directly or indirectly, from the transmitter clock, the transmission is termed synchronous . The block length is only 8 data-bits, permitting different clocks with only approximate synchronism to be used. For this reason, frame synchronisation is often referred to as character synchronisation in asynchronous systems.
Synchronous transmission is used for high data rate transmission.
The timing is generated by sending a separate clock signal, or embedding the timing information into the transmission. This information is used to synchronise the receiver circuitry to the transmitter clock. Because the clocks are synchronised, much longer block lengths are possible. The type of framing, character or block, is sometimes used to indicate whether a system uses asynchronous or synchronous transmission. This terminology does obscure the important difference between the different methods of timing control.