120:. It 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. SDLC was released in 1975, based on work done for
506:, with each one's output connected to the next's 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.
529:
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
441:
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
517:
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) constitute a "go-ahead" sequence (also called EOP, end of poll) giving a secondary permission to transmit. A secondary which wishes to
530:
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.)
518:
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.
195:
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
525:
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.
513:
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.
473:
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
437:
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
208:. 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
521:
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.
154:
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.
457:
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).
803:
All communication within the ATC controller unit shall be SDLC-compatible command-response protocol, support 0-bit stuffing, and operate at a data rate of 614.4 Kilobits per second.
460:
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.
482:
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
131:
470:
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.
829:
181:, 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).
174:
162:
502:
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
213:
424:
Extended (modulo-128) sequence numbers and the corresponding SNRME U frame, were added to SDLC after the publication of the HDLC standard.
454:
8 (off)/9 (on): Wrap mode. Enter local loopback, connecting the secondary's input to its own output for the duration of the test.
854:
451:
4 (off)/5 (on): Monitor mode. Disable all frame generation, becoming silent, but do not stop carrier or loop mode operation.
864:
849:
817:
665:
128:
786:
765:
746:
727:
448:
2 (off)/3 (on): Beacon test. Disable all output, causing the next recipient to lose carrier (and begin beaconing).
796:
221:
166:
109:
178:
801:. v01.02.17b. Washington, DC: Institute of Transportation Engineers. November 16, 2006. p. 96.
859:
101:
466:
Several U frames are almost entirely unused in HDLC, existing primarily for SDLC compatibility:
205:
25:
302:
HDLC renamed some SDLC frames. The HDLC names were incorporated into later versions of SDLC:
689:
216:(DDCMP), Burroughs Network Architecture (BNA) with Burroughs Data Link Control (BDLC), and
98:
127:
SDLC operates independently on each communications link in the network and can operate on
8:
420:
Some features were added in HDLC, and subsequently added back to later versions of SDLC.
776:
244:
frames not a multiple of 8 bits long are illegal in SDLC, but optionally legal in HDLC.
782:
761:
742:
723:
717:
143:
704:
139:
113:
79:
490:
The TEST U frame was not included in early HDLC standards, but was added later.
486:
allows multiple secondaries to respond without having to poll them individually.
232:
HDLC is mostly an extension of SDLC, but some features were deleted or renamed.
117:
572:
16:
Computer communications protocol from IBM's
Systems Network Architecture (SNA)
843:
737:
Friend, George E.; Fike, John L; Baker, H. Charles; Bellamy, John C (1988).
503:
135:
151:
147:
708:
256:
asynchronous balanced mode, and the associated SABM and SABME U frames,
253:
asynchronous response mode, and the associated SARM and SARME U frames,
499:
274:
Also not in SDLC are later HDLC extensions in ISO/IEC 13239 such as:
74:
815:
663:
201:
192:
158:
95:
442:
diagnostic modes. The secondary echoes the byte in its response.
217:
778:
Data and computer communications: networking and internetworking
498:
A special mode of SDLC operation which is supported by e.g. the
209:
188:
510:
293:
the "unnumbered information with header check" (UIH) U frame.
197:
184:
825:(Technical report) (3rd ed.). Document No. GA27-3093-2.
741:(2nd ed.). Indianapolis: Howard W. Sams & Company.
673:(Technical report) (4th ed.). Document No. GA27-3093-3.
830:
Cisco page on
Synchronous Data Link Control and Derivatives
170:
177:(ADCCP). The latter standards added features such as the
121:
105:
834:
756:
Pooch, Udo W.; Greene, William H; Moss, Gary G (1983).
247:
HDLC optionally allows addresses more than 1 byte long.
819:
IBM Synchronous Data Link
Control: General Information
433:
Two U frames in SDLC which do not exist in HDLC are:
250:
HDLC has an option for a 32-bit frame check sequence.
719:
227:
816:IBM Communication Products Division (March 1979).
774:
664:IBM Communication Products Division (June 1986).
841:
415:
259:and several other frame types created for HDLC:
478:respond to any frame with the poll bit set, it
290:an information field in mode set U frames, and
175:Advanced Data Communication Control Procedures
659:
657:
655:
653:
651:
649:
240:Features present in HDLC, but not SDLC, are:
428:
235:
214:Digital Data Communications Message Protocol
287:a frame format field preceding the address,
268:the nonreserved (NR0 through NR3) U frames.
690:"System Network Architecture: An Overview"
646:
775:Hura, Gurdeep S.; Mukesh Singhal (2001).
687:
667:Synchronous Data Link Control: Concepts
842:
736:
627:
588:
583:
581:
138:facilities, on switched or dedicated,
760:. Boston: Little, Brown and Company.
755:
640:
614:
601:
297:
715:
547:
262:the selective reject (SREJ) S frame,
112:(SNA). SDLC is used as layer 2, the
795:
578:
566:
560:
509:When a secondary is powered off, a
13:
14:
876:
809:
758:Telecommunications and Networking
739:Understanding Data Communications
722:. Indianapolis, IN: Cisco Press.
228:Differences between SDLC and HDLC
187:used SDLC as a base protocol for
278:15- and 31-bit sequence numbers,
835:Bitbus/fieldbus community site.
633:
620:
607:
594:
553:
540:
1:
681:
445:0: Stop all diagnostic modes.
416:HDLC extensions added to SDLC
265:the reset (RSET) command, and
191:, still popular in Europe as
161:standard has been adopted by
88:Synchronous Data Link Control
20:Synchronous Data Link Control
855:Systems Network Architecture
575:, accessed 15. October 2009.
493:
395:Request initialization mode
167:High-Level Data Link Control
110:Systems Network Architecture
7:
865:Telecommunication protocols
850:Computer network technology
781:. Indianapolis: CRC Press.
389:Request initialization mode
284:8-bit frame check sequence,
61:; 50 years ago
10:
881:
281:the set mode (SM) U frame,
179:Asynchronous Balanced Mode
429:SDLC features not in HDLC
310:
307:
236:HDLC features not in SDLC
73:
55:
47:
39:
31:
24:
688:McFadyen, J. H. (1976).
533:
484:to the broadcast address
333:Nonsequenced information
319:Nonsequenced acknowledge
339:Unnumbered information
325:Unnumbered acknowledge
102:communications protocol
716:Odom, Wendell (2004).
206:National Semiconductor
169:(HDLC) in 1979 and by
26:Communication protocol
124:in the early 1970s.
798:ITS Cabinet Standard
643:, pp. 309–321).
104:first introduced by
709:10.1147/sj.151.0004
697:IBM Systems Journal
409:Request disconnect
146:circuits, and with
21:
403:Request disconnect
367:Disconnected mode
298:Naming differences
19:
413:
412:
347:Nonsequenced poll
85:
84:
872:
826:
824:
805:
792:
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752:
733:
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637:
631:
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611:
605:
598:
592:
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576:
573:PC Lube and Tune
570:
564:
557:
551:
544:
353:Unnumbered poll
305:
304:
69:
67:
62:
22:
18:
880:
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630:, p. 191).
625:
621:
617:, p. 303).
612:
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604:, p. 302).
599:
595:
591:, p. 188).
586:
579:
571:
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558:
554:
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541:
536:
496:
431:
418:
300:
238:
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114:data link layer
108:as part of its
80:Data link layer
65:
63:
60:
17:
12:
11:
5:
878:
868:
867:
862:
860:Link protocols
857:
852:
838:
837:
832:
827:
811:
810:External links
808:
807:
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793:
787:
772:
766:
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747:
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728:
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375:Command reject
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365:
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361:Request online
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129:point-to-point
118:protocol stack
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9:
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813:
804:
800:
799:
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790:
788:0-8493-0928-X
784:
780:
779:
773:
769:
767:0-316-71498-4
763:
759:
754:
750:
748:0-672-27270-9
744:
740:
735:
731:
729:1-58720-094-5
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381:Frame reject
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366:
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352:
349:
346:
343:
342:
338:
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315:
314:
308:Original name
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116:, in the SNA
115:
111:
107:
103:
100:
97:
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89:
81:
78:
76:
72:
58:
54:
50:
46:
42:
38:
34:
30:
27:
23:
818:
802:
797:
777:
757:
738:
718:
700:
696:
666:
635:
622:
609:
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568:
555:
542:
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520:
516:
508:
504:ring network
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301:
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239:
231:
183:
156:
126:
91:
87:
86:
56:Introduction
48:Developer(s)
43:Data framing
32:Abbreviation
703:(1): 4–23.
628:Friend 1988
589:Friend 1988
152:half-duplex
148:full-duplex
844:Categories
682:References
641:Pooch 1983
615:Pooch 1983
602:Pooch 1983
132:multipoint
548:Odom 2004
500:Zilog SCC
494:Loop mode
311:New name
144:four-wire
75:OSI layer
561:ITS 2006
202:Motorola
193:fieldbus
159:de facto
140:two-wire
96:computer
218:ARPANET
94:) is a
64: (
40:Purpose
785:
764:
745:
726:
210:DECnet
204:, and
189:BITBUS
99:serial
823:(PDF)
693:(PDF)
671:(PDF)
534:Notes
511:relay
438:mode.
220:with
212:with
198:Zilog
185:Intel
157:This
783:ISBN
762:ISBN
743:ISBN
724:ISBN
476:must
378:FRMR
372:CMDR
222:IMPs
171:ANSI
150:and
136:loop
92:SDLC
66:1974
59:1974
35:SDLC
705:doi
480:may
400:RQD
392:RIM
386:RQI
358:ROL
344:NSP
330:NSI
316:NSA
173:as
165:as
163:ISO
142:or
134:or
122:IBM
106:IBM
51:IBM
846::
701:15
699:.
695:.
648:^
580:^
563:).
550:).
406:RD
364:DM
350:UP
336:UI
322:UA
224:.
200:,
791:.
770:.
751:.
732:.
711:.
707::
639:(
626:(
613:(
600:(
587:(
559:(
546:(
90:(
68:)
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