EN ISO/IEC 7498-1:1995

SLOVENSKI STANDARD
01-december-1997
Information technology - Open Systems Interconnection - Basic Reference Model:
The Basic Model (ISO/IEC 7498-1:1994)
Information technology - Open Systems Interconnection - Basic Reference Model: The
Basic Model (ISO/IEC 7498-1:1994)
Informationstechnik - Kommunikation Offener Systeme - Basis-Referenzmodell:
Basismodell (ISO/IEC 7498-1:1994)
Technologies de l'information - Modele de référence de base pour l'interconnexion de
systemes ouverts (OSI): Le modele de base (ISO/IEC 7498-1:1994)
Ta slovenski standard je istoveten z: EN ISO/IEC 7498-1:1995
ICS:
35.100.01 Medsebojno povezovanje Open systems
odprtih sistemov na splošno interconnection in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERNATIONAL
ISO/IEC
STANDARD
7498-l
Second edition
1994-l l-l 5
Corrected and reprinted
1996-06-l 5
Information technology - Open Systems
Interconnection - Basic Reference Model:
The Basic Model
Technologies de I’informa tion - Mod&/e de rt? f&ence de base pour
/‘in terconnexjon de s ys tkmes ouverts (OS/): Le mod&/e de base
Reference number
ISO/l EC 7498-l : 1994(E)
ISO/IEC 7498=1:1994(E)
Contents
Page
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 Scope
Definitions .
..........................................................................................................................................................
3 Notation
....................................................................................
4 Introduction to Open Systems Interconnection (OSI)
4.1 Definitions .
Open System Interconnection Environment .
4.2
.........................................................................................................
4.3 Modelling the OS1 Environment
Concepts of a layered architecture .
Introduction .
..........................................................................................................................
512 Principles of layering
Communication between peer-entities .
514 Identifiers .
Properties of service-access-points .
55 .
56 Data-units .
The nature of the (N)-service .
5.8 Elements of layer operation .
Routing .
...................................................................................................................
5110 Quality Of Service (QOS)
Introduction to the specific OS1 layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
61 Specific layers
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The principles used to determine the seven layers in the Reference Model
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
63 Layer descriptions
. . . . . . . . . .~.
614 Combinations of connection-mode and connectionless-mode
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65 . Configurations of OS1 Open Systems
...................................................................................
7 Detailed description of the resulting OS1 architecture
...............................................................................................................................
7.1 Application Layer
7.2 Presentation Layer .
......................................................................................................................................
7.3 Session Layer
7.4 Transport Layer .
75 Network Layer .
7:6 Data Link Layer .
.....................................................................................................................................
7.7 Physical Layer
0 ISO/IEC 1994
All rights reserved. Unless otherwise specified, no part of this publication may be
reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopying and microfilm, without permission in writing from the publisher.
ISO/IEC Copyright Office l Case postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
ii
ISO/IEC 7498=1:1994(E)
0 ISO/lEC
8 Management aspects of OSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81 . Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2 Introduction .
8.3 Categories of management activities .
...............................................................................
8.4 Principles for positioning management functions
9 Compliance and Consistency with this reference model .
91 Definitions .
.................................................................. 55
912 Application of consistency and compliance requirements
- Brief explanation of how the layers were chosen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Annex A
Annex B - Alphabetical index to definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . .
ISO/IEC 7498=1:1994(E) 0 ISO/IEC
Foreword
IS0 (the International Organization for Standardization) and IEC (the Inter-
national Electrotechnical Commission) form the specialized system for worldwide
standardization. National bodies that are members of IS0 or IEC participate in the
development of International Standards through technical committees established
by the respective organization to deal with particular fields of technical activity.
IS0 and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with
IS0 and IEC, also take part in the work.
In the field of information technology, IS0 and IEC have established a joint
technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the
joint technical committee are circulated to national bodies for voting. Publication
as an International Standard requires approval by at least 75 % of the national
bodies casting a vote.
International Standard ISO/IEC 7498-l was prepared by Joint Technical
Committee ISO/IEC JTC 1, Information technology, in collaboration with ITU-T.
The identical text is published as ITU-T Recommendation X.200.
This second edition, along with parts 2, 3 and 4, cancels and replaces the first
edition (IS0 7498: 1984), which has been technically revised.
ISO/IEC 7498 consists of the following parts, under the general title Information
technology - Open Systems Interconnection - Basic Reference Model:
- Part 1: The Basic Model
- Part 2: Security Architecture
Part 3: Naming and addressing
Part 4: Management framework
Annex B forms an integral part of this part of ISO/IEC 7498. Annex A is for
information only.
iv
ISO/IEC 7498=1:1994(E)
Introduction
This reference model provides 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. It also
identifies areas for developing and improving standards and provides a common reference for maintaining consistency among
all related standards. The text was developed jointly with ITU-T 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.
V
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ISO/IEC 7498-l : 1994(E)
INTERNATIONAL STANDARD
CCITT RECOMMENDATION
INFORMATION TECHNOLOGY - OPEN SYSTEMS INTERCONNECTION -
BASIC REFERENCE MODEL: THE BASIC MODEL
Scope
1.1 The purpose of this 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.
1.2 The term Open Systems Interconnection (OSI) qualifies standards for the exchange of information among
one another for this purpose by virtue of their mutual use of
systems that are “open” to the applicable standards.
does not imply
1.3 The fact that a system is open any particular systems implementation, technology or means of
interconnection, but refers to the mutual recognition and support 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 implementation 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 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 ad vances in technology and expansion in user
This flexibility is also intended to allow the ph
demands. ased transition from existin g implementati .ons to OS1 standards.
1.6 While the scope of the general architectural principles required for OS1 is very broad, this Reference Model 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 attention are described briefly (see 4.2).
1.7 The description of the Basic Reference Model of OS1 is developed in stages:
1.8 Clause 4 establishes the reasons for Open Systems Interconnection, defines what is being connected, the scope
of the interconnection, and describes the modelling principles used in OSI.
1.9 Clause 5 describes the general nature of the architecture of the Reference Model; namely that it is layered,
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 the 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.
An indication of how the layers were chosen is given in Annex A to this Basic Reference Model.
1.14
Additional aspects of this Reference Model beyond the basic aspects are described in several parts. The first
1.15
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 Reference 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 cases where a feature is explicitly marked (optional) in the Basic 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).
ITU-T Rec. X.200 (1994 E) 1
ISO/IEC 7498-1 : 1994(E)
1.18 This Reference Model does not specify services and protocols for OSI. It is neither an implementation
specification for systems, nor a basis for appraising the conformance of implementations.
1.19 For standards which meet the OS1 requirements, a small number of practical subsets are defined from optional
functions, to facilitate implementation and compatibility.
2 Definitions
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 are introduced in clause 5. An (N)-, (N+l)- and (N-l)- notation is used to identify and relate adjacent
layers:
(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,
(N+l)-service.
3.2 Clause 6 introduces names for individual layers. When referring to these layers by name, the (N)-, (N+l)-
and (N-l)- prefixes are replaced by the names of the layers, for example transport-protocol, session-entity, and
network-service.
4 Introduction to Open Systems Interconnection (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.
41 . Definitions
the associated software, peripherals, terminals, human
4.1.1 real system: A set of one or more computers,
that forms an autonomous whole capable of performing
operators, physical processes, information transfer means, etc.,
information processing and/or information transfer.
standards in its
4.1.2 real open system: A real system which complies with the requirements of OS1
systems.
communication with other real
4.1.3 open system: The representation within the Reference Model of those aspects of a real open system that are
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, functions, 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 Local System Environment (LSE): An abstract representation of that part of the real system that is not
pertinent to OSI.
NOTE - The LSE may include functions necessary for non-OS1 communication.
4.1.7 A specific utilization of part or all of the capabilities of a given application
application-process-invocation:
process in support of a specific occasion of information processing.
4.1.8 application-process-type: A description of a class of application processes in terms of a set of information
processing capabilities.
2 ITU-T Rec. X.200 (1994 E)
ISO/IEC 7498-l : 1994(E)
42 . 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 processing and/or information transfer.
An application process is an element within a real open system which performs the information processing for
4.2.2
a particular application.
4.2.3 Application processes can represent manual processes, computerized processes or physical processes.
4.2.4 Some examples of application processes that are applicable to this open system definition are the following:
a person operating a banking terminal is a manual application-process;
a FORTRAN program executing in a computer center and accessing a computerized
a remote database is
b)
application-process; the remote database management systems server is also an application -process; and
program executing
a process control in a dedicated computer attached to some industrial equipment
C)
li .nked into a plan .t control system is a physical application-process.
4.2.5 An application process represents a set of resources, 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 processing
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 between 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 interactions with other application
process invocations. Such coordination is outside the scope of this Reference Model.
exchange functioning of
4.2.7 OS1 is concerned with the of information between open systems (and not the internal
each individual real open system).
4.2.8 As shown in Figure 1, the physical for Open Systems Interconnection provides the means for the
transfer of information between open systems.
Physical media
TlSO2830-94/dOl
Figure 1 - Open systems connected by physical media
4.2.9
OS1 is concerned only with the interconnection of systems. All other aspects of systems which are not related
to interconnection are outside the scope of OSI.
ITU-T Rec. X.200 (1994 E) 3
ISO/IEC 7498-l : 1994(E)
4.2.10 OSI 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, OS1 is concerned with the
interconnection aspects of cooperation l) 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 in 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 standards are employed in communication with other open systems.
43 . Modelling the OS1 Environment
4.3.1 The development of OS1 standards, i.e. standards for the interconnection of real open systems, is assisted by
the use of abstract models. To specify the external behavior of interconnected 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 behavior of these open systems. Only the external behavior of open
systems is retained for the definition of standards for real open systems. The description of the internal behavior of open
systems is provided in the Basic 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.
4.3.2 This abstract modelling is used in two steps.
4.3.3 First, basic elements of open systems and some key decisions concerning their organization and functioning,
are developed. This constitutes the Basic Reference Model of Open Systems Interconnection described in this
Recommendation I Part of this International Standard.
4.3.4 Then, the detailed and precise description of the functioning of the open system is developed in the framework
formed by the Basic Reference Model. This constitutes the services and protocols for OS1 which are the subject of other
Recommendations and/or International Standards.
4.3.5 It should be emphasized that the Basic Reference Model does not, by itself, specify the detailed and precise
functioning of the open system and, therefore, it does not specify the external behavior of real open systems and does not
imply the structure of the implementation of a real open system.
4.3.6 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 similiar appearance to concepts commonly found in
real systems. Therefore, real open systems need not be implemented as described by the Model.
l) Cooneration 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
a)
between OS1 application processes;
data representation, which concerns all aspects of the creation and maintenance of data descriptions and data
b)
transformations for reformatting data exchanged between open systems;
data storage, which concerns storage media, and file and database systems for managing and providing access to data
d
stored on the media;
process and resource management, which concerns the means by which OS1 application processes are declared,
d)
initiated and controlled, and the means by which they acquire OS1 resources;
integrity and security, which concern information processing constraints that have to be preserved or assured during
d
the operation of the open systems; and
program support, which concerns the definition, compilation, linking, testing, storage, transfer, and access to the
f)
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 0%
This Basic Reference Model covers the elements of OS1 aspects of these activities which are essential for early development of OS1
standards.
ITU-T Rec. X.200 (1994 E)
ISO/IEC 7498-1: 1994(E)
4.3.7 Throughout the remainder of this Basic Reference Model, only the aspects of real systems and application-
processes which lie within the OS1 Environment (OSIE) are considered. Their interconnection is illustrated throughout
this Reference Model as depicted in Figure 2.
4.3.8 The extent of the application of the OSIE concept through the use of OS1 standards may result in subsets of the
OSIE which corresponds to partially disjoint sets of real open systems, which are not physically cauable of OS1
I
communication between them.
Aspects of application-processes of concern to OSI,
i.e. applications-entities (see 7.1)
open system s
Aspects of real
open systems of
concern to OSI
r
I
TlSO2840-94/dO2
Physical media for OSI
Associations
Figure 2 - Basic elements of OS1
Concepts of a layered architecture
51 . Introduction
5.1.1 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, connections, 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 connections, transmission of data, and error functions. Then, routing aspects are introduced and
finally, management aspects are discussed.
51.2 The concepts described in clause 5 are those required to describe the Reference Model of Open Systems
Interconnection. However, not all of the concepts described are employed in each layer of the Reference Model.
ITU-T Rec. X.200 (1994 E)
ISO/IEC 7498-l : 1994(E)
5.1.3 Four elements are basic to the Reference Model (see Figure 2):
open systems;
a)
b) the application-entities which exist within the OS1 Environment (see 7.1);
the associations (see 5.3) which join the application-entities and permit them to exchange information;
C)
and
d) the physical media for OSI.
NOTE - Security aspects which are also general architectural elements of protocols are discussed in CCITI’ Rec. X.800 I
IS0 7498-2.
. Principles of layering
5.2.1 Definitions
a hierarchical division of an open system which interacts only with
5.2.1.1 (N)-subsystem: An element in directly
in the next higher division or the next lower division of that open system.
elements
5.2.1.2 (N)-layer: A subdivision of the OS1 architecture, constituted by subsystems of the same rank (N).
5.2.1.3 peer-(N)-entities: Entities within the same (N)-layer.
5.2.1.4 sublayer: A subdivision of a layer.
5.2.1.5 (N)-service: A capability of the (N)-layer and the layers beneath it, which is provided to (N+l)-entities at the
boundary between the (N)-layer and the (N+l)-layer.
5.2.1.6 (N)-facility: A part of an (N)-service.
5.2.1.7 (N)-function: A part of the activity of (N)-entities.
5.2.1.8 (N)-service-access-point, (N)-SAP: The point at which (N)-services are provided by an (N)-entity to an
(N+l)-entity.
5.2.1.9 (N)-protocol: A set of rules and formats (semantic and syntactic) which determines the communication
behavior of (N)-entities n the performance of (N)-functions.
5.2.1.10 (N)-entity-type: A description of a class of (N)-entities in terms of a set of capabilities defined for the
(N)-layer.
5.2.1.11 (N)-entity: An active element within an (N)-subsystem embodying a set of capabilities defined for the
(N)-layer that corresponds to a specific (N)-entity-type (without any extra capabilities being used).
5.2.1.12 (N)-entity-invocation: A specific utilization of part or all of the capabilities of a given (N)-entity (without
any extra capabilities being used).
5.2.2 Description
5.2.2.1 The basic structuring technique in the Reference Model of Open Systems Interconnection is layering.
According to this technique, each open system is viewed as logically composed of an ordered set of (N)-subsystems,
represented for convenience in the vertical sequence shown in Figure 3. Adjacent (N)-subsystems communicate through
their common boundary. (N)-subsystems of the same rank (N) collectively form the (N)-layer of the Reference Model
of Open Systems Interconnection. There is one and only one (N)-subsystem in an open system for layer N. An
(N)-subsystem consists of one or several (N)-entities. Entities exist in each (N)-layer. Entities in the same (N)-layer are
termed peer-(N)-entities. Note that the highest layer does not have an (N+l)-layer above it and the lowest layer does not
have an (N-Q-layer below it.
5.2.2.2 Not all peer-(N)-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). Communication among peer-(N)-entities which reside in the same (N)-subsystem is provided by the LSE and
therefore is out of the scope of OSI.
6 ITU-T Rec. X.200 (1994 E)
ISO/IEC 7498-l : 1994(E)
Open
OPen
system A system B
Hicrhest laver I I I
Physical media for OSI
TiSO2850-94/dO3
Figure 3 - Layering in cooperating open systems
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 description 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 instances of this type can be referred to by an individual name. Each nameable
instance and the type to which this instance belongs carry distinguishable names.
For example, given that a programmer has written a computer program, that programmer has generated a type of something
where instances of that type are created every time that particular program is invoked 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 of that program.
The general concept of instantiation applies within OSI: Consider now an (N)-entity in the OS1 context. It too, has two
aspects, a type and a collection of invocations. The type of an (N)-entity is defined by description of the specific set of (N)-layer
functions it is able to perform. An invocation of that type of (N)-entity is a specific invocation of whatever it is within the relevant
open system that provides the (N)-layer functions called for by its type for a particular occasion of communication. It follows from
these observations that (N)-entities refer only to the properties of an association between peer (N)-entities, while an (N)-entity-
invocation refers to the specific, dynamic occasions of actual information exchange.
It is important to note that actual communication occurs only between (N)-entity-invocations at all layers. In the
connection-mode (see 5.3.3), it is only at connection establishment time (or its logical equivalent during a recovery process) that
(N)-entities are explicitly relevant. An actual connection is always made with a specific (N)-entity-invocation, although the request
for connection is often made to an arbitrary (N)-entity (of a specific type). If an (N)-entity-invocation is aware of the name of its
peer-(N)-entity-invocation, it is able to request another connection to that (N)-entity-invocation.
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 communication services of the layer. The
detailed definition or additional characteristics of a sublayer are for further study.
5.2.2.3 Except for the highest layer, each (N)-layer provides (N+l)-entities in the (N+l)-layer with an (N)-service at
(N)-SAP(s). The properties of (N)-SAPS are described in 5.5. The highest layer is assumed to represent all possible uses
of the (N)-service which are provided by the lower layers.
NOTE - Not all open systems provide the initial source or final destination of data. Such open systems need not contain
the higher layers of the architecture (see Figure 12).
ITU-T Rec. X.200 (1994 E)
ISO/IEC 7498-l : 1994(E)
5.2.2.4 Each service provided by an (N)-layer may be tailored by the selection of one or more (N)-facilities which
determine the attributes of that service. When a single (N)-entity cannot by itself fully support a service requested by an
(N+l)-entity it calls upon the co-operation of other (N)-entities to help complete the service request. In order to
co-operate, (N)-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 communicate directly via the
physical media which connect them.
‘ii”“““’ ” ““’ . . .:
‘.,‘. .,\;,,l;I~;~~,, .,,,, ,, ! :“y:,. .,; .:~~:.‘;~:i::(~~ ,,, : ,:,,, ,,,: :, ,,, ~,:-
(N)-layer ‘1’. ,I,, ,y,:: ::‘:,r: 1,‘:::
., .: .“.,. _.,
\ .: .“‘, . . . . _,.,,: :;I.: ,/,, “:,,: .,,. :., .:. ., .
.,_._
TlSO2860-94/dO4
Figure 4 - (N + l)-entities in the (N + l)-layer
communicate through the (N)-layer
5.2.2.5 The services of an (N)-layer are provided to the (N+l)-layer using the (N)-functions performed within the
(N)-layer and as necessary the services available from the (N-1)-layer.
NOTE - This does not preclude the case where no protocol action is required in the (N)-layer to support a given
(N)-facility because it is already available at the (N-1)-service boundary. However, null functionality of the complete (N)-protocol is
not allowed.
5.2.2.6 An (N)-entity may provide services to one or more (N+l)-entities and use the services of one or more
(N-1)-entities. An (N)-service-access-point is the point at which a pair of entities in adjacent layers use or provide
services (see Figure 7).
5.2.2.7 Cooperation between (N)-entities is governed by one or more (N)-protocols. The entities and protocols within
a layer are illustrated in Figure 5.
(N)-entities
TIS 02870~94/dO5
(N)-protocol
Figure 5 - (N)-protocols between (IV)-entities
8 IT&T Rec. X.200 (1994 E)
ISO/IEC 7498-1 : 1994(E)
Communication between peer-entities
53 .
5.3.1 Definitions
(N)-association: A cooperative relationship among (N)-entity-invocations.
5.3.1.1
5.3.1.2 (N)-connection: An association requested by an (N+l)-entity for the transfer of data betweeen two or more
(N+l)-entities. The association is established by the (N)-layer and provides explicit identification of a set of (N)-data-
transmissions and agreement concerning the (N)-data-transmission services to be provided for the set.
(N)-connection-endpoint: A terminator at one end of an (N)-connection within an (N)-service-access-point.
5.3.1.3 -
5.3.1.4 multi-endpoint-connection: A connection with more than two connection-endpoints.
5.3.1.5 correspondent (N)-entities: (N)-entities with an (N-1)-connection between them.
5.3.1.6 (N)-relay: An (N)-function by means of which an (N)-entity forwards data received from one peer-(N)-entity
to another peer-(N)-entity.
5.3.1.7 (N)-data-source: An (N)-entity that sends (N-1)-service-data-units (see 5.6.1.7) on an (N-l)-connection.2)
5.3.1.8 (N)-data-sink: An (N)-entity that receives (N-1)-service-data-units on an (N-1)-connnection2)
5.3.1.9 (N)-data-transmission: An (N)-facility which conveys (N)-service-data-units from one (N+l)-entity to one
or more (N+l)-entities.
5.3.1.10 (N)-duplex-transmission: (N)-data-transmission in both directions at the same time.2)
5.3.1.11 (N)-half-duplex-transmission: (N)-data-transmission in either direction, one direction at a time; the choice of
direction is controlled by an (N+1)-entity.2)
5.3.1.12 (N)-simplex-transmission: (N)-data-transmission in one preassigned direction2)
5.3.1.13 (N)-data-communication: An (N)-function which transfers (N)-protocol-data-units (see 5.6.1.3) according to
an (N)-protocol, over one or more (N-l)-connections.2)
5.3.1.14 (N)-two-way-simultaneous-communication: (N)-data-communication in both directions at the same time.
5.3.1.15 (N)-two-way-alternate-communication: (N)-data-communication in both directions, one direction at a time.
5.3.1.16 (N)-one-way-communication: (N)-data-communication in one preassigned direction.
5.3.1.17 (N)-connection-mode transmission: (N)-data-transmission in the context of an (N)-connection.
5.3.1.18 (N)-connectionless-mode transmission: (N)-data-transmission not in the context of an (N)-connection and
not required to maintain any logical relationship between (N)-service-data-units.
Description
5.3.2
5.3.2.1 For information to be exchanged between two or more (N+l)-entities, an association is established between
them in the (N)-layer using an (N)-protocol.
NOTE - Classes of protocols may be defined within the (N)-protocols.
5.3.2.2 The rules and formats of an (N)-protocol are instantiated in an (N)-subsystem by an (N)-entity. An (N)-entity
may support one or more (N)-protocols. (N)-entities may support (N)-protocols which are connection-mode or
connectionless-mode or both. (N)-entities when supporting connection-mode maintain the binding of (N)-connections to
the appropriate (N+l)-entities at the appropriate (N)-SAPS. (N)-entities when supporting connectionless-mode maintain
a binding with the appropriate (N)-SAPS for delivering the connectionless data to the (N+l)-entities.
5.3.2.3 (N+l)-entities can communicate only by using the services of the (N)-layer. There are instances where services
provided by the (N)-layer do not permit direct access between all of the (N+l)-entities which have to communicate. If
this is the case communication can still occur if some other (N+l)-entities can act as relays between them (see Figure 6).
2, These definitions are not for use in this Basic Reference Model, but are for use in other OS1 standards.
ITU-T Rec. X.200 (1994 E) 9
ISO/IEC 7498-l : 1994(E)
Relay entity
TlSO2880-94/dO6
Figure. 6 - ~Communication through a relay
5.3.2.4 The fact that communication is relayed by a chain of (N+l)-entities is known neither by the (N)-layer nor by
the (N+2)-layer.
5.3.3 Modes of communication
5.3.3.1 Introduction
5.3.3.1.1 An (N)-layer may offer a connection-mode service, a connectionless-mode service, or both, to the (N+l)-layer,
using the service or services provided by the (N-1)-layer. Any instance of transmission between the (N+l)-entities must
utilize the same mode of (N)-service.
5.3.3.1.2 Both the (N)-connection-mode service and the (N)-connectionless-mode service are characterised by the
facilities which they offer to, and the quality of service seen by, the (N+l)-entities. For both the (N)-connection-mode
service and the (N)-connectionless-mode service, functions may be provided by the (N)-layer to enhance the facilities
offered to, and the quality of service seen by the (N+l)-entities over those which are offered to the (N)-layer by the
(N-1)-layer and, if necessary, to convert between one mode of service and another.
5.3.3.1.3 Since connection-mode transmission and connectionless-mode transmission are complementary concepts, they
are best understood in juxtaposition, particularly since connectionless-mode transmission is defined most easily in
relationship to the concept of a connection.
5.3.3.1.4 In order for (N+l)-entities to be able to communicate using an (N)-connection-mode service or an
(N)-onnectionless-mode service it is essential that a prearranged association exists between them, constituted by the
pre-knowledge which it is essential that each (N+l)-entity has of the others in order at least to initiate the use of the
service. This association is established in ways which are not detailed in this Basic Reference Model and comprises four
elements:
a) knowledge of the addresses of the peer-(N)-entities involved;
b) knowledge of a protocol agreed by the peer-(N)-entities for use at least to initiate communication;
c) knowledge of the availability for communication of the peer-(N)-entities;
d) knowledge of the quality of service available from the (N)-service.
NOTE - The pre-knowledge constituting a prearranged association can be acquired in many ways; some examples are
listed below:
from information acquired manually when contracts are exchanged with a service provider;
a)
b) from information which a network administration may provide in a directory or enquiry database;
from information that may be learned from previous instances of communication;
C)
d) from information that may be provided dynamically through the operation of management protocols.
The total pre-knowledge constituting a prearranged association is likely to be acquired in a combination of the above
ways.
10 ITU-T Rec. X.200 (1994 E)
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