SIST EN 27498:1998
(Main)Information processing systems - Open Systems Interconnection - Basic reference model (ISO 7498:1984 and Addendum 1:1987)
Information processing systems - Open Systems Interconnection - Basic reference model (ISO 7498:1984 and Addendum 1:1987)
Informationsverarbeitungssysteme - Kommunikation Offener Systeme - Basis-Referenzmodell (ISO 7498:1984, Ausg. 1 + Zusatz:1987)
Systemes de traitement de l'information - Interconnexion des systemes ouverts - Modele de référence de Base (ISO 7498:1984, éd. 1 + Addendum 1:1987)
Information processing systems - Open Systems Interconnection - Basic reference model (ISO 7498:1984 and Addendum 1:1987)
General Information
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Information processing systems - Open Systems Interconnection - Basic reference model (ISO 7498:1984 and Addendum 1:1987)Informationsverarbeitungssysteme - Kommunikation Offener Systeme - Basis-Referenzmodell (ISO 7498:1984, Ausg. 1 + Zusatz:1987)Systemes de traitement de l'information - Interconnexion des systemes ouverts - Modele de référence de Base (ISO 7498:1984, éd. 1 + Addendum 1:1987)Information processing systems - Open Systems Interconnection - Basic reference model (ISO 7498:1984 and Addendum 1:1987)35.100.01Medsebojno povezovanje odprtih sistemov na splošnoOpen systems interconnection in generalICS:Ta slovenski standard je istoveten z:EN 27498:1989SIST EN 27498:1998en01-januar-1998SIST EN 27498:1998SLOVENSKI
STANDARD
SIST EN 27498:1998
SIST EN 27498:1998
SIST EN 27498:1998
International Standard 7498 INTERNATIONAL ORGANIZATION FOR STANDARDlZATION.MEXAYHAPOAHAR OPrAHM3AUMR il0 CTAHAAPTH3AUMM.ORGANlSATlON INTERNATIONALE DE NORMALISATION Information processing systems - Open Systems * Interconnection - Basic Reference Model Systèmes de traitement de l'information - Interconnexion de systèmes ouverts - Modèle de Référence de base First edition - 1984-10-15 UDC 681.3.01 Ref. No. 607498-1984 (E) - Descriptors : data processing, information interchange, open system interconnection, reference model ~; c O 1- Price based on 40 pages SIST EN 27498:1998
Foreword IS0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out through IS0 technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, govern- mental and non-governmental, in liaison with ISO, also take part in the work. Draft International Standards adopted by the technical committees are circulated to the member bodies for approval before their acceptance as International Standards by the IS0 Council. They are approved in accordance with IS0 procedures requiring at least 75 % approval by the member bodies voting. International Standard IS0 7498 was prepared by Technical Committee ISO/TC 97, Information processing systems. NOTE - Possible questions arising using International Standard IS0 7498 should be directed to subcommittee ISO/TC 97/SC 21 - Information Processing Systems - Information Retrieval, Transfer and Management for Open Systems interconnection - through its Secretariat (ANSI). Subcommittee ISO/TC 97/SC 21 handles such questions and maintains a register of answers to questions of general interest. Information about the register and its availability may be obtained from : Secretariat of iSO/TC 97/SC 21 or IS0 Central Secretariat American National Standards institute 1430 Broadway Case postale 56 USA Switzerland 1, rue de Varembé NEW YORK, N.Y. 10018 CH-1211 GENEVA 20 O International Organization for Standardization, 1984 0 Printed in Switzerland SIST EN 27498:1998
Contents O Introduction , . . . . . . . . . . . . . <.<.<. . 1 Scope and field of application . . . . ,. <._. 2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . <.<. 4 Introduction to Open Systems Interconnection (OsIl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . <.<. 4.2 Open Systems Interconnection environment . . . . . . . . . . . . . . . . . . . . . . . 4.3 Modelling the OS1 environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Concepts of a layered architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 '"''.~'.".''.' 5.2 Principles of layering . 5.3 Communication between peer-entities . . . . . . . . . . 5.4 Identifiers . . . . . . . . . . 5.5 Properties of service-access-points . . . . . . . . . . . . . . . . . . . . 5.6 Data-units . . . . . , . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Elements of layer operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Routing .,. <<,. . <.<.<.<<<.<<. 5.9 Management Aspects of OS1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Introduction to the specific OS1 layers ._.,._ <<. 6.1 Specific layers. . . . . . . . . . . . . <.<.<.<<.<. ciples used to determine the seven layers of the Reference . <._._. .__.__._. <<. 6.3 Layer descriptions . . . . . . . . . . . 7 Detailed description of the resulting OS1 architecture . . . . . . . . . . , . , . , , . . . . .,.,. <.<. 7.1 Application Layer . <_.<._._._.<._. 7.2 Presentation Layer <<.<_.<<.<<<._._._._.<_. Page 1 2 2 2 2 2 2 4 4 4 5 7 8 10 10 12 16 16 18 18 20 20 20 20 22 iii SIST EN 27498:1998
7.3 Session Layer . . 7.4 Transport Layer . . 7.5 Network Layer . 7.6 Data Link Layer. . . 7.7 Physical Layer. . . . . Annexes A Brief explanation of how the layers were chosen . . . 6 Alphabetical index to definitions . 23 26 29 33 34 38 39 iv SIST EN 27498:1998
INTERNATIONAL STANDARD IS0 7498-1984 (E) Information processing systems - Open Systems Interconnection - Basic Reference Model O Introduction e 0.1 About this standard The purpose of this International Standard Reference Model of Open Systems Interconnection is to provide a common basis for the coordination of standards development for the purpose of systems interconnection, while allowing existing standards to be placed into perspective within the overall Reference Model. The term Open Systems Interconnection (OSI) qualifies stan- dards for the exchange of information among systems that are "open" to one another for this purpose by virtue of their mutual use of the applicable standards. The fact that a system is open does not imply any particular systems implementation, technology or means of interconnec- tion, but refers to the mutual recognition and support of the ap- plicable standards. It is also the purpose of this International Standard to identify areas for developing or improving standards, and to provide a common reference for maintaining consistency of all related standards. It is not the intent of this International Standard either to serve as an implementation specification, or to be a basis for appraising the conformance of actual implementa- tions, or to provide a sufficient level of detail to define precisely the services and protocols of the interconnection architecture. Rather, this International Standard provides a conceptual and functional framework which allows international teams of ex- perts to work productively and independently on the development of standards for each layer of the Reference Model of OSI. The Reference Model has sufficient flexibility to accommodate advances in technology and expansion in user demands. This flexibility is also intended to allow the phased transition from existing implementations to OS1 standards. NOTE - The Reference Model is expected to be subject to future ex- pansion. Some anticipated directions of expansion are indicated by notes or footnotes in this International Standard. While the scope of the general architectural principles required for OS1 is very broad, this International Standard is primarily concerned with systems comprising terminals, computers and associated devices and the means for transferring information between such systems. Other aspects of OS1 requiring atten- tion are described briefly (see 4.2). The justification for development of standards shall follow nor- mal administrative procedures even though such standards are identified in the Reference Model. As standards emerge to meet the OS1 requirements, a small ?umber of practical subsets should be defined by the standards developers from optional functions, to facilitate implementa- tion and compatibility. The description of the Reference Model of OS1 given in this In- ternational Standard is developed in stages : Clause 4 establishes the reasons for Open Systems Inter- connection, defines what is being connected, the scope of the interconnection and, describes the modelling principles used in OSI; Clause 5 describes the general nature of the architecture of the Reference Model: namely that it is layered, what layer- ing means, and the principles used to describe lavers; Clause 6 names, and introduces the specific layers of the ar- chitecture; and Clause 7 provides the description of the specific layers. An indication of how the layers were chosen is given in annex A to this International Standard. The Reference Model serves as a framework for the definition of services and protocols which fit within the boundaries established by the Reference Model. In those few cases where a feature is explicitly marked (op- tional) in the Reference Model it should remain optional in the corresponding service or protocol (even if at a given instant the two cases of the option are not yet documented). 1 SIST EN 27498:1998
IS0 7498-1984 (E) 0.2 Related OS1 standards Concurrently with the preparation of this International Stan- dard, work is in progress within IS0 on the development of OS1 standards in the following areas : a) virtual terminal protocols; b) file transfer, access and management protocols; Ci job transfer and manipulation protocols; di common application services and protocols; e) presentation layer services and protocols; f) Session Layer services and protocols; g) Transport Layer services and protocols; h) Network Layer services and protocols; j) Data Link Layer services and protocols; k) Physical Layer services and protocols; and mi OS1 management protocols. The first five items in this list relate to the Application and Presentation Layers of the Reference Model. In addition, liaison is maintained with CCITT in the develop- ment of OS1 standards. 1 Scope and field of application This International Standard describes the Reference Model of Open Systems Interconnection. It establishes a framework for coordinating the development of existing and future standards for the interconnection of systems and is provided for reference by those standards. This International Standard does not specify services and pro- tocols for OSI. It is neither an implementation specification for systems, nor a basis for appraising the conformance of im- plementations. 2 Definitions Definitions of terms are included at the beginning of individual clauses and sub-clauses. An index of these terms is provided in an annex B for easy reference. 3 Notation Layers are introduced in clause 5. An (Ni-, (N+ 1)- and (N - 1)- notation is used to identify and relate adjacent layers : (NI-layer : any specific layer; (N + 1)-layer : the next higher layer; f N - 1)-layer : the next lower layer. 2 This notation is also used for other concepts in the model which are related to these layers, for example (Ni-protocol, (N + 1 )-service. Clause 6 introduces names for individual layers. When referring to these layers by name, the (NI-, iN + 1)- and (N - 1)- prefixes are replaced by the names of the layers, for example transport- protocol, session entity, and network-service. 4 Introduction to Open Systems Interconnection (OsIl NOTE - The general principles described in clauses 4 and 5 hold for all layers of the Reference Model, unless layer specific statements to the contrary are made in clauses 6 and 7. 4.1 Definitions m 4.1.1 real system : A set of one or more computers, the associated software, peripherals, terminals, human operators, physical processes, information transfer means, etc., that forms an autonomms whole capable of performing information processing and/or information transfer. 4.1.2 real open system : A real system which complies with the requirements of OS1 standards in its communication with other real systems. 4.1.3 open system : The representation within the Reference Model of those aspects of a real open system that are Dertinent to OSI. 4.1.4 application-process : An element within a real open system which performs the information processing for a par- ticular application. 4.2 Open Systems Interconnection environment In the concept of OSI, a real system is a set of one or more 0 computers, associated software, peripherals, terminals, human operators, physical processes, information transfer means, etc., that forms an autonomous whole capable of performing information processing and/or information transfer. An application-process is an element within an open system which performs the information processing for a particular ap- plication. Application-processes can represent manual processes, com- puterized processes or physical processes. Some examples of application-processes that are applicable to this open system definition are the following : a) a person operating a banking terminal is a manual application-process; b) a FORTRAN program executing in a computer centre and accessing a remote database is a computerised application-process; the remote database management systems server is also an application-process; and SIST EN 27498:1998
IS0 7498-1984 (E) c) a process control program executing in a dedicated com- puter attached to some industrial equipment and linked into a plant control system is a physical application-process. OS1 is concerned with the exchange of information between open systems (and not the internal functioning of each in- dividual real open system). As shown in figure 1, the physical media for Open Systems In- terconnection provides the means for the transfer of informa- tion between open systems. NOTE -- At this point, only telecommunications media have been con- sidered The use of other interconnection media is for further study. OS1 is concerned only with interconnection of systems. All other aspects of systems which are not related to interconnec- tion are outside the scope of OSI. OS1 is concerned not only with the transfer of information be- tween systems, i.e. transmission, but also with their capability to interwork to achieve a common (distributed) task. In other words, OS1 is concerned with the interconnection aspects of cooperation') between systems, which is implied by the expres- sion "systems interconnection". The objective of OS1 is to define a set of standards to enable real open systems to cooperate. A system which complies with the requirements of applicable OS1 standards in its cooperation with other systems is termed a real open system. ------- Physical media Figure 1 - Open systems connected by physical media 1 i Cooperation among open systems involves a broad range of activities of which the following have been identified : a) interprocess communication, which concerns the exchange of information and the synchronization of activity between OS1 application- processes; b) data representation, which concerns all aspects of the creation and maintenance of data descriptions and data trarisformations for reformatting data exchanged between open systems; ci data storage, which concerns storage media, and file and database systems for managing and providing access to data stored on the media, d) process and resource management, which concerns the means by which OS1 appliration-processes are declared, initiated and controlled, aiiri the means by which they acquire OS1 resources; e) integrity and security, which concern information processing constraints that have to be be preserved or assured during the operation of the open systems; and f) program support, which concerns the definition, compilation, linking, testing, storage, transfer, and access to the programs executed by OS1 application-processes. Some of these activities may imply exchange of information between the interconnected open systems and their interconnection aspects may, therefore, be of concern to OSI. This International Standard covers the elements ot OS1 aspects of these activities which are essential for early development of OS1 standards. 3 SIST EN 27498:1998
1 IS0 7498-1984 (E) 4.3 Modelling the OS1 environment The development of OS1 standards, i.e. standards for the inter- connection of real open systems, is assisted by the use of abstract models. To specify the external behaviour of intercon- nected real open systems, each real open system is replaced by a functionally equivalent abstract model of a real open system called an open system. Only the interconnection aspects of these open systems would strictly need to be described. However to accomplish this, it is necessary to describe both the internal and external behaviour of these open systems. Only the external behaviour of open systems is retained as the standard of behaviour of real open systems. The description of the inter- nal behaviour of open systems is provided in the Reference Model only to support the definition of the interconnection aspects. Any real system which behaves externally as an open system can be considered to be a real open system. This abstract modelling is used in two steps. First, basic elements of open systems and some key decisions concerning their organization and functioning, are developed. This constitutes the Reference Model of Open Systems Inter- connection described in this International Standard. Then, the detailed and precise description of the functioning of the open system is developed in the framework formed by the Reference Model. This constitutes the services and protocols for OS1 which are the subject of other International Standards. It should be emphasized that the Reference Model does not, by itself, specify the detailed and precise functioning of the open system and, therefore, it does not specify the external behaviour of real open systems and does not imply the struc- ture of the implementation of a real open system. 1 ~ The reader not familiar with the technique of abstract modelling is cautioned that those concepts introduced in the description of open systems constitute an abstraction despite a similar ap- pearance to concepts commonly found in real systems. Therefore real open systems need not be implemented as described by the Model. Throughout the remainder of this International Standard, only the aspects of real systems and application-processes which lie within the OS1 environment are considered. Their interconnec- tion is illustrated throughout this International Standard as depicted in figure 2. 5 Concepts of a layered architecture 5.1 Introduction Clause 5 sets forth the architectural concepts that are applied in the development of the Reference Model of Open Systems Interconnection. Firstly, the concept of a layered architecture (with layers, entities, service-access-points, protocols, connec- tions, etc.) is described. Secondly, identifiers are introduced for entities, service-access-points, and connections. Thirdly, service-access-points and data-units are described. Fourthly, elements of layer operation are described including connec- tions, transmission of data, and error functions. Then, routing aspects are introduced and finally, management aspects are discussed. The concepts described in clause 5 are those required to describe the Reference Model of Open Systems Interconnec- tion. However, not all of the concepts described are employed in each layer of the Reference Model. Aspects of application-processes of concern to OSI, I i.e., application-entities (see 7.1) Open system S Aspects of real open systems of concern to OS1 t \ Physical media for OS1 Figure 2 - Basic elements of OS1 1 4 SIST EN 27498:1998
Four elements are basic to the Reference Model (see figure 2) : ai open systems; b) vironment; the application-entities which exist within the OS1 en- CI the connections (see 5.3) which join the application- entities and permit them to exchange information (see note 1); and di the physical media for OSI. NOTES 1 This Basic Reference Model of Gpen Systems Interconnection is based on the assumption that a connection is required for the transfer of data. An addendum to this International Standard is currently being developed to extend the description to cover the connectionless forms of data transmission which may be found in a wide variety of data com- munications techniques (for example local area networks, digital radio, etc.) and applications (for example remote sensing and banking). 2 Security aspects which are also general architectural elements of protocols are not discussed in this International Standard. 0 5.2 Principles of layering 5.2.1 Definitions 5.2.1.1 (NI-subsystem : An element in a hierarchical division of an open system which interacts directly only with elements in the next higher division or the next lower division of that open system. 5.2.1.2 (NI-layer : A subdivision of the OS1 architecture, constituted by subsystems of the same rank (NI. IS0 7498-1984 5.2.1.3 (NI-entity : An active element within i NI-subsystem. 5.2.1.4 peer-entities : Entities within the same layer. 5.2.1.5 sublayer : A subdivision of a layer, E) an 5.2.1.6 (NI-service : A capability of the (NI-layer and the layers beneath it, which is provided to (N + 1)-entities at the boundary between the (NI-layer and the (N + 1)-layer. 5.2.1.7 (NI-facility : A part of an (NI-service. 5.2.1.8 (NI-function : A part of the activity of (Ni-entities. 5.2.1.9 (NI-service-access-point : The point at which (NI-services are provided by an (NI-entity to an (N + lbentity. 5.2.1.10 (NI-protocol : A set of rules and formats (semantic and syntatic) which determines the communication behaviour of (NI-entities in the performance of (Ni-functions. 5.2.2 Description The basic structuring technique in the Reference Model of Open Systems Interconnection is layering. According to this technique, each open system is viewed as being logically com- posed of ail ordered set of subsystems, represented for conve nience in the vertical sequence shown in figure 3. Adjacent subsystems communicate through their common boundary. Subsystems of the same rank (Ni collectively form the (NI-layer of the Reference Model of Open Systems Interconnection. An i NI-subsvstem consists of one or several (NI-entities. Entities rxist in each layer. Entities in the same layer are termed peer- entities. Note that the highest layer does not have an (N + 1) layer above it and the lowest layer does not have an (N -- 1)-layer below it. Open system A Open Open system B system C Open system S I Physical media for OS1 Figure 3 - Layering in co-operating open systems 5 SIST EN 27498:1998
IS0 7498-1984 (E) Not all peer (NI-entities need or even can communicate. There may be conditions which prevent this communication (for example: they are not in interconnected open systems, or they do not support the same protocol subsets). NOTES 1 The distinction between the type of some object and an instance of that object is a distinction of significance for OSI. A type is a descrip- tion of a class of objects. An instance of this type is any object that conforms to this description. The instances of the same type constitute a class. A type, and any instançes of this type can be referred to by an individual name. Each nameable instance and the type to which this in- stance belongs should carry distinguishable names. For example, given that a programmer has written a computer pro- gram, that programmer has generated a type of something where in- stances of that are created every time that particular program is invok- ed into execution by a computer. Thus, a FORTRAN compiler is a type and each occasion where a copy of that program is invoked in a data processing machine one displays an instance. Consider now an (Ni-entity in the OS1 context. It too, has two aspects, a type and a collection of instances. The type of an (Ni-entity is defined by the specific set of (NI-layer functions it is able to perform. An in- stance of that type of (Ni-entity is a specific invocation of whatever it is within the relevant open system that provides the (Ni-layer functions called for by its type for a particular occasion of communication. It follows from these observations that (Ni-entity types refer only to the properties of an association between peer (Ni-entities, while an (Ni-entity instance refers to the specific, dynamic occasions of actual information exchange. It is important to note that actual communication occurs only between (Ni-entity instances at all layers. It is only at connection establishment time (or its logical equivalent during a recovery process) that (Ni-entity types are explicitly relevant. Actual connections are always made to specific (NI-entity instances, although a request for connection may well be made for arbitrary (NI-entity instances of a specified type. Nothing in this International Standard, however, precludes the request for a connection with a specific (named) instance of a peer (Ni-entity. If an (NI-entity instance is aware of the name of its peer (Ni-entity in- stance, it should be able to request another connection to that (Ni-entity instance. 2 It may be necessary to further divide a layer into small substructures called sublayers and to extend the technique of layering to cover other dimensions of OSI. A sublayer is defined as a grouping of functions in a layer which may be bypassed. The bypassing of all the sublayers of a layer is not allowed. A sublayer uses the entities and connections of its layer. The detailed definition or additional characteristics of a sublayer are for further study. Except for the highest layer, each (Ni-layer provides (N + 1)-entities in the (N + 1)-layer with (NI-services. The highest layer is assumed to represent all possible uses of the services which are provided by the lower layers. NOTES 1 Not all open systems provide the initial source or final destination of data. Such open systems need not contain the higher layers of the ar- chitecture isee figures 6 and 13). 2 Classes of service may be defined within the (Ni-services. The precise definition of the term ”classes of service” is for further study. Each service provided by an (NI-layer may be tailored by the selection of one or more (NI-facilities which determine the at- tributes of that service. When a single (Ni-entity cannot by itself fully support a service requested by an (N + 1 )-entity it calls upon the co-operation of other (Ni-entities to help com- plete the service request. In order to co-operate, (NI-entities in any layer, other than those in the lowest layer, communicate by means of the set of services provided by the (N - 1 )-layer (see figure 4). The entities in the lowest layer are assumed to com- municate directly via the physical media which connect them. The services of an (NI-layer are provided to the (N + 1 )-layer, using the (NI-functions performed within the (N)-layer and as necessary the services available from the (N - 1 )-layer. An (NI-entity may provide services to one or more (N+ 1)-entities and use the services of one or more (N - 1)-entities. An (NI-service-access-point is the point at which a pair of entities in adjacent layers use or provide services isee figure 7). Co-operation between (NI-entities is governed by one or more (NI-protocols. The entities and protocols within a layer are il- lustrated in figure 5. (N + 1)-layer / i Ni-layer Figure 4 - (N + 1)-entities in the (N,+ 1)-layer communicate through the (Nl-layer 6 SIST EN 27498:1998
IS0 7498-1984 (E) (Ni-entities (Ni-protocol Figure 5 - (NI-protocols between (NI-entities 5.3 Communication between peer-entities e 5.3.1 Definitions 5.3.1 .I (NI-connection : An association established by the (NI-layer between two or more (N + 1 bentities for the transfer of data. 5.3.1.2 (NI-connection-endpoint : A terminator at one end of an (NI-connection within an (NI-service-access-point. 5.3.1.3 multi-endpoint-connection : A connection with more than two connection-endpoints. 5.3.1.4 correspondent (NI-entities : (N)-entities with an (N - 1 )-connection between them. 5.3.1.5 (NI-relay : An (NI-function by means of which an (NI-entity forwards data received from one correspondent (NI-entity to another correspondent (NI-entity. 5.3.1.6 (NI-data-source : An (NI-entity that sends (N - 1 )-service-data-units (see 5.6.1.7) on an (N - 1)-connection.1) 5.3.1.7 (NI-data-sink : An (NI-entity that receives (N - II-service-data-units on an (N - l)-connection.l) 5.3.1.8 (NI-data-transmission : An (NI-facility which con- veys (NI-service-data-units from one (N + 1)-entity to one or more (N + 1 bentities. 1) 5.3.1.9 (NI-duplex-transmission : (NI-data transmission in both directions at the same time.1) 5.3.1.10 (NI-half-duplex-transmission : (NI-data transmis- sion in either direction, one direction at a time; the choice of direction is controlled by an (N + 1)-entity. 1) 5.3.1.11 (NI-simplex-transmission : ( NI-data-transmission in one pre-assigned direction. 1) 5.3.1 .I2 (NI-data-communication : An (NI-function which transfers (NI-protocol-data-units (see 5.6.1.3) according to an (NI-protocol, over one or more (N - l)-connedions.l) 5.3.1.13 (NI-two-way-simultaneous communication : (NI-data-communication in both directions at the same time. 5.3.1.14 (NI-two-way-alternate communication : (NI-data-communication in both directions, one direction at a time. 5.3.1 .I5 (NI-one-way-communication : (Ni-datacommuni- cation in one pre-assigned direction. 5.3.2 Description For informatio
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