1、INTERNATIONAL TELECOMMUNICATION UNIONCCITT I.321THE INTERNATIONALTELEGRAPH AND TELEPHONECONSULTATIVE COMMITTEEINTEGRATED SERVICESDIGITAL NETWORK (ISDN)OVERALL NETWORK ASPECTSAND FUNCTIONS,ISDN USER-NETWORK INTERFACESB-ISDN PROTOCOL REFERENCE MODELAND ITS APPLICATIONRecommendation I.321Geneva, 1991FO
2、REWORDThe CCITT (the International Telegraph and Telephone Consultative Committee) is the permanent organ ofthe International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariffquestions and issuing Recommendations on them with a view to standardizing tel
3、ecommunications on a worldwidebasis.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 lai
4、d down in CCITT Resolution No. 2 (Melbourne, 1988).Recommendation I.321 was prepared by Study Group XVIII and was approved under the Resolution No. 2procedure on the 5 of April 1991._CCITT NOTES1) In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelec
5、ommunication Administration 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
6、 photocopying and microfilm, without permission in writing from the ITU.Preamble to B-ISDN RecommendationsIn 1990, CCITT SG XVIII approved a first set of Recommendations on B-ISDN. These are:I.113 Vocabulary of terms for broadband aspects of ISDNI.121 Broadband aspects of ISDNI.150 B-ISDN asynchrono
7、us transfer mode functional characteristicsI.211 B-ISDN service aspectsI.311 B-ISDN general network aspectsI.321 B-ISDN Protocol Reference Model and its applicationI.327 B-ISDN functional architectureI.361 B-ISDN ATM layer specificationI.362 B-ISDN ATM Adaptation Layer (AAL) functional descriptionI.
8、363 B-ISDN ATM Adaptation Layer (AAL) specificationI.413 B-ISDN user-network interfaceI.432 B-ISDN user-network interface Physical Layer specificationI.610 Operation and maintenance principles of B-ISDN accessThese Recommendations address general B-ISDN aspects as well as specific service- and netwo
9、rk-orientedissues, the fundamental characteristics of the asynchronous transfer mode (ATM), a first set of relevant ATM orientedparameters and their application at the user-network interface as well as impact on operation and maintenance of the B-ISDN access. They are an integral part of the well es
10、tablished I-Series Recommendations. The set of Recommendationsare intended to serve as a consolidated basis for ongoing work relative to B-ISDN both within CCITT and in otherorganizations. They may also be used as a first basis towards the development of network elements.CCITT will continue to furth
11、er develop and complete these Recommendations in areas where there areunresolved issues and develop additional Recommendations on B-ISDN in the I-Series and other series in the future.Recommendation I.321 1Recommendation I.321Recommendation I.321B-ISDN PROTOCOL REFERENCE MODEL AND ITS APPLICATION1 I
12、ntroduction1.1 GeneralRecommendation I.320 contains the description of the ISDN protocol reference model (PRM). ThisRecommendation is based on the ISDN protocol reference model as defined in Recommendation I.320, and wherevernot explicitly indicated, it should conform with Recommendation I.320. The
13、purpose of this Recommendation is to takeinto account the functionalities of B-ISDN, which may eventually be incorporated into Recommendation I.320, as anexpansion of the existing ISDN protocol reference model. The Protocol Reference Model in this Recommendation iscalled “B-ISDN PRM” (protocol refer
14、ence model of the broadband aspects of ISDN). By applying this B-ISDN PRM, 4 describes functions associated with individual layers of B-ISDN.1.2 Relationship with the OSI reference model (X.200)The B-ISDN layered model reflects the principles of layered communication defined inRecommendation X.200 t
15、he reference model of open systems interconnection (OSI) for CCITT applications.OSI is a logical architecture and as such defines a set of principles including protocol layering, Layer servicedefinition, service primitives, modularity and independence. In general, these principles seem appropriate t
16、o thebroadband environment. However, the principle of layer independence has not been fully applied in this B-ISDN PRM.The OSI reference model has seven layers, each with specific functions and offering defined services to thelayer above and utilizing services of the layer below. This logical archit
17、ecture should be applicable to the broadbandnetwork and user applications.The exact relationship between the lower layers of the OSI model and the ATM Adaptation Layer (AAL), theATM Layer and the Physical Layer of the B-ISDN PRM are for further study.2 The B-ISDN protocol reference modelThe B-ISDN p
18、rotocol reference model is shown in Figure 1/I.321; it is composed of a user plane, a controlplane and a management plane.Above the Physical Layer, the ATM Layer provides call transfer for all services and the AAL providesservice-dependant functions to the layer above the AAL.The layer above the AAL
19、 in the control plane provides call control and connection control. The managementplane provides network supervision functions. Functional descriptions of the Physical Layer, the ATM Layer and theAAL are given in the following section. Further study is required on the functions of the layers above t
20、he AAL.2 Recommendation I.321T1810451-90FIGURE 1/I.321Management planeControl planeControl planeHigher layers Higher layersATM adaptation layerATM layerPhysical layerLayermanagementPlane managementB-ISDN protocol reference model3 Description of the planes3.1 User planeThe user plane, with its layere
21、d structure, provides for user information flow transfer, along with associatedcontrols (e.g. flow control, and recovery from errors, etc.).3.2 Control planeThis plane has a layered structure and performs the call control and connection control functions; it deals withthe signalling necessary to set
22、 up, supervise and release calls and connections.The distinction, if any, between local and global control plane functions in the broadband environment is forfurther study.3.3 Management planeThe management plane provides two types of functions, namely Layer Management and plane managementfunctions.
23、3.3.1 Plane management functionsThe plane management performs management functions related to a system as a whole and providescoordination between all the planes. Plane management has no layered structure.3.3.2 Layer Management functionsLayer Management performs management functions (e.g. meta-signa
24、lling) relating to resources and parametersresiding in its protocol entities. Layer Management handles the operation and maintenance (OAM) information flowsspecific to the layer concerned. Additional details are provided in Recommendation Q.940.Recommendation I.321 3Note A possible merger of plane m
25、anagement and Layer Management functions is for further study.4 Functions of the individual layers of the B-ISDN PRMThe functions of each layer, the primitives exchanged between layers, and primitives exchanged between thelayers and the management plane are described below. The information flows des
26、cribed do not imply a specific physicalrealization. Figure 2/I.321 illustrates the layers of the PRM, and identifies the functions of the Physical Layer, the ATMLayer, and the AAL.Higher layer functions Higher layersConvergence CSAALSegmentation and reassembly SARGeneric flow controlCell header gene
27、ration/extractionCell VPI/VCI translationCell multiplex and demultiplexATMCell rate decouplingHEC header sequence generation/verificationCell delineationTransmission frame adaptationTransmission frame generation/recoveryTCBit timingPhysical mediumPMCS Convergence sublayerPM Physical mediumSAR Segmen
28、tation and reassembly sublayerTC Transmission convergenceFIGURE 2/I.321Functions of the B-ISDN in relation to theprotocol reference model4 Recommendation I.3214.1 Terminology4.1.1 idle call (Physical Layer)Cell which is inserted/extracted by the Physical Layer in order to adapt the cell flow rate at
29、 the boundarybetween the ATM Layer and the Physical Layer to the available payload capacity of the transmission system used.4.1.2 valid cell (Physical Layer)Cell whose header has no errors or has been modified by the cell header error control (HEC) verificationprocess.4.1.3 invalid cell (Physical La
30、yer)Cell whose header has errors and has not been modified by the cell HEC verification process (discarded at thePhysical Layer).4.1.4 assigned cell (ATM Layer)Cell which provides a service to an application using the ATM Layer service.4.1.5 unassigned cell (ATM Layer)ATM Layer cell which is not an
31、assigned cell.4.2 Physical LayerThe Physical Layer consists of two sublayers. The physical medium (PM) sublayer includes only physicalmedium dependant functions. The transmission convergence (TC) sublayer performs all functions required to transforma flow of cells into a flow of data units (e.g. bit
32、s) which can be transmitted and received over a physical medium. Theservice data unit (SDU) crossing the boundary between the ATM Layer and the Physical Layer is a flow of valid cells.The ATM Layer is unique, i.e. independent of the underlying Physical Layer. The data flow inserted in the transmissi
33、onsystem payload is physical medium independent and self-supported; the Physical Layer merges the ATM cell flow withthe appropriate information for cell delineation, according to the cell delineation mechanism described inRecommendation I.432 and carries operations and maintenance (OAM) information
34、relating to this cell flow.4.2.1 Physical medium sublayer functionsThe physical medium sublayer provides bit transmission capability including bit transfer and bit alignment. Itincludes line coding and electrical-optical transformation.4.2.1.1 Physical mediumThe transmission functions are highly dep
35、endent on the medium used and are outside the scope of thisRecommendation.4.2.1.2 Bit timingThe principal function is the generation and reception of waveforms suitable for the medium, the insertion andextraction of bit timing information and line coding (if required). The primitives identified at t
36、he border between the PMand TC sublayers are a continuous flow of logical bits or symbols with this associated timing information.4.2.2 Transmission convergence sublayer functions4.2.2.1 Transmission frame generation and recoveryThis function performs the generation and recovery of transmission fram
37、e.Recommendation I.321 54.2.2.2 Transmission frame adaptationThis function performs the actions which are necessary to structure the cell flow according to the payloadstructure of the transmission frame (transmit direction) and to extract this cell flow out of the transmission frame(receive directio
38、n). The transmission frame may be a cell equivalent (i.e. no external envelope is added to the cell flow),an SDH envelope, a Recommendation G.703 envelope, etc.4.2.2.3 Cell delineationCell delineation prepares the cell flow in order to enable the receiving side to recover cell boundariesaccording to
39、 the self-delineating mechanism defined in Recommendation I.432. In the transmit direction, the ATM cellstream is scrambled. In the receive direction, cell boundaries are identified and confirmed (using the HEC mechanism)and the cell flow is descrambled.4.2.2.4 HEC sequence generation and cell heade
40、r verificationIn transmit direction, the HEC sequence is calculated and inserted in the header. In receive direction, cellheaders are checked for errors and, if possible, header errors are corrected. Cells whose headers are determined to beerrored and non-correctable are discarded.4.2.2.5 Cell rate
41、decouplingCell rate decoupling includes insertion and suppression of idle cells, in order to adapt the rate of valid ATMcells to the payload capacity of the transmission system.4.2.3 OAM related to the Physical LayerThe required OAM functions relating to the Physical Layer are outlined in Recommenda
42、tions I.432 and I.610.4.2.4 Physical Layer primitivesBetween the Physical Layer and the ATM Layer, two primitives are currently defined:PH-DATA request: The ATM Layer requests the Physical Layer that the SDU associated with thisprimitive should be transported to its peer.PH-DATA-indication: The ATM
43、Layer is notified by the Physical Layer that the SDU associated with theprimitive coming from its peer is available.Note The Physical Layer will provide a clock to the ATM Layer. This clock is derived from the line rate ofthe Physical Layer (e.g. a bit clock at 155.52 Mbit/s for the user network int
44、erface standardized inRecommendation I.432).For certain applications, there may be a need to indicate to the ATM Layer that an idle cell has been discardedby the Physical Layer. Two possible applications are generic flow control (GFC) and the shaping of traffic flows.The exact definitions of the pri
45、mitives needed are for further study.4.3 ATM LayerThe characteristics of the ATM Layer are independent of the physical medium.6 Recommendation I.3214.3.1 ATM Layer functions4.3.1.1 Cell multiplexing and demultiplexingIn the transmit direction, the cell multiplexing function combines cells from indiv
46、idual virtual paths (VPs) andvirtual channels (VCs) into a non-continuous composite cell flow. In the receive direction, the cell demultiplexingfunction directs individual cells from a non-continuous composite cell flow to the appropriate VP or VC.4.3.1.2 Virtual path identifier (VPI) and virtual ch
47、annel identifier (VCI) translationThis function occurs at ATM switching fields and/or cross-connect nodes (including B-NT2). The value of theVPI and/or VCI fields of each incoming ATM cell is mapped into a new VPI and/or VCI value. This mapping functioncould be null.4.3.1.3 Cell header generation/ex
48、tractionThese functions apply at points where the ATM Layer is terminated.In the transmit direction, the cell header generation function receives a cell information field from a higherlayer and generates an appropriate ATM cell header except for the header error control (HEC) sequence. This function
49、could also include the translation from a service access point (SAP) identifier to a VP and VC identifier.In the receive direction, the cell header extraction function removes the ATM cell header and passes the cellinformation field to a higher layer. This function could also include a translation of a VP and VC identifier into an SAPidentifier.4.3.1.4 Generic flow controlWhen GFC is applied at the ATM Layer, the flow control information is carried in assigned and unassignedcells. Cells carrying this information are generated at the ATM Lay
