Протокол связи мэйнфреймов IBM Binary Synchronous Communication (Bisync)

IBM mainframe communications protocol
Binary Synchronous Communication (BSC or Bisync) is an IBM character oriented, half duplex link protocol, announced in 1967 after the introduction of System/360. It replaced the synchronous transmit receive (STR) protocol used with second generation computers. The intent was that common link management rules could be used with three different character encodings for messages. Six bit Transcode looked backward to older systems; USASCII with 128 characters and EBCDIC with 256 characters looked forward. Transcode disappeared very quickly but the EBCDIC and USASCII dialects of Bisync continued in use. At one time Bisync was the most widely used communications protocol and is still in limited use in 2013.

Much Bisync traffic is point to point. Point to point lines can optionally use contention to determine the master station. In this case one device can transmit ENQ to bid for control. The other device can reply ACK0 to accept the bid and prepare to receive, or NAK or WABT to refuse. In some cases connection of a terminal to multiple hosts is possible via the dial telephone network. Multi drop is part of the initial Bisync protocol. A master station, normally a computer, can sequentially poll terminals which are attached via analog bridges to the same communication line. This is accomplished by sending a message consisting only of an ENQ character addressed to each device in turn. The selected station then transmits a message to the master or reply with EOT to indicate that it has no data to transmit.

The original purpose of Bisync was for batch communications between a System/360 mainframe and another mainframe or a Remote Job Entry (RJE) terminal such as the IBM 2780 or IBM 3780. The RJE terminals support a limited number of data formats: punched card images in and out and print line images to the terminal. Some non IBM hardware vendors such as Mohawk Data Sciences used Bisync for other purposes such as tape to tape transmission. A programmer can easily emulate an RJE terminal or other device. IBM offered assembler language macros to provide programming support. During the System/360 era, these access methods were BTAM (Basic Telecommunications Access Method) and QTAM (Queued Telecommunications Access Method) – which was later replaced by Telecommunications Access Method (TCAM). IBM introduced VTAM (Virtual Telecommunications Access Method) with the System/370. Teleprocessing monitors such as IBM's CICS and third party software such as Remote DUCS (display unit control system) and Westi platforms used Bisync line control to communicate with remote devices. The academic computing network Bitnet, together with connecting networks in other geographic areas, used Bisync to connect 3000 computer systems at its peak. Financial network S. W. I. F. T. used BSC protocol for communication between Regional Center and Institution (bank) server over leased line. In a mid 1990 BSC was replaced by the X.25 infrastructure. Some important systems use Bisync data framing with a different link control protocol. Houston Automatic Spooling Priority (HASP) uses Bisync half duplex hardware in conjunction with its own link control protocol to provide full duplex multi datastream communication between a small computer and a mainframe running HASP. In Bisync terms, this is conversational mode. Some early X.25 networks tolerated a connection scheme where transparent Bisync data frames encapsulated HDLC LAPB data and control packets. as of 2012, several vendors encapsulate Bisync transmissions within TCP/IP data streams.

Bisync began to be displaced in the 1970s by Systems Network Architecture (SNA) which allows construction of a network with multiple hosts and multiple programs using telecommunications. X.25 and the Internet Protocol are later protocols which, like SNA, provide more than mere link control.

Other computer vendors offered their own variety of byte oriented protocols similar to Bisync. Some widely used protocols include Digital Equipment Corporation's Digital Data Communications Message Protocol, and Burroughs Corporation's Poll and Select Protocol.