To achieve synchronization between the transmitting and receiving devices, synchronization characters for the beginning and end of the character are added to each transmitted character. Receiving side scanning of the received signal is done in the middle of the digital characters. Synchronous control is based on the physical layer and is applied to asynchronous and synchronous transmission. Asynchronous transmission, also known as start-stop operation, works with no fixed timing pattern. The transmitting unit determines when the transmission starts and communicates this to the receiving unit. The transmission starts and ends with start bits and stop bits. In between are the information bits.
Synchronous transmission is a clock-based method that is based on bit synchron ization or byte synchronization. In synchronous transmission, the information bits are always sent only at specified times. In bit synchronous transmission control, the clock pulses of the synchronous control are used for the synchronization of the data and control characters.