Data transmission in which a relatively large set of data is organized into a frame or block, with one or more synchronization bits or bit patterns used to identify the beginning and end of a logical block of data.T1 transmission, for example, is synchronized through framing bits that occur at the beginning of each frame. E-1 transmission is synchronized through the use of a separate time slot zero (0). Synchronous modems coordinate the receiving terminal on the rate of transmission of the data from the sending terminal. Synchronous data protocols such as Synchronous Data Link Control (SLDC) and High-Level Data Link Control (HDLC) use a specific bit pattern to form synchronizing characters that are integral to each frame. Through the receipt of the synchronizing bits or characters, a receiving device can match its speed of data receipt to the rate of data transmission across the circuit. Thereby, each bit of data and control information can be distinguished at the physical layer. Higher layer protocols sort out when to expect what information, in which data fields, and in what sequence, based on an agreed upon protocol such as frame relay or Internet Protocol (IP). Synchronous transmission is much more efficient than asynchronous transmission, as only a few framing bits and synchronizing bits surround a large block of data. See also asynchronous transmission, block, E-1, frame, frame relay, HDLC, IP, protocol, SDLC, SYNTRAN, and T1.
The transmission of data in which both stations are synchronized. Codes are sent from the transmitting station to the receiving station to establish the synchronization, and the data are then transmitted in continuous streams. Modems that transmit at 1,200 bps and higher often convert the asynchronous signals from a computer's serial port into synchronous transmission over the transmission line. Contrast with asynchronous transmission.