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DATA PRO

Data Networking

2783

Standards

ISO Reference Model
for Open Systems
Interconnection (OSI)
In this report:

Datapro Summary

OSI Standards Progress ..... 6

The goal of Open Systems Interconnection (OS!) was designed to enable dissimilar computers in multivendor environments to share information transparently. The OSI structure
calls for cooperation among systems of different manufacture and design. There are seven
layers of the OSI model that communicate between one end system and another. The layers
cover nearly all aspects of information flow, from applications-related services provided at
the Application Layer to the physical connection of devices to the communications medium
at the Physical Layer. All seven layers have long since been defmed and ISO protocols
ratified for each layer, though extensions have been made occasionally. Although the model
has changed the way we look at networking, the dream of complete OSI-compliance has not
come to fruition. The causes are varied, but this is essentially because OSI protocols are too
expensive and too complex compared with other protocols that have become de facto standards in their own right. Even so, it is important to understand the model because, although
the complete stack of protocols is not much used today, the model has formed the way we
think of the structure of networks, and the model itself is always referred to in intemetworking matters.

OSI Management ................ 8
OSI and the Future •....••....•.. 9

Note: This report explains the OSI SevenLayer Reference Model at
all layers; compares OSI
to other architectures;
rationalizes the need for
standards testing and verification; examines the
case for OSI; profiles
major testing organizations; and outlines OSI
Management standards
and status.

-By Marina Smith
Senior Analyst
email: smithma@mcgraw-hill.com

1997 The McGraw-HUI Companies, Inc. Reproduction Prohibited.
Dalapro Information Services Group. Dellan NJ 08075 USA

@ August

standardization effort and confront the issue of
incompatibility head-on. The purpose of TC97!
SC 16 was to develop a model and define the
protocols and interfaces required to support an
open system. The goal of OS! was, and still is,
to enable dissimilar computers in multivendor
environments to share information transparently.
With this capability, it was thought that global
digital networks could become a reality. As we
shall see, however, much of the OSI goal has in
fact come about through widespread use of de
facto protocols such as TCPIIP and from multivendor initiatives formed to ensure interoperability such as the ATM Forum, a standards-setting body in its own right.

The Open System
The ISO defines a system as a set of one or more
computers and associated software, peripherals,
tenninals, human operators, physical processes,
information transfer means, etc., which form an
autonomous whole capable of performing information processing and/or information transfer.
An open system is one that obeys OSI standards
in its communication with other systems.

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ISO Reference Model
for Open Systems·
Interconnection (OSI)

2783
Standards

An application process is an element within a system that
performs information processing for a particular application. The
application process can be manual (a person operating a banking
terminal), computerized (a program executing in a computer center and accessing a remote database), or physical (a process control program executing in a dedicated cQIllPuter attachedto industrial equipment and linked to a plant control system).
The OSI structure calls for cooperation among systems of different manufacture and design. This includes coordinating activities such as the following:
• Interprocess communications-the synchronization between
OSI application processes and the exchange of information
• Data representation-the creation and maintenance of data
descriptions and transformations for reformatting data exchanged between systems
• Data storage-storage media, file systems, and database systems for providing access to and management of stored data
• Process and resource management-how application processes
are declared, initiated, controlled, and acquired
• Integrity and security-information processing constraints that
must be ensured during open systems operations
• Program support-the definition, compilation, testing, linking,
storage, and transfer of and access to programs executed by the
application processes
The OSI model is concerned only with the exchange of information between open systems.

Data Networking

distinct groups. Communications-oriented functions are sepa. rated from user-oriented functions; features which move informa~on across a network are distinct from features which handle and
format information.
There are seven layers of the OSI model that communicate
between one end system and another end system. The layers
cover nearly all aspects of information flow, from applicationsrelated services provided at the Application Layer to the connectioJ;l of devices to the communications medium at the Physical
Layer. Below the Physical Layer, the media itself corresponding
to '1..ayer 0"-such as wire, cable, or through-the-air communication-is not addressed by the model. Application, Presentation,
Session, Transport, Network, Data Link, and Physical Layers
have been defined (see Table "The Seven Layers of OSI"). The
table describes the OS.! model's seven layers and their purposes.
In the model, information flows down from Layer 7 to Layer 1,
and then out over a physical transmission medium. At the receiving end, the information flows into another end system and up
from Layer I to Layer 7, until it is received by a user.
The seven layers can be divided into two functional groups:
the Transport Platform (Layers I to 4) and the Application Platform (Layers 5 to 7). The Transport Platform's function is to get
data from one system to another without errors. The Application
Platform's function is to interpret the data stream and present it to
the user in a usable form (see Figure "Application and Transport
Division").
Each layer contributes functions to the communications task.
For example, the Link Layer enables communications across a
single physical connection, while the Network Layer provides
end-to-end routing and data relay. Services at the upper-layer interface--providing communications to the next~higher layerare provided by each layer, usually described by a service specification for the layer. Services at each layer are provided by a
layer entity. Each layer entity communicates with its peer at the
same layer on another system, providing services specified in the
service specification.

The Seven Layers of 051
Layer

Name

Purpose

Application

Applications and application interfaces for
OSI networks. Provides access to lower-layer
functions and services.

Presentation

Negotiates syntactic representation for the
Presentation Layer and performs data
transformations.

Session

Coordinates connection and interaction
between applications. Establishes a dialog,
manages and synchronizes the direction of
data flow.

Transport

Ensures end-to-end data transfer between
applications, data integrity, and service
quality. Assembles data packets for routing by
Layer 3.

Network

Routes and relays data units among network
nodes.

Data Unk

Transfers data units from one network node
to another over a transmission circuit.
Ensures data integrity between nodes.

Physical

Delimits and encodes the bits onto the
physical medium.

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Data Networking

ISO Reference Model
for Open Systems
Interconnection (051)

Standards Organizations
Several standards testing and
verification bodies have been
organized by vendor consortiums, govemment agencies,
and independent organizations. They have found that
developing conformance
specifications, producing testing suites, and conducting
comprehensive testing are
complicated, expensive, and
time consuming. Regional differences can stymie attempts
at verification. The trend for
these organizations, therefore,
is to cooperate with each
other, sharing resources and
expertise. A primary objective
is demonstrating interoperability among different vendors;
i.e., proving that standards
really work and fostering enduser interest. Many agencies
have tried vainly to involve
more end users but are
backed primarily by the
vendors.

ANSI (American National
Standards Institute)
ANSI
11 W.42nd Street
New York NY 10036, U.S.A.
Tel:+12126424900
Fax: +12123980023
http://www.ansi.org

ATMForum
World Headquarters
2570 West EI Camino Real
Suite 304
Mountain View, CA 940401313, U.S.A.
Phone: +1 4159496700
Fax: +14159496705
http://www.atmforum.com

European Office
Boulevard Saint-Michel 78
1040 Brussels, BELGIUM
Phone: +32 2 732 8505
Fax: +32 2 732 8485

Asia-Pacific Office
Hamamatsucho Suzuki
Bldg.3F
1-2-11 Hamamatsucho,
Minato-ku
Tokyo 105, JAPAN
Phone: +8133438 3694
Fax: +81 3 3438 3698

IEEE (The Institute Of
Electrical And Electronics
Engineers)
The Institute Of Electrical And
Electronics Engineers, Inc.
1828 l Street NW, Suite 1202
Washington DC 20036-5104,
U.S.A.
Tel: +1 908 981 0060
Fax: +19089810027
http://www.ieee.org

IBM's SNA is also a layered architecture, following rules of
layering similar to OSI and other layered architectures. There are
good reasons for layering: layering simplifies change; components inside a layer can be changed without affecting any other
layers in that node. Layers are like structured programming-but
for teleprocessing systems. Because there are rigid interfaces between levels, fewer people need to react to changes, allowing
them to be implemented faster. There is no better way of achieving complex functions. Layering allows each network function to
be made "transparent," unaware and independent of other functions at other layers, thus enabling any layer to be modified without changing the entire monolithic architecture.
Each layer may support one of several different protocols
designed for specific network applications; the choice of a specific protocol is optional, allowing users to tailor networks to
their own design. Each layer defines functions crucial to the communications process at that layer, independent of the other layers.
However, a layer may perform functions hinging on functions
performed in the layers immediately above or below. A layer can
only communicate with another device or network node at its peer
layer. Messages exchanged between peer layers are "enveloped"
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2783

Standards

IEEE Corporate Office
345 E. 47th Street
New York, NY, 10017, U.S.A.
Tel: +1 2127057900
IEEE Operations Center
445 Hoes Lane
Piscataway, NJ, 08855-1331,
U.S.A.
Tel: +1 908981 0060
IEEE European Operations
Center (Brussels)
Tel: +32 2 770 2242
Fax: 32 2 770 8505
E-mail: memserviceeurope@ieee.org

ISO (International
Organization for
Standardization)
ISO Central Secretariat
1, rue de Varembe
Case Postale 56
CH-1211 Geneva 20
Switzerland
Tel: + 4122 749 0111
Fax: + 4122 733 34 30
E-mail:
INTERNET: central@iso.ch
X.4OO: c=ch; a=4oonet; p=iso;
e=isocs; s=central
http://www.iso.ch
Please note: Copies of ISO
standards can be ordered
from local standards offices.

ITU (International
Telecommunications Union)
Place des Nations
CH - 1211 Geneva 20
Switzerland
Tel: +412273051 11
Fax (Group 3): +41 22 733
7256
Fax: (Group 4): +41 22 730
6500
http://www.itu.ch

European Computer
Manufacturers Association
(ECMA)
Rue du Rhone 114
CH-1204 Geneva, Switzerland
Tel: +41 22 849 6000
Fax: +41 22 849 6001
Telex: 413237 ECMA CH
http://www.ecma.ch

Electronic Industries
Association (EIA)
Electronic Industries
Association
2500 Wilson Boulevard
Arlington, VA 22201-3834
Tel: +1 703 907 7500
Fax: +1 703 907 7501
http://www.eia.org

with messages from other layers and passed through these other
layers on the way to their destination, picking up and then shedding these other protocol layers along the way. For example, if
layer seven at one end system must send a message to layer seven
at another, it must travel down through six layers at its own end
and then up through six layers at the other, until it reaches layer
seven at its opposite (peer) layer.
Each network node (a network user, computer, terminal) is
equipped with this layer mechanism. However, not all intermediate nodes need all seven layers. Network nodes, in particular,
must only route and transmit data packets-functions at the bottom three layers of the OS! model. Layer 4 through 7 functions
are not required and, therefore, not included in network node software. Data packets processed in these nodes reach only Layer 3
and are then routed elsewhere (see Figure "Message Movement
Among OS] l.o.yerS'). A node communicates with its peer in another node sending or receiving data. Data transfer is routed from
Layer 7 down to Layer 1 at the transmitting node, then along the
network to Layer 1 at the receiving node, and finally from Layer I
up to Layer 7. Peer layers communicate by the same method.

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ISO Reference .Model
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Interconnection (051)

2783

Standards

does not provide services to a layer above it. Some of the services
provided by this layer, other than information transfer, are the
following:

Figure
Application and Transport Division

• Identifying intended communications partners

7
Application

6
Presentation

• Determining current availability of the intended partners

Application
Platform

• Agreeing on responsibility for error recovery
• Agreeing on procedures for controlling data integrity

The Presentation Layer (Layer 6) allows an application to interpret the meaning of information exchanged, Information is formatted and translated at this layer. Aspects of Layer 6 include data
syntax, which is the data to be transferred between layers, and the
presentation image syntax, which is the data structure thatapplication entities refer to in their dialog, or the set of actions that may
be performed on the data structure.
Services provided to the Presentation Layer include the
following:

Session

4
Transport

3
2
Data Unk

• Establishing the authority to communicate

The Presentation Layer

Network

Data Networking

Transport
Platform

1
Physical

• Transforming data syntax, primarily code and character set
conversion
• Transforming and selecting the presentation syntax, the adaptation and modification of the presentation data (the OSI view)
Functions within the Presentation Layer include session establishment request; data transfer; negotiation and renegotiation of
data syntax and presentation image syntax; and session termination request.

The Session Layer
The seven layers are divided into two functional groups.

The Session Layer (Layer 5) allows cooperating presentation
entities to organize and synchronize their dialog and to manage
data exchange. It provides the following services:

• Session-connection establishment-creation of an exchange
between presentation entities

The Layers
A number of objectives were considered by the reference model's
designers: to limit the number of layers to make the system engineering task of describing and integrating the layers as simple as
possible; to create boundaries between layers at points where the
description of services can be small and the number of interactions across each boundary is minimized; and to collect similar
functions in the same layer. Table "The Seven Layers of OSf'
summarizes the OSI Reference Model's layers; more detailed
descriptions follow for each layer.

The App6cation Layer
The Application Layer (Layer 7) is the highest layer, providing
the means for the application process to access the OSI environment. Its function is to serve as the passageway between application processes using Open Systems Interconnection to exchange
information; consequently, all application process parameters are
made known to the OSI environment through this layer.
All services directly usable by the application process (i.e.,
systems and applications management functions) are provided by
the Application Layer. It differs from the other layers in that it

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• Session-connection release
• Normal data exchange
• Expedited data exchange
• Interaction management-allowing presentation entities to
take turns exercising control functions
• Session-connection synchronization
• Exception reporting-permitting the presentation entities to be
notified of exceptional situations
• Activity management

The Transport Layer
The Transport Layer (Layer 4) provides transparent data flow
between session entities, freeing the Session Layer from responsibility for cost-effective and reliable data transfer. Layer 4 provides information interchange according to a user-specified reliability level and end-to-end control. Transport protocols transfer
information from one end of a physical connection to another and
ensure that it is delivered correctly_ Layer 4 protocols are used
after a route has been established through the network by the
network-layer protocol.
The services provided by this layer include the following:
• Transport-connection establishment to complete a connection
between session entities

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Data Networking

ISO Reference Model
for Open Systems
Interconnection (OSI)

2783

Standards

Figure
Message Movement Among OSI Layers

Message moves from Layer 7 through ~
the other layers to reach Layer 7
at the opposite end

,....---------,
7

Application

6
Presentation

Session

4
Transport

3
Network

I-......;..=.;.;:.;..:.:.----I-~

3
Network

- - - - - - - - -

2
Data Link

3
Network

---------~~------_4

I-------+~-

Data Link

- - - - - - --

Data Link
---------~~------_4

1
Physical
L . - -__r------L~

1
Physical
- - - - - - - - -

Transmission
Media

'--v-----'

---------~~--~--~

'--v-----'

End System

Network Node

End System

Intermediate nodes in an OSI network require only bottom-layer functions of the OSI modeL Note how peer layers communicate only with their
peers; i.e., Layer I talks to other Layer Is but not to Layer 2s.

• Data transfer, in accordance with the agreed quality of service
• Transport-connection release

• Noting errors for reporting unrecoverable errors to the transport layer
• Sequencing network control data units

The European Computer Manufacturers Assn. (ECMA) has
defined this layer in its Transport Protocol standard, ECMA-72.
In the early 199Os, as the popularity of the OSI protocols began to wane and TCPIIP began to take over, a number of hybrid
stacks were developed whereby data streams could "cross over"
from one type of stack to another, in either direction. The crossover point was at the Transport Layer in all cases. This allowed
applications to reach their intended destinations whichever system they were using, and encouraged interoperability. Although
this was in keeping with the aims of the ISO, it made possible
migration to the newly-popular TCPIIP stack, and aided the eventual near-demise of the OSI stack as a complete set of protocols.

The Network Layer
The Network Layer (Layer 3) provides the means to establish,
maintain, and terminate connections between systems. Its basic
service is providing transparent data transfer between transport
entities.
The services provided by this layer encompass the following:
• Establishing network connections for transporting data between transport entities through network addresses
• Identifying connection endpoints
• Transferring network service data units
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• Row control
• Releasing the network connection
The Network Layer is where routers and, nowadays, some LAN
switches operate. They are indifferent to the type of network and
can therefore be used to pass data from one type of network to
another, for example from Ethernet to Token-Ring. Although
many routers have OSI protocol support, in fact it is little used, IP
being by far the preferred protocol for this layer. Given the importance of the Internet, which today runs almost entirely on IP, this
can only increase. However, the remarks made earlier as to how
the model has formed the way we think about networks holds
particularly true for this layer and Layer 2-nobody can talk coherently about networking without mentioning these layers.

2783

Staodards

Layers are sometimes divided into sublayers, for several reasons. Layer functions are often divided into separate modules to
handle the service interface of the layer beneath it. This avoids
"rewriting" the entire layer. The Data Link Layer of the IEEE 802
Local Area Network (LAN) standards is divided into a Logical
Link Control (LLC) sublayer and a Media Access Control (MAC)
sublayer. The MAC sublayer depends on characteristics of the
underlying" Physical Layer. Any layer may originate a message to
fulfill its responsibilities. The message may not bypass any layer
en route to its destination. If a message leaves the node, it will end
up in another node at the same layer that originated the message.
It is at Layer 2 that bridges and most LAN switches reside. All
such devices have a MAC address table for all the end stations
(which all have MAC addresses).
The Physical Layer
The lowest of the OSI layers is Physical Layer I. It provides the
electrical, mechanical, functional, and procedural characteristics
for activation, maintenance, and deactivation of a physical connection. Physical Layer standards specify physical interfaces
(connectors) connected by a physical medium.
Services provided by this layer include the following:
• Activating and deactivating physical connections
• Data circuit identification
• Sequencing
• Transmitting physical service data units either synchronously
or asynchronously
• Fault condition notification

Abstract Syntax Notation One (ASN.1)
ASN.I is a specification language adopted for the OSI Reference
Model, giving standards developers a common method for defining syntax. ASN.I is somewhat analogous to grammatical rules
defining the English language, with the exception that it is not
procedural. Just as English grammar specifies notation (punctuation symbols) and word classifications (such as nouns and verbs),
ASN.I specifies the rules that help standards developers define
complex data types in terms of simple building blocks.
ASN.I was first formally described and published in 1984, in
the ITU-T X.409 standard entitled "Message Handling Systems:
Presentation Syntax and Notation." It is now described (in less
readable fashion) in two later documents: ITU-T X.208 (ISO
8824), entitled "Specification of Abstract Syntax Notation One
(ASN.l)," and X.2OO (ISO 8825), "Basic Encoding Rules for
Abstract Syntax Notation One (ASN.l)."
According to ASN.I, each fragment of information must possess a type and a value. For example:
.• Device-Status could be a type (in this case, it is a Boolean tyj,e)
• Zero or One are the possible values
This is specified in ASN.I notation as such:
Device-Status ::= Boolean
Boolean ::= I (or 0)
This is a very simple example; ASN.I is a powerful grammar,
capable of specifying very complex data types. Hence, it will
continue to be the grammar of choice for specifying open systems
standards and protocols.

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ISO Reference Model
for OpenSyatems
Interconnection (OSI)

Data NetworKing

OSI Standards Progress
There are four stages in the developmerit cycle: working paper;
committee draft (CD), previously known as a draft proposal; draft
international standard (DIS); and international standard (IS). A
working paper is developed in the first stage. When it matures and
contains well-developed technical concepts, it is registered as a
CD. Passage advances the CD to the DIS level, and the document
is considered sufficiently stable to serve as the basis of initial
implementations. At the DIS level, the document is distributed for
a 180-day ballot. The DIS may require multiple ballots. A successful ballot elevates the DIS to the level of IS and completes
ISO's process.
The entire process usually takes between four and eight years.
Here we can see one of the reasons for the lack of popularity for
the protocols involved. The ITU-T was formerly the CCITT,
under whose aegis these standards were set. The CCITT has now
been devolved into the International Telecommunications Union
(ITU). Telecommunications standards can be set over a long
period of time as the voice and WAN world needs to ensure high
quality and complete interoperability between systems. Data networking, on the other hand, in many cases exists in isolated
LANs, and though these are almost always today connected to the
outside world in some manner, a gateway of some sort, often IP,
gives enough connectivity. For Wide Area Networks (WANs) the
same needs may apply as for voice but there is a lot of crossover
between the types of device used and the WAN manufacturers
have adopted the same standards as are used in LANs in many
cases. With the new standard, Asynchronous Transfer Mode
(ATM), the standard was set for WANs, but became developed by
the LAN vendors far more quickly. Given the fast-moving nature
of LAN developments, with low-cost high-volume products
appearing in ever greater numbers and new high-specification
models ensuring replacements in ever-shorter times, it is easy to
see that a standards process that takes up to eight years to complete is not workable. LAN standards today are proposed one
year and a first version out the next. Again, the ISO process that
demands a complete standard before the first release is ignored.
as soon as a halfway working standard can be given out, it is, with
a further release the year after. A case in point is the new LAN
standard for virtual LANs, being developed by the IEEE 802.lq
working group-the first release, planned for mid- to late 1997,
has only the facilities for port-switching LANs, whereas the idea
of completely separating the logical from the physical LAN
(which is what a virtual LAN can do) needs far more than that to
be efficient.
A list of standards organizations, together with contact details,
associated with the OSI Reference Model and other standards
mentioned here is given at the end of this report.

OSI Applications Standards
In the early 1990s, applications at Layer 7 were considered the
driving force of OSI acceptance. In particular, the ISO electronic
mail standard for message handling systems (MHSs)-or
X.400-was becoming popular in commercial implementations.
Two versions, one in 1984, and another in 1988, were unratified
draft international standards. The standard was given a big boost
in 1989, when the Aerospace Industry Assn. (AlA) adopted X.400
to interconnect its diverse electronic mail networks. Gateways to
proprietary E-Mail systems were also developed that year, and
dozens of vendors rolled out X.400-based products. In addition,
the ISO adapted XAOO as the message medium for electronic data
interchange (EDI). In this context, X.400 was to be used as the
communications method to· store and forward trade documents
and business forms conforming to ANSI X 12, the European EDIFACT, and de facto EDI standards. Alas, MHSIX.400 proved

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ISO Reference Model
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costly and complicated to implement, and was overtaken by
other, simpler, electronic mail standards, particularly, since the
mid-199Os, Internet mail based on IP.
One component of successful E-Mail internetworking is directory services (OS), with the ISO version known as X.500. Again,
this seems to be unworkable and, while no international standard
has taken over, Novell's NOS and Banyan's StreetTalk provide
most of the directory functionality in use today. Microsoft is also
in the process of developing a similar function. In today's open
networking environment, reliance on proprietary software is unusual (unless as pre-standard releases) and it can only be concluded that X.500 proved more or less unworkable, although
Banyan declared its StreetTalk to be based very much upon X.500
and obviously drew upon it for a lot of the ideas.
X.500 specifies an on-line directory for message communications, ultimately allowing network providers to map a common,
interconnected directory of worldwide users. X.500 dictates naming conventions, how users access directory information, and
what services are available.
The OSI protocol pair for office automation, Office Document
Architecture (aDA) and Office Document Interchange Format
(ODIF), has been an international standard since 1988 (ISO
8613). It was also specified in the various governments' GOSIP
procurement standards until the demise of GOSIP worldwide in
the early 199Os. aDA and ODIF facilitate the exchange of office
documents-such as letters, memoranda, and business reportsamong dissimilar systems. Moreover, the standard specifies the
formatting and exchange of compound documents-those containing combinations of text, images, and graphics. Several ISO
working groups are attempting to strengthen and extend the standard in such areas as the inclusion of audio, spreadsheet data,
color graphics, document security, and various layout and presentation styles.
The OSI standard for sending and sharing data files-File
Transfer, Access, and Management (FTAM)-is also a finalized
international standard. It is a Layer 7 component of the Manufacturing Automation Protocol (MAP), which was developed from
networking efforts in the manufacturing industry. It spread
quickly in Europe and made significant progress in domestic
business applications, but the file protocol used with Transmission Control Protocol/lnternet Protocol (TCPIIP)-File Transfer
Protocol (FfP)-was also well established in the U.S., however,
and generally more popular than FTAM. FTPhas since taken over
from FTAM for most file transfer and is the established way of
downloading large files from Internet sites. It was thought that
FfAM would become the standard for EDI transfers but, again, it
became apparent that it was too cumbersome, and that was when
X.400 became popular for that. The difference in use between
X.400 (or any messaging system) and FTAM (or any other file
transfer protocol such as FfP), is that file transfer is real time
while X.400 is store and forward. With any store-and-forward
system, there can be long delays while the information gets
through the network-with FfAM and FTP any delays during
transmission are usually brief, but are variable depending upon
bottlenecks on a large system such as the Internet.
The Layer 7 protocol for network management, Common
Management Information Protocol (CMIP), is now an International Standard. CMIP is a communications protocol between the
agent process, and management agents at each managed OSI
node. The real work of managing network processes is located
within each node's managed objects at individual OSI layers. In
other words, each layer must have its own network management
system, which OSI does not specify. CMIP allows a centralized
management process to either modify the value of an attribute, or
request its value (read its status) at each of the layers. While
CMIP has been more or less superseded by the Simple Network

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Management Protocol (SNMP) in the LAN, CMIP is still popular
for WAN management, showing once again how different the two
worlds can be. SNMP has taken up the structure for management
that OSI set up with CMIP and now does the job of managing
networks perfectly well. (For more information, see the OSI
Management section featured later in this report.)
Other Layer 7 protocols include distributed Transaction Processing (TP), designed to interconnect different transaction computing systems across OSI networks; Remote Database Access
(RDA), a protocol for integrating database management systems;
and Manufacturing Message Specification (MMS), ISO 9506, a
manufacturing protocol that requires extensions for specific
manufacturing device types.
Connection Methods
Every layer of the OSI Reference Model, except the Physical
Layer, supports both connection and connectionless mode (this is
one of the reasons the protocols are so complex and expensive to
implement-{)ther technologies are one or the other). Connection-oriented service requires a connection establishment phase, a
data transfer phase, and a connection termination phase; a logical
connection is set up between end systems prior to data exchange.
These phases define the necessary sequence of events for successful data transmission. Connection-oriented service capabilities include data sequencing, flow control, and transparent
error handling.
In a connectionless service, such as Switched Multi-megabit
Data Service (SMDS), each Protocol Data Unit (PDU) isjndependently routed to the destination; no connection establishment
activities are required, since each data unit is independent of the
previous or subsequent one. Connectionless-mode service transfers data units without regard to establishing or maintaining connections. In connectionless mode, transmission delivery is uncertain due to the possibility of errors. This appears contrary to the
goal of network design--users want to ensure that messages
reach their destination. In reality, connectionless-mode communication simply shifts responsibility for message integrity to a
higher layer, which checks integrity only once, rather than requiring checks at each lower layer. Alternatively, each data unit might
contain the error recovery mechanism.
In connection-oriented networking, such as ATM, a connection is established for the whole data stream just once, and all
packets or cells follow that path. There is now also what might be
thought of as a hybrid never dreamed of in OSI-IP switching,
where the first packet is examined but the rest follow the path of
the first through the switch, despite the connectionless nature of
the protocols involved. This depends on having a switched network. Multiprotocol Over ATM (MPOA) will perform the same
function when standardized late in 1997.
Lower-Layer Protocols
Lower-layer OSI protocols for Layers 1 through 3 are welldefined veterans and in many cases borrowed from existing EIA,
IEEE, or ITU-T standards_ Connectionless communications at the
lower layers of the OSI model is well established and is found, for
example, in LANs and metropolitan area networks (MANs).
While the original OSI model-described in ISO 7498-was
connection oriented, the ISO foresaw the need for connectionless service and issued an addendum to that protocol (ISO 74981
ADI). The ISO standard for Network Layer service, ISO 8348,
contains connectionless service (in ADl) in addition to the connection mode.

OSI Security
By definition, an open system is one that encourages communications between different applications or users. Unfortunately, an

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open system can also encourage illegal eavesdropping and information theft or destruction. Notorious examples of white-<:ollar
crime, corporate espionage, and network intrusions by computer
worms and viruses have alarmed information processing professionals, and raised a general awareness of computer security issues. The concepts of information security and open systems are
antithetical; nevertheless, the ISO has taken steps to provide a
secure environment within the OSI Reference Model.
International Standard 7498, Part 2 addresses a security
architecture within the general OSI model. It describes security
measures that can be provided by specific layers in the model.
Specific security standards are not yet defined, however, but
are under study by working group JTC1, Subcommittee 27
for Information Technology Security Standards, plus other subcommittees.
SC2l, concerned with maintaining and defining the upper
three layers of the OSI Reference Model, stabilized several network management and security standards in 1991. The Security
model is composed of six frameworks that work together across
all seven layers of the OSI Reference Model: authentication,
access control, security audit, nonrepudiation, confidentiality,
and integrity.

OSI and MAP/TOP
The Manufacturing Automation Protocol and Technical Office
Protocol (TOP) were originally developed by General Motors and
Boeing Computer Services, respectively; to automate manufacturing functions on the factory floor and in the "back office." Both
are based on the OSI Reference Model, using formal standards
for each layer where possible. MAP, in particular, is probably the
best-known example of a formal multilevel OSI implementation
and is achieving substantial industry acceptance.
Version 3.0 added a Presentation Layer to the profile and
implemented a version of the Manufacturing Message Specification (MMS), the protocol for transferring factory and robotics
information, ISO 9506. Other Layer 7 protocols specified are
FfAM, Network Management, and Directory Service. Middle
layers implement ISO connection-oriented protocols, although
these must be bypassed for time-critical applications. At the lower
transport layers, MAP specifies the IEEE 802.4 token bus system
employing a Type F coaxial connection to a 75-ohm cable. Although MAP was taken up widely in some countries, notably
Japan, it has not been further pursued generally. LANs have
moved on and few new LANs use token bus or coaxial cable now
(except that in the new field of home LANs, coaxial cable is often
employed for its ease of use, but standard Ethernet is universally
installed here).

OSI and TCP/IP
Transmission Control ProtocollInternet Protocol was developed
by the U.S. government's Defense Advanced Research Projects
Agency (DARPA) for its research network, ARPANET. By 1986,
TCPIIP had gained a following of commercial users seeking a
protocol that could be used as a common denominator for multivendor computer networks. TCP and IP are actually two separate
protocols, occupying middle layers number Four (Transport) and
number Three (Network), respectively, of the OSI Reference
Model.
TCPIIP has been implemented on almost every type of computer and is especially successful in commercial Ethernet LAN
environments. The reason TCPIIP is so popular is because it is
free and its development was paid for by the U.S. government. It
avoids the connection-orientedlconnectionless dilemma by essentially avoiding it. OSI provides a richer set of network options,
but these may not be compatible in different networks. Users
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cannot communicate across different networks if they implement
different options at these layers.
Despite OSI's early promise, a majority ·of networks no:w use
TCPIIP for LAN interconnectivity. The Internet, the. tool and
plaything "Of the 1990s that is ARPANET's grandchild, runs
almost exclusively on IP, though ATM is hidden now in the infrastructure, and we can expect this to increase as ever more bandwidth is needed. TCPIIP is also used for Ethernet LANs, and
Novell has recently withdrawn its proprietary IPX from new NetWare installations, preferring instead to migrate customers to
pure IP.

Advantages and Disadvantages to OSI·Based
Network Management
In addition to solving the problem of managing heterogeneous
environments, OSI-based network management played a part in
bringing about a new phenomenon-the unbundling of network
management from network products. In a proprietary enVlron- .
ment, a given vendor's products are primarily manageable only
by products developed by that vendor. The promise of OS I helped
to split that one-to-one relationship, making it possible for any
OSI-based network management system (NMS) to manage any
OSI Management-conformant device. Despite being widely supported, CMIP has in the end lost out to SNMP in the LAN environment, but the initiative encouraged the widespread publication
of private Management Information Bases (MIBs) so that heterogeneous networks could be managed under SNMP. In the WAN,
CMIP is both widespread and popular. Here, there is not such an
open environment due to lack of customer demand for it, and such
interoperability as there is must be reliable. WAN management is
vital--LAN management is still often, wrongly, treated as a
lUXUry.
The market (both vendors and users)widely criticized ISO for
moving too slowly in its efforts to ratify OSI Management standards. Vendors are wisely unwilling to develop products based on
standards that are not yet final. In an effort to open the door to
new OSI~based network management system products, SC21
WG4 defined groups of network management functions that were
to be covered within OSI-based network management: fault, configuration, performance, accounting, and security management
specifications. These principles, if not the actual protocols, have
been established as the expected field to be covered by any network management system (NMS), and this is one of the areas that
OSI has changed networking profoundly.
Another disadvantage of standards-based network management is that OSI standards merely provide a menu of options.
There are numerouS gaps and ambiguities in OSI Management
standards that could be interpreted differently, leading to incompatible implementations.

Standards Documents
OSI Management standards can be broadly categorized into four

areas:
1.

Functions--what network management is, according to OSI

Services--how network management functions are accomplished

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3.
4.

ISO Reference Model
for Open Systems
Interconnection (OSI)

Information Structure-terms and categories describing
what is managed (e.g., "management information")
Protocols-describe means of transporting network management information

Taken together, these four areas describe a generic package for
network management systems, and how these products relate to
the network devices they manage (called managed objects in OSI
terminology).
A blueprint document, Management Framework OMNIPoint,
places the OSI Management environment in perspective by
describing terms and the scope of OSI network management.

OSI Management Functions
OSI Management functions are described in the Systems Management standards (IS 10040, IS 10164-1 through 10164-7, and
N 10164-8 through 10164-12). Management using three models:
1.

The Organizational Model---describes ways OSI Management can be distributed administratively

The Information Model-provides guidelines for defining
managed objects and their interrelationships, classes, and
names

The Functional Model-describes network management
functions

The Functional Model outlines how ISO has partitioned network
management into five functional areas: fault management, configuration and name management, performance management,
accounting management, and security management. ISO originally described each of these areas in its own standard. Further
studies revealed that functions overlapped; therefore, ISO reorganized the documents in December 1988 into their present Systems Management form.
Fault management provides the detection, isolation, and correction of abnormalities in network operation. Configuration
and name management facilities permit network managers to
control the configuration of the system, network, or layer entities.
Changed configurations may isolate faults, alleviate congestion,
or meet changing user needs. Performance management enables
the network manager to monitor and evaluate the performance of
the system, network, and layer entities. Data from performance
management may be used to initiate configuration changes and
diagnostic testing to allow a satisfactory level of performance.
Accounting management facilities help determine and allocate
costs for the use of a network manager's communications resources. Security management facilities permit the management
of those services providing access protection of communications
resources.

Services

(If"

Services are described, in part, in the Common Management Information Protocol (CMIP) standard, IS 9596. Services use
primitives, or command types, to accomplish network management functions. Examples of CMIP commands include GET,
SET, GET REPORT, CREATE, and DELETE. While service
primitives are somewhat abstract, they are important building
blocks for composite commands used by network management
applications to obtain vital data on the status and activity of network devices.
Common Management Infonnation Services (CMIS) include
a detailed abstract model of open systems management services.
These fall into three categories--event notification, information
transfer, and control. Event notification allows one system to notify another that some event of importance has occurred.

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Information Structure
The most important standards in this category are Structure of
Management Information (SMI), Parts I, 2, and 4 (CD 1Ol6?-I,
-2, and -4). (Part 3 is not missing; rather, ISO merged Part 3 mto
Part 4.) Included in these standards is an explanation of the
object-oriented paradigm, used to model a network in terms of
object classes and attributes. In object-oriented environments, a
variable (for example, a variable called Bridge) is defined both in
terms of the operations that can be performed on it and the values
of attributes it can possess. For example, Bridge can have an
attribute such as Status, which may have a value of Busy; a network management system may obtain this value via a Get operation, or alter it via a Set operation.
Objects (including their attributes and operations) are stored
in a MIB, sometimes called an Object Library. The SMI documents just listed provide syntax and semantics for information in
the MIB; however, no single ISO standard defines exactly what
the OSI MIB will contain, nor how vendors and users can register
objects in the standard MIB.
In the TCPIIP world, an Internet Standard MIB exists for
objects managed using SNMP. This MIB functions in an analogous role to the proposed OSI MIB, although the administration
and rules governing the two are sure to differ.
MIB includes all information needed to make management
decisions. MIB is a conceptual repository of all OSI management
data in an OSI environment. The MIB concept does not imply any
form of physical or logical storage for management infonnation,
however, and its implementation is outside the scope of OSI standards. Rather, the SMI defines the abstract syntax and the semantics of information, so that it can be represented in OSI protocol
exchanges.

Protocols
Common Management Information Protocol, IS 9596, is the primary OSI Management protocol. CMIP specifies procedures for
the exchange of basic management information between open
systems interconnected by OSI protocols.
X.500.-The Directory
The Directory is a related standard designed to manage namerelated infonnation concerning protocol layers and network
nodes. These services connect the actual names used in the network with names and addresses understood by human users. The
Directory is defined in CD 9594.

OSI and the Future
The world needs a network of computers, similar to standards for
international telephony, to link users across oceans and continents. A few years ago, most industry analysts perceived OSI as
the answer. Although endorsed by such prominent vendors as
IBM, DIGITAL, and Hewlett-Packard, OSI's once-bright future
as the premier means of interconnecting multivendor computer
networks is now dimmed. Where OSI was too complex, slow, and
expensive, TCPIIP stepped in to fill the gaps. The OSI Reference
Model also had some technical glitches and holes that prevented
it from being widely implemented. These will probably never
now be repaired. As an internetworking protocol, TCPIIP has
proved its worth and is a popular and commercially succ~ssful
method of linking users across diverse networks. OSI IDlddlelayer protocols 3 through 5, the altematives to TCPIIP, are not as
simple to implement in the real world. SNMP, the network management protocol for TCPIIP networks, is also a proven, comm7rcially successful solution. As long as vendors and users requIre
practical networking products, they will continue using TCPIIPbased protocols-standard or not.

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In reality, OSI and other layered architectures .do not serve
every application and are not a panacea. Proprietary architectures
, will continue to thrive alongside both de j(lCto and de jure standards-based networks, especially for closed user groups (where
internetworking is not a requirement) or in time-sensitive applications intolerant of layered protocols' high overhead,
All others who desire internetworking must realize that the
associated protocols are still evolving-nothing is truly cast in
iron. In market-based economies, products that do .not satisfy
market needs will not gain widespread favor. Therefore, prospective users must evaluate OSI protocols and their adoption with an

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. eye toward future standards developments. As it stands now, OSI
will not be the interconnection standard of the future .. Significant
portions of it, however, have most certainly contributed to the
evolution of international standards. The OSI 7-Layer model itself, however, has proved itself over and over. This view of data
communication has become universally accepted, with even
SNA-which is layered differently-often explained in the OSI
model's terms. While the standards and protocols may not
develop, the structure has added greatly to a common understanding of networking and has now also stood the test of time. -

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(

ISO Reference Model for
Open Systems
Interconnection (OSI)
In this report:

Datapro Summary

051 Standards Progress ..... 6

The goal of Open Systems Interconnection (OSI) is to enable dissimilar computers in mul-

051 Management ..•..•.........• 9
051 and the Future .....••..... 11

Note: This report updates
the OSI's status at all
seven layers; compares
OSI to other architectures;
rationalizes the need for
standards testing and verification; profiles major
testing organizations; and
outlines OSI Management
standards and status.

tivendor environments to share information transparently. The OSI structure calls for cooperation among systems of different manufacture and design. With this capability, global
digital networks can become a reality. There are seven layers of the OSI model that communicate between one end system and another. The layers cover nearly all aspects of information flow, from applications-related services provided at the Application Layer to the physical connection of devices to the communications medium at the Physical Layer. Although
all seven layers have long since been defined and ISO protocols ratified for each layer, the
ISO committees must keep refining and extending specific sections of the model by rewriting existing definitions and adding new protocols.

Analysis
The proliferation of computerized data processing systems in the late 1960s produced a need for
compatible data communications networks in
the 1970s. Several proprietary network architectures were developed for mainframe-to-tenninal
communications, including IBM's SNA in 1974.
Although many of these proprietary architectures were based on a layered model, none were
compatible with any other. The CClTf's X.25
host interface to the packet networks standard
was ratified in 1976 but is not a complete network architecture. In 1977 the International Organization for Standardization (ISO) formed
ISO Technical Committee 97 (TC97), Subcemmittee 16 (SC16), to embark on a worldwide
standardization effort and confront the issue of
incompatibility head-on. The pUIpOse of TC97/
SC 16 was to develop a model and define the protocols and interfaces required to support an open
system. The goal of OSI was, and still is, to enable dissimilar computers in multivendor environments to share infonnation transparently.
With this capability, global digital networks can
become a reality.

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The Open System
The ISO defines a system as a set of one or more
computers and associated software, peripherals,
terminals, human operators, physical processes,
infonnation transfer means, etc., which fonn an
autonomous whole capable of perfonning information processing and/or infonnation transfer.
An open system is one that obeys OSI standards
in its communication with other systems.
An application process is an element within a
system that performs infonnation processing for
a particular application. The application process
can be manual (a person operating a banking terminal), computerized (a program executing in a
computer center and accessing a remote database), or physical (a process control program executing in a dedicated computer attached to industrial equipment and linked to a plant control
system).
The OSI structure calls for cooperation
among systems of different manufacture and design. This includes coordinating activities such
as the following:
• Intetprocess communications-the synchronization between OSI application processes
and the exchange of infonnation
• Data representation-the creation and maintenance of data descriptions and transfonnations for refonnatting data exchanged between systems

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• Data storage---storage media, file systems, and database systems for providing access to and management of stored data
• Process and resource management-how application processes
are declared, initiated, controlled, and acquired
• Integrity and security-information processing constraints that
must be ensured during open systems operations
• Program support-the definition, compilation, testing, linking,
storage, and transfer of and access to programs executed by the
application processes
The OSI model is concerned only with the exchange of information between open systems.

The Layering Concept
Layering is a basic structuring technique used in the OSI model.
Each layer is composed of an ordered set of subsystems, with
logically related functions grouped together. The OSI model
breaks down internetworking activities between systems into two
distinct groups. Communications-oriented functions are separated from user-oriented functions; features which move information across a network are distinct from features which handle and
format information.
There are seven layers of the OSI model that communicate
between one end system and another end system. The layers
cover nearly all aspects of information flow, from applicationsrelated services provided at the Application Layer to the connection of devices to the communications medium at the Physical
Layer. Below the Physical Layer, the media itself corresponding
to "Layer 0"-such as wire, cable, or through-the-air communication-is currently not addressed by the model. Application,
Presentation, Session, Transport, Network, Data Link, and Physical Layers have been dermed (see Table 1). The model described
in Table 1 is OSI's seven layers with their purposes. In Table I,
information flows down from Layer 7 to Layer I, and then out
over a physical transmission medium. At the receiving end, the
information flows into another end system and up from Layer 1 to
Layer 7, until it is received by a user.

The seven layers can be divided into two functional groups:
the Transport Platform (Layers 1 to 4) and the Application Platform (Layers 5 to 7). The Transport Platform's function is to get
data from one system to another without errors. The Application
Platform's function is to interpret the data stream and present it to
the user in a usable form (see Figure 1).
Each layer contributes functions to the communications task.
For example, the Link Layer enables communications across a
single physical connection, while the Network Layer provides
end-to-end routing and data relay. Services at the upper-layer interface-providing communications to the next-higher layerare provided by each layer, usually described by a service specification for the layer. Services at each layer are provided by a
layer entity. Each layer entity communicates with its peer at the
same layer on another system, providing services specified in the
service specification.
Layers are sometimes divided into sublayers, for several reasons. Layer functions are often divided into separate modules to
handle the service interface of the layer beneath it. This avoids
"rewriting" the entire layer. For example, the Link Layer of the
IEEE 802 Local Area Network (LAN) standards is divided into a
Logical Link Control (LLC) sublayer and a Media Access Control (MAC) sublayer. The MAC sublayer depends on characteristics of the underlying Physical Layer. Any layer may originate a
message to fulfill its responsibilities. The message may not bypass any layer en route to its destination. If a message leaves the
node, it will end up in another node at the same layer that originated the message.
IBM's SNA is also a layered architecture, following rules of
layering similar to OSI and other layered architectures. There are
good reasons for layering: layering simplifies change; components inside a layer can be changed without affecting any other
layers in that node. Layers are like structured programming-but
for teleprocessing systems. Because there are rigid interfaces between levels, fewer people need to react to changes, allowing
them to be implemented faster. There is no better way of achieving complex functions. Layering allows each network function to

Table 1. The Seven Layers of OSI
Layer

Name

.Purpose

Application

Applications and application interfaces for
OSI networks. Provides access to lower-layer
functions and services.

Presentation

Negotiates syntactic representation for the
Presentation Layer and performs data
transformations.

Session

Transport

Coordinates connection and interaction
between applications. Establishes a dialog,
manages and synchronizes the direction of
dataflow.
Ensures end-ta-end data transfer between
applications, data integrity, and service
quality. Assembles data packets for routing
by Layer 3.

Network

Routes and relays data units among network
nodes.

Data Link

Transfers data units from one network node
to another over a transmission circuit.
Ensures data integrity between nodes.

Physical

Delimits and encodes the bits onto the
physical medium.

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(
Standards Organizations and
Testing Agencies
Although standards are indispensable for computer internetworking, they are useless
unless vendor claims of compatibility can be tested and
users can be ensured of 00quiring products that comply
fully with the standards. In
general, vendor claims of
standards compatibility are
suspect unless verified by an
impartial testing agency. Additionally, compatibility claims
can mean different things to
different people. For users, it
is a good idea to probe vendor
claims of standards compatibility to determine what is
compatible with what, and at
what levels.
Several standards testing and
verification bodies have been
organized both here and
abroad by vendor consortiums, government agencies,
and independent organizations. They have found that
developing conformance
specifications, producing testing suites, and conducting
comprehensive testing are
complicated, expensive, and
time consuming. Regional differences can stymie attempts
at verification. The trend for
these organizations, therefore,
is to cooperate with each
other, sharing resources and

expertise. A primary objective
is demonstrating interoperability among different vendors;
i.e., proving that standards
really work and fostering enduser interest. Many agencies
have tried vainly to involve
more end users but are
backed primarily by the vendors.
In the U.S., primary testing
and certification agencies include the Corporation for
Open Systems (COS), the
National Institute of Science
and Technology (NIST), and
Bell Communications Research (Bellcore). Several
smaller organizations and certain vendors, however, also
offer testing services. Europe
is represented by the Standards Promotion & Application
Group (SPAG); Japan by the
Promoting Conference for OSI
(POSI). Standards organizations are listed below:
American National Standards Institute (ANSI)
1430 Broadway
New York, NY 10018
(212) 642-4900
ANSI X3 Secretariat
Computer and Business
Equipment Manufacturers
Assn. (CBEMA)

be made "transparent," unaware and independent of other functions at other layers, thus enabling any layer to be modified without changing the entire monolithic architecture.
Each layer may support one of several different protocols designed for specific network applications; the choice of a specific
protocol is optional, allowing users to tailor networks to their own
design. Each layer defines functions crucial to the communications process at that layer, independent of the other layers. However, a layer may perform functions hinging on functions performed in the layers immediately above or below. A layer can
only communicate with another device or network node at its peer
layer. Messages exchanged between peer layers are "enveloped"
with messages from other layers and passed through these other
layers on the way to their destination, picking up and then shedding these other protocol layers along the way. For example, if
layer seven at one end system must send a message to layer seven
at another, it must travel down through six layers at its own end

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Customer Service Hot Line:
(800) 521-CORE
Corporation for Open Systems(COS)
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Telex: WU16503157578 MCI
UW
European Computer Manufacturers Assn. (ECMA)
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CH-1204 Geneva, Switzerland
(+41) 22 735 36 34
Telex: 413237 ECMA CH
Electronic Industries Assn.
(EIA)
1722 Eye Street NW, Suite
300
Washington, DC 20006
(202) 457-4900
International Organization
for Standardization (ISO)
1, Rue de Varembe
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C8-1211 Geneva 20, Switzerland
(+41) 22 33 34 30
International Telegraph and
Telephone Consultative
Committee (CCITT)
General Secretariat
International Telecommunications Union (ITU)

Place des Nations
CH-1211 Geneva 20, Switzerland
(+41) 22 99 51 11
Fax: (+41) 22 33 72 56
Telex: 421 000 UIT CH
Institute of Electrical and
Electronics Engineers
(IEEE)
Headquarters
345 E. 47th Street
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(212) 70~7900
Fax (publications): (212) 70~
7682
IEEE Service Center
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445 Hoes Lane, P.O. Box
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Piscataway, NJ 0885~1331
(908) 981-1393
Fax: (908) 981-9667
Telex: 833-233
OSINET
Mr. Jerry Mulvenna, Chairman
OSINET Steering Committee
NIST Building 225, Room
B217
Clopper Road
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Standards Promotion & Application Group SA (SPAG)
Avenue Louise 149, Box 7
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National Institute of Standards and Technology
(NIST)
U.S. Department of Commerce
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(301) 97~2000

and then up through six layers at the other, until it reaches layer
seven at its opposite (peer) layer.
Each network node (a network user, computer, terminal) is
equipped with this layer mechanism. However, not all intermediate nodes need all seven layers. Network nodes, in particular,
must only route and transmit data packets-functions at the bottom three layers of the OSI model. Layer 4 through 7 functions
are not required and, therefore, not included in network node software. Data packets processed in these nodes reach only Layer 3
and are then routed elsewhere (see Figure 2). A node communicates with its peer in another node sending or receiving data. Data
transfer is routed from Layer 7 down to Layer 1 at the transmitting node, then along the network to Layer 1 at the receiving
node, and finally from Layer 1 up to Layer 7. Peer layers communicate by the same method.
.

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does not provide· services to a layer above it. Some of the services.
provided by this layer, other than information transfer,are the'-<_
following:
• .Identifying intended communications partners
• Determining current availability of the intended partners

Figure 1.
ApplictJlion tuUl Transport Divisions

7
AppHcation

6
Presentation

Application
Platform

5
Session

4
Transport

3
Network
2

Transport
Platform

Data Link
1

Physical
The seven layers are divided into two junctional groups.

The message initiated at the Application Layer is passed from
layer to layer, through the various OSI layers, encapsulating control information in the process. A fully encapsulated message enters the cable at Layer 1. The procedure is reversed at the receiving end. Each item of control information is processed at its
appropriate layer,and the message itself passes upto Layer 7.
Data transfer essentially is a packaging process at the transmitting
node and an unpackaging process at the receiving node.
The Lare,.
A number of objectives were considered by the reference model's
designers: to limit the number of layers to make the system engineering task of describing and integrating the layers as simple as
possible; to create boundaries between layers at points where the
description of services can be small and the number of interactions across each boundary is minimized; and to colleCt similar
functions in the same layer. Table 1 summarizes the OSI Reference Model's layers; more detailed descriptions follow for each
layeL
.
The Application Layer
The Application Layer (Layer 7) is the highest·layer, providing
the means for the application process to access the OSI environment. Its function is to serve as the passageway between application processes using Open Systems Interconnection to exchange
information; consequently, all application process parameters are
made known to the OSI environment through this layer.
All services directly usable by the application process (Le.,
systems and applications management functions) are provided by
the Application Layer. It differs from the other layers in that it
JUNE 1993

Data Networking

• Establishing the authority to communicate.
• Agreeing on responsibility for error recovery
• Agreeing on procedures for controlling datil integrity
The Presentation Layer
The Presentation Layer (Layer 6) allows an application·to interpret the meaning of information exchanged. Information is formatted and translated at this layer. Aspects of Layer 6 include data
syntax, which is the data to be transferred between layers, and the
presentation image syntax, which is the data structure that application entities refer to in their dialog, or the set of actions that may
be perfomied on the data structure.
Services provided to the Presentation Layer include the following:
• Transforming data syntax, primarily code and character set
conversion
• Transforming and selecting the presentation syntax, the adaptation and modification of the presentation data (the OSI view)

Functions within the Presentation Layer include session establishmentrequest; data transfer; negotiation and renegotiation of
data syntax and presentation image syntax; and sessiontermination request.
The ~OD Layer
The Session Layer (Layer 5) allows cooperating presentation entities to organize and synchronize their dialog and to manage data
exchange. It provides the following services:
• Session-connection establishment-creation of an exchange
between presentation entities
• Session-connection release
• Normal data exchange
• Expedited data exchange
• Interaction management-allowing presentation entities to
take turns exercising control functions

• Session-connection synchronization
• Exception reporting-permitting the presentation entities to be
notified of exceptional situations
• Activity management
The Transport Layer
The Transport Layer (Layer 4) provides transparent data flow
between session entities, freeing the Session Layer from responsibility for cost-effective and reliable data transfer. Layer 4 provides information interchange according to Ii user-specified reliability level and end-to-end control. Transport protocols transfer
information from one end of a physical connection to another and
ensure that it is delivered correctly. Layer 4 protocols are used
after a route has been established through the network by· the
network-layer protocol.

The services provided by this layer include the following:
• Transport-connection establishment to complete a connection
between session entities

1993 McGraw-HIlI. Incorporated. Reproduction Prohibited.
Datapro Infonnation Services Group; Delran NJ 08075 USA

."----

ISO Reference Model for
Open Systems
IntercOlHWCtlon (OSII

Data Networking

2783

Standards

Figure 2.

Message Movement Among OSI Layers

Message moves from Layer 7 through - - - - - - . .
the other layers to reach Layer 7
at the opposite end

7
Application

~------------------------------------------

6
Presentation

~------------------------------------------

5
Session

4
Transport

~------------------------------------------

3
Network

--------------

2
Data Link

- -------------

1
Physical

ion~

Transmiss
Media \......_ _

..
..
..
..

-------------

3
Network

..
..

)

y--~

~-------------~

2
Data Link

~-------------

1
Physical

End System

6
Presentation

5
Session

4
Transport

3
Network
2
Data Link
1
Physical

~-------------

7
Application

)

Network Node

\._--,.-~)

End System

Intermediate nodes in an OSI network require only bottom-layer functions o/the OSI model. Note how peer layers communicate only with their
peers; i.e.• Layer J talks to other lAyer Is but not to lAyer 2s.

• Data transfer, in accordance with the agreed quality of service
• Transport-connection release
The European Computer Manufacturers Assn. (ECMA) has defined this layer in its Transport Protocol standard, ECMA-72.
This standard has gained the support of a number of North American and European computer manufacturers.
The Network Layer
The Network Layer (Layer 3) provides the means to establish,
maintain, and tenninate connections between systems. Its basic
service is providing transparent data transfer between transport
entities.

The services provided by this layer encompass the following:
• Establishing network connections for transporting data between transport entities through network addresses

The Data Link Layer
Data Link Layer 2 provides the procedural and functional means
to establish, maintain, and release data link connections between
two network nodes or network entities and to transfer data fmmes
(or packets). This layer also detects and may correct errors that
occur in the Physical Layer.
Services provided by the Data Link Layer to the Network
Layer include data link connection, sequencing, error notification, flow control, and data unit transfer.
The Physical Layer
The lowest ofthe OSI layers is Physical Layer 1. It provides the
electrical, mechanical, functional, and proceduml chamcteristics
for activation, maintenance, and deactivation of a physical connection. Physical Layer standards specify physical interfaces
(connectors) connected by a physical medium.

Services provided by this layer include the following:

• Identifying connection endpoints
• Transferring network service data units
• Noting errors for reporting unrecovemble errors to the transport layer
• Sequencing network control data units

• Activating and deactivating physical connections
• Data circuit identification

• Flow control
• Releasing the network connection

• Fault condition notification

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• Sequencing
• Transmitting physical service data units either synchronously
or asynchronously

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Standards

Abstract Syntax Notation One (ASN.1)
ASN.l is a specification language adopted for the OSI Reference
Model, giving standards developers a common method for defining syntax. ASN.l is somewhat analogous to grammatical rules
defining the English language, with the exception that it is not
procedural. Just as English grammar specifies notation (punctuation symbols) and word classifications (such as nouns and verbs),
ASN.l specifies the rules that help standards developers define
complex data types in terms of simple building blocks.
ASN.l was first formally described and published in 1984, in
the CCITI X.409 standard entitled "Message Handling Systems:
Presentation Syntax and Notation." It is now described (in less
readable fashion) in two later documents: CCITI X.208 (ISO
8824), entitled "Specification of Abstract Syntax Notation One
(ASN.l)," and X.2OO (ISO 8825), "Basic Encoding Rules for
Abstract Syntax Notation One (ASN.l)."
According to ASN.1, each fragment of information must possess a type and a value. For example:
• Device-Status could be a type (in this case, it is a Boolean
type)
• Zero or One are the possible values
This is specified in ASN.1 notation as such:
Device-Status ::= Boolean
Boolean ::- 1 (or 0)
This is a very simple example; ASN.1 is a powerful grammar,
capable of specifying very complex data types. Hence, it will
continue to be the grammar of choice for specifying open systems
standards and protocols.

OSI Standards Progress
There are four stages in the development cycle: working paper;
committee draft (CD), previously known as a draft proposal;
draft international standard (DIS); and international standard (IS).
A working paper is developed in the first stage. When it matures
and contains well-developed technical concepts, it is registered as
a CD. Passage advances the CD to the DIS level, and the document is considered sufficiently stable to serve as the basis of initial implementations. At the DIS level, the document is distributed for a 180-day ballot. The DIS may require multiple ballots. A
successful ballot elevates the DIS to the level of IS and completes
ISO's process.
The entire process usually takes between four and eight years.
A list of standards organizations associated with the OSI Reference Model is given at the end of this report.

The Evolution of OSI CommiHees
In the spring of 1977, ISO Technical Committee 97 (TC97)
formed a special subcommittee (SC16) charged with developing
an architectural model that would extend from applications-layer
communications clear down to the connection with the physical
interface. The first draft of the seven-layer OSI Reference Model
was completed in 1978. Between 1978 and 1983, the Basic Reference Model and many of the standards for the individual layers
approached or attained draft international standard status. By the
end of 1984, SC16 was reorganized to form Subcommittee 21
(SC21). Working groups within SC16 were also realigned.
The OSI Basic Reference Model became an international standard in 1984. During 1985 a number of vendors demonstrated
products that implemented these standards and, by the end of
1986, many of these products were commercially introduced.
In July 1987 the Joint Technical Committee for Information
Technology (JTel) was formed when ISOITC97 joined forces
JUNE 1993

ISO Reference MOctei for
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Interc~lon (OSI)

Data Networking

with Technical Committee 83 (TC83) of the International Electrotechnical Commission (IEC). The me is a coalition of industrial
standards bodies that is co-located with the ISO in Geneva, Switzerland. The new JTCI held its first meeting in 1987. The standardization activities of SC21 report to JTC I.
SC21 is composed of member bodies (MBs) from 23 different
countries. Each MB has its own national standards organization;
for example, ANSI represents the United States in JTCI. The
individuals or "national correspondents" comprising the MB
delegations come from different groups including user organizations, manufacturing firms, government agencies, and common
carriers or PTfs. As such, they bring varying perspectives and
concerns to the committee sessions.
When an OSI committee or working group produces a document such as a CD, the document is circulated among the MBs for
a vote and to the liaison organizations (LOs) for review. LOs are
independent organizations which also have a vested interest in
OSI development. LOs provide comments on the content of OSI
documents but do not have voting privileges.

Status of OSI Protocols
Protocol standards for all seven layers of the OSI model have
been approved; however, OSI committees are refining and extending some standards as required and may add new standards at
specific layers (particularly Layer 7). Additionally, other standards groups-such as the CCITI, ANSI, and IEEE-may adopt
OSI protocol standards as their own and vice versa. Consequently, many OSI standards are known by more than one standard designation. Table 2 shows some major ISO protocols approved for each OSI layer and lists corresponding appellations
from ANSI, the CCITI, and the ECMA, where applicable.

OSI Applications Standards
ISO committees are working hard at Layer 7, the Application
Layer. In fact, OSI application standards are perceived as potentially powerful and versatile and are the driving force for OSI
market acceptance. We devote considerable space reviewing
some of the most important ones here.
In particular, the ISO electronic mail standard for message
handling systems (MHSs)-or CCITI X.400-is becoming popular in commercial implementations. Two versions, one in 1984,
and another in 1988, are draft international standards that have
not been ratified. The standard was given a big boost in 1989,
when the Aerospace Industry Assn. (AlA) adopted X.400 to interconnect its diverse electronic mail networks. Gateways to proprietary E-Mail systems were also developed that year, and dozens
of vendors have rolled out X.400-based products. Most public
E-Mail carriers have also adopted the standard and are migrating
to the 1988 version.
In addition, the ISO is adapting X.400 as the message medium
for electronic data interchange (EDI). In this context, X.400
would be used as the communications method to store and forward trade documents and business forms conforming to ANSI
X12, the European EDIFACT, and de facto EDI standards.
One component of successful E-Mail intemetworking is directory services (OS), commonly known as CCITI X.500. X.500
specifies an on-line directory for message communications, ultimately allowing network providers to map a common, interconnected directory of worldwide users. X.500 dictates naming conventions, how users access directory information, and what
services are available.
Since the 1988 standard is not flexible, SC21 WG4 is working
to ease the transition to the new 1992 version. Older 1985 X.500
systems will require a software modification to work with the
1992 version. Realistically, the vision of a worldwide messaging
directory probably will not be realized until the late 199Os.

1993 MoGraw-HiII, Incorporated. Reproduction Prohibited.
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Data Networking

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Open S,stems

Table 2. ISO Protocols and Equivalent Standards
Layer

ISO

8571 (FTAM)
10021 (MHS)
9041 (VT)
10026 (OTP)

Application

ANSI (1)

CCITT

ECMA

X.400

9594 (OS)
8613 (OOA)
9579 (ROA)
9596 (CMIP)

X.500
T.410 Series, T.73

6
Presentation

8823 (connection)
9596 (connectionless)

X.226

5
Session

8327 (connection)
9548 (connectionless)

X.225

X3.153

ECMA-75

4
Transport

8073 (TPO-TP4)
(connection)
860218072
(connectionless)

X.224

X3.140

ECMA-72

3
Network

8208 (Layers 1-3)
8348 (connection)
8473 (connectionless)
9542 (15-15)
8878 (use w/8208)
8880 (LAN)
8881 (X.25 on LANs)

X.25
X.213

7n6(LAPB)

X.25

2
OataUnk

Physical

ECMA-101

ECMA-92
(X.25)

3309 (HOLC)
8802.2-.7 (LAN)
(IEEE 802.2-.7)
9314 (FOOl)
2110 (EIA-2320)
4902 (EIA-449)
2593
4903

X3.66

ECMA-40
ECMA-82, -81, -90, -89

X3.148, X3.139, X3.166
V.24, V.28
V.24, V.28
V.35
X-Series interfaces,
Other V-Series

(1) No ANSI standards exist by policy in most instances, as U.S. follows International standards.

(C-

The OSI protocol pair for office automation, Office Document
Architecture (ODA) and Office Document Interchange Format
(ODIF), has been an international standard since 1988 (ISO
8613). It is also specified in the U.S. government's GOSIP standard. ODA and ODIF facilitate the exchange of office documents-such as letters, memoranda, and business reportsamong dissimilar systems. Moreover, the standard specifies the
formatting and exchange of compound documents-those containing combinations of text, images, and graphics. Several ISO

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working groups are attempting to strengthen and extend the Standard in such areas as the inclusion of audio, spreadsheet data,
color graphics, document security, and various layout and presentation styles.
The OSI standard for sending and sharing data files-File
Transfer, Access, and Management (FfAM)-is also a finalized
international standard. It is a Layer 7 component of the Manufacturing Automation Protocol (MAP), which was developed from
networking efforts in .the manufacturing industry. FfAM has
spread quickly in Europe and has made significant progress in
domestic business applications. The file protocol used with
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Transmission Control Protocol/lntemet Protocol (TCP/IP)-File
Transfer Protocol (FfP)-is also well established in the U.S.,
however, and generally more popular than FTAM.
The Layer 7 protocol for network management, Common
Management Information Protocol (CMIP), is now an International Standard. CMIP is a communications protocol between the
agent process and management agents at each 'managed OSI
node. The real work of managing network processes is located
within each node's managed objects at individual OSI layers; in
other words, each layer must have its own network management
system, which OSI does not specify. CMIP allows a centralized
management process to either modify the value of an attribute or
request its value (read its status) at each of the layers. Definitions
and descriptions of management structures and managed information are contained in other OSI standards yet to be completed.
(For more information, see the OSI Management section featured
later in this report.)
Other Layer 7 protocols include distributed Transaction Processing (TP), designed to interconnect different transaction computing systems across OSI networks; Remote Database Access
(RDA), a protocol for integrating database management systems;
and Manufacturing Message Specification (MMS), ISO 9506, a
manufacturing protocol that requires extensions for specific manufacturing device types.
Connection Methods
Every layer of the OSI Reference Model, except the Physical
Layer, supports connection and connectionless mode. Connection-oriented service requires a connection establishment phase, a
data transfer phase, and a connection termination phase; a logical
connection is set up between end systems prior to data exchange.
These phases define the necessary sequence of events for successful data transmission. Connection-oriented service capabilities include data sequencing, flow control, and transparent error
handling.
In a connectionless service, such as new Switched Multimegabit Data Service (SMDS), each Protocol Data Unit is independently routed to the destination; no connection establishment
activities are required, since each data unit is independent of the
previous or subsequent one. Connectionless-mode service transfers data units without regard to establishing or maintaining connections. In connectionless mode, transmission delivery is uncertain due to the possibility of errors. This appears contrary to the
goal of network design-users want to ensure that messages
reach their destination. In reality, connectionless-mode communication simply shifts responsibility for message integrity to a
higher layer, which checks integrity only once, rather than requiring checks at each lower layer. Alternatively, each data unit might
contain the error recovery mechanism.
Lower-Layer Protocols
Lower-layer OSI protocols for Layers I through 3 are well-defmed veterans and in many cases borrowed from existing EIA,
IEEE, or CCITT standards. Connectionless communications at
the lower layers of the OSI model is well established and is found,
for example, in LANs and metropolitan area networks (MANs).
While the original OSI model-described in ISO 7498-was
connection oriented, the ISO foresaw the need for connectionless
service and issued an addendum to that protocol (ISO
7498/ADI). The ISO is now working to update the Connectionless Addendum, and CCITT SG VII pursues a parallel process.
The CCITT, however, has been reluctant to insert connectionlessmode data transmission concepts into CCITT X.200-its version
of the OSI model. The ISO standard for Network Layer service,
ISO 8348, contains connectionless service (in AD I) in addition to
the connection mode.

JUNE 1993

ISO Reference Model for
Open Systems

Data Networking

Interconnection (OSI)

OSI Security
By definition, an open system is one that encourages communications between different applications or users. Unfortunately, an
open system can also encourage illegal eavesdropping and information theft or destruction. Recently, notorious examples of
white-collar crime, corporate espionage, and network intrusions
by computer worms and viruses have alarmed information processing professionals and raised a general awareness of computer
security issues. The concepts of information security and open
systems are antithetical; nevertheless, the ISO has taken steps to
provide a secure environment within the OSI Reference Model.
International Standard 7498, Part 2 addresses a security architecture within the general OSI model. It describes security measures that can be provided by specific layers in the model. Specific security standards are not yet defined, however, but are
under study by working group JTCI, Subcommittee 27 for Information Technology Security Standards, plus other subcommittees.
SC21, concerned with maintaining and defining the upper
three layers of the OSI Reference Model, stabilized several network management and security standards in 1991. The Security
model is composed of six frameworks that work together across
all seven layers of the OSI Reference Model: authentication, access control, security audit, nonrepudiation, confidentiality, and
integrity. SC21 is working to establish two of the six security
standards as Draft International Standards (OISs), and the remaining four standards, which are working drafts, will progress
to CD status.

OSI and MAP/TOP
The Manufacturing Automation Protocol and Technical Office
Protocol (TOP) were originally developed by General Motors and
Boeing Computer Services, respectively, to automate manufacturing functions on the factory floor and in the "back office."
Both are based on the OSI Reference Model, using formal standards for each layer where possible. MAP, in particular, is probably the best-known example of a formal multilevel OSI implementation and is achieving substantial industry acceptance. Many
vendors now offer MAP 3.0 products, which have nearly eliminated proprietary "shop floor" automated factory solutions.
Today, manufacturing networking standards are directed by
the MAPITOP users group. MAP Version 3.0 was released in June
1988 and will remain free from major changes until 1994. Version
3.0 added a Presentation Layer to the profile and implemented a
version of the Manufacturing Message Specification (MMS), the
protocol for transferring factory and robotics information, ISO
9506. The ISO is currently working to extend MMS in support of
realtime applications. Other Layer 7 protocols specified are
FTAM, Network Management, and Directory Service. Middle
layers implement ISO connection-oriented protocols, although
these must be bypassed for time-critical applications. At the lower
transport layers, MAP specifies the IEEE 802.4 token bus system
employing a Type F coaxial connection to a 75-ohm cable.

OSI and TCP/IP
Transmission Control Protocol/lntemet Protocol was developed
by the U.S. government's Defense Advanced Research Projects
Agency (DARPA) for its research network, ARPANET. By 1986,
TCP/IP had gained a following of commercial users seeking a
protocol that could be used as a common denominator for multivendor computer networks. TCP and IP are actually two separate
protocols, occupying middle layers number Four (Transport) and
number Three (Network), respectively, of the OSI Reference
Model.

1993 McGraw-Hill. Incorporated. Reproduction Prohibited.
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Data Networking

ISO Reference Model for

Open Syatem.
Interconnection (Ostl

TCP/IP has been implemented on almost every type of computer and is especially successful in commercial Ethernet LAN
environments. The reason TCP/IP is so popular is because it is
free and its development was paid for by the U.S. government. In
fact, it is actually more complex than TP4. It avoids the connection-orientedlconnectionless dilemma by essentially avoiding it.
OSI provides a richer set of network options, but these may not be
compatible in different networks. Users cannot communicate
across different networks if they implement different options at
these layers.
Already, some proprietary stripped-down versions of OSI
have been developed to operate over TCP/IP, and some pundits
believe that TCP/IP will evolve to resemble OSI in the future.
TCP/IP's future could have been jeopardized, since the U.S. government mandated OSI compliance in government procurements,
had it not been for Novell's introduction earlier this year of a new
version of its NetWare network operating software that supports
TCP/IP.
A majority of users still use TCP/IP networks for LAN interconnectivity. However, the consensus is that TCP/IP is not the
ultimate solution-a feat attributed to OSI. The trend is toward a
migration to OS I-based applications running on a TCP/IP infrastructure. As a result, more vendors, including Unisys and Amdahl, are introducing products that support multiple protocols.

OSI Management
Since the first draft of the seven-layer ISO model was produced in
1978, extensions to the basic model have been developed to more
adequately represent all of the functions required by large-scale,
multivendor networking environments. OSI Management is an
extension to the original reference model that specifies transfer of
network management information in the Application Layer and
support for network management functions at Layers 4, 5, and 6.
Advantages to OSI·Based Network
Management
OSI-based network management continues to capture attention as
the premier solution for multivendor network management. Vendors such as AT&T, Digital Equipment, Hewlett-Packard, and
NCR are now designing their network management architectures
to accommodate OSI Management standards and protocols.
In addition to solving the problem of managing heterogeneous
environments, OSI-based network management will bring about
a new phenomenon-unbundling network management from network products. In a proprietary environment, a given vendor's
products are primarily manageable only by products developed
by that vendor. Widespread use of OSI will split that one-to-one
relationship, making it possible for any OS I-based network management system (NMS) to manage any OSI Management-conformant device.
Disadvantages to OSI·Baseci Network
Management
The market (both vendors and users) has widely criticized ISO for
moving too slowly in its efforts to ratify OSI Management standards. Indeed, the greatest disadvantage to OS I-based network
management is that the demand for it far exceeds the available
products-and vendors are wisely unwilling to develop products
based on standards that are not yet final. In an effort to open the
door to new OSI-based network management system products,
SC21 WG4 is currently working to finalize fault, configuration,
performance, accounting, and security management specifications. These standards will assist in differentiating OSI-based systems from Simple Network Management Protocol (SNMP)-based
products.

«>

1993 McGraw-Hili, IncO!pOrated. Reproduction Prohibited.
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2783

Standards

Another disadvantage to standards-based network management is that OSI standards merely provide a menu of options.
There are numerous gaps and ambiguities in OSI Management
standards that could be interpreted differently, leading to incompatible implementations. Industry consensus is the only hope for
interoperable implementations. Currently, this consensus is building around the Network Management Forum and the Network
Management Special Interest Group (NMSIG) of the OIW, sponsored by NIST and the IEEE. The NMSIG is developing Implementation Agreements (lAs) based on emerging network management standards. lAs are being introduced in phases that
coincide with ISO/IEC standards as they progress from CD to
international standards. The OIW NM Phase I IA became stable
in December 1990.
To further simplify government procurement of network management products, NIST introduced a proposal in 1991, called the
Government Network Management Profile (GNMP). GNMP will
also be introduced in phases that will cross-reference the latest
GOSIP versions. GNMP Phase I, II, and III will address the following categories of management information:
• Phase I-IEEE 802 LAN standards, X.25, ISDN, FDOI, modems, mUltiplexers, bridges, and the physical link of the OSI
model.
• Phase II-protocol software operating in Layers 3 to 7, routers,
terminal servers, MTAs, PBX, and circuit switches.
• Phase I1I--applications, services, operating systems, computers, networks, and database management systems.
GNMP Phase I specifies CMISIP, management definitions in
GNMP section 4, and five systems management functions: object
management function, state management function, attributes for
representing relationships, alarm reporting, and event reporting.
Since SNMP is already widely implemented, it is likely that
SNMP will be deployed to manage routers. Future versions of
GNMP will specify a network management architecture incorporating both SNMP and GNMP protocols.
Standards Documents
OSI Management standards can be broadly categorized into four
areas:
1. Functions--what network management is, according to OSI
2. Services--how network management functions are accomplished
3. Information Structure-terms and categories describing
what is managed (e.g., "management information")
4. Protocols--describe means o!transporting network management information
Thken together, these four areas describe a generic package for
network management systems, and how these products relate to
the network devices they manage (called managed objects in OSI
terminology).
A blueprint document, Management Framework OMNIPoint,
places the OSI Management environment in perspective by describing terms and the scope of OSI network management.
051 Management Functions
OSI Management functions are described in the Systems Management standards (IS 10040, IS 10164-1 through 10164-7, and
N 10164-8 through 10164-12). Management using three models:
1. The Organizational Model-describes ways OSI Management can be distributed administratively

JUNE 1993

ISO Reference Model for

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Open Systems
Interconnection 10SI)

Standards

Figure 3.
ISDN Through OSI Eyes

Application-related functions

Encryption/decryption

etc•

Compression/expansion

....

>as

1~
.2> c:

Session
connection
establishment

:I:.2

Session
connection
release

Layer 4
connection
multiplexing

Routing/
relaying

Session
Session
Session
connection
transport conmanagement
synchronization nection mappina
Layer 4
connection
establishment

Network
connection
establishment

Data link
connection
establishment

Data link
congestion
release

Physical
layer connection
activation

Physical layer
connection
deactivation

Layer 4
connection
release

etc.

Error
detection/
recovery

Flow
control

Network
Network
connection connection
release
multiplexing

Congestion
control

Addressing

etc.

Flow
control

Error
control

Sequence
control

Framing
synchronization

etc.

Bit transmission

Channel
structure
multiplex

Segmenting
blocking

etc.

ISDN functions allocated according to Ioyering principles ofRecommendation X.200.

The Infonnation Model-provides guidelines for defining
managed objects and their interrelationships, classes, and
names
3. The Functional Model-describes network management
functions

The Functional Model outlines how ISO has partitioned network
management into five functional areas: fault management, configu~tion and name management, perfonnance management, accountmg management, and security management. ISO originally
described each of these areas in its own standard. Further studies
revealed that functions overlapped; therefore, ISO reorganized
the documents in December 1988 into their present Systems
Management fonn.
Fault management provides the detection, isolation, and correction of abnonnalities in network operation. Configuration and
name management facilities pennit network managers to control
the configuration of the system, network, or layer entities.
Changed configurations may isolate faults, alleviate congestion,
or meet changing user needs. Perfonnance management enables
the network manager to monitor and evaluate the perfonnance of
the system, network, and layer entities. Data from perfonnance
management may be used to initiate configuration changes and
diagnostic testing to allow a satisfactory level of perfonnance.
Accounting management facilities help detennine and allocate
costs for the use of a network manager's communications resources. Security management facilities pennit the management
of those services providing access protection of communications
resources.

JUNE 1993

Service.
Services are described, in part, in the Common Management Infonnation Protocol (CMIP) standard, IS 9596. Services use primitives, or command types, to accomplish network management
functions. Examples of CMIP commands include GET, SET,
GET REPORT, CREATE, and DELETE. While service primitives are so~ewhat abstract, they are important building blocks
for composite commands used by network management applications to obtain vital data on the status and activity of network
devices.
Common Management Infonnation Services (CMIS) include
a detailed abstract model of open systems management services.
These fall into three categories-event notification, infonnation·
transfer, and control. Event notification allows one system to notify another that some event of importance has occurred.

Information Stftlcture
The most important standards in this category are Structure of
Management Infonnation (SMI), Parts 1,2, and 4 (CD 10165-1,
-2, and -4). (part 3 is not missing; rather, ISO merged Part 3 into
Part 4.) Included in these standards is an explanation of the object-oriented paradigm, used to model a network in tenns of object classes and attributes. In object-oriented environments a
variable (for example, a variable called Modem) is defined ~th
in tenns of the operations that can be perfonned on it and the
values of attributes it can possess. For example, Modem can have
an attribute such as Status, which may have a value of On-Line or
Off-Line; a network management system may obtain this value
via a Get operation or alter it via a Set operation.
Objects (including their attributes and operations) are stored
in a Management Information Base (Mffi), sometimes called an
Object Library. The SMI documents just listed provide syntax

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Datapro Information Servlc;es Group. 0eI1'lIll NJ 08076 USA

Data Networking

ISO Reference Model for
Open Systems
Interconnection lOSI)

and semantics for information in the MIB; however, as yet no
single ISO standard defines exactly what the OSI MIB will contain, nor how vendors and users will register objects in the standard MIB. SC21 WG4 is currently working to finalize the SMI,
providing guidelines that can be used to define management objects and their attributes. The final SMI will ensure interoperability among OSI-based network management systems.
In the TCP/IP world, an Internet Standard MIB exists for objects managed using SNMP. This MIB functions in an analogous
role to the proposed OSI Mm, although the administration and
rules governing the two are sure to differ.
MIB includes all information needed to make management
decisions. Mm is a conceptual repository of all OSI management
data in an OSI environment. The MIB concept does not imply any
form of physical or logical storage for management information,
however, and its implementation is outside the scope of OSI standards. Rather, the SMI defines the abstract syntax and the semantics of information so that it can be represented in OSI protocol
exchanges.

Protocols
Common Management Information Protocol, IS 9596, is the primary OSI Management protocol. CMIP specifies procedures for
the exchange of basic management information between open
systems interconnected by OSI protocols. CMIP is intended to be
a general-purpose management protocol suitable for the management of both OSI resources and the real resources used to provide
communications services.

X.500-The Directory
The Directory is a related standard designed to manage namerelated information concerning protocol layers and network
nodes. These services connect the actual names used in the network with names and addresses understood by human users. The
Directory is defined in CD 9594 and several other OSI working
drafts. CD 9594 attained DIS in March 1988.

2783
Standards

means of interconnecting multivendor computer networks is now
uncertain. The tremendous growth of the Internet has given
TCP/IP a firm base in the United States and the protocol has also
been making significant strides in Europe. OSI applications protocols, such as CCITI X.400, X.500, and EDI, are still very popular, however, and lend support to OSI.
The OSI Reference Model has some glitches and holes that
prevent it from being widely implemented. In the U.S., OSI and
TCPIIP proponents are badly divided. As an internetworldng protocol, TCP/IP has proved its worth and is a popular and commercially successful method oflinking users across diverse networksparticularly LANs. OSI middle-layer protocols 3 through 5, the
alternatives to TCP/IP, are not as simple to implement in the real
world. SNMP, the network management protocol for TCP/IP networks, is also a proven, commercially successful solution. As
long as vendors and users require practical networking products,
they will continue using TCP/IP-based protocols-standards or
not.
OSI will most likely evolve to better serve user needs, with an
OSI-TCP/IP hybrid a likely compromise. A possible scenario for
wider OSI acceptance is that TCP/IP middle layers will migrate to
resemble OSI, at least functionally. In many commercial networking applications, however, vendors are blending different
protocol stacks from different sources to match user needs. For
instance, one vendor's network protocol might graft together different layers from OSI, TCP/IP, and IBM's SNA.
In reality, OSI and other layered architectures do not serve
every application and are not a panacea. Proprietary architectures
will continue to thrive alongside OSI-based networks, especially
for closed user groups (where internetworking is not a requirement) or in time-sensitive applications intolerant of layered protocols' high overhead.
All others who desire internetworking must realize that the
associated protocols are still evolving-nothing is truly cast in
iron. In market-based economies, products that do not satisfy
market needs will not gain widespread favor. Therefore, prospective users must evaluate OSI protocols and their adoption with an
eye toward future standards developments. As it stands now, OSI
will not be the interconnection standard of the future. Significant
portions of it, however, will most certainly contribute to the evolution of an international standard. -

(
1993 McGraw-Hili, Incorporated. Reproduction Prohibited.
Datapro Infonnation Services Group. Delran NJ 08075 USA

JUNE 1993

DATAPRO

Data Networking

2783

Standards

ISO Reference Model for
Open Systems
Interconnection (OSI)
In this report:

Synopsis

The Open
System ........................... 2

Editor's Note
The goal of Open Systems Interconnection (OSI) is to enable dissimilar
computers in multi vendor environments to share information transparently. The OSI structure calls for ,
cooperation among systems of different manufacture and design. With
this capability, global digital networks can become a reality.

OSI Standards
Progress ...................... 10
OSI and Other
Network
Architectures ................ 14
Testing and Verification
Agencies ...................... 16
OSI
Management ................ 18
OSI and the
Future ........................... 21

There are seven layers of the OSI
model that communicate between
one end system and another. The
layers cover nearly all aspects of information flow, from applicationsrelated services provided at the
Application Layer to the physical
connection of devices to the communications medium at the Physical
Layer.

@ 1991 McGraw-Hili, Incorporated. Reproduction Prohibited.
Datapro Information Services Group. Delran NJ 08075 USA

Although all seven layers have long
since been defined and ISO protocols
ratified for each layer, the ISO committees must keep refining and redefining specific sections of the model
by rewriting existing definitions and
adding new protocols.
Report Highlights
This report updates the OSI's status
at all seven layers; gives the status of
OSI standards progress; compares
OSI to other architectures, including
ISDN, SNA, DECnet, MAP/TOP,
and TCP/IP; rationalizes the need
for standards testing and verification; profiles major testing organizations; and outlines OSI Management
standards and status.

AUGUST 1991

2783
Standards

Analysis

ISO Reference Model for
Open Systems
Interconnection (OSI)

Data Networking

TheOSI structure calls for cooperation
among systems of different manufacture and design. This includes coordinating activities such as
the following:
• Interprocess communications-the synchronization between OSI application processes and
the exchange of information

The proliferation of computerized data processing
systems in the late 1960s produced a need for compatible data communications networks in the
1970s. Several proprietary network architectures
were developed for mainframe-to-terminal communications, including IBM's SNA in 1974. Although many of these proprietary architectures
were based on a layered model, none were compatible with any other. The CCITT's X.25 host interface to the packet networks standard was ratified
in 1976 but is not a complete network architecture.
In 1977, the International Organization for Standardization (ISO) formed ISO Technical Committee 97 (TC97), Subcommittee 16 (SC16), to
embark on a worldwide standardization effort and
confront the issue of incompatibility head-on. The
purpose of TC97/SC16 was to develop a model and
define the protocols and interfaces required to support an open system. The goal of Open Systems
Interconnection (OSI) was, and still is, to enable
dissimilar computers in multivendor environments
to share information transparently. With this capability, global digital networks can become a reality.

The Open System
The ISO defines a system as a set of one or more
computers and associated software, peripherals,
terminals, human operators, physical processes,
information transfer means, etc., which form an
autonomous whole capable of performing information processing and/or information transfer. An
open system is one that obeys OSI standards in its
communication with other systems.
An application process is an element within a
system that performs information processing for a
particular application. The application process can
be manual (a person operating a banking terminal),
computerized (a program executing in a computer
center and accessing a remote database), or physical (a process control program executing in a dedicated computer attached to industrial equipment
and linked to a plant control system).
AUGUST 1991

• Data representation-the creation and maintenance of data descriptions and transformations
for reformatting data exchanged between systems
• Data storage-storage media, file systems, and
database systems for providing access to and
management of stored data
• Process and resource management-how application processes are declared, initiated, controlled, and acquired
• Integrity and security-information processing
constraints that must be ensured during open
systems operations
• Program support-the definition, compilation,
testing, linking, storage, and transfer of and access to programs executed by the application
processes
The OSI model is concerned only with the exchange of information between open systems.
The Layering Concept

Layering is a basic structuring technique used in
the OSI model. Each layer is composed of an ordered set of subsystems, with logically related functions grouped together. The OSI model breaks
down internetworking activities between systems
into two distinct groups. Communicationsoriented functions are separated from useroriented functions; features which move
information across a network are distinct from features which handle and format information.
There are seven layers of the OSI model that
communicate between one end system and another
end system. The layers cover nearly all aspects of
information flow, from applications-related services provided at the Application Layer to the connection of devices to the communications medium
at the Physical Layer. Below the Physical Layer,
the media itself corresponding to "Layer 0"-such
as wire, cable, or through-the-air communicationis currently not addressed by the model.
~

1991 McGraw-HiII,lncorporated. Reproduction Prohibited.
Datapro Information Services Group. Delran NJ 08075 USA

Data Networking

ISO Reference Model for
Open Systems
Interconnection (OSI)

Standards Organizations and
Testing Agencies

r
'I r

',-

Although standards are
indispensable for computer internetworking,
they are useless unless
vendor claims of compatibility can be tested and
users can be assured of
acquiring products that
comply fully with the standards. In general, vendor
claims of standards compatibility are suspect unless verified by an
impartial testing agency.
Additionally, compatibility
claims can mean different
things to different people.
For users, it is a good
idea to probe vendor
claims of standards compatibility to determine
what is compatible with
what, and at what levels.
Several standards testing
and verification bodies
have been organized both
here and abroad by vendor consortiums, government agencies, and
independent organizations. They have found
that developing conformance specifications, producing testing suites, and
conducting comprehensive testing is complicated, expensive, and
time consuming. Regional
differences can stymie
attempts at verification.
The trend for these organizations, therefore, is to
cooperate with each
other, sharing resources

and expertise. A primary
objective is demonstrating
interoperability among
different vendors; i.e.,
proving that standards
really work and fostering
end-user interest. Many
agencies have tried vainly
to involve more end users
but are backed primarily
by the vendors.
In the U.S., primary testing and certification agencies include the
Corporation for Open
Systems (COS), the National Institute of Science
and Technology (NIST),
and Bell Communications
Research (Bellcore). Several smaller organizations
and certain vendors, however, also offer testing
services. Europe is represented by the Standards
Promotion & Application
Group (SPAG); Japan by
the Promoting Conference for OSI (POSI). Standards organizations are
listed below:
American National Standards Institute (ANSI)
1430 Broadway
New York, NY 10018
(212) 642-4900
ANSI X3 Secretariat
Computer and Business
Equipment Manufacturers
Association (CBEMA)
Suite 500, 311 First
Street, NW

2783

Standards

Washington, DC 200012178
(202) 737-8888
Bell Communications Research (Bellcore)
60 New England Avenue
Piscataway, NJ 088544196
(908) 699-2000
Customer Service Hot
Line:
(800) 521-CORE
Corporation for Open
Systems (COS)
Suite 400, 1750 Old
Meadow Road
McLean, VA 22102-4306
(703) 883-2700
Telex: WUI 6503157578
MCIUW
European Computer
Manufacturers Association(ECMA)
Rue du Rhone 114
CH-1204 Geneva, Switzerland
(+41) 22 735 36 34
Telex: 413237 ECMA CH
Electronic Industries Association (EIA)
1722 Eye Street NW,
Suite 300
Washington, DC 20006
(202) 457-4900
Intemational Organization for Standardization
(ISO)
1, Rue de Varembe
Case Postale 56
CS-1211 Geneva 20,
Switzerland
(+41) 22 33 34 30
Intemational Telegraph
and Telephone Consultative Committee (CCITT)
General.8ecretariat
International Telecommunications Union (ITU)
Place des Nations

CH-1211 Geneva 20,
Switzerland
(+41)22995111
Fax: (+41) 22 33 72 56
Telex: 421 000 UIT CH
Institute of Electrical and
Electronics Engineers
(IEEE)
Headquarters
345 E. 47th Street
New York, NY 10017
(212) 705-7900
Fax (publications): (212)
705-7682
IEEE Service Center
(published standards:)
445 Hoes Lane, P.O. Box
1331
Piscataway, NJ 088551331
(908) 981-1393
Fax: (908) 981-9667
Telex: 833-233
OSINET
Mr. Jerry Mulvenna,
Chairman
OSINET Steering Committee
NIST Building 225, Room
B217
Clopper Road
Gaithersburg, MD 20899
Standards Promotion"
Application Group sa
(SPAG)
Avenue Louise 149, Box 7
1050 Brussels, Belgium
(+32) 2 535 0811
Telex: 20307 SPAG B
National Institute of Standards and Technology
(NIST)
U.S. Department of Commerce
Gaithersburg, MD 20899
(301) 975-2000

("
1991 McGraw-Hili. Incorporated. Reproduction Prohibited.
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AUGUST 1991

ISO Reference Mode' for
Open S,stems
Interconnection (OSI)

2783
Standards

(Analysis continued)

model. Application, Presentation, Session, Transport, Network, Data Link, and Physical Layers
have been defined (see Table 1). The model represented by Table 1 is only one end system, in this
case the transmitting end; most networks have at
least two end systems. In Table 1, information
flows down from Layer 7 to Layer 1, and then out
over a physical transmission medium. At the receiving end, the information flows into another
7, until it
end
. system and up from Layer 1 to Layer
.
IS received by a user.
The seven layers can be divided into two
functional groups: the Transport Platform (Layers
1 to 4) and the Application Platform (Layers 5 to
7). The Transport Platform's function is to get data
from one system to another without errors. The
Application Platform's function is to interpret the
datastream and present it to the user in a usable
form (see Figure 1).
Each layer contributes functions to the communications task. For example, the Link Layer enables communications across a single physical
connection, while the Network Layer provides endto-end routing and data relay. Services at the upper
Figure 1.
Application and Transport Divisions
7
Application
6
Presentation

Application
Platform

5
Session

4
Transport

3
Network

Transport
Platform

2
Data Link

1
Physical

Data Networking

layer interface-providing connection to the nexthigher layer-are provided by each layer, usually
described by a service specification for the layer.
Services at each layer are provided by a layer entity. Each layer entity communicates with its peer
at the same layer on another system, providing services specified in the service specification.
Layers are sometimes divided into sublayers,
for several reasons. Layers often need sublayers to
handle the service interface of the layer beneath it.
This avoids "rewriting" the entire layer. For example, the Link Layer of the IEEE 802 Local Area
Network (LAN) standards is divided into a Logical
Link Control (LLC) sublayer and a Media Access
Control (MAC) sub layer. The MAC sublayer depends on characteristics of the underlying physical
layer. Layer independence and modularity are promoted by ensuring that layer entities on one system
are not permitted to communicate with nonpeers
on another OSI system.
IBM's SNA is also a layered architecture, following rules of layering common to OSI and other
layered architectures. Any layer may originate a
message to fulfill its responsibilities. The message
may not bypass any layer en route to its destination. If a message leaves the node it will end up in
another node at the same layer that originated the
message.
There are good reasons for layering: layering
simplifies change; components inside a layer can
be changed without affecting any other layers in
that node. Layers are like structured programmingbut for teleprocessing systems. Because there
are rigid interfaces between levels, fewer people need to react to changes, allowing them to be
implemented faster. There is no better way of
achieving complex functions. Layering allows each
network function to be made "transparent," unaware and independent of other functions at other
layers, thus enabling any layer to be modified without changing the entire monolithic architecture.
Each layer may support one of several different protocols designed for specific network applications; the choice of a specific protocol is optional,
allowing users to tailor networks to their own design. Each layer defines functions crucial to the
communications process at that layer, independent
ofthe other layers. However, a layer may perform
functions hinging on functions performed in the
layers immediately above or below. A layer can

The seven layers are divided into two functional groups.
AUGUST 1991

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ISO Reference Mode. for
Open Systems
Interconnection (OSII

Data Networking

2783

Standards

Figure 2.
Message Movement among OS] Layers

Message moves from Layer 7 through ~
the other layers to reach Layer 7
at the opposite end
r----------,

r-----~~~

Application

6
Presentation

Presentation

5
Session

Session

4
Transport

6
5

Network

Data Link

1
Physical

'---y---J

Network Node

End System

Transmission
Media

'---y---J
End System

Intermediate nodes in an OS] network require only bottom-layer functions ofthe OSI model. Note how peer layers communicate only with their peers; i.e., Layer I talks to other Layer Is but not to Layer 2s.

only communicate with another device or network
node at its peer layer. Messages exchanged between
peer layers are "enveloped" with messages from
other layers and passed through these other layers
on the way to their destination, picking up and
then shedding these other protocol layers along the
way. For example, if layer seven at one end system
must send a message to layer seven at another, it
must travel down through six layers at its own end
and then up through six layers at the other, until it
reaches layer seven at its opposite (peer) layer.
Each network node (a network user, computer, terminal) is equipped with this layer mechanism. However, not all intermediate nodes need all
seven layers. Network nodes, in particular, must
only route and transmit data packets-functions at
the bottom three layers of the OSI model. Layer
four through seven functions are not required and
therefore not included in network node software.
Data packets processed in these nodes reach only
Layer 3 and are then routed elsewhere (see Figure
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Datapro Information Services Group. Delran NJ 08075 USA

2). A node communicates with its peer in the node
sending or receiving data. Interfaces within nodes
allow them to accept, process, and route data. Data
transfer is routed from Layer 7 down to Layer 1 at
the transmitting node, then along the network to
Layer 1 at the receiving node, and finally from
Layer 1 up to Layer 7. Peer layers communicate by
the same route.
The message initiated at the transmitting
node (Layer 7) is passed from layer to layer, each
layer adding control information, if required, and
acting in accord with control information from its
peer in the receiving node. A fully prepared message enters the cable at Layer 1. The procedure is
reversed at the receiving end. Each item of control
information stops at its appropriate layer, and the
message itself passes up to Layer 7. Data transfer
essentially is a cumulating process at the transmitting node and a diminishing process at the receiving node.

AUGUST 1991

ISO Reference Model for·
Open S,stems
Interconnection (OSI)

2783
Standards

Data Networking

Table 1. The Seven Layers of OSI
Layer

Name

Purpose

Application

Applications and application interfaces for
OSI networks. Provides access to lower
layer functions and services.

Presentation

Formats data received from Layer 7; includes terminal standards, display rules.

Session

Coordinates connection and interaction
between applications. Establishes a dialog, manages and synchronizes the direction of data flow, and terminates the
session.

Transport

Ensures end-to-end data transfer between applications, data integrity, and
service quality. Assembles data packets
for routing by Layer 3.

Network

Routes and relays data units among network nodes.

Data Link

Transfers data units from one network
node to another over a transmission circuit. Ensures data integrity between
nodes.

Physical

Sends the bit stream to the transmission
medium.

The Layers

A number of objectives were considered by the reference model's designers: to limit the number of
layers to make the system engineering task of describing and integrating the layers as simple as possible; to create boundaries between layers at points
where the description ofservices can be small and
the number of interactions across each boundary is
minimized; and to collect similar functions in the
same layer. Table I summarizes the OSI Reference
Model's layers; more detailed descriptions follow
for each layer.
The Application Layer
The Application Layer (Layer 7) is the highest
layer, providing the means for the application process to access the OSI environment. Its function is
to serve as the passageway between application
processes using Open Systems Interconnection to
exchange information; consequently, all application process parameters are made known to the
OSI environment through this layer.
All services directdy usable by the application
process (i.e., systems and applications management
functions) are provided by the Application Layer.
It differs from the other layers in that it does not

AUGUST 1991

provide services to a layer above it nor is it associated with a service-access point. Some of the services provided by this layer, other than
information transfer, are the following:
•

Identifying intended communications partners

• Determining current availability of the intended partners
• Establishing the authority to communicate
• Agreeing on responsibility for error recovery
• Agreeing on procedures for controlling data integrity
The Application Layer actually consists of two sublayers. The uppermost sublayer consists of Specific
Application Service Elements (SASEs), such as
message handling system (X.400), FTAM, Directory Services (X.500), DTP, and Virtual Terminal
(VT). The lower sublayer consists of Common Application Service Elements (CASEs), which provide
specific services for the upper applications. Examples of CASE protocols include Commitment Concurrency and Recovery Service Element (CCR) and
Remote Operation Service Element (ROSE).
The Presentation Layer
The Presentation Layer (Layer 6) allows an application to interpret the meaning of information exchanged. Information is formatted and translated
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Data Networking

ISO Reference Model for
Open S,stems
Interconnection (OSI)

2783
Standards

Table 2. ISO Protocols and Equivalent Standards
Layer
7
Application

6
Presentation
5
Session
4
Transport
3
Network

2
Data Link

(

1
Physical

ISO
8571 (FTAM)
10021 (MHS)
9041 (VT)
10026 (OTP)
9594 (OS)
8613 (OOA)
9579 (ADA)
9596 (CMIP)
8823 (connection)
9596 (connectionless)
8327 (connection)
9548 (connection less)
8073 (TPO-TP4)
(connection)
8602/8072
(connection less)
8208 (Layers 1-3)
8348 (connection)
8473 (connectionless)
9542 (IS-IS)
8878 (use w/8208)
8880 (LAN)
8881 (X.25 on LANs)
7776 (LAPB)
3309 (HOLC)
8802.2-.7 (LAN)
(IEEE 802.2-.7)
9314 (FOOl)
2110 (EIA-2320)
4902 (EIA-449)
2593
4903

CCITT

ANSI

ECMA

X.400
X.500
T.410 Series, T.73

ECMA-101

X.226
X.225

X3.153

ECMA-75

X.224

X3.140

ECMA-72

X.25
X.213
ECMA-92
(X.25)
X.25
X3.66

ECMA-40
ECMA-82, -81, -90, -89

X3.148, X3.139, X3.166
V.24, V.28
V.24, V.28
V.35
X-Series interfaces,
Other V-Series

at this layer. Aspects of Layer 6 include data syntax, which is the data to be transferred between
layers, and the presentation image syntax, which is
the data structure that application entities refer to
in their dialog, or the set of actions that may be
performed on the data structure.
Services provided to the Presentation Layer
include the following:
• Transforming data syntax, primarily code and
character set conversion
• Transforming and selecting the presentation
image syntax, the adaptation and modification
of the presentation image (the OSI view of the
data structure)

The Session Layer
The Session Layer (Layer 5) allows cooperating
presentation entities to organize and synchronize
their dialog and to manage data exchange. It provides the following services:
•

Session-connection establishment-creation of
an exchange between presentation entities

• Session-connection release
•

Normal data exchange

•

Quarantine service-in which data units sent
by a presentation entity are withheld from the
receiving presentation entity until released by
the sending presentation entity

• Expedited data exchange
Functions within the Presentation Layer include
session establishment request; data transfer; negotiation and renegotiation of data syntax and presentation image syntax; special data transformations, such as compression; and session
termination request.

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•

Interaction management-allowing presentation entities to take turns exercising control
functions

•

Session-connection synchronization

AUGUST 1991

2783
Standards

ISO Reference Model for
Open System.
Interconnection (OSI)

Data Networking

Table 3. OSI Network Management Standards
Current Status

Finalization Date

Common Management Information Service (CMIS) (IS 9595)

International Standard

Final 11/89

CMIS CancelGet Addendum (AD 9595-1)

Addendum

Final 11/90

CMIS Add/Remove Addendum (AD 9595-2)

Addendum

Final 11/90

Support for Allomorphism (Amendment to CMIS) (CD 9595)

New Work Items

DIS expected in 11/91; IS expected in 11/92

Access Control (Amendment to CMIS) (CD 9595)

New Work Items

DIS expected in 11/91; IS expected in 11/92

Common Management Information Protocol (CMIP) (IS 9596)

International Standard

Final 11/89

CMIP CancelGet Addendum (AD 9596-1)

Addendum

Final 11/90

CMIP Add/Remove Addendum (AD 9596-2)

Addendum

Final 11/90

Support for Allomorphism (Amendment to CMIP) (CD 9596)

New Work Items

DIS expected in 11/91; IS expected in 11/92

PICS Proforma (Amendment to CMIP) (CD 9596)

New Work Items

DIS expected in 11/91; IS expected in 11/92

OSI Systems Management Overview (DIS 10040)

Draft International Standard

IS 5/91

Object Management Function (DIS 10164-1)

Draft International Standard

IS 5/91

State Management Function (DIS 10164-2)

Draft International Standard

IS 5/91

Attributes for Representing Relationships (DIS 10164-3)

Draft International Standard

IS 5/91

Alarm Reporting Function (DIS 10164-4)

Draft International Standard

IS 5/91

Event Management Function (DIS 10164-5)

Draft International Standard

IS 5/91

Log Control Function (DIS 10164-6)

Draft International Standard

IS 5/91

Security Alarm Reporting Function (DIS 10164-7)

Draft International Standard

IS 5/91

Title
CMIS/CMIP

OSI Systems Management Functions

Security Audit Trail Function (CD 10164-8)

Committee Draft·

DIS 4/91

Objects and Attributes for Access Control (CD 10164-9)

Committee Draft·

DIS 4/91

Accounting Meter Function (CD 10164-10)

Committee Draft"

DIS 4/91

Workload Monitoring Function (CD 10164-11)

Committee Draft·

DIS 4/91

Confidence and Diagnostic Testing Classes.
Test Management Function (N4078. 10164-X)

Committee Draft·

DIS expected 8/91; IS expected
8/92

Measurement Summarization (N4081. 10164-X)

Committee Draft·

DIS expected 8/91; IS expected
8/92

Response Time Monitoring (N4079. 10164-X)

Committee Draft·

DIS expected 8/91; IS expected
8/92

Software Management Function (10164-X)

New Work Item

DIS expected 7/92; IS expected
7/93

Time Management Function (10164-X)

New Work Item

DIS expected 8/92; IS expected
8/93

Structure of Management Information (SMI)
Management Information Model (DIS 10165-1)

Draft International Standard

IS 5/91

Definition of Management Information (DIS 10165-2)

Draft International Standard

IS 5/91

Guidelines for Definition of Managed Objects (DIS 10165-4·')

Draft International Standard

IS 5/91

Generic Managed Objects (N4075)

Working Draft

DIS registration dates to be
determined

·Draft proposals (DPs) are now referred to as Committee Drafts (CDs).

* ·Document DIS 10165-3 was merged with DIS 10165-2 and will not appear in future standards listings.

• Exception reporting-permitting the presentation entities to be notified of exceptional situations

An example of a working Layer 5 protocol outside
the scope of OSI is the Department of Defense
Transmission Control Protocol (TCP).

AUGUST 1991

1991 McGraw-HiII,lncorporated. Reproduction Prohibited.
Datapro Information Services Group. Delran NJ 08075 USA

D,ata Networking

(

ISO Reterence Model for
Open Systems
Interconnection (OSI)

2783

Standards

Table 3. OSI Network Management Standards (Continued)
Title

Current Status

Finalization Date

Performance Management (N4981)

Working Draft