1、STD=ITU-T RECMN X-Li5-ENGL L77b 48b2571 b27777 OLi m INTERNATIONAL TELECOMMUNICATION UN ION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU x.45 (I 0/96) SERIES X: DATA NETWORKS AND OPEN SYSTEM COMM U N I CATI ON Public data networks - Interfaces Interface between Data Terminal Equipment (DTE)
2、 and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks, designed for efficiency at higher speeds ITU-T Recommendation X.45 (Previously CCITT Recommendation) STD-ITU-T RECMN XoqS-ENGL L77b VAb2571 Ob27780 AbT ITU-T X-SERIES RECOM
3、MENDATIONS DATA NETWORKS AND OPEN SYSTEM COMMUNICATION Transmission, signalling and switching X.90-X. 149 X. 150-X. 179 Administrative arrangements X. 180-X. 199 OPEN SYSTEM INTERCONNECTION X.200-X.299 Model and notation X.200-X.209 Service definitions X.210-X.219 Connection-mode protocol specificat
4、ions X.220-X.229 Connectionless-mode protocol specifications X.230-X.239 PICS profomas X.240-X.259 Protocol Identification X.260-X.269 Security Protocols X.270-X.279 Layer Managed Objects X.280-X.289 Conformance testing X.290-X.299 INTERWORKING BETWEEN NETWORKS x.300-x.399 General x.300-x.349 Satell
5、ite data transmission systems x.350-x.399 MESSAGE HANDLING SYSTEMS x.400-x.499 DIRECTORY x.500-x.599 OS1 NETWORKING AND SYSTEM ASPECTS X.600-X.699 Networking X.600-X.629 Efficiency X.630-X.649 Naming, Addressing and Registration X.650-X.679 Abstract Syntax Notation One (ASN. i) X.680-X.699 Systems M
6、anagement framework and architecture X.700-X.709 Management Communication Service and Protocol X.710-X.719 Stnicture of Management Information X.720-X.729 SECURITY X.800-X. 849 OS1 APPLICATIONS X.850-X.899 Commitment, Concurrency and Recovery X.850-X.859 Transaction processing X.860-X.879 Remote ope
7、rations X. 880-X. 899 OPEN DISTRIBUTED PROCESSING X.900-X.999 OS1 MANAGEMENT x.700-x.799 Management functions x.730-x.799 For fither details, please refer to ITU-T List of Recommendations. STD-ITU-T RECMN X-Li5-ENGL L99b = Li8b259L Ob29781 7Tb m FOREWORD The ITLJ-T (Telecommunication Standardization
8、 Sector) is a permanent organ of the International Telecommunication 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 St
9、andardization Conference (WTSC), which meets every four years, establishes 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
10、 No. 1 (Helsinki, March 1-12, 1993). IT-T Recommendation X.45, was prepared by ITLJ-T Study Group 7 (1993-1996) and was approved by 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
11、telecommunication administration and a recognized operating agency. 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 from the ITU. Recom
12、mendation X.45 (10/96) 1 7 8 9 10 11 CONTENTS Scope References Terms and definitions . Abbreviations . Convention . Description of services . 6.2 Service parameters Architecture of the protocol . 7.1 Service provided by the underlying layer . 7.2 Link Flow Control 7.3 Link set-up 7.4 Link acknowledg
13、ement . 7.5 Multiplexing . 7.6 VC sequence and retransmission control 7.7 VC-flow control 7.8 VC segmentation and reassembly . 7.9 VC signalling Description of the DTE/DCE interface (physical layer) 6.1 General Coding of frames and packets 9.1 General 9.2 Description of fiames . 9.3 Description of p
14、ackets Meaning of the fiame and packet fields Coding ofthe Type fieid . Link and VC-level procedures . 9.4 9.5 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 10.13 10.14 10.15 Identification of the link connection . Generic status request procedure Future upgrades with upward compati
15、bility . Flow control at the link layer Link set-up and disconnection Procedure for positive and negative acknowledgements of PDU . Multiplexing / demultiplexing Numbering of packets at the VC layer Re-ordering of data packets Flow control at the VC layer . Segment definition Error correction . Data
16、 transfer . Interrupt procedure . Reset of a virtual circuit Signalling coding . 11.1 11.2 11.3 Addressing 11.4 Message functional definitions and contents General message format and information elements coding Handling of coding errors . Page 1 1 1 2 2 2 2 3 4 5 5 5 5 5 5 5 6 6 6 6 6 7 9 15 16 17 1
17、7 17 17 18 18 20 21 25 26 26 26 27 27 29 29 31 31 35 70 71 11 Recommendation X.45 (10196) 12 13 14 Page Procedures for permanent virtual circuit service 73 Signalling procedures for switched virtual circuits 73 13.1 Set-up and clearing procedures . 73 13.2 Procedure for restart . 75 13.3 Effects of
18、clearing and restart procedures on transfer of packets . 76 Procedures for user facilities 76 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10 14.11 14.12 14.13 14.14 14.15 14.16 14.17 Incoming calls barred . Outgoing calls barred Packet size negotiation facility . Assured / non-assured data trans
19、fer negotiation facility . Throughput negotiation facilities Closed user group related facilities . Fast select Reverse charging Reverse charging acceptance Local charging prevention Network User Identification (NUI) related facilities Transit network selection related facilities . Hunt group Call r
20、edirection and call deflection related facilities Called line address modified notification . Transit delay selection and indication . Alternative addressing facilities 76 76 76 77 77 78 81 82 82 82 83 84 84 85 87 88 88 Annex A -Additional information for interworking between X.25 and X.45 protocols
21、 90 A . 1 Throughput mapping 90 A.2 Data transfer option 90 A.3 Signalling coding mapping . 90 Appendix I . Subscription parameters 94 1.1 Subscription parameters per interface . 94 1.2 Subscription parameters per PVC . 95 . 111 Recommendation X.45 (1 0/96) - SUMMARY This Recommendation specifies th
22、e interface between packet mode DTEs and public data networks for physical links which have data rates higher than those of the scope of Recommendation X.25. Full interworking compatibility between X.25 DTEs and X.45 DTEs is provided. in particular credit based flow control is available for both dir
23、ections of each virtual circuit at each DTEDCE interface. For X.45 DTEs an unassured virtual circuit option is offered for virtual circuits where data losses are preferred to extra delays in case of transmission errors. Protocol of this Recommendation can be used on underlying layers which are funct
24、ionally equivalent to HDLC layer 2.a, in particular on ATM-VPs and VCs used with the CPCS of AAL 5. iv Recommendation X.45 (10/96) - - Recommendation X.45 INTERFACE BETWEEN DATA TERMINAL EQUIPMENT (DTE) FOR TERMINALS OPERATING IN THE PACKET MODE AND CONNECTED TO PUBLIC DATA NETWORKS, DESIGNED FOR EF
25、FICIENCY AT HIGHER SPEEDS AND DATA CIRCUIT-TERMINATING EQUIPMENT CE) (Geneva, 1996) 1 Scope The purpose of this Recommendation is to speciS. procedures to be used for supporting X.25 compatible services at higher access speeds than Recommendation X.25 itself. Also, in order to add a new flexibility
26、regarding the trade-off between error correction and variations of propagation delay, it includes a non-assured data transfer option. It should be noted that this Recommendation is not intended to be a replacement of Recommendation X.25. It is an alternative that is more suitable than Recommendation
27、 X.25 for higher speed operation in particular in the multi-Mbit/s range of access speeds. The use of the interface specified in this Recommendation is a network option; it may not be implemented in all networks. This Recommendation specifies a combination of protocol layers which still has high lev
28、el of efficiency when the product of bit rate times propagation delay reaches the equivalent of hundreds or even thousand of packets. This high level of efficiency is relative to the amount of overhead per data packet, the capacity to reach high throughputs in data octets and data packets per second
29、, the window sizes for reaching these throughputs, and effects of transmission errors on effective data rates and on end-to-end delays. In particular, the mode of operation specified in this Recommendation for transmission error conditions is such that a transmission error on a packet of one virtual
30、 circuit entails no retransmission and no processing delay for other virtual circuits. For this, the sharing out of functions between the link layer of the protocol (layer 2) and the virtual circuit layer (layer 3) is different from that of X.25. Although the X.45 protocol is different from the X.25
31、 protocol, the X.25 service is fully provided by this Recommenda- tion, ensuring that there is a full interoperability between an X.45 DTE and an X.25 DTE connected either to the same public data network or to different public data networks. An important feature of the X.45 protocol is that it is sy
32、mmetrical except for those signalling aspects which are inherently non-symmetrical (e.g. cause codes, addressing, etc.). Hence its applicability could be extended to multi-point links. 2 References The following Recommendations, and other references contain provisions which, through reference in thi
33、s text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision: all users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent
34、edition of the Recommendations and other references listed below. A list of the currently valid IT-T Recommendations is regularly published. - ITU-T Recommendation X.25 (1996), Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the
35、 packet mode and connected to public data networks by dedicated circuit. ITU-T Recommendation 4.931 (1993), ISDN user-network interface lqyer 3 specification for basic call control. - 3 Terms and definitions For the purpose of this Recommendation the following definitions apply: 3.1 frame: PDU meani
36、ngful for the entire link. Recommendation X.45 (10/96) 1 3.2 3.3 message: Signalling PDU. packet: PDU meaningful for only one virtual circuit and transmitted over that particular virtual circuit. 4 Abbreviations For the purposes of this Recommendation, the following abbreviations apply: AAL ATM CPCS
37、 DCE DTE HDLC LAN MAC PDU PVC Rsrvd SAP SDU svc vc VP ATM Adaptation Layer Asynchronous Transfer Mode Common Part Convergence Sublayer Data Circuit-terminating Equipment Data Terminal Equipment High level Data Link Control Local Area Network Media Access Control Protocol Data Unit Permanent Virtual
38、Circuit Reserved Service Access Point Service Data Unit Switched Virtual Circuit Virtual Circuit Virtual Path See also Tables 9-1,9-2 and 11-1. 5 Convention No special convention is defined for the purpose of this Recommendation. 6 Description of services 6.1 General Services offered by a network ac
39、ross an X.45 interface are the same as those across an X.25 interface. In addition, support of non-assured data transfer may be offered as an option on a per virtual circuit basis for calls where a bounded transit delay is more important than the absence of data loss. This may be the case for instan
40、ce where data are encapsulated connectionless packets which are subject to end-to-end error protection. Compatibility of services between Recommendation X.25 and this Recommendation implies that virtual circuits can be established between an X.45 interface and an X.25 interface or between two X.45 i
41、nterfaces with the same functional properties as between two X.25 interfaces. Annex A provides additional specifications to ensure exhaustively the interworking between Recommendation X.25 and this Recommendation. For communications between two X.45 interfaces belonging to different networks, servic
42、es will be restricted to those of Recommendation X.25 if Recommendation X.75 is used at a network-to-network interface. 2 Recommendation X.45 (10196) 6.2 Service parameters Service parameters recapitulated hereafter are related to link and VC-level procedures. Service parameters related to signallin
43、g procedures, i.e. user facilities, are described in clause 14. Appendix I provides a list of subscription parameters which includes service parameters and user facilities when applicable. 6.2.1 Access rate The access rate is the maximum transfer rate at which the DTE may sendreceive data tofrom the
44、 network. It is deter- mined by the speed of the underlying media that is selected from a set supported by the network at subscription time. NOTE - If the underlying media provides asymmetrical speeds, then two access rates are defined one for each direction of data transmission. 6.2.2 Maximum PDU s
45、ize For the whole DTE/DCE interface, and for each direction of data transmission, the maximum PDU size is determined by the receiver which signals this maximum PDU size to the sender during link Set-up. 6.2.3 Maximum packet size For each virtual circuit and for each direction of data transmission, t
46、he maximum packet size is determined at subscription time in case of a permanent virtual circuit or by the receiver which signals this maximum packet size to the sender during call Set-up in case of switched virtual circuit. 6.2.4 Tbroughput Throughput for a virtual circuit is the number of user dat
47、a transferred in one direction per unit time. Throughput for one direction of transmission is an inherent characteristic of the virtual circuit related to the amount of resources available to this virtual circuit. The optimal conditions to maximise the throughput include the following : 1) the under
48、lying media at the local and remote DTE/DCE interfaces do not constrain the throughput; 2) flow control procedures at the local and remote DTEDCE interfaces do not constrain the throughput; 3) the traffic characteristics of other virtual circuits do not constrain the throughput; 4) the receiving DTE
49、 is not flow controlling the DCE such that the throughput is not attainable. The throughput is expressed in bits per second. In case of permanent virtual circuit, the throughput is determined at subscription time for each direction of data transmission. In case of switched virtual circuit, the throughput is determined during call set-up for each direction of data transmission. 6.2.5 Assuredlnon-assured data transfer The data transfer option: assured or non-assured is selected for each virtual circuit, at subscription time in case of permanent virtual circuit or during call set-up in
