A mode of serial transmission for digital modems, ISDN terminal adapters, Channel Service Unit/Data Service Units (CSU/DSUs), and other telecommunications devices.
Synchronous transmission uses clocking circuitry at both the transmitting station and the receiving station to ensure that communication is synchronized. This is in contrast to asynchronous transmission, in which start and stop bits are added to the beginning and end of each frame.
Devices that communicate with each other synchronously use either separate clocking channels to ensure synchronization between them or some kind of special signal code embedded in the signal for self-clocking purposes. Separate clocking lines are generally used when the distance between the data terminal equipment (DTE) and data communications equipment (DCE) is fairly short. Typically, the receiving station (such as a modem, a common form of DCE) provides the clocking signal to the transmitting station (usually a computer or a terminal).
The alternative is to use signal preamble, a special group of bytes (usually 8 bytes) called a SYNC signal that alerts the receiver that data is coming, synchronizes the clocks at the two devices, and starts the transmission. Special predefined voltage transition patterns familiar to both the transmitting and receiving stations are contained within the signal and are used to maintain synchronization between the devices. The receiver must extract this embedded information from the signal and use it to maintain synchronization between it and the transmitting station.
Synchronous transmission interfaces are generally about 20 percent faster and somewhat more reliable than comparable asynchronous interfaces.