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Synchronous Data Link Control (SDLC) is a computer
communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchro ...
. It is the layer 2 protocol for IBM's
Systems Network Architecture Systems Network Architecture (SNA) is IBM's proprietary networking architecture, created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes formats and protocols but, in itself, is not a pi ...
(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. In the United States and Canada, SDLC can be found in traffic control cabinets. In 1975, IBM developed the first bit-oriented protocol, SDLC,PC Lube and Tune
accessed 15. October 2009.
from work done for IBM in the early 1970s.. This
de facto ''De facto'' ( ; , "in fact") describes practices that exist in reality, whether or not they are officially recognized by laws or other formal norms. It is commonly used to refer to what happens in practice, in contrast with '' de jure'' ("by l ...
standard has been adopted by ISO as High-Level Data Link Control (HDLC) in 1979 and by
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as Advanced Data Communication Control Procedures (ADCCP). The latter standards added features such as the
Asynchronous Balanced Mode High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides both ...
, 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
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facilities, on switched or dedicated, two-wire or four-wire circuits, and with
full-duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
and
half-duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
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 Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
used SDLC as a base protocol for BITBUS, still popular in Europe as
fieldbus Fieldbus is the name of a family of industrial computer networks used for real-time distributed control. Fieldbus profiles are standardized by the International Electrotechnical Commission (IEC) as IEC 61784/61158. A complex automated industrial ...
and included support in several controllers (i8044/i8344, i80152). The 8044 controller is still 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 included
Zilog Zilog, Inc. is an American manufacturer of microprocessors and 8-bit and 16-bit microcontrollers. It is also a supplier of application-specific embedded system-on-chip (SoC) products. Its most famous product is the Z80 series of 8-bit microp ...
,
Motorola Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorola ...
, and
National Semiconductor National Semiconductor was an American semiconductor manufacturer which specialized in analog devices and subsystems, formerly with headquarters in Santa Clara, California. The company produced power management integrated circuits, display dr ...
. 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
Digital Data Communications Message Protocol Digital Data Communications Message Protocol (DDCMP) is a byte-oriented 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 pr ...
(DDCMP), Burroughs Network Architecture (BNA) with Burroughs Data Link Control (BDLC), and
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with IMPs..


Differences between SDLC and HDLC

HDLC is mostly an extension of SDLC, but some features were deleted or renamed.


HDLC features not in SDLC

Features present in HDLC, but not SDLC, are: * frames not a multiple of 8 bits long are illegal in SDLC, but optionally legal in HDLC. * HDLC optionally allows addresses more than 1 byte long. * HDLC has an option for a 32-bit frame check sequence. * asynchronous response mode, and the associated SARM and SARME U frames, * asynchronous balanced mode, and the associated SABM and SABME U frames, * and several other frame types created for HDLC: ** the selective reject (SREJ) S frame, ** the reset (RSET) command, and ** the nonreserved (NR0 through NR3) U frames. Also not in SDLC are later HDLC extensions in ISO/IEC 13239 such as: * 15- and 31-bit sequence numbers, * the set mode (SM) U frame, * 8-bit frame check sequence, * a frame format field preceding the address, * an information field in mode set U frames, and * the "unnumbered information with header check" (UIH) U frame.


Naming differences

HDLC renamed some SDLC frames. The HDLC names were incorporated into later versions of SDLC:


HDLC extensions added to SDLC

Some features were added in HDLC, and subsequently added back to later versions of SDLC. * Extended (modulo-128) sequence numbers and the corresponding SNRME U frame, were added to SDLC after the publication of the HDLC standard.


SDLC features not in HDLC

Two U frames in SDLC which do not exist in HDLC are: * BCN (Beacon): When a secondary loses carrier (stops receiving any signal) from the primary, it begins transmitting a stream of "beacon" responses, identifying the location of the communication failure. This is particularly useful in SDLC loop mode. * CFGR (Configure for test) command and response: The CFGR command contains a 1-byte payload which identifies some special diagnostic operation to be performed by the secondary. The least significant bit indicates that the diagnostic mode should start (1) or stop (0). A payload byte of 0 stops all diagnostic modes. The secondary echoes the byte in its response. ** 0: Stop all diagnostic modes. ** 2 (off)/3 (on): Beacon test. Disable all output, causing the next recipient to lose carrier (and begin beaconing). ** 4 (off)/5 (on): Monitor mode. Disable all frame generation, becoming silent, but do not stop carrier or loop mode operation. ** 8 (off)/9 (on): Wrap mode. Enter local loopback, connecting the secondary's input to its own output for the duration of the test. ** 10 (off)/11 (on): Self-test. Perform local diagnostics. CFGR response is delayed until the diagnostics complete, at which time the response is 10 (self-test failed) or 11 (self-test successful). ** 12 (off)/13 (on): Modified link test. Rather than echoing TEST commands verbatim, generate a TEST response consisting of a number of copies of the first byte of the TEST command. Several U frames are almost entirely unused in HDLC, existing primarily for SDLC compatibility: * Initialization mode, and the associated RIM and SIM U frames, are so vaguely defined in HDLC as to be useless, but are used by some peripherals in SDLC. * Unnumbered poll (UP) is almost never used in HDLC, its function having been superseded by asynchronous response mode. UP is an exception to the usual rule in normal response mode that a secondary must receive the poll flag before transmitting; while a secondary ''must'' respond to any frame with the poll bit set, it ''may'' respond to a UP frame with the poll bit clear if it has data to transmit. If the lower-level communication channel is capable of avoiding collisions (as it is in loop mode), UP ''to the broadcast address'' allows multiple secondaries to respond without having to poll them individually. The TEST U frame was not included in early HDLC standards, but was added later.


Loop mode

A special mode of SDLC operation which is supported by e.g. the Zilog SCC but was not incorporated into HDLC is SDLC loop mode. In this mode, a primary and a number of secondaries are connected in a unidirectional
ring network A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node – a ring. Data travels from node to node, with each node along the way handling ever ...
, with each one's transmit output connected to the next's receive input. Each secondary is responsible for copying all frames which arrive at its input so that they reach the rest of the ring and eventually return to the primary. Except for this copying, a secondary operates in half-duplex mode; it only transmits when the protocol guarantees it will receive no input. When a secondary is powered off, a
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connects its input directly to its output. When powering on, a secondary waits for an opportune moment and then goes "on-loop" inserting itself into the data stream with a one-bit delay. A similar opportunity is used to go "off-loop" as part of a clean shutdown. In SDLC loop mode, frames arrive in a group, ending (after the final flag) with an all-ones idle signal. The first seven 1-bits of this (the pattern 01111111) constitutes a "go-ahead" sequence (also called EOP, end of poll) giving a secondary permission to transmit. A secondary which wishes to transmit uses its 1-bit delay to convert the final 1 bit in this sequence to a 0 bit, making it a flag character, and then transmits its own frames. After its own final flag, it transmits an all-ones idle signal, which will serve as a go-ahead for the next station on the loop. The group starts with commands from the primary, and each secondary appends its responses. When the primary receives the go-ahead idle sequence, it knows that the secondaries are finished and it may transmit more commands. The beacon (BCN) response is designed to help locate breaks in the loop. A secondary which does not see any incoming traffic for a long time begins sending "beacon" response frames, telling the primary that the link between that secondary and its predecessor is broken. Because the primary also receives a copy of the commands it sent, which are indistinguishable from responses, it appends a special "turnaround" frame at the end of its commands to separate them from the responses. Any unique sequence which will not be interpreted by the secondaries will do, but the conventional one is a single all-zero byte. This is a "runt frame" with an address of 0 (reserved, unused) and no control field or frame check sequence. (Secondaries capable of full-duplex operation also interpret this as a "shut-off sequence", forcing them to abort transmission.)


Notes


References

* * * * * *


External links

*{{cite techreport , url=http://www.bitsavers.org/pdf/ibm/datacomm/GA27-3093-2_SDLC_General_Information_Mar79.pdf , title=IBM Synchronous Data Link Control: General Information , edition=3rd , date=March 1979 , id=Document No. GA27-3093-2 , author=IBM Communication Products Division
Cisco page on Synchronous Data Link Control and DerivativesBitbus/fieldbus community site.
Systems Network Architecture Link protocols Telecommunication protocols