1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T Y.1251TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (08/2002) SERIES Y: GLOBAL INFORMATION INFRASTRUCTURE AND INTERNET PROTOCOL ASPECTS Internet protocol aspects Architecture, access, network capabilities and resource management General architectural mo
2、del for interworking ITU-T Recommendation Y.1251 ITU-T Y-SERIES RECOMMENDATIONS GLOBAL INFORMATION INFRASTRUCTURE AND INTERNET PROTOCOL ASPECTS GLOBAL INFORMATION INFRASTRUCTURE General Y.100Y.199 Services, applications and middleware Y.200Y.299 Network aspects Y.300Y.399 Interfaces and protocols Y.
3、400Y.499 Numbering, addressing and naming Y.500Y.599 Operation, administration and maintenance Y.600Y.699 Security Y.700Y.799 Performances Y.800Y.899 INTERNET PROTOCOL ASPECTS General Y.1000Y.1099 Services and applications Y.1100Y.1199 Architecture, access, network capabilities and resource manageme
4、nt Y.1200Y.1299 Transport Y.1300Y.1399 Interworking Y.1400Y.1499 Quality of service and network performance Y.1500Y.1599 Signalling Y.1600Y.1699 Operation, administration and maintenance Y.1700Y.1799 Charging Y.1800Y.1899 For further details, please refer to the list of ITU-T Recommendations. ITU-T
5、Rec. Y.1251 (08/2002) i ITU-T Recommendation Y.1251 General architectural model for interworking Summary This Recommendation specifies a general architectural model for interworking that can be used to analyze, categorize and describe cases of interworking between networks. Two interworking methods
6、are identified, namely Service Interworking and Network Interworking. Source ITU-T Recommendation Y.1251 was prepared by ITU-T Study Group 13 (2001-2004) and approved under the WTSA Resolution 1 procedure on 13 August 2002. Keywords Interworking, Network Interworking, Service Interworking. ii ITU-T
7、Rec. Y.1251 (08/2002) 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 studying technical, operating and
8、 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 groups which, in turn, produce Re
9、commendations 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 ISO and IEC. NOTE In this Reco
10、mmendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the us
11、e of a claimed Intellectual Property Right. ITU takes no position 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
12、, ITU had not received notice of intellectual property, protected 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 2002 A
13、ll rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. Y.1251 (08/2002) iii CONTENTS Page 1 Scope 1 2 References. 1 3 Definitions 1 4 Abbreviations 2 5 Interworking General considerations. 2 5.1 The problem
14、space 2 5.2 Possible solutions . 2 6 Protocol adjacency relationships 4 6.1 Layering principle 4 6.2 Peering principle. 5 7 Generic interworking model. 5 8 Application of the model to service interworking and network interworking 6 8.1 Service interworking 6 8.2 Network interworking 7 9 Related prin
15、ciples . 9 ITU-T Rec. Y.1251 (08/2002) 1 ITU-T Recommendation Y.1251 General architectural model for interworking 1 Scope This Recommendation specifies a generic interworking model that can be used to analyze, categorize and describe cases of interworking between networks. 2 References The following
16、 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 references are subject to revision; all users of this Recom
17、mendation 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 a document within this Recommendation does not
18、give it, as a stand-alone document, the status of a Recommendation. 1 ITU-T Recommendation I.510 (1993), Definitions and general principles for ISDN interworking. 2 ITU-T Recommendation G.805 (2000), Generic functional architecture of transport networks. 3 ITU-T Recommendation Y.1001 (2000), IP fram
19、ework A framework for convergence of telecommunications network and IP network technologies. 4 ITU-T Recommendation Y.1401 (2000), General requirements for interworking with Internet Protocol (IP)-based networks. 3 Definitions For the purposes of this Recommendation the following definitions are emp
20、loyed. Consistency with definitions in other Recommendations has been taken into account and is noted where appropriate. 3.1 interworking: The term interworking is used to express interactions between networks, between end systems, or between parts thereof, with the aim of providing a functional ent
21、ity capable of supporting an end-to-end communication. The interactions required to provide a functional entity rely on functions and on the means to select these functions. NOTE This definition is identical to that given in ITU-T Rec. I.510, see 1. 3.2 service interworking: In service interworking,
22、 the Interworking Function (IWF) of Figure 4 terminates the protocol used in network 1 and translates (i.e., mapping) the Protocol Control Information (PCI) to the PCI of the protocol used in network 2 for User, Control and Management Plane functions to the extent possible. In general, since not all
23、 functions may be supported in one or other of the networks, the translation of PCI may be partial or non-existent. However, this should not result in any loss of user data since the payload is not affected by PCI conversion at the service interworking IWF. NOTE This definition is identical to that
24、given in ITU-T Rec. Y.1401, see 4. 3.3 network interworking: In network interworking, the PCI of the protocol used in network 1 and network 2 and the payload information are transferred transparently by an IWF of Figure 4. Typically the IWF encapsulates (known as tunnelling in some specifications) t
25、he information which is transmitted by means of an adaptation function and transfers it transparently to the other network. NOTE This definition is identical to that given in ITU-T Rec. Y.1401, see 4. 2 ITU-T Rec. Y.1251 (08/2002) 3.4 adaptation function: A processing function which adapts the clien
26、t layer network characteristic information into a form suitable for transport over a trail1in the server layer network. NOTE This definition is taken from ITU-T Rec. G.805, see 2. 4 Abbreviations This Recommendation uses the following abbreviations: IP Internet Protocol IWF Interworking Function IWU
27、 Interworking Unit PCI Protocol Control Information 5 Interworking General considerations 5.1 The problem space The objective of interworking is to provide end-to-end connectivity across two separate networks, N1 and N2, which cannot be directly interconnected due to the deployment of different prot
28、ocols in each of these networks. Simply, the problem to be solved is illustrated in Figure 1. Y.1251_F01Network 1 Network 2?N1 stack not equal N2 stackFigure 1/Y.1251 The interworking problem The lack of direct interconnectivity will arise when the protocol stacks in Network 1 (N1) are different fro
29、m those in Network 2 (N2), and thus where one, or more protocol discontinuity will occur in an interworking path created to span both N1 and N2. This discontinuity may occur within a single given layer, some set of given layers, or within all layers between the respective networks. 5.2 Possible solu
30、tions Two solutions are possible to achieve end-to-end connectivity. One is to effect a semantically equivalent translation between Network 1 (N1) and Network 2 (N2). The semantic equivalence can be achieved by mapping equivalent service characteristics from N1 to N2. The basic concept is shown in F
31、igure 2, below. _ 1See ITU-T Rec. G.805, for further definitions of trail, etc. In this Recommendation, the use of the term “layer“ or “protocol layer“ is synonymous with the use of the term “layer network“ in ITU-T Rec. G.805. ITU-T Rec. Y.1251 (08/2002) 3 Y.1251_F02Network 1 Network 2Mapping ofser
32、vice characteristicsFigure 2/Y.1251 Interworking by service mapping/translation between N1 and N2 This solution is only possible if there is sufficient similarity in service characteristics between N1 and N2. In this solution, there is a distinct discontinuity between the protocol layer(s) of N1 and
33、 N2. Thus, there is a protocol discontinuity between the user of network N1 and users of network N2. Some services of N1 may not be available to users of N2 and vice versa, due to lack of exact equivalence between all the services of N1 and all the services of N2. This solution is known as Service I
34、nterworking. The other way to achieve end-to-end connectivity is to superimpose one network on top of the other. This can be achieved by encapsulating, via an adaptation function, the relevant protocol(s), one or more, of one network on top of the protocols of the other. This is shown simplistically
35、 in Figure 3 for providing the users of N2 with access to N1. In this solution the users of N2 have to deal with the protocol layer(s) of both N2 and N1 (to some or all extent depending on the exact interworking arrangements), when intercommunication between users of N2 and N1 is required. Y.1251_F0
36、3Network 1Network 2Figure 3/Y.1251 Interworking by superimposition (Encapsulation) This second method encapsulates the protocol(s) of N1 within the uppermost protocol of N2. Users of N2, wishing to communicate with users of N1 must use the new stack comprising the N2 stack plus the additional elemen
37、ts of the N1 stack used for the interworking. Theoretically, this new stack may be regarded as a new logical network N3, since users of N2 still communicate amongst themselves using the original N2 stack. This solution is known as Network Interworking. It may also sometimes be called interworking by
38、 encapsulation. In this case there is no protocol discontinuity between users of N1 and users of N2 at the superimposed interworking layer(s). Equally possible, but not shown in the figure, protocols of N2 could be superimposed over N1. The functions for interworking between N1 and N2, for either se
39、rvice interworking or network interworking, are provided by an interworking function (IWF) considered to notionally exist between the N1 and N2, as shown in Figure 4. The exact physical location of the interworking unit (IWU) containing the IWF is an implementation issue, but could be contained with
40、in N1, N2, or as an independent unit. 4 ITU-T Rec. Y.1251 (08/2002) Y.1251_F04Protocol stack 1User planeControl planeManagement planeNetwork 1 Network 2IWF Protocol stack 2User planeControl planeManagement planeFigure 4/Y.1251 Interworking function 6 Protocol adjacency relationships The two methods
41、of interworking described in clause 5 result in the need to position protocols in two different ways. The service interworking method results in the establishment of a horizontal positioning relationship between the protocols of N1 and N2. The network interworking method results in a vertical positi
42、oning relationship between the protocols of N1 and N2. These basic positioning relationships are shown in Figure 5. Two basic principles are involved, namely a vertical layering principle and a horizontal peering principle. Note, these principles are identical to those described in ITU-T Rec. Y.1001
43、, see 3. Y.1251_F05Protocol AProtocol BProtocol XPeering relationshipLayering relationshipService mappingj greater equal 1), or at several layers simultaneously including or excluding the layer N itself. It should also be noted that the number of layer protocols in each stack may be different. In ge
44、neral, mappings between features may span several layers. It cannot be assumed that mappings performed purely at a peer level are sufficient, or that each peer layer is independent of the underlying layer(s). For example, a particular feature at layer N in N1 may be required to be mapped to non-peer
45、 layer in N2. These considerations may apply particularly to QoS related features and/or aspects. Printed in Switzerland Geneva, 2002 SERIES OF ITU-T RECOMMENDATIONS Series A Organization of the work of ITU-T Series B Means of expression: definitions, symbols, classification Series C General telecom
46、munication statistics Series D General tariff principles Series E Overall network operation, telephone service, service operation and human factors Series F Non-telephone telecommunication services Series G Transmission systems and media, digital systems and networks Series H Audiovisual and multime
47、dia systems Series I Integrated services digital network Series J Cable networks and transmission of television, sound programme and other multimedia signals Series K Protection against interference Series L Construction, installation and protection of cables and other elements of outside plant Seri
48、es M TMN and network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits Series N Maintenance: international sound programme and television transmission circuits Series O Specifications of measuring equipment Series P Telephone transmission
49、quality, telephone installations, local line networks Series Q Switching and signalling Series R Telegraph transmission Series S Telegraph services terminal equipment Series T Terminals for telematic services Series U Telegraph switching Series V Data communication over the telephone network Series X Data networks and open system communications Series Y Global information infrastructure and Internet protocol aspects Series Z Languages and general software aspects for telecommunication systems *22806*
