Binary Synchronous Transmission

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Binary Synchronous Transmission (Bisync) is an IBM link protocol, developed in the 1960 and popular in the 1970s and 1980s.

Binary Synchronous Transmission has been largely replaced in IBM environments with SDLC. Bisync was developed for batch communications between a System 360 mainframe and the IBM 2780 and IBM 3780 Remote Job Entry (RJE) terminals. It supports RJE and on-line terminals in the CICS/VSE environment. It operates with EBCDIC or ASCII character sets. It requires that every message be acknowledged (ACK) or negatively acknowledged (NACK) so it has high transmission overhead. It is typically character oriented and half-duplex, although some of the bisync protocol flavours or dialects support binary transmission and full-duplex operation.

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Framing

Bisync uses a framing approach Byte oriented. The beginning of a frame is signalled by spacial character SYN (synchronization). The body of the frame is wrapped between two special sentinel characters: STX (start of text) and ETX (End of text). The beginning of the header is signalled by the special sentinel SOH (Start of header). It is possible that the body contains the special character ETX. In that case it would be a problem to transmit the correct frame. The solution is known as character stuffing. Another special sentinel DLE (Data link escape) is transmitted before every occurrence of ETX. This way ETX is transmitted as being part of the message to be sent. With the same behaviour the protocol can transmit DLE characters in the body of the frame.

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References

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
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