Synchronous serial communication describes a serial communication protocol in which "data is sent in a continuous stream at constant rate."[1]
Synchronous communication requires that the clocks in the transmitting and receiving devices are synchronized – running at the same rate – so the receiver can sample the signal at the same time intervals used by the transmitter. No start or stop bits are required. For this reason "synchronous communication permits more information to be passed over a circuit per unit time"[2] than asynchronous serial communication. Over time the transmitting and receiving clocks will tend to drift apart, requiring resynchronization.