1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU X.25 (I 0196) SERIES X: DATA NETWORKS AND OPEN SYSTEM COM M U N I CATI ON Public data networks - Interfaces Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for ter
2、minals operating in the packet mode and connected to public data networks by dedicated circuit ITU-T Recommendation X.25 (Previously “CCITT Recommendation“) STD-ITU-T RECMN X.25-ENGL 199b 48b259L Ob280b1 882 ITU-T X-SERIES RECOMMENDATIONS DATA NETWORKS AND OPEN SYSTEM COMMUNICATION Services and faci
3、lities X.1-X.19 Transmission, signalling and switching X.50-X.89 Network aspects Maintenance Administrative arrangements Model and notation Service definitions Connection-mode protocol specifications Connectionless-mode protocol specification PICS proformas Protocol Identification Security Protocols
4、 Layer Managed Objects Conformance testing General Satellite data transmission systems MESSAGE HANDLING SYSTEMS DIRECTORY 3SI NETWORKING AND SYSTEM ASPECTS Networking Efficiency Naming, Addressing and Registration Abstract Syntax Notation One (ASN. I) Systems Management framework and architecture Ma
5、nagement Communication Service and Protocol Structure of Management Information Management functions )PEN SYSTEM INTERCONNECTION NTERWORKING BETWEEN NETWORKS OS1 MANAGEMENT SECURITY OS1 APPLICATIONS Commitment, Concurrency and Recovery Transaction processing Remote operations OPEN DISTRIBUTED PROCES
6、SING X.90-X. 149 X.150-X.179 X. 180-X. 199 X.200-X.299 X.200-X.209 X.2 10-X.2 19 X.220-X.229 X.230-X.239 X.240-X.259 X.260-X.269 X.270-X.279 X.280-X.289 X.290-X.299 X.300-X.399 X.300-X.349 X.350-X.399 X.400-X.499 X.500-X.599 X.600-X.699 X.600-X.629 X.630-X.649 X.650-X.679 X.680-X.699 X.700-X.799 X.7
7、00-X.709 X.710-X.719 X.720-X.729 X.730-X.799 X.800-X.849 X.850-X.899 X.850-X.859 X.860-X.879 X.880-X.899 X.900-X.999 Forfirther details, please refer to ITU-T List of Recommendations. FOREWORD The IT-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunica
8、tion Union (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommen- dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, e
9、stablishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the IT-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). IT-T Recommendation X.25 wa
10、s revised by ITU-T Study Group 7 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 5th of October 1996. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication I administration and a recognized operating ag
11、ency. O ITU 1997 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing ftom the IT. Recommendation X.25 (10/96) 1 CONTENTS Page 1 DTEDCE interface chara
12、cteristics (physical layer) 1.1 X.21 interface . 1.2 X.21 bis interface 1.3 V-Series interface . 1.4 X.31 interface . 2 Link access procedures across the DTEDCE interface . Scope and field of applications . LAPB elements of procedures Description of the LAPB procedure . 2.1 2.2 Framing aspects 2.3 2
13、.4 2.5 MultiLink Procedure (MLP) (Subscription-time selectable option) . Description of the packet layer DTEDCE interface 3.2 Basic structure of packets . 3.3 Procedure for restart . 3 3.1 Logical channels . 3.4 Error handling . Procedures for virtual circuit services 4.1 Procedures for virtual call
14、 service 4.2 Procedures for permanent virtual circuit service 4.3 Procedures for data and interrupt transfer . 4.4 Procedures for flow control 4.5 Effects of clear, reset and restart procedures on the transfer of packets . 4.6 Effects of the physical layer and the data link layer on the packet layer
15、 4 5 Packet formats i l 5.1 General 5.2 Call Set-up and clearing packets . 5.3 Data and interrupt packets 5.4 Flow control and reset packets 5.5 Restart packets 5.6 Diagnostic packet 5.7 Packets required for optional user facilities Procedures for optional user facilities (packet layer) . 6.1 TOA/NP
16、I address subscription . 6.2 Extended and super extended packet sequence numbering facilities . 6.3 D-bit modification 6.4 Packet retransmission . 6.5 Incoming calls barred . 6.6 Outgoing calls barred 6.7 One-way logical channel outgoing . 6.8 One-way logical channel incoming 6.9 Non-standard defaul
17、t packet sizes 6 2 2 3 3 4 4 4 5 9 23 35 45 45 45 45 47 47 47 49 49 52 56 57 58 58 60 74 74 82 84 86 86 86 86 88 88 88 89 89 89 89 ii Recommendation X.25 (10/96) STD-ITU-T RECMN X.25-ENGL 177b 9 48bZ.571 Ub280bi 571 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17 6.18 6.19 6.20 6.2 1 6.22 6.23 6.24 6.25 6.2
18、6 6.27 6.28 Non-standard default window sizes Default throughput classes assignment . Throughput class negotiation facilities . Closed user group related facilities . Bilateral closed user group related facilities . Fast select Fast select acceptance . Reverse charging Reverse charging acceptance Fl
19、ow control parameter negotiation Local charging prevention Network user identification (NUI) related facilities . Charging information ROA related facilities . Hunt group Call redirection and call deflection related facilities . Called line address modified notification . Transit delay selection and
20、 indication . Alternative addressing related facilities 7 Formats for facility fields . 7.1 General 7.2 Coding of the facility code fields 7.3 Coding of the facility parameter fields . Annex A - Range of logical channels used for virtual calls and permanent virtual circuits . Annex B - Packet layer
21、DTEDCE interface state diagrams . Symbol definition of the state diagrams . Order definition of the state diagrams interface as perceived by the DCE . Annex D - Packet layer DCE time-outs and DTE time-limits D.l DCE time-outs D.2 DTE time-limits Annex E - Coding of X.25 network generated diagnostic
22、fields in clear, reset and restart indication, and diagnostic packets Annex F - Subscription-time optional user facilities that may be associated with a network user identifier in B.l B.2 Annex C - Actions taken by the DCE on receipt of packets in a given state of the packet layer DTEDCE conjunction
23、 with the NUI ovemde facility Annex G - IT-T specified DTE facilities to support the OS1 Network service and other purposes . G.l Introduction G.2 Coding of the facility code fields G.3 Coding of the facility parameter field . Appendix I - Examples of data link layer transmitted bit patterns by the
24、DCE and the DTE . Appendix II - An explanation of how the values for N1 in 2.4.9.5 are derived . 11.1 DTE N1 . 11.2 DCE N1 11.3 General DCE N1 calculations . Recommendation X25 (10/96) Page 89 90 90 91 92 95 95 98 98 98 98 99 100 100 100 101 104 104 105 107 107 109 109 119 121 121 121 124 134 134 13
25、4 137 139 140 140 140 140 145 146 146 146 147 iii Page 149 III.1 Introduction 149 111.2 MLP reset initiated by either the DCE or the DTE . 149 111.3 MLP reset initiated by both the DCE and the DTE simultaneously . 150 Appendix III . Examples of multilink resetting procedures . , Appendix IV IV . 1 I
26、V.2 IV.3 IV.4 IV.5 IV.6 IV.7 IV.8 lV.9 . . Information on addresses in call Set-up and clearing packets . Main address and complementary address . Addresses in call request packet . Addresses in incoming call packets Addresses in call accepted packets . Addresses in call connected packets . Addresse
27、s in clear request packets Addresses in clear indication packets . Addresses in clear confirmation packets . Addresses in call redirection and call deflection related facilities 150 150 151 151 152 152 153 153 153 153 Appendix V . Guidelines for transmission over channels with long round trip delay
28、andor transmission rates higher than 64 O00 bit/s 153 V.l Preamble . 153 V.2 Common guidelines 154 V.3 Guidelines for channels with long round trip delays operating at 64 O00 bit/s 154 V.4 Guidelines for circuits with long round trip delays operating at 1920 kbit/s 155 Appendix VI - Format for NUI p
29、arameter field 155 157 Appendix VI1 - Examples of the use of multi-selective reject option . iv Recommendation X.25 (10196) SUMMARY This Recommendation specifies the protocol to be used between a Data Terminal Equipment (DTE) and a Packet Switched Public Data Network (PSPDN), when the access is made
30、 by a dedicated circuit. In reference to the OS1 model, three layers are described: the physical layer, the data link layer and the packet layer. The service offered is compliant with the OS1 network service and permits the transmission of data without loss nor duplication. The access speeds and the
31、 throughputs may go up to 2 Mbit/s. Recommendation X.25 (10/96) V STD-ITU-T RECMN X.25-ENGL L77b U 48b257L Ob28Ub7 2TO Recommendation X.25 INTERFACE BETWEEN DATA TERMINAL EQUIPMENT (DTE) AND DATA IN THE PACKET MODE AND CONNECTED TO PUBLIC DATA NETWORKS BY DEDICATED CIRCUIT (Geneva, 1976; amended at
32、Geneva, 1980; Malaga-Torremolinos, 1984; Melbourne, 1988; Helsinki 1993, revised in 1996) CIRCUIT-TERMINATING EQUIPMENT CE) FOR TERMINALS OPERATING The establishment in various countries of public data networks providing packet switched data transmission services creates a need to produce standards
33、to facilitate international interworking. The IT-T, considering (a) that Recommendation X.l includes specific user classes of service for data terminal equipments operating in the packet mode, defines categories of access, Recommendation X.2 defines user facilities, Recommendations X.2 1 and X.2 1 b
34、is define DTE/DCE physical layer interface characteristics, Recommendation X.92 defines the hypothetical reference connections for packet switched data transmission service and Recommendation X.96 defines call progress signals; (b) that data terminal equipments operating in the packet mode will send
35、 and receive network control information in the form of packets; (cl that certain data terminal equipments operating in the packet mode will use a packet interleaved synchronous data circuit; (d) the desirability of being able to use a single data circuit to a Data Switching Exchange (DSE) for all u
36、ser facilities; (e) that Recommendation X.2 specifies which of the various data transmission services and optional user facilities described in this Recommendation are “essential” and have thus to be made available internationally, and which are not; (0 the need for defining an international Recomme
37、ndation for the exchange between DTE and DCE of control information for the use of packet switched data transmission services; (8) that this definition is made in Recommendation X.32 with regard to the access through a public switched telephone network, an Integrated Services Digital Network (ISDN),
38、 or a circuit switched public data network; (h) that Recommendation X.3 1 defines the support of packet-mode terminal equipment by an Integrated Services Digital Network (ISDN); (i) that, when this Recommendation is used to support the Network Service defined in Recommendation X.213 ISOAEC 8348, the
39、 physical, data link and packet layers correspond to the Physical, Data link and Network Layers respectively, as defined in Recommendation X.200; u that this Recommendation includes all the features necessary to support the services included in Recommendation X.213 I ISOAEC 8348 as well as other fea
40、tures; that Recommendation X.223 defines the use of X.25 packet layer protocol to provide the OS1 connection mode Network service; (k) that the necessary elements for an interface Recommendation should be defined independently as: - Physical layer: The mechanical, electrical, functional and procedur
41、al characteristics to activate, maintain and deactivate the physical link between the DTE and the DCE; - Data link layer: The link access procedure for data interchange across the link between the DTE and the DCE; - Packet layer: The packet format and control procedures for the exchange of packets c
42、ontaining control information and user data between the DTE and the DCE, Recommendation X.25 (10/96) 1 - STD=ITU-T RECMN X.25-ENGL 1996 48b2571 Ob280b9 073 unanimously declares that for public data networks accessed via dedicated circuits by data terminal equipments operating in the packet mode: (1)
43、 the mechanical, electrical, functional and procedural characteristics to activate, maintain and deactivate the physical link between the DTE and the DCE should be as specified in clause 1 below, DTE/DCE interface characteristics; (2) specified in clause 2 below, Link access procedure across the DTE
44、/DCE integace; the link access procedure for data interchange across the link between the DTE and the DCE should be as (3) should be as specified in clause 3 below, Description of thepacket layer DTE/DCE interface; the packet layer procedures for the exchange of control information and user data at
45、the DTEDCE interface (4) Procedures for virtual circuit services; the procedures for virtual call and permanent virtual circuit services should be as specified in clause 4 below, (5) Packet formals; the format for packets exchanged between the DTE and the DCE should be as specified in clause 5 below
46、, (6) user facilities; the procedures for optional user facilities should be as specified in clause 6 below, Procedures for optional (7) the formats for optional user facilities should be as specified in clause 7 below, Formats for facilityfields. NOTE - This Recommendation fully specifies the behav
47、iour of the DCE. In addition, a minimum set of requirements is specified for the DTE. Additional guidance for the design of DES is available in IS0 standards: IS0 7776 (data link layer) and ISOAEC 8208 (packet layer). It is not required by this Recommendation that these ISOIIEC standards be used. If
48、 using these IS0 standards, note must be taken that their scope is expanded beyond that of just interfacing with packet switched public data networks. It should also be noted that this Recommendation uses the term DTE to refer to the equipment to which the DCE interfaces. In ISOIIEC 8208, distinctio
49、n is made between a DTE and a packet switched private data network, which are both considered as DES in this Recommendation. Finally, the procedures in this Recommendation may be selected for use for cases other than packet-mode operation when accessing a hblic Data Network by dedicated circuit. In such cases, it may not be possible nor necessary to apply the capabilities in this Recommendation exactly as defined herein. For example, the procedures for layer 2 addressing in clause 2 or for optional user facilities in clause 6 may need to be modified for the s
