1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T X.42TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (10/2003) SERIES X: DATA NETWORKS AND OPEN SYSTEM COMMUNICATIONS Public data networks Interfaces Procedures and methods for accessing a public data network from a DTE operating under control of a general
2、ized polling protocol ITU-T Recommendation X.42 ITU-T X-SERIES RECOMMENDATIONS DATA NETWORKS AND OPEN SYSTEM COMMUNICATIONS PUBLIC DATA NETWORKS Services and facilities X.1X.19 Interfaces X.20X.49 Transmission, signalling and switching X.50X.89 Network aspects X.90X.149 Maintenance X.150X.179 Admini
3、strative arrangements X.180X.199 OPEN SYSTEMS INTERCONNECTION Model and notation X.200X.209 Service definitions X.210X.219 Connection-mode protocol specifications X.220X.229 Connectionless-mode protocol specifications X.230X.239 PICS proformas X.240X.259 Protocol Identification X.260X.269 Security P
4、rotocols X.270X.279 Layer Managed Objects X.280X.289 Conformance testing X.290X.299 INTERWORKING BETWEEN NETWORKS General X.300X.349 Satellite data transmission systems X.350X.369 IP-based networks X.370X.399 MESSAGE HANDLING SYSTEMS X.400X.499 DIRECTORY X.500X.599 OSI NETWORKING AND SYSTEM ASPECTS
5、Networking X.600X.629 Efficiency X.630X.639 Quality of service X.640X.649 Naming, Addressing and Registration X.650X.679 Abstract Syntax Notation One (ASN.1) X.680X.699 OSI MANAGEMENT Systems Management framework and architecture X.700X.709 Management Communication Service and Protocol X.710X.719 St
6、ructure of Management Information X.720X.729 Management functions and ODMA functions X.730X.799 SECURITY X.800X.849 OSI APPLICATIONS Commitment, Concurrency and Recovery X.850X.859 Transaction processing X.860X.879 Remote operations X.880X.899 OPEN DISTRIBUTED PROCESSING X.900X.999 For further detai
7、ls, please refer to the list of ITU-T Recommendations. ITU-T Rec. X.42 (10/2003) i ITU-T Recommendation X.42 Procedures and methods for accessing a public data network from a DTE operating under control of a generalized polling protocol Summary This Recommendation defines procedures and a framework
8、for providing access to a Public Data Network (PDN) by a DTE operating under the control of a generalized polling protocol, via the use of a Packet Assembly/Disassembly (PAD) device situated within (or associated with) the PDN. The PAD supporting the generalized polling protocol aspect is referred t
9、o as a “GPAD“. The generalized polling environment provides support for access, along with basic requirements necessary within the PAD to enable the selection and modification of the protocol Aspect. This Recommendation has been developed in realization that a well-established base of DTEs operating
10、 under control of a generalized polled protocol needs to access a PDN. Also, procedures are defined that allow Internet Protocol (IP) enabled devices to access their corresponding networks over the generalized polling protocol defined under ITU-T Rec. X.42. The revision is a result of comments recei
11、ved from implementers. Several clarifications are introduced that remove ambiguities in the areas of bit, field and frame sequencing; event conditions; and checksum algorithms. Source ITU-T Recommendation X.42 was approved by ITU-T Study Group 17 (2001-2004) under the ITU-T Recommendation A.8 proced
12、ure on 29 October 2003. ii ITU-T Rec. X.42 (10/2003) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for st
13、udying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study gr
14、oups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with
15、 ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provis
16、ions (to ensure e.g. interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such
17、 words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position
18、 concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected
19、 by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2004 All rights reserved. No part of this publication may be reproduced
20、, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. X.42 (10/2003) iii CONTENTS Page 1 Scope 1 2 References. 2 3 Definitions 3 4 Abbreviations 4 5 GPAD framework. 5 5.1 Assumptions . 6 6 General configuration . 8 6.1 Data stations . 8 6.2 Transmission considerations 8
21、6.3 Protocol sequence. 8 7 Elements of procedure 8 7.1 Definition of PDUs and PDU components. 8 7.2 Procedures 11 7.3 Full/Half-duplex considerations . 13 7.4 Timeout function considerations 13 7.5 Link end points association 14 7.6 Error detection and recovery 14 7.7 Addressing conventions 14 8 Str
22、ucture and encoding of Protocol Data Units (PDUs). 14 8.1 PDU types. 14 8.2 Data transparency . 17 9 Classes of procedures . 18 9.1 X.42 PAD configuration. 18 9.2 Normal Poll Mode class . 18 9.3 Selective Poll Mode class Description of procedures . 19 9.4 Poll priority mode class 20 9.5 Encapsulatio
23、n Mode Class Description and procedures . 20 10 Proxy operation. 21 Annex A Point to Multipoint (PMP) network. 21 Annex B Explanatory notes on the implementation of frame check sequences . 21 B.1 Longitudinal Redundancy Check (LRC) 21 B.2 Diagonal Redundancy Check (DRC) . 22 Annex C 22 Annex D Proxy
24、 Operation X.42 PAD with proxy terminal IP code 23 Appendix I 24 ITU-T Rec. X.42 (10/2003) 1 ITU-T Recommendation X.42 Procedures and methods for accessing a public data network from a DTE operating under control of a generalized polling protocol 1 Scope This Recommendation defines the procedures an
25、d methods for accessing a Public Data Network (PDN) from a DTE operating under the control of a generalized polling protocol via the use of a Packet Assembly/Disassembly (PAD) device situated within (or associated with) the PDN. This Recommendation is the first of a set of Recommendations produced t
26、o facilitate asynchronous access to a PDN via Point-to-Multipoint (PMP) circuits. This Recommendation addresses the most important aspect of communications between the PAD and a subnetwork of terminals connected via PMP circuits the multidrop data link protocol providing medium access arbitration, d
27、ata link addressing and data protection for transmission over a PMP circuit. For those terminal devices and PMP environments, which require expanded networking capabilities and which, in addition to PMP medium arbitration, transmission error detection, and station addressing, require transparent tra
28、nsmission of network layer PDUs within user data of X.42 information frames, an encapsulation class of procedures is defined. In the case of IP-based networks, an X.42 PAD is therefore considered as a network edge device, which provides connectivity between PMP-based, IP-enabled terminals and IP-bas
29、ed devices on the opposite side of the network. There exist large networks of terminals connected to PDNs, characterized by low data throughput requirements both in terms of channel bandwidth and packet per second rate. Such a traffic profile mandates the need for a reduction of cost sensitive netwo
30、rk architecture components, which traditionally grow as a function of the number of remote terminals. In a practical sense, this amounts to a requirement for various forms of concentration and clustering of switching equipment, tail circuits, virtual communications channels and trunks, such that the
31、 total number does not grow in direct proportion to the number of end users terminals. Major savings in cost and resource utilization can be achieved when effective sharing of data communications channels is applied. The majority of the networks which utilize PMP technology are implemented using ine
32、xpensive analogue circuits and asynchronous data transmission. This configuration is prevalent in many transaction processing applications with request/response type traffic, consisting of a small number of short bidirectional messages. To be economically viable, applications running over PMP circui
33、ts typically require very low unit cost data communications. Such low costs can only be achieved through major economies of scale leading to a lower telecommunications resource usage. These PDN resources, in the case of those complying with ITU-T Rec. X.25, are: multidrop ports; Telecommunications m
34、odems; X.25 DTEs, with an efficient concentration ratio of PAD X.25 DTEs to the total number of terminals; X.25 SVCs, with an efficient concentration ratio of X.25 SVCs to the total number of terminals; Call Establishment Requests, with a low ratio of total number of call requests to the total numbe
35、r of terminals; and Non-informational traffic, as the request/response characteristics of the traffic inherently induces piggy-back acknowledgments. 2 ITU-T Rec. X.42 (10/2003) This Recommendation directly addresses these needs by providing simple, low complexity and bandwidth-efficient network reso
36、urce management. Introduction of this protocol aspect enables a low cost offering, and opens PDN transport to a broad existing terminal base. See Figure 1. X.042_F01Public Data NetworkPADThe depiction of the PAD as outside of thenetwork is for diagrammatic purposes. Asoutlined in the X-series Recomm
37、endations, thePAD may lie outside or inside dependingupon network characteristics.Overview and presentation ofgeneralized polling protocol PADaspect (“GPAD“)Remote X.25DTEX.42PAD aspectAsingleVCsupportsbothuserdatatomultipleterminalsandcontrolforasingleX.42aspectD1D3D2D4X.25D5D7D6D1D3D2D4D5D7D6Figur
38、e 1/X.42 GPAD environment 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other referen
39、ces are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to
40、 a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation. ITU-T Recommendation X.3 (2000), Packet Assembly/Disassembly facility (PAD) in a public data network. ITU-T Recommendation X.5 (1996), Facsimile Packet Assembly/Disassembly facility (F
41、PAD) in a public data network. ITU-T Recommendation X.8 (1994), Multi-aspect PAD (MAP) framework and service definition. ITU-T Recommendation X.25 (1996), Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and conne
42、cted to public data networks by dedicated circuit. ITU-T Recommendation X.28 (1997), DTE/DCE interface for a start-stop mode Data Terminal Equipment accessing the Packet Assembly/Disassembly facility (PAD) in a public data network situated in the same country. ITU-T Rec. X.42 (10/2003) 3 ITU-T Recom
43、mendation X.29 (1997), Procedures for the exchange of control information and user data between a Packet Assembly/Disassembly (PAD) facility and a packet mode DTE or another PAD. ITU-T Recommendation X.38 (1996), G3 facsimile equipment/DCE interface for G3 facsimile equipment accessing the Facsimile
44、 Packet Assembly/Disassembly facility (FPAD) in a public data network situated in the same country. ITU-T Recommendation X.39 (1996), Procedures for the exchange of control information and user data between a Facsimile Packet Assembly/Disassembly (FPAD) facility and a packet mode Data Terminal Equip
45、ment (DTE) or another FPAD. ITU-T Recommendation X.200 (1994) | ISO/IEC 7498-1:1994, Information technology Open Systems Interconnection Basic Reference Model: The basic model. ITU-T Recommendation X.213 (2001) | ISO/IEC 8348:2002, Information technology Open Systems Interconnection Network service
46、definition. ITU-T Recommendation X.263 (1998) | ISO/IEC TR 9577:1999, Information technology Protocol identification in the Network Layer. ITU-T Recommendation X.340 (1993), General arrangements for interworking between a Packet-Switched Public Data Network (PSPDN) and the international telex networ
47、k. 3 Definitions This Recommendation defines the following terms: 3.1 access information path: The communication link between a DTE and a GPAD. 3.2 access interface: A physical interface where a DTE is connected to a GPAD to use the functions it provides. 3.3 binary numbers: Base 2 numbers, represen
48、ted by the digits 0 and 1 followed by the letter “b“. 3.4 broadcasting: Global addressing or delivery of messages via the outbound information path, utilizing an address unique from all other addresses assigned to terminals on a particular GPAD port such that it is recognized by all terminals as its
49、 own. Broadcasting is always Unsolicited. Broadcasting does not generate any inbound acknowledgment traffic. 3.5 byte complement: Defined by complementing all eight bits of a byte. 3.6 drop: A station/terminal device connected to a PMP circuit operating in secondary mode. 3.7 generalized polling protocol packet assembly/disassembly facility: A facility that provides an access to a Public Data Network for a DTE which operates under control of a generalized polling protocol. 3.8 hexadecimal numbe
