1、ITU-T RECMN*X.200 94 m YB62591 0594439 T50 m INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU x.200 (07/94) DATA NETWORKS AND OPEN SYSTEM COMMUNICATIONS AND NOTATION OPEN SYSTEMS INTERCONNECTION - MODEL INFORMATION TECHNOLOGY - OPEN SYSTEMS INTERCONNECTION
2、- BASIC REFERENCE MODEL: THE BASIC MODEL ITU-T Recommendation X.200 (Previously “CCIlT Recommendation”) ITU-T RECMN*X.200 94 = 4862591 0594420 772 Foreword ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommunications. The ITU Telecommunicati
3、on Standardization Sector (ITU-T) is a permanent organ of the ITU. Some 179 member countries, 84 telecom operating entities, 145 scientific and industrial organizations and 38 international organizations participate in ITU-T which is the body which sets world telecommunications standards (Recommenda
4、tions). The approval of Recommendations by the Members of ITU-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, 1993). In addition, the World Telecommunication Standardization Conference (WTSC), which meets every four years, approves Recommendations submitted to it and esta
5、blishes the study programme for the following period. 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. The text of ITU-T Recommendation X.200 was approved on Ist of July 1994. The identical text
6、is also published as ISO/IEC Intemational Standard 7498-1. NOTE In this Recommendation. the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. O ITU 1994 All rights reserved. No part of this publication may be re
7、produced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMN*X-200 94 4862593 0594421 607 9 CONTENTS Page 1 Scope 1 2 Definitions 2 3 Notation 4 Introduction to Open Systems Interconnection (
8、OSI) 4.1 Definitions . 4.2 Open System Interconnection Environment . 4.3 Modelling the OS1 Environment . Concepts of a layered architecture . 5 5.1 Introduction . 5.2 Principles of layering 5.3 Communication between peer-entities 5.4 Identifiers 5.5 Properties of service-access-points . 5.6 Data-uni
9、ts 5.7 The nature of the (N)-service 5.8 5.9 Routing 5.10 Introduction to the specific OS1 layers . 6.1 Specific layers . 6.2 6.4 6.5 Elements of layer operation Quality Of Service (QOS) . 6 The principles used to determine the seven layers in the Reference Model . 6.3 Layer descriptions . Combinati
10、ons of connection-mode and connectionless-mode Configurations of OS1 Open Systems . 7.1 Application Layer . 7.3 Session Layer 7.4 Transport Layer . 7.5 Network Layer 7.6 Data Link Layer 7.7 Physical Layer . Management aspects of OS1 . 8.1 Definitions . 8.2 Introduction . 8.3 Categories of management
11、 activities . A . 8.4 Principles for positioning management functions . 7 Detailed description of the resulting OS1 architecture . 7.2 Presentation Layer 8 2 2 2 3 4 5 5 6 9 13 14 15 16 16 27 27 28 28 29 30 30 31 32 32 33 34 37 41 46 49 52 52 53 53 54 9 Compliance and Consistency with this reference
12、 model 54 9.1 Definitios . 54 9.2 Application of consistency and compliance requirements 55 56 57 Annex A - Brief explanation of how the layers were chosen Annex B - Alphabetical index to definitions . ITU-T Rc X.200 (1994 E) i Summary This reference model provides a common basis for the coordinatio
13、n of standards development for the purpose of systems interconnection, while allowing existing standards to be placed into perspective within the overall reference model. It also identifies areas for developing and improving standards and provides a common reference for maintaining consistency among
14、 all related standards. The text was develo jointly with ISO/IEC and the main intent of this revision is to introduce the joint text, which incorporates inclusion of the concept of connectionless transmission, in addition to a number of technical and editorial refinements. ii ITU-T RW. X.200 (1994 E
15、) ITU-T RECMN*X.ZOO 74 m 4abm owt 481 m ISO/IEC 7498-1 : 1994m) INTERNATIONAL STANDARD CCITT RECOMMENDATION INFORMATION TECHNOLOGY - OPEN SYSTEMS INTERCONNECTION - BASIC REFERENCE MODEL: THE BASIC MODEL 1 scope 1.1 The purpose of this Reference Model of Open Systems Interconnection is to provide a c
16、ommon basis for the coordination of standards development for the purpose of systems interconnection, while allowing existing standards to be placed into perspective within th overall Reference Model. 1.2 The term Open Systems Interconnection (OSI) qualifies standards for the exchange of information
17、 among systems that are “open” to one another for this purpose by virtue of their mutual use of the applicable standards. 1.3 The fact that a system is open does not imply any particular systems implementation, technology or means of interconnection, but refers to the mutual recognition and support
18、of the applicable standards. 1.4 It is also the purpose of this Reference Model 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 Reference Model either to serve as an implemen
19、tation specification, or to be a basis for appraising the conformance of actual implementations, or to provide a sufficient level of detail to define precisely the services and protocols of the interconnection architecture. Rather, this Reference Model provides a conceptual and functional framework
20、which allows international teams of experts to work productively and independently on the development of standards for each layer of the Reference Model for OSI. 1.5 The Reference Model has sufficient flexibility to accommodate advances in technology and expansion in user demands. This flexibility i
21、s also intended to allow the phased transition from existing implementations to OS1 standards. 1.6 While the scope of the general architectural principles required far OS1 is very broad, this Reference Model is primarily concerned with systems comprising terminals, computers, and associated devices
22、and the means for transferring information between such systems. Other aspects of OS1 requiring attention are described briefly (see 4.2). 1.7 1.8 of the interconnection, and describes the modelling principles used in OSI. The description of the Basic Reference Model of OS1 is developed in stages: C
23、lause 4 establishes the reasons for Open Systems Interconnection, defines what is being connected, the scope 1.9 what layering means, and the principles used to describe layers. , 1.10 Clause 6 names, and introduces the specific layers of the architecture. 1.11 Clause 7 provides the description of t
24、he specific layers. 1.12 Clause 8 provides the description of Management Aspects of OSI. 1.13 Clause 9 specifies compliance and consistency with the OS1 Reference Model. 1.14 An indication of how the layers were chosen is given in Annex A to this Basic Reference Model. 1.15 Additional aspects of thi
25、s Reference Model beyond the basic aspects are described in several parts. The first part describes the Basic Reference Model. The second part describes the architecture for OS1 Security. The third part describes OS1 Naming and Addressing. The fourth describes OS1 System Management. 1.16 The Basic R
26、eference Model serves as a framework for the definition of services and protocols which fit within the boundaries established by the Reference Model. 1.17 In those few c.es where a feature is explicitly marked (optional) in the Basic Reference Model it should remain optional in the corresponding ser
27、vice or protocol (even if at a given instant the two cases of the option are not yet documented). Clause 5 describes the general nature of the architecture of the Reference Model; namely that it is layered, ITU-T RC. X.200 (1994 E) 1 ITU-T RECHN*X-ZOO 94 m 4862593 0594424 318 ISO/IEC 7498-1 : 1994(E
28、) 1.18 specification for systems, nor a basis for appraising the conformance of implementations. 1.19 functions, to facilitate implementation and compatibility. This Reference Model does not specify services and protocols for OSI. It is neither an implementation For standards which meet the OS1 requ
29、irements, a small number of practical subsets are defined from optional 2 Definitions i Definitions of terms are included at the beginning of individual clauses and sub-clauses. An index of these terms is provided in Annex B for easy reference. 3 Notation 3.1 layers: Layers are introduced in clause
30、5. An (N)-, (N+l)- and (N-1)- notation is used to identify and relate adjacent (N)-layer: any specific layer; (N+l)-layer: the next higher layer; (N-1)-layer: the next lower layer. This notation is also used for other concepts in the model which are related to these layers, for example (N)-protocol,
31、 (N+1 )-service. 3.2 Clause 6 introduces names for individual layers. When refemng to these layers by name, the (N)-, (N+l)- 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 Interconnectio
32、n (OSI) 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 4.1.1 real system: A set of one or more computers, the associated software, peripherals, termin
33、als, human operators, physical processes, information transfer means, etc., that forms an autonomous whole capable of performing information processing andor information transfer. 4.1.2 real open system: A real system which complies with the requirements of OS1 standards in its communication with ot
34、her real systems. 4.1.3 pertinent to OSI. 4.1.4 application process: An element within a real open system which performs the information processing for a particular application. 4.1.5 Open System Interconnection Environment (OSIE): An abstract representation of the set of concepts, elements, functio
35、ns, services, protocols, etc., as defined by the OS1 Reference Model and the derived specific standards which, when applied, enable communications among open systems. 4.1.6 pertinent to OSI. open system: The representation within the Reference Model of those aspects of a real open system that are Lo
36、cal System Environment (LSE): An abstract representation of that part of the real system that is not NOTE -The LSE may include functions necessary for non-OS1 communication. 4.1.7 process in support of a specific occasion of information processing. 4.1.8 processing capabilities. application-process-
37、invocation: A specific utilization of part or all of the capabilities of a given application application-process-type: A description of a class of application processes in terms of a set of information 2 ITU-T RS. X.200 (1994 E) ITU-T RECMN*X.200 94 = 4862593 0594425 254 W ISO/IEC 7498-1 : 1994) 4.2
38、 Open System Interconnection Environment 4.2.1 In the concept of OSI, a real system is a set of one or more computers, associated software, peripherals, terminals, human operators, physical processes, information transfer means, etc., that forms an autonomous whole capable of performing information
39、processing and/or information transfer. 4.2.2 a particular application. An application process is an element within a real open system which performs the information processing foy 4.2.3 Application processes can represent manual processes, computerized processes or physical processes. 4.2.4 Some ex
40、amples 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 center and accessing a remote database is a computerized application-process; t
41、he remote database management systems server is also an application-process; and c) a process control program executing in a dedicated computer attached to some industrial equipment and linked into a plant control system is a physical application-process. 4.2.5 An application process represents a se
42、t of resoufces, including processing resources, within a real open system that may be used to perform a particular information processing activity. An application process may organize its interactions with other application processes in whatever way is necessary to achieve a particular information p
43、rocessing goal: no constraints are imposed by this Reference Model either on the form of these interactions or on the possible relationships that may exist between them. 4.2.6 The activity of a given application process is represented by one or more application process invocations. Cooperation betwe
44、en application processes takes place via relationships established among application process invocations. At a particular time, an application process may be represented by none, one or more application process invocations. An application process invocation is responsible for coordinating its intera
45、ctions with other application process invocations. Such coordination is outside the scope of this Reference Model. 4.2.7 each individual real open system). OS1 is concerned with the exchange of information between open systems (and not the internal functioning of 4.2.8 transfer of information betwee
46、n open systems. As shown in Figure 1, the physical media for Open Systems Interconnection provides the means for the 4.29 to interconnection are outside the scope of OSI. OS1 is concemedonly with the interconnection of systems. All other aspects of systems which are not related ITU-T Rm. X.2QO (1994
47、 E) 3 ITU-T RECMN*X.200 94 m 4862593 0594426 390 m ISOJIEC 7498-1 : 1994(E) 4.2.10 OS1 is concerned not only with the transfer of information between systems, i.e. transmission, but also with their capability to interwork to achieve a common (distributed) task. In other words, OST is concerned with
48、the interconnection aspects of cooperation) between systems, which is implied by the expression “systems interconnection.” 4.2.11 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 i
49、n its cooperation with other systems is termed a real open system. 4.2.12 The design intent of the OS1 standards is to specify a set of standards that make it possible for autonomous systems to communicate. Any equipment which communicates in conformance with all applicable OS1 protocol standards is a real world equivalent of the model concept “open system”. Equipment that is in the “terminal” category, that is, one that requires human intervention for the dominant parts of information processing, may satisfy the conditions of the previous sentences when the appropriate OS1 stan
