1、INTERNATIONAL TELECOMMUNICATION UNIONCCITT I.324THE INTERNATIONALTELEGRAPH AND TELEPHONECONSULTATIVE COMMITTEEINTEGRATED SERVICESDIGITAL NETWORK (ISDN)OVERALL NETWORK ASPECTSAND FUNCTIONS,ISDN USER-NETWORK INTERFACESISDN NETWORK ARCHITECTURERecommendation I.324Geneva, 1991FOREWORDThe CCITT (the Inte
2、rnational Telegraph and Telephone Consultative Committee) is a permanent organ of theInternational Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariffquestions and issuing Recommendations on them with a view to standardizing telecommunications on a worldw
3、idebasis.The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approvesRecommendations prepared by its Study Groups. The approval of Recommendations by the members of CCITT betweenPlenary Assemblies is covered by the procedure laid down in CCITT Resolution
4、No. 2 (Melbourne, 1988).Recommendation I.324 was prepared by Study Group XVIII and was approved under the Resolution No. 2procedure on the 25 of October 1991._CCITT NOTES1) In this Recommendation, the expression “Administration” is used for brevity to indicate both atelecommunication Administration
5、and a recognized private operating agency.2) A list of abbreviations used in this Recommendation can be found in Annex A. ITU 1991All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm,
6、 without permission in writing from the ITU.Recommendation I.324 1Recommendation I.324Recommendation I.324ISDN NETWORK ARCHITECTURE(Melbourne, 1988; amended at Geneva, 1991)1 General1.1 Basic philosophyThe objective of this Recommendation is to provide a common understanding of the CCITT studies on
7、thegeneral architecture of an ISDN from the functional point of view. The model is not intended to require or exclude anyspecific implementation of an ISDN, but only to provide a guide for the specification of ISDN capabilities.1.2 DefinitionsA number of terms used in this Recommendation are describ
8、ed in more detail in other Recommendations. Toassist the understanding of the reader, the following particular definitions apply in this Recommendation:1) reference configurations are conceptual configurations which are useful in identifying various possiblearrangements in an ISDN. The reference con
9、figurations are based on association rules of functionalgroupings and reference points. Detailed descriptions of reference configurations for ISDN connectiontypes are given in other I-Series Recommendations. For user-to-network access they are defined inRecommendation I.411 and for interworking betw
10、een networks they are defined in the I.500-Series ofRecommendations.2) functional groupings are sets of functions which may be needed in ISDN arrangements. The relationshipbetween generic functions and specific functions allocated to particular entities (or functional groupings)in the ISDN are expla
11、ined in Recommendation I.310.3) reference points are the conceptual points at the conjunction of two functional groupings. In a particularexample, a reference point may correspond to a physical interface between pieces of equipment, or inother examples, there may not be any physical interface corres
12、ponding to the reference point. Interfaceswill not be defined by CCITT for an ISDN unless the corresponding reference points have been alreadyspecified.2 General architecture of an ISDNIn practical ISDN implementations some of the ISDN functions will be implemented within the same networkelements, w
13、hereas other specific ISDN functions will be dedicated to specialized network elements. Various differentISDN implementations are likely to be realized depending on national conditions.A basic component of an ISDN is a network for circuit switching of end-to-end 64 kbit/s connections. Inaddition to
14、this connection type, depending on national conditions and evolution strategies, the ISDN will or will notsupport other connection types, such as packet mode connection types and n 64 kbit/s circuit mode connection types,and other broadband connection types.2 Recommendation I.3242.1 Basic architectu
15、ral modelA basic architectural model of an ISDN is shown in Figure 1/I.324. This shows the eight main switching andsignalling functional capabilities of an ISDN: ISDN local connection related functions (CRF), see 4.2.2.1; 64 kbit/s circuit switched functional entities; 64 kbit/s circuit non-switched
16、 functional entities. (The identification and definition for 8, 16, 32 kbit/sswitched or non-switched functional entities is left for further study.); packet switching functional entities; frame mode functional entities; common channel inter-exchange signalling functional entities, for example confo
17、rming to CCITTSignalling System No. 7; switched functional entities at rates greater than 64 kbit/s; non-switched functional entities at rates greater than 64 kbit/s.These components need not be provided by distinct networks, but may be combined as appropriate for aparticular implementation.High lay
18、er functions (HLF) which may be implemented within (or associated with) an ISDN may be accessedby means of any of the above-mentioned functional entities. Those functional entities could be implemented totallywithin an ISDN or be provided by dedicated networks or specialized service providers. Both
19、cases may provide the sameISDN teleservices (see Recommendation I.210) from the users point of view.Recommendation I.324 3T1812710-91TEa)b) b)d)c)a)b)c)d)FIGURE 1/I.324Low layer capabilities (BLLF and ALLF)64 kbit/s circuitnon-switchedcapabilities64 kbit/scircuit-switchedcapabilitiesPacket switching
20、capabilitiesFrame modecapabilitiesCommonchannel signallingcapabilitiesTE orserviceproviderUser-networksignallingTEBLLFALLFBHLFAHLFTerminal equipmentBasic low layer functionsAdditional low layer functionsBasic high layer functionsAdditional high layer functionsIn certain national situations, ALLF may
21、 also be implemented outside the ISDN, in special nodes or in certain categoriesof terminals.These functions may either be implemented within ISDN or be provided by separate networks. Possible applications forbasic high layer functions and for additional high layer functions are contained in Recomme
22、ndation I.210.For signalling between international ISDNs, CCITT Signalling System No. 7 shall be used.Basic architectural model of an ISDN 64 kbit/snon-switchedcapabilities 64 kbit/sswitchedcapabilitiesHigh layercapabilities(BHLF and AHLF) The ISDN local functional capabilities correspond to functio
23、ns provided by a local exchange and possibly including otherequipment, such as electronic cross connect equipment, muldexes, etc.User-to-user(user-to-network) signalling4 Recommendation I.3242.2 Architectural components of the ISDNRecommendation I.310 describes the functions of an ISDN. These functi
24、ons are by their nature static(i.e. time-independent). The relative distribution and allocation of these functions is the subject of the architecture of theISDN and is described in this Recommendation. The dynamic aspects of these functions are modelled inRecommendation I.310 as executive processes.
25、Therefore the key components in this architectural model are: what functions are contained in the ISDN, wherethey are located, and what is the relative topology for their distribution in the ISDN.3 Aspects of the architecture of the ISDNThe architecture of the ISDN includes low layer capabilities an
26、d high layer capabilities. These capabilitiessupport services both within the ISDN and via interworking (see 5) to other networks.3.1 Low layer capabilitiesFrom the main functional capabilities of the ISDN, as shown in Figure 1/I.324, four main functionalcapabilities require further description.3.1.
27、1 Circuit switching capabilitiesCircuit-switched connections with information transfer rates up to 64 kbit/s are carried by B-channels at theISDN user-network interfaces and switched at 64 kbit/s by the circuit-switching functional entities of the ISDN. Circuitswitching can also be applied to inform
28、ation transfer rates greater than 64 kbit/s.Signalling associated with circuit switched connections is carried by the D-channel at the ISDN user-networkinterface and processed by the local CRF (see 4.2.2.1). User-to-user signalling could be carried through the commonchannel signalling functional ent
29、ities (in the transit connection elements).User bit rates of less than 64 kbit/s are rate adapted to 64 kbit/s, as described in Recommendation I.460, beforeany switching can take place in the ISDN. Multiple information streams from a given user may be multiplexed togetherin the same B-channel, but f
30、or circuit switching an entire B-channel will be switched to a single user-network interface.This multiplexing should be in accordance with Recommendation I.460. Furthermore, circuit switched data services withbit rates less than 64 kbit/s (in accordance with Recommendation X.1 user classes of servi
31、ce) may be handled by adedicated circuit switched public data network to which the user gains access by means of an ISDN connection.Circuit switching capabilities are based on 64 kbit/s switching. Connection types at higher bit rates could alsobe provided on a semi-permanent basis. Switched connecti
32、ons at these bit rates could also be provided by broadbandswitching functional entities.3.1.2 Packet switching capabilitiesA number of packet mode bearer services are described in the I.230-Series of Recommendations. Differentnetwork solutions and corresponding architectures may be adopted in differ
33、ent countries to support these services.Recommendation I.324 5Recommendations I.310 (ISDN-Network functional principles), I.462 (Support of packet mode terminalequipment by an ISDN) and Q.513 (Exchange interfaces for operations, administration and maintenance) constitute thebasis for the description
34、 of packet switching functions in an ISDN.Two types of functional groupings are involved in the provision of packet switched bearer services by anISDN: packet handling functional groupings, which contain functions relating to the handling of packet callswithin the ISDN; interworking functional group
35、ings, which ensure interworking between ISDN and packet switched datanetworks.The solutions which could be used to access packet bearer services are: via the B-channel, with the following cases: circuit (switched or semi-permanent) access through ISDN to an interworking function within aPSPDN, circu
36、it (switched or semi-permanent) access associated with packet handling functions and/orinterworking functions in the ISDN, circuit (switched or semi-permanent) access associated with packet handling functions within theISDN; via the D-channel, with the following cases: packet handling functions and
37、interworking functions within the ISDN, packet handling functions in the ISDN (without interworking functions).Note This classification does not preclude a combination of the solutions described above.Depending on national considerations, the ISDN packet handling and interworking functions can becen
38、tralized or distributed. The following cases may be encountered: packet handling and interworking functions are not integrated in the local CRF (e.g., they are located in atransit CRF); packet handling functions are integrated in the local CRF; packet handling functions and/or interworking functions
39、 are integrated in the local CRF.3.1.3 Frame mode capabilitiesFrame mode bearer services are described in Recommendation I.233. They aim to support a wide range of dataapplications and rates from very low, to high (typically 2 Mbit/s). A typical application might be interconnectionbetween local area
40、 networks (LANs).Frame mode capabilities provide the order preserving bidirectional transfer of service data units (layer2 frames) from one S or T reference point to another. The data units are routed through the network on the basis of anattached label. This label is a logical identifier with local
41、 significance (termed DLCI in the protocol description).6 Recommendation I.324The user-network interface structure at an S or T reference point allows for the establishment of multiplevirtual calls and/or permanent virtual circuits to many destinations. This bearer capability is generally available
42、on thefollowing ISDN access arrangements: point-to-multipoint (passive bus) and point-to-point (NT2).All control-plane procedures (signalling) associated with frame mode bearer services, if needed, are performedin a logical separate manner using protocol procedures that are integrated across all ISD
43、N telecommunication services.The user-plane procedures at layer 1 are based on Recommendations I.430/I.431. Layer 2 procedures are basedon the core functions of Recommendations Q.922. These layer 2 core functions allow for the statistical multiplexing ofuser information flows immediately above layer
44、 1 functions.3.1.4 Other switching capabilitiesFor the support of broadband connections, other switching capabilities in addition to the above-mentionedswitching capabilities could be employed (see Recommendation I.327 for B-ISDN functional architecture).3.1.5 Transmission capabilitiesIn addition to
45、 the normal transmission capabilities of the IDN (integrated digital network), the followingtransmission aspects are important when considering the architecture of the ISDN. Services which do not requireunrestricted digital information transfer capability, such as telephony, may also employ non-tran
46、sparent channels (e.g.,packetized voice, digital speech interpolation). Channels at 8, 16, 32 kbit/s may be used in the transmission part of thenetwork; they may be used to support some services (e.g., voice-band connection types). They may also be used in caseswhere a B-channel (at the S or T refer
47、ence point) carries user data streams at bit rates lower than 8, 16, 32 kbit/srespectively.3.2 High layer capabilitiesNormally the high layer functions (HLFs) are involved only in the terminal equipment, but for the support ofsome services, provision of HLFs could be made via special nodes in the IS
48、DN belonging to the public network or tocentres operated by other organizations and accessed via ISDN user-network or inter-network interfaces. Some servicessuch as Message Handling service (MHS) may be used on a large scale and the relevant functional entities could beprovided within the ISDN excha
49、nges. For both cases the protocols used to activate such services should be identical andintegrated with the general user procedures defined for the activation of ISDN teleservices.4 Location of functions in the ISDN4.1 OverallIn considering an ISDN call (i.e. an instance of a telecommunication service) two major functional areas areinvolved:i) the customer equipment (TE and optional customer network);ii) the public ISDN connection type.Recommendation I.411 describes the functional groupings and reference configurations for the customernetwork
