Binary Synchronous Communications
Binary Synchronous Communication (BSC or Bisync) is an
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[1] and is still in limited use in 2013.[2][3]
Framing
Bisync differs from protocols that succeeded it in the complexity of message framing. Later protocols use a single
Char | EBCDIC (hexadecimal) |
USASCII (hexadecimal) |
Transcode (hexadecimal) |
Description |
---|---|---|---|---|
SYN | 32 | 16 | 3A | Synchronous idle |
SOH | 01 | 01 | 00 | Start of heading |
STX | 02 | 02 | 0A | Start of text |
ETB | 26 | 17 | 0F | End of transmission block |
ETX | 03 | 03 | 2E | End of text |
EOT | 37 | 04 | 1E | End of transmission |
ENQ | 2D | 05 | 2D | Enquiry |
NAK | 3D | 15 | 3D | Negative acknowledgement |
DLE | 10 | 10 | 1F | Data link escape |
ITB | 1F | 1F (US) | 1D (US) | Intermediate block check character |
ACK0 and ACK1 (even/odd affirmative acknowledgement) are encoded as two characters—DLE '70'x, and DLE / for EBCDIC, DLE 0 and DLE 1 for USASCII, DLE - and DLE T for Transcode. WABT (wait before transmit) was encoded as DLE ", DLE ?, or DLE W.
All frame formats begin with at least two
A normal block ending character (ETB or ETX) is followed by a
Pad characters are required following a line turn-around—NAK, EOT, ENQ, ACK0, ACK1. If the transmission ends with EOT or ETX the pad follows the BCC. This pad is either all '1' bits or alternating '0' and '1' bits. The next transmission begins with a pad character which can be either of the above or a SYN.
An optional heading containing control information can precede data in a frame. The content of the heading is not defined by the protocol but is defined for each specific device. The heading, if present, is preceded by an SOH (start of heading) character and followed by an STX (start of text).[4]
Text data normally follows the heading, begun by the STX, and terminated by ETX (end of text) or ETB (end transmission block).
Normal data frames do not allow certain characters to appear in the data. These are the block ending characters: ETB, ETX and ENQ and the ITB and SYN characters. The number of unique characters that can be transmitted is therefore limited to 59 for Transcode, 123 for USASCII, or 251 for EBCDIC.
Transparent data framing provides an unrestricted alphabet of 64, 128 or 256 characters. In transparent mode block framing characters such as ETB, ETX, and SYN are preceded by a DLE character to indicate their control significance (The DLE character itself is represented by the sequence DLE DLE). This technique became known as character stuffing, by analogy with bit stuffing.
Link control
The link control protocol is similar to STR. The designers attempted to protect against simple transmission errors. The protocol requires that every message be acknowledged (ACK0/ACK1) or
Error recovery is by retransmission of the corrupted frame. Since Bisync data packets are not serial-numbered, it's considered possible for a data frame to go missing without the receiver realizing it. Therefore, alternating ACK0s and ACK1s are deployed; if the transmitter receives the wrong ACK, it can assume a data packet (or an ACK) went missing. A potential flaw is that corruption of ACK0 into ACK1 could result in duplication of a data frame.
Error protection for ACK0 and ACK1 is weak. The Hamming distance between the two messages is only two bits.
The protocol is half-duplex (2-wire). In this environment, packets or frames of transmission are strictly unidirectional, necessitating 'turn-around' for even the simplest purposes, such as acknowledgments. Turn-around involves
- the reversal of transmission direction,
- quiescing of line echo,
- resyncing.
In a 2-wire environment, this causes a noticeable round-trip delay and reduces performance.
Some datasets support full-duplex operation, and full-duplex (4-wire) can be used in many circumstances to improve performance by eliminating the turn-around time, at the added expense of 4-wire installation and support. In typical full-duplex, data packets are transmitted along one wire pair while the acknowledgements are returned along the other.
Topology
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.
Applications
The original purpose of Bisync was for batch communications between a System/360
IBM offered assembler language macros to provide programming support. During the System/360 era, these
The academic computing network
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[update], several vendors encapsulate Bisync transmissions within TCP/IP data streams.
Disposition
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.
Devices
A large number of devices use the Bisync protocol, some of these are:
- IBM 3270 Display Terminal Subsystem control units.
- IBM 2780Data Transmission Terminal.
- IBM 2703 Transmission Control.
- IBM HASP workstations.
- IBM 1130 Computing System.
- IBM 2922 Programmable Terminal.
Comparable protocols
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,[5] and Burroughs Corporation's Poll and Select Protocol.
See also
References
- ^ Scuilli, Joseph A. (Oct 26, 1981). "Terrestrial to Satellite Switching Creates Options". Computerworld. Retrieved Aug 27, 2012.
- ^ Cisco. "Binary Synchronous and Asynchronous Communications (Bisync/Async)". Retrieved Oct 23, 2013.
- ^ Gartner. "Binary Synchronous Communications (BSC)". IT Glossary. Retrieved Oct 23, 2013.
- ^ IBM Corporation. General Information - Binary Synchronous Communications (PDF).
- ^ Peterson, Larry; Davie, Bruce (2012). Computer Networks: A Systems Approach. Elsevier. Retrieved August 17, 2023.
Further reading
- Detailed discussion of Bisync link control by Charles A Wilde (new link)
- "Bisync, BSC". Connectivity Knowledge Platform. Made IT. Retrieved 2006-07-06. A detailed description of the protocol.
- Bisync & STR programming for IBM 1130
- "Data Communications Protocols". Telecom Corner Technical Reference Site. TBI/WebNet, Inc. October 2004. Retrieved 2006-07-06.
- "What is Bisync? A Short History Lesson". Serengeti Systems. Archived from the original on 2009-07-02. Retrieved 2006-07-06.
- IBM Corporation. "Bisync DLC Character Codes in Communications Trace on OS/400 or i5/OS System". Archived from the original on 2013-01-26. Retrieved 2012-06-07.
- IBM Corporation. General Information - Binary Synchronous Communications, first edition (PDF).
- IBM Corporation. General Information - Binary Synchronous Communications, third edition, October 1970 (PDF).