1、I N T E R N AT I 0 N A L S TA N D A R D ISO/IEC 7776 Second edition 1995-07-01 Information technology - Telecommunications and information exchange between systems - High-level data link control procedures - Description of the X.25 LAPB-compatible DTE data link procedures Technologies de /informatio
2、n - Telecommunications et echange dinformation entre systemes - Procedures de commande de liaison de donnees de haut niveau - Description des procedures de liaison de donnees E77D compatibles X.25 LAPB National Standard of Canada CA N/CSA -ISO/!IE (2-7776-96 l International Standard ISO/IEC 7776 : 1
3、995 has been adopted, without modification, as CAN/CSA-ISO/IEC-7 776-96, which has been approved as a National Standard of Canada by the Standards Council of Canada. This Standard supersedes CAN/CSA-Z243.53-88 (adopted ISO/IEC 7776 : 1986). April 1996 I Reference number ISO/IEC 7776:1995(E) ISO/IEC
4、7776 : 1995 (E) Contents Page . Foreword . 111 Introduction iv 1 2 Scope 1 Normative references. . . . . . . . . . . . . . . . . . . . . . . . . . . . . , , . . . . . . . . . . . 2 Framescture . 2 Elements of procedures . . . . . . . . . . . , . . . , . . . . -. . . -. . . . . . . . . . . . . . . 7
5、Description of the procedure . , . , , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Multilink procedure (MLP) . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . 2 1 Conformance 28 Annexes A PICSProforma _* 30 B Repeated requirements from other InternationaI
6、 Standards . . . . . . . . . . . . 39 0 ISO/IEC 1995 All rights reserved. Unless otherwise specified, no part of this publication niay be reproduced or utilized in any form or by any means, electronic or mechanical. incIuding photocopying and microfilm, without permission in writing from the publish
7、er. ISO/IEC Copyright Office Case PostaIe 56 CH-1211 Genkve 20 Switzerland 11 0 ISO/IEC ISOlIEC 7776 : 1995 (E) Foreword IS0 (the International Organization for Standardization) and IEC (the Inter- national Electrotechnical Commission) form the specialized system for worldwide standardization. Natio
8、nal bodies that are members of IS0 or IEC participate in the development of International Standards through technical committees established by the respective organization to deal with particular fields of technical activity. IS0 and IEC technical committees collaborate in fields of mutual interest.
9、 Other international organizations, governmental and non-governmental, in liaison with IS0 and IEC, also take part in the work. In the field of information technology, IS0 and IEC have established a joint technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint technica
10、l committee are circulated to national bodies for voting. Publication as an International Standard requires approval by at least 75 % of the national bodies casting a vote. International Standard ISO/IEC 7776 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology. This secon
11、d edition cancels and replaces the first edition (IS0 7776:1986), and consolidates Amendment I : 1992, as well as Technical Corrigenda 1, 2 and 3. Annex A forms an integral part of this International Standard. Annex B is for information only. . 111 ISOIIEC 7776 : 1995 (E) 0 rSO/rEC Introduction This
12、 document provides the ISO/IEC description of the ITU-T Recommendation X.25 WB interface operation as viewed by the DTE. It is the DTE counterpart of the X.25 LAPB DCE description. This document also provides the ISO/IEC description of how two DTEs are capable of communicating directly with one anot
13、her at the Data Link layer using the X.25 LAPB procedures without an intervening public data network. The Data Link layer provides the DTE with three basic functions: a) link initialization: necessary for the DTE to begin communication in a known state; b) flow control: control of the flow of frames
14、 between the DE and the other station (DCE or DTE) to ensure that they are not sent more quickly than they can be received: and c) error control: provided in two forms: 1) a cyclic redundancy check (CRC) using a 16-bit polynomial to detect mutilated frames, and 2) use of sequence numbers to ensure a
15、gainst losing entire frames. (The Data Link layer endeavours to ensure correct receipt of all frames by retransmission of mutilated or missing frames.) This International Standard repeats requirements of other International Standards. Annex B contains a list of these repeated requirements and refere
16、nces to the corresponding International Standards. To evaluate conformance of a particular implementation, it is necessary to have a statement of which capabilities and options have been implemented. Such a statement is called a “Protol Implementation Conformance Statement” (PICS), as defined in ISO
17、/IEC 9646-1. This International Standard provides such a PICS proforma in compliance with the relevant requirements, and in accordance with the relevant guidance, given in iSO/EC 9646-2. iv INTERNATIONAL STANDARD ISOflEC ISO/IEC 7776 : 1995 (E) Information technology - Telecommunications and informa
18、tion exchange between systems - High-level data link control procedures - Description of the X.25 LAPB- compatible DTE data link procedures 1 Scope This International Standard defines an application of the following HDLC standards: ISO/IEC 3309, ISO/IEC 4335, IS0 7478. and ISOiIEC 7809. When there i
19、s difficulty in the interpretation of a reworded requirement from one of the other International Standards, the original requirement of ISO/IEC 3309, ISOilEC 4335, IS0 7478 or ISO/IEC 7809 is definitive. It also defines the structure, elements and procedures for the operation of a DTE using the X.25
20、 LAPB protocol as specified in ITU-T Recommendation X.2S.1) The procedures are applicable to data interchange between a DTE and a DCE, or between two DTEs. The procedures are defined for use on duplex links, using synchronous transmission or stadstop transmission. Clause 3 describes two frame struct
21、ures: one for basic (modulo 8) operation and one for extended (modulo 128) operation. Basic (modulo 8) operation is the ISO/IEC balanced asynchronous class of procedure with optional functions 2 and 8 (BAC, 2, 8). Extended (modulo 128) operation is the ISOfiEC balanced asynchronous class of procedur
22、e with optional functions 2, 8 and 10 (BAC, 2,8, 10). For those DTE/DCE: connections that support both basic (modulo 8) operation and extended (modulo 128) operation, the choice is made at subscription-time only. For those DTE/reniote DTE connections that support both basic (modulo 8) operation and
23、extended (modulo 128) operation, the choice is made by bilateral agreement. NOTE - The procedure herein described as basic (moduIo 8) operation is the only one available in all public data networks. Clause 3 also describes two methods for encoding the frames, as sequences of bits when synchronous tr
24、ansmission is used, and as sequences of octets when start/stop transmission is used. The start/stop encoding specifies optional mechanisms, for use in environments that are sensitive to transmission of octets with values that could be interpreted as lSO/IEC 646 control characters, and/or in environm
25、ents that support transfer of only seven data bits per start/stop character. The choice of encoding is made by bilateral agreement, or other suitable means, to suit the data transmission characteristics of the environment. Clause 4 describes the elements of procedures. Some aspects are only operable
26、 for the basic (modulo 8) operation and some for the extended (modulo 128) operation. Clauses 5 and 6 describe the single link procedure (SLP which is derived from the frame structure and the elements of procedures, and an optional niultilink procedure (MLP), respectively. The SLP is used for data i
27、nterchange over a single data link and the MLP is used for data interchange over a multiple of parallel SLPs. An MLP is required if the effects of individual SLP failures are not to disrupt the higher layer operation. An MLP can also be used over a single SLP by prior bilateral agreement. For DTE/DC
28、E connections the choice of an MLP operation or not is made at subscription-time only. For DTE/remote DTE connections, the choice is made by bilateral agreement. Where choices among alternative actions are indicated in the procedures, a recommended choice is usually indicated. Unless specifically st
29、ated otherwise, the choice of action does not affect interoperability with other implementations of this International Standard although efficiency of operation may be affected. Where such choices do affect inreroperztbility, the procedures explicitly state that prior bilateral 1) on the 1993 ITU-T
30、Recomniendarion X.25 Future revisions of this International Standard will be inde in accordance with revisions of ITU-T Recommendation X.25. The present version is based 1 ISOllEC 7776 : 1995 (E) Flag Address F A 011 I1 1 io 8-bits 0 ISO/EEC Control FCS Flag C FCS F 8-bi 16-bits 01 11 11 10 agreemen
31、t on the choice of procedure with the remote end is needed. An attempt has been made to minimize such choices consistent with the need to satisfy a broad range of applications. A basic requirement for all hiplenientations of this International Standard is that they be capable of responding, as speci
32、fied, to any actions taken at the remote end that are permitted by this International Standard (except possibly for those procedures whose use involves prior bilateral agreement.) Flag Address Control Information FCS nag - F A C I FCS F 011 111 10 $-bits %bits N-bits 16-bits 01 11 11 LO Clause 7 cov
33、ers the Static Conformance requirements, the Dynamic Conformance requirements, and the Protocol Implementation Conformance Statement (TICS). 2 Normative references. The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard.
34、 At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards listed below. Members of IEC and I
35、S0 maintain registers of currently valid International Standards. ISO/IEC 646: 199 1, Information technology - IS0 7-bit coded cliaracter set for irlforniotion interchange. ISO/IEC 3309: 1993, Information technology - Telecommunications and information exchange between systems - High-level data link
36、 control (HDLC) procedures -Frame structure. ISO/IEC 4335: 1993, Information technology - Telecommitriicarioirs and iiflotmatioii exchange betweem systems - High-level dara link control (HDLC) procedures - Elements of procedure. IS0 7478: 1987. Itformariati pi-ocessitig systems - Data comrnruiicatio
37、ti - Mrrlrilink procedures. ISO/IEC 7809: 1993, InJormation tccliiiology - Telecommuriicatiotrs atid irlformatiotr cxcharige bcnuecti systems - High-level data link control procedures (HDLC) - Classes of procedures. ISO/IEC 9646-1 : 1994, Information technology - Open Systems Interconnection - Confo
38、rmance testing methodology and framework - Part 1: General concepts. ISO/IEC 9646-2: 1994, Information technology - Open Systems Interconnection - Confomance testing methodology and framework - Part 2: Abstract Test Suite specification. ITU-T Recommendation X.25, Interface between data termirial cqr
39、ril;rnzprit (DE) und data circuit-rerminating equiiiment (DCE) for terminals operating itz the packet nzode and cortriecred to public dutu tiehwrks by dedilicated circuit. 3 Frame structure All transmissions on a SLP are in frames conforming to one of the fomiats of table 1 for basic (modulo 8) oper
40、ation, or alternatively one of the formats of table 2 for extended (modulo 128) operation. The flag preceding the address field is defined as the opening flag. The flag foIlowing the FCS field is defined as the closing flag. All transmissions from the DCEhemote DTE are expected to use this frame str
41、ucture. Bit order of transmission Bit order of transmission Table 1 - Frame formats -Basic (modulo 8) operation 16 to 1 12345678 12345678 12345678 1234S678 FCS = Frame Check Sequence l2345678 12345678 12345678 16 to 1 12345678 2 0 ISO/IEC Flag Address Control F A C 0111 1110 8-bits *-bits ISO/IEC 77
42、76 : 1995 (E) FCS nag FCS F 16-bits 01 f 11 110 Table 2 -Frame formats -Extended (modulo 128) operation nag Address Control Information F A C I 0111 11 10 %bits 8-bits N-bits Bit order of transmission FCS flag FCS F 16-bits 01 11 11 10 Bit order of transmission * 16 for frame formats that contain se
43、quence numbers; 8 for frime formats that do not contain sequence numbers. 3.1 Flag sequence All frames shall start and end with the flag sequence consisting of one “0“ bit followed by six contiguous “1“ bits and one “0“ bit. The DTE hunts continuously for this sequence on a bit-by-bit basis, and thu
44、s uses the flag sequence for frame synchronization. The DTE/DCE/remote DTE may send one or more complete flag sequences between frames. The DTE shall only send complete eight-bit flag sequences when sending multiple flag sequences (see 3.10). A single flag may be used as both the closing flag for on
45、e frame and the opening flag for the next frame. 3.2 Address field The address field shall consist of one octet. The address field identifies the intended receiver of a command frame and the transmitter of a response frame. The coding of the address field is described in 5.1. 3.3 Control field For b
46、asic (modulo 8) operation, the control field shall consist of one octet. For extended (modulo 128) operation, the control fieId shall consist of two octets for frame fomiats that contain sequence numbers, and one octet for frame fomiats that do not contain sequence numbers. The content of this field
47、 is described in 4.1. 3.4 Information field The information field of a frame, when present, follows the control field (see 3.3) and precedes the frame check sequence (see 3.6). (See 4.3.9 and 6.2 for the various codings and groupings of bits in the infomiation field that are defined for use in this
48、International Standard.) The coding and grouping of bits received from a higher Iayer are unrestricted. except for requirements that are imposed by the higher layer itself. For start/stop transmission there shall be eight (8) infomiation bits between the start element and the stop element: the infor
49、mation field is therefore constrained to be octet-aligned. See 4.3.9 and 5.7.3 with regard to the maximum infomiation field length. 3.5 Transparency 3.5.1 Synchronous transmission A DTE, when transmitting, shall examine the frame content berween the two flag sequences including the address, control, information and FCS fields and shall insert a “0“ bit after a11 sequences of five contiguous “1“ bits (including the Iast five bits of the FCS) to ensure that a flag sequence is not simulated. A DTE, when receiving, shall examine the frame content and shall discard any “0“ bit which direct
