Synchronous Data Link Control
Encyclopedia
Synchronous Data Link Control (SDLC) is a computer
communications protocol
. It is the layer 2 protocol for IBM's Systems Network Architecture
(SNA). SDLC supports multipoint links as well as error correction. It also runs under the assumption that an SNA header is present after the SDLC header. SDLC was mainly used by IBM mainframe and midrange systems; however, implementations exist on many platforms from many vendors. The use of SDLC (and SNA) is becoming more and more rare, mostly replaced by IP-based protocols or being tunneled through IP (using AnyNet or other technologies).
In 1975, IBM developed the first bit-oriented protocol, SDLC, from work done for IBM
in the early 1970s. This de facto
standard has been adopted by ISO as High-Level Data Link Control
(HDLC) in 1979 and by ANSI
as Advanced Data Communication Control Procedures
(ADCCP). The latter standards added features such as the Asynchronous Balanced Mode
, frame sizes that did not need to be multiples of bit-octets, but also removed some of the procedures and messages (such as the TEST message).
SDLC operates independently on each communications link, and can operate on point-to-point
multipoint
or loop
facilities, on switched or dedicated, two-wire
or four-wire
circuits, and with full-duplex and half-duplex operation.
A unique characteristic of SDLC is its ability to mix half-duplex secondary stations with full-duplex primary stations on four-wire circuits, thus reducing the cost of dedicated facilities.
Intel used SDLC as a base protocol for BITBUS, still popular in Europe fieldbus
and included support in several controllers (i8044/i8344, i80152). The 8044 controller is in production by third party vendors. Other vendors putting hardware support for SDLC (and the slightly different HDLC) into communication controller chips of the 1980s include Zilog, Motorola, and National Semiconductor. As a result, a wide variety of equipment in the 1980s used it and it was very common in the mainframe centric corporate networks which were the norm in the 1980s. The most common alternatives for SNA with SDLC were probably DECNET
with DDCMP, BDLC and ARPA
with IMPS.
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
communications protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
. It is the layer 2 protocol for IBM's Systems Network Architecture
Systems 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). SDLC supports multipoint links as well as error correction. It also runs under the assumption that an SNA header is present after the SDLC header. SDLC was mainly used by IBM mainframe and midrange systems; however, implementations exist on many platforms from many vendors. The use of SDLC (and SNA) is becoming more and more rare, mostly replaced by IP-based protocols or being tunneled through IP (using AnyNet or other technologies).
In 1975, IBM developed the first bit-oriented protocol, SDLC, from work done for IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
in the early 1970s. This de facto
De facto
De facto is a Latin expression that means "concerning fact." In law, it often means "in practice but not necessarily ordained by law" or "in practice or actuality, but not officially established." It is commonly used in contrast to de jure when referring to matters of law, governance, or...
standard has been adopted by ISO as High-Level Data Link Control
High-Level Data Link Control
High-Level Data Link Control is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization...
(HDLC) in 1979 and by ANSI
Ansi
Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....
as Advanced Data Communication Control Procedures
Advanced Data Communication Control Procedures
In telecommunication, Advanced Data Communication Control Procedures is a bit-oriented data link layer protocol used to provide point-to-point and point-to-multipoint transmission of data frames that contain error control information. It places data on a network and ensures proper delivery to a...
(ADCCP). The latter standards added features such as the Asynchronous Balanced Mode
Asynchronous Balanced Mode
Asynchronous Balanced Mode is a communication mode of HDLC and derivative protocols, supporting peer-oriented point-to-point communications between two nodes, where either node can initiate transmission....
, frame sizes that did not need to be multiples of bit-octets, but also removed some of the procedures and messages (such as the TEST message).
SDLC operates independently on each communications link, and can operate on point-to-point
multipoint
or loop
Current loop
A current loop describes two different electrical signalling schemes.- Digital :For digital serial communications, a current loop is a communication interface that uses current instead of voltage for signaling...
facilities, on switched or dedicated, two-wire
Two-wire circuit
In telecommunication, a two-wire circuit is characterized by supporting transmission in two directions simultaneously, as opposed to four-wire circuits, which have separate pairs for transmit and receive. In either case they are twisted pairs. Telephone lines are almost all two wire, while trunks...
or four-wire
Four-wire circuit
In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path...
circuits, and with full-duplex and half-duplex operation.
A unique characteristic of SDLC is its ability to mix half-duplex secondary stations with full-duplex primary stations on four-wire circuits, thus reducing the cost of dedicated facilities.
Intel used SDLC as a base protocol for BITBUS, still popular in Europe fieldbus
Fieldbus
Fieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158....
and included support in several controllers (i8044/i8344, i80152). The 8044 controller is in production by third party vendors. Other vendors putting hardware support for SDLC (and the slightly different HDLC) into communication controller chips of the 1980s include Zilog, Motorola, and National Semiconductor. As a result, a wide variety of equipment in the 1980s used it and it was very common in the mainframe centric corporate networks which were the norm in the 1980s. The most common alternatives for SNA with SDLC were probably DECNET
DECnet
DECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s...
with DDCMP, BDLC and ARPA
ARPA
Arpa and ARPA may refer to:Arpa* Arpa River in Armenia* Areni, Armenia - formerly called Arpa* Arpi, Armenia, also called Arpa* Turkish for Akhurian River in Turkey and Armenia* Italian for harp, sometimes used in scoresARPA...
with IMPS.