Binary Synchronous Communications
Encyclopedia
Binary Synchronous Communication (BSC or Bisync) is an IBM 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 alphabets for encoding messages. Six-bit Transcode looked backwards to older systems; USASCII
with 128 characters and EBCDIC
with 256 characters looked forward. Transcode disappeared very quickly but the EBCDIC dialect of Bisync still has limited use in the early years of the 21st century.
(DDCMP), Point-to-Point Protocol
(PPP), etc. all have different framing schemes but only one frame format exists within a specific protocol. Bisync had five different framing formats. Normal data framing restricted the number of different characters that could be included in a text block to: Transcode 59, USASCII 123, EBCDIC 251. Transparent data framing provided an unrestricted alphabet of 64, 128 or 256 characters. The framing protocol varied with the message content:
All of these frame formats begin with at least two SYNC bytes. The binary form of the SYNC byte has the property that no rotation of the byte is equal to the original.
This allows the receiver to find the beginning of a frame by searching the received bit stream for the SYNC pattern. When this is found, tentative byte synchronization has been achieved. If the next character is also a SYNC, character synchronization has been achieved. The receiver then searches for a character that can start a frame. Characters outside of this set are described as "leading graphics". They are sometimes used to identify the sender of a frame.
The beginning of a data frame is signalled by the special character SYN (synchronization). The body of the frame is wrapped between two special sentinel characters: STX (start of text) and ETX (End of text).
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 SYNC characters. A long data frame should contain an inserted SYNC byte every two seconds to indicate that character synchronization is still present. The receiver deletes this character.
A normal block ending character (ETB or ETX) is followed by some kind of check sum. For USASCII, this is a one character longitudinal redundancy check
; for Transcode and EBCDIC, the check sum is a two character cyclic redundancy check
. A data frame may contain an intermediate check sum preceded by an ITB character. This ability to include intermediate checksums in a long data frame allowed a considerable improvement of the error detection probability.
Transparent data frames allowed all 256 EBCDIC characters to be transmitted. Block framing characters such as ETB, ETX, and SYNC were preceded by a DLE character to indicate their control significance. This technique became known as character stuffing, by analogy with bit stuffing
.
Error protection for ACK0 and ACK1 is weak. The Hamming distance
between the two messages is only two bits.
(NAK), so transmittal of small packets has high transmission overhead. The protocol can recover from a corrupted data frame, a lost data frame, and a lost acknowledgment.
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.
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
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.
Multi-drop was part of the initial bisync protocol. A master station (control unit) would poll several terminals which are attached via analog bridges to the same communication line. The selected station could then transmit a message to the master. The master could address a message to a specific station. Leading graphics mentioned above were used for station selection.
and another mainframe or an Remote Job Entry
(RJE) terminal such as the IBM 2780 and IBM 3780. The RJE terminals supported 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. RPG for System/36
and System/38
included support for bisync. A programmer could easily emulate an RJE terminal or other device.
IBM offered assembler language macros to provide programming support. During the S/360 era, these primitive access methods were BTAM (basic teleprocessing access method) and QTAM
(queued teleprocessing access method). IBM introduced VTAM
(virtual teleprocessing access method) with the S/370 and, subsequently, CICS
environment in support of the remote 3270
. The 3270s were further supported by Remote DUCS
(display unit control system) and WestiTAM
(Westinghouse telecommunications access method) application platforms (both written by the same architect). CICS
went on to fold in SDLC
under the SNA umbrella.
networks tolerated a connection scheme where transparent bi-sync data frames encapsulated HDLC LAPB
data and control packets.
(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.
Link protocol
In communications, a link protocol is a method and specification for transmission of data from one node on a local network or network link to another node on the same link...
, announced in 1967 after the introduction of System/360
System/360
The IBM System/360 was a mainframe computer system family first announced by IBM on April 7, 1964, and sold between 1964 and 1978. It was the first family of computers designed to cover the complete range of applications, from small to large, both commercial and scientific...
. 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 alphabets for encoding messages. Six-bit Transcode looked backwards to older systems; USASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
with 128 characters and EBCDIC
EBCDIC
Extended Binary Coded Decimal Interchange Code is an 8-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems....
with 256 characters looked forward. Transcode disappeared very quickly but the EBCDIC dialect of Bisync still has limited use in the early years of the 21st century.
Framing
BiSync differs from all popular protocols that succeeded it in the complexity of message framing. Later protocols used a single framing scheme for all messages sent by the protocol. HDLC, Digital Data Communications Message ProtocolDigital Data Communications Message Protocol
Digital Data Communications Message Protocol is a communications protocol devised by Digital Equipment Corporation in 1974 to allow communication over point-to-point network links for the company's DECnet Phase I network protocol...
(DDCMP), Point-to-Point Protocol
Point-to-Point Protocol
In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
(PPP), etc. all have different framing schemes but only one frame format exists within a specific protocol. Bisync had five different framing formats. Normal data framing restricted the number of different characters that could be included in a text block to: Transcode 59, USASCII 123, EBCDIC 251. Transparent data framing provided an unrestricted alphabet of 64, 128 or 256 characters. The framing protocol varied with the message content:
- Normal data: restricted character set
- Transparent data: unrestricted character set
- Single character link control phrase: EOT, NACK etc.
- DLE stick: ACK0, ACK1, WACK, RVI, DLE-EOT
- Forward abort: STX ... ENQ
-
- a. Temporary Transmit Delay: (TTD) encoded STX ENQ
All of these frame formats begin with at least two SYNC bytes. The binary form of the SYNC byte has the property that no rotation of the byte is equal to the original.
This allows the receiver to find the beginning of a frame by searching the received bit stream for the SYNC pattern. When this is found, tentative byte synchronization has been achieved. If the next character is also a SYNC, character synchronization has been achieved. The receiver then searches for a character that can start a frame. Characters outside of this set are described as "leading graphics". They are sometimes used to identify the sender of a frame.
The beginning of a data frame is signalled by the special character SYN (synchronization). The body of the frame is wrapped between two special sentinel characters: STX (start of text) and ETX (End of text).
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 SYNC characters. A long data frame should contain an inserted SYNC byte every two seconds to indicate that character synchronization is still present. The receiver deletes this character.
A normal block ending character (ETB or ETX) is followed by some kind of check sum. For USASCII, this is a one character longitudinal redundancy check
Longitudinal redundancy check
In telecommunication, a longitudinal redundancy check or horizontal redundancy check is a form of redundancy check that is applied independently to each of a parallel group of bit streams...
; for Transcode and EBCDIC, the check sum is a two character cyclic redundancy check
Cyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
. A data frame may contain an intermediate check sum preceded by an ITB character. This ability to include intermediate checksums in a long data frame allowed a considerable improvement of the error detection probability.
Transparent data frames allowed all 256 EBCDIC characters to be transmitted. Block framing characters such as ETB, ETX, and SYNC were preceded by a DLE character to indicate their control significance. This technique became known as character stuffing, by analogy with bit stuffing
Bit stuffing
In data transmission and telecommunication, bit stuffing is the insertion of noninformation bits into data...
.
Error protection for ACK0 and ACK1 is weak. The Hamming distance
Hamming distance
In information theory, the Hamming distance between two strings of equal length is the number of positions at which the corresponding symbols are different...
between the two messages is only two bits.
Link control
The link control protocol is similar to STR. The designers attempted to protect against simple transmission errors. The EBCDIC CRC-16 used to protect data frames is reasonably strong; the Transcode CRC-12 is somewhat weaker. The protocol requires that every message be acknowledged (ACK) or negatively acknowledgedNegative-acknowledge character
* In telecommunications, a negative-acknowledge character is a transmission control character sent by a station as a negative response to the station with which the connection has been set up....
(NAK), so transmittal of small packets has high transmission overhead. The protocol can recover from a corrupted data frame, a lost data frame, and a lost acknowledgment.
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.
The protocol is half-duplex
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
(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,
- and 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
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
(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.
Routing
Much bisync traffic was strictly point-to-point. In some cases connection of a terminal to multiple hosts was possible via the dial telephone network.Multi-drop was part of the initial bisync protocol. A master station (control unit) would poll several terminals which are attached via analog bridges to the same communication line. The selected station could then transmit a message to the master. The master could address a message to a specific station. Leading graphics mentioned above were used for station selection.
BiSync applications
The original purpose of bisync was for batch communications between a System/360 mainframeMainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
and another mainframe or an Remote Job Entry
Remote Job Entry
Remote job entry is the term used to describe the process of sending jobs to Mainframe computers from remote workstations, and by extension the process of receiving output from mainframe jobs at a remote workstation....
(RJE) terminal such as the IBM 2780 and IBM 3780. The RJE terminals supported 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. RPG for System/36
System/36
The IBM System/36 was a minicomputer marketed by IBM from 1983 to 2000. It was a multi-user, multi-tasking successor to the System/34. Like the System/34 and the older System/32, the System/36 was primarily programmed in the RPG II language...
and System/38
System/38
The System/38 was a midrange computer server platform manufactured and sold by the IBM Corporation. The system offered a number of innovative features, and was the brainchild of IBM engineer Dr. Frank Soltis...
included support for bisync. A programmer could easily emulate an RJE terminal or other device.
IBM offered assembler language macros to provide programming support. During the S/360 era, these primitive access methods were BTAM (basic teleprocessing access method) and QTAM
QTAM
Queued Telecommunications Access Method is an alternative to the simpler Basic Telecommunications Access Method communications access method, introducing built-in queuing. It was developed by IBM as part of DOS/360 and OS/360 and used mainly to transmit batches of data...
(queued teleprocessing access method). IBM introduced VTAM
VTAM
Virtual Telecommunications Access Method is IBM's software package that provides communications via telecommunication devices for mainframe environments. It is the implementation of Systems Network Architecture for mainframes...
(virtual teleprocessing access method) with the S/370 and, subsequently, CICS
CICS
Customer Information Control System is a transaction server that runs primarily on IBM mainframe systems under z/OS and z/VSE.CICS is a transaction manager designed for rapid, high-volume online processing. This processing is mostly interactive , but background transactions are possible...
environment in support of the remote 3270
IBM 3270
The IBM 3270 is a class of block oriented terminals made by IBM since 1972 normally used to communicate with IBM mainframes. As such, it was the successor to the IBM 2260 display terminal. Due to the text colour on the original models, these terminals are informally known as green screen terminals...
. The 3270s were further supported by Remote DUCS
DUCS (software)
DUCS was one of two early local teleprocessing packages for IBM's DOS/VSE environment. DUCS was an acronym for Display Unit Control System.- The product :DUCS provided an interface and access method for programmers to 'talk' to monitors...
(display unit control system) and WestiTAM
Westi (software)
Westi was one of two early local teleprocessing packages for IBM's DOS/VSE environment. Westi stood for Westinghouse Terminal Interactive.- The product :...
(Westinghouse telecommunications access method) application platforms (both written by the same architect). CICS
CICS
Customer Information Control System is a transaction server that runs primarily on IBM mainframe systems under z/OS and z/VSE.CICS is a transaction manager designed for rapid, high-volume online processing. This processing is mostly interactive , but background transactions are possible...
went on to fold in SDLC
SDLC
SDLC can refer to:* Systems Development Life Cycle, known also as "System Design Life Cycle"* "Software Development Life Cycle" synonym to Software development process* Synchronous Data Link Control, an IBM Protocol...
under the SNA umbrella.
Pseudo-Bisync applications
Some important systems used bi-sync data framing with a different link control protocol. HASP used bi-sync half-duplex hardware in conjunction with their own link control protocol to provide full-duplex multi-datastream communication between a small computer and a mainframe running HASP. Some early X.25X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
networks tolerated a connection scheme where transparent bi-sync data frames encapsulated HDLC LAPB
LAPB
Link Access Procedure, Balanced implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in and ISO/IEC 7776...
data and control packets.
Disposition
Bisync was first displaced by Systems Network ArchitectureSystems Network Architecture
Systems Network Architecture is IBM's proprietary networking architecture created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes the protocol and is, in itself, not actually a program...
(SNA) which allows construction of a network with multiple hosts and multiple programs using telecommunications. X.25
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
and the Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
are later protocols which, like SNA, provide more than mere link control.