In order for the receiving side to understand the message, the clock must be synchronized between the parts. The clock synchronization may be implicit, as in setting the same data rate to read and write on the bus, or achieved by sharing the clock line from one side using an additional line, to explicitly synchronize the transmit data rate. Serial protocols that don't foresee shared clock lines are called asynchronous.
Symbol synchronization should instead be explicit. As we expect to send and receive information in byte form, the beginning of each eight bit sequence should be marked either through special preamble sequences on the data line, or by turning the clock on and off at the right time. The symbol ...
