Bisynchronous communications, or binary synchronous communications (also known as bisync or BSC), is one of the two commonly used methods of encoding data for transmission between devices in IBM mainframe environments.
As in synchronous communication, bisync communication requires that both sending and receiving devices be synchronized before transmission of data begins. Data characters are gathered in a package called a frame, which is marked by two synchronization bits, hence the term bisynchronous.
Each frame contains leading and trailing characters that allow the computers to synchronize their clocks. The structure of a bisynchronous communications frame starts with initial synchronizing characters, followed by optional header characters, then the data message, which is preceded by a start-of-transmission (STX) character and followed by an end-of-transmission (ETX) character. The ETX is followed by a block check character (BCC), which verifies the accuracy of the transmission. The BCC is a one- or two-character result of a transmission verification algorithm performed on the block of data being transmitted.
While bisynchronous communication is still used extensively in IBM environments, the more modern protocol is Synchronous Data Link Control (SDLC), the data link layer of IBM’s Systems Network Architecture (SNA). SDLC is a more efficient method than the older bisync protocol when it comes to packaging data for transmission between computers. Packets of data are sent over the line without the overhead created by synchronization and other padding bits.
