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2、ecommendationG0G08 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation X.81 was published in Fascicle VIII.3 of the Blue Book. This file is an extract from theBlue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file
3、 are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No
4、part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle VIII.3 - Rec. X.81 1Recommendation X.81INTERWORKING BETWEEN AN ISDN CIRCUIT -SWITCHED AND A CIRCUIT-S
5、WITCHEDPUBLIC DATA NETWORK (CSPDN)(Melbourne, 1988)The CCITT,considering(a) that I-Series Recommendations describe the integrated services digital network (ISDN);(b) that interface characteristics, line multiplexing and inter-exchange signalling for use in CSPDNs aredescribed by Recommendations such
6、 as X.26/X.27/X.50/X.51/Q.761 to Q.766/X.60/X.61/X.71/X.80;(c) that Recommendation X.200 describes the reference model for Open System Interconnection;(d) that Recommendation X.213 describes the Network Service Definition for Open Systems Interconnectionfor CCITT applications;(e) that Recommendation
7、 X.300 defines the general principles for interworking between public networks, andbetween public networks and other networks for the provision of data transmission services;(f) that Recommendation X.301 defines the general arrangements for call control within a subnetwork andbetween subnetworks for
8、 the provision of data transmission services;(g) that Recommendation X.302 defines the general arrangements for internal network utilities within asubnetwork and between subnetworks for the provision of data transmission services;(h) that Recommendation X.305 describes functionalities of subnetworks
9、 relating to the support of the OSINetwork Service;(i) that Recommendation X.10 describes categories of access to PSPDNs and ISDNs for the provision of datatransmission services;(j) that the I.230 Series Recommendations describe the bearer services supported by an ISDN;(k) that Recommendation X.30 d
10、escribes the support of X.21, X.21 bis and X.20 bis based DTEs on theISDN;(l) the need for arrangements when interworking between ISDN circuit switched and CSPDNs for theprovision of data transmission services.unanimously declares the viewthat the scope of this Recommendation is intended to cover in
11、terworking between an ISDN using circuit-switched connection types and a CSPDN.CONTENTS0 Introduction1 Scope and field of application2 References3 Definitions4 Abbreviations2 Fascicle VIII.3 - Rec. X.815 General aspects5.1 CSPDN5.2 ISDN (CS)5.3 Call control between the CSPDN and ISDN6 Specification
12、of interworking functions6.1 Interworking functions for identical data transmission services6.1.1 Physical link characteristics and interworking functions assigned to layer 16.1.2 Support of the OSI-NLS6.1.3 Signalling conversion (protocol mapping)6.1.4 Protocol mapping for supplementary services6.1
13、.5 Mapping of service signals and causes6.2 Interworking functions for non-identical data transmission services6.2.1 Physical link characteristics and interworking functions assigned to layer 16.2.2 Support of the OSI-NLS6.2.3 Signalling conversion (protocol mapping)6.2.4 Protocol mapping for supple
14、mentary services6.2.5 Mapping of service signals and causes7 Operation and maintenance0 IntroductionThis Recommendation is one of a set of Recommendations produced to facilitate considerations of interworkingbetween networks. It is based on Recommendation X.300, which defines the general principles
15、for interworking betweenpublic networks, and between public networks and other networks for the provision of data transmission services.Recommendation X.300 indicates in particular how collections of physical equipment can be represented as“Subnetworks“ for consideration in interworking situations.T
16、his Recommendation describes the interworking arrangements between ISDNs (circuit switched bearer) andCSPDNs for the provision of data transmission services.1 Scope and field of applicationThe purpose of this Recommendation is to describe the detailed arrangements for the interworking betweenCSPDNs
17、and ISDNs (CS) for the provision of data transmission services. These arrangements are applicable only to theinterworking involving transmission capabilities, and not to interworking involving communication capabilities asdescribed in Recommendation X.300.a) within the scope of interworking between
18、an ISDN using circuit-switched connection types and a CSPDNthe following cases of network interworking can be identified:i) where the terminals connected to the interworking networks use identical data transmission servicesand have identical high layer capabilities. The identity of data transmission
19、 services in both networksassumes that both terminals involved in a communication belong to the same user class of service;ii) where the terminals connected to the interworking networks use non-identical data transmissionservices but have identical high layer capabilities. In this case two terminals
20、 involved in acommunication may belong to different user classes of service, e.g. user classes of service 4 and 30.b) Not within the scope of this Recommendation is asynchronous mode of operation at the network-to-network interface (ISDN-to-CSPDN interface). In the case, that the ISDN supports the c
21、onnection ofasynchronous mode terminals via an appropriate terminal adaptor (TA) (see Recommendation X.30) andthat interworking has to be provided to asynchronous terminals connected to a CSPDN, then theinterworking arrangements as for synchronous mode operation will be used. Asynchronous to synchro
22、nousconversion may be provided within the TA for ISDN-connected terminals and at the terminal or at theCSPDN for CSPDN-connected terminals.Note - The typing of subnetworks in this Recommendation is based on the support for the OSI connection-mode Network Service and is therefore only valid in this c
23、ontext.Fascicle VIII.3 - Rec. X.81 32 References1 Recommendation X.12 Recommendation X.23 Recommendation X.104 Recommendation X.205 Recommendation X.216 Recommendation X.21 bis7 Recommendation X.258 Recommendation X.279 Recommendation X.3010 Recommendation X.5011 Recommendation X.5112 Recommendation
24、 X.6013 Recommendation X.6114 Recommendation X.7115 Recommendation X.30016 Recommendation X.32117 I.230 and I.250 Series Recommendations18 Recommendation G.70319 Recommendation G.70820 Recommendation G.81121 Recommendation Q.761-Q.7663 DefinitionsThis Recommendation makes use of the following terms
25、defined in the Recommendation as indicated:Terms Defined in RecommendationBearer service I.230 SeriesData transmission service X.10; X.300OSI network layer service X.213User class of service X.1Supplementary services I.250 Series; X.24 AbbreviationsDTE Data terminal equipmentDCE Data circuit-termina
26、ting equipmentSS No. 7 Common channel signalling system number 7CSPDN Circuit switched public data networkISDN Integrated services digital networkIWF Interworking functionsLAPB Link access protocol, balancedOSI-NLS Open system interconnection - Network layer serviceHDLC High level data link controlT
27、A Terminal adaptorTE Terminal equipmentUC User class of service4 Fascicle VIII.3 - Rec. X.81Abbreviations of SS No. 7 messages and X.71 signalsSS No. 7 messages:ACM Address completeANS AnswerIAM Initial addressINF InformationINR information messageREL ReleaseRLC Release completeRLSD ReleasedX.71 sig
28、nals:CC Call connectedCCF Call confirmationCDI Called line identificationCLI Calling line identificationCLEAR Clearing signalCLEAR C. Clear confirmationSEL Selection signalsTTC Transit through connectAdditional information contained in SS No. 7 messages and X.71 signalsCDI Called line identification
29、CDIR Called line identification requestCLI Calling line identificationCLIR Calling line identification request5 General aspectsThis Recommendation, in describing interworking arrangements between two subnetworks for the provision ofdata transmission services, adheres to the general principles of Rec
30、ommendation X.300. The environments of these twosubnetworks are described in the following sections.5.1 CSPDNThe CSPDN provides circuit switched data transmission services as defined in Recommendations X.1 and X.2for the provision of data transmission services, the CSPDN may be accessed by DTEs by t
31、he categories of access B asdefined in Recommendation X.10. Other possibilities of access, which are not relevant to this Recommendation aredescribed in Recommendation X.321.5.2 ISDNThe ISDN provides circuit switched data transmission services/bearer services/supplementary services as definedin Reco
32、mmendation X.1, the I.230 Series and I.250 Series. For the provision of data transmission services the ISDN maybe accessed by TE2s by the categories of access S as defined in Recommendation X.10 and by TEs as defined in theI.230 Series. (circuit-mode 64 kbit/s unrestricted, unstructured). Other poss
33、ibilities of access which are not relevant tothis Recommendation are described in Recommendation X.321.5.3 Call control between the CSPDN and ISDNThe general arrangements for call control between the CSPDN and ISDN circuit switched are as defined inRecommendation X.301. Network utilities used betwee
34、n the CSPDN and ISDN circuit switched are as defined inRecommendation X.302 (not visible for users).Fascicle VIII.3 - Rec. X.81 56 Specification of interworking functionsThe interworking functions specified hereafter have been grouped in accordance with their assignmentto OSI-model-layers 1 to 3.6.1
35、 Interworking functions for identical data transmission servicesReference configuration of the interworking between a circuit switched ISDN and a CSPDN using identicalbearer services.As can be seen from Figure 1/X.81, the following terminal to terminal interworking cases may arise for whichnetwork i
36、nterworking capabilities are necessary for user classes 3-7/categories S and B of Recommendation X.10.ISDN CSPDN(Categories S1-S5) (Categories B1-B5)TE2 + TA X.21 DTE X.21TE2 + TA X.21 DTE X.21 bisTE2 + TA X.21 bis DTE X.21TE2 + TA X.21 bis DTE X.21 bisTE1 (see Note in Figure 1/X.81)TE2 and DTE invo
37、lved in an end-to-end communication must be of the same user class. In this case a CSPDNGateway must also support the same user class of service.6.1.1 Physical link characteristics and interworking functions assigned to layer 16.1.1.1 Location of the interworking functionsThe interworking functions
38、may be provided by the ISDN, or by the CSPDN or separately between theinterworking networks.The interworking links may be provided as multiplexed lines offering a multiplicity of channels at the conversionpoint, or as individual channels. Consequently, the conversion facility has to provide layer 1
39、multiplexing/demultiplexingfunctions in the case where multiplexed lines are connected to it. Multiplexing may take place only on the ISDN side oronly on the CSPDN side or on both sides of the location, where the interworking functions are provided. The question asto whether the interconnection line
40、s will be multiplexed or not will depend on the location of the IWF, i.e. on whetherthe conversion facilities are installed at the data switching exchange or at the ISDN exchange. In either case the locationof the IWF should be considered from an economic viewpoint regardless of6 Fascicle VIII.3 - R
41、ec. X.81whether the conversion facilities are designed as separate hardware and software-based modules or as integrated parts ofeither an ISDN exchange or a data switching exchange (see also Figure 2/X.81).Depending on the location of the IWF (see Figure 2/X.81), the electrical characteristics on on
42、e or both sides ofthe IWF shall comply with the current Recommendations on interface characteristics as follows:a) Recommendation G.703/G.708 when offering the 2048/1544 kbit/s interface for 32/24 x 64 kbit/s channelson the ISDN side;b) Recommendation G.703/X.27 when offering the 64 kbit/s interface
43、 for data channels multiplexed inaccordance with the X.50 or the X.51 or the X.51 multiplex scheme on the CSPDN side;c) any interface recommendation applicable to individual data channels offered on the CSPDN side at speedsspecified for user classes of service 3-7 and their associated 6 + 2 and 8 +
44、2 envelope bit rates.Fascicle VIII.3 - Rec. X.81 76.1.1.2 Timing requirementsSince interworking takes place between two networks, the ISDN and the CSPDN, phase and/or clock adjustmentmust take place. The timing requirements of the IWF are the same, as specified in Recommendation G.811 for theinterco
45、nnection of digital link.6.1.1.3 Bit rate adaptionSince switching in the ISDN is provided only for 64 kbit/s channels and no standards are currently available forswitching of subchannels, the bit rates of user classes of service 3-7 have to be adapted to 64 kbit/s.The bit rate adaption mechanism sha
46、ll be in accordance with Recommendation X.30, 2.1.1 and 2.2.1. On theCSPDN side of the IWF there will be the need to allocate the information which is contained within 40 bit framesreceived from the ISDN on a 64 kbit/s bearer into either 6 + 2 or 8 + 2 envelopes.The same applies to an incoming envel
47、ope stream received from a data network. The bit positions within theenvelopes have to be allocated to the relevant positions in an outgoing 40 bit frame to be sent to the ISDN.Concerning the layer 1 relay functions of an IWF for rate adaption, the following two configurations, presentedby Figure 3/
48、X.81, have been identified.The bit rate adaption for both directions is restricted to the transfer of information and status. Framing bits,housekeeping bits and any other bits will not be transferred by the IWF. Mapping of housekeeping functions is a subjectof further study.(1) Interworking with an
49、X.50-structured CSPDN(8 bit envelope structure)For interworking with a CSPDN utilizing the 8 bit envelope structure as identified inRecommendation X.50, 1.1 a). This implies the suppression of each fourth status bit on transition fromCSPDN to ISDN and replication of each third status bit on transition from ISDN to CSPDN is necessary.(2) Interworking with an X.51-structured CSPDN(10 bit envelope structure)For interworking with a CSPDN utilising the 10 bit envelope structure as identified inRecommendation X.51, the data and status bits are
