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IEEE TRANSACTIONS .ON COMMUNICATIONS, VOL. COM-28, NO. 4,A PRIL 1 980

4 25

OS1 Reference Model-The I S 0 Model of Architecture for
Open Systems Interconnection
HUBERT ZIMMERMANN
(Invited Paper)

A bstructxonsidering the urgency of the need for standards which
initial discussions revealed [6]
that
a
consensus could
be
would
allow
constitution of heterogeneous
computernetworks,
I S0
reached rapidly on a layered architecture which would satisfy
c reated a new subcommittee for “Open Systems Interconnection”( IsO/
most requirementsofOpenSystemsInterconnectionwith
TC97/SC16) in 1977. The first priority subcommittee 16 was to develop
of
thecapacityof
being expandedlater t o meet newrequirean architecture for open systems interconnectionwhich could serve as a
ments.decided
SC16
t o give the highest priority the
to
framework for the
definition of s tandard protocols. As a result 18 months
of
of studies and
discussions, SC16 adopted layered architecture comprisingdevelopment ofa standard Model of Architecturewhich would
a
seven layers (Physical, Data Link, Network, Transport, Session, Presconstitutetheframeworkforthe
development ofstandardentation, Application). July
and
In
1979 the specifications of this
protocols.
After
less than 18 m onthsof discussions, this
architecture, established by SC16, were passed under the name of “OS1
task was completed, the
and
I S0 Model of Architecture
R eference Model” to Technical Committee 97 “Data Processing“ along
called the Reference Model of Open Systems Interconnection
withrecommendations to start
officially, on this basis, a’set of protocols
[7] was transmittedby SC16 t o itsparent TechnicalComstandardization projects to cover the most urgent needs. These recommendations were adoptedby TC97 at the end 1979 as t he basis for the mittee on “ Data Processing” (TC97) along with recommendaof
following development of standards for Open Systems Interconnection
tions t oofficially start a number of projects for developing
within ISO. T he OS1 Reference Model was also recognized by CCITT
on this basis an initial setofstandardprotocolsforOpen
Rapporteur’sGroup
on “Layered Model for
Public
Data
Network
Systems
Interconnection.
These recommendations were
Services.”
This paper presents themodel of architecture for Open Systems Interadopted by TC97 at the end of1979 the basis for following
as
connection developed by SC16. Some indications are also given on the
development of standards for Open Systems Interconnection
initial setof protocols which will likely be developed in this OS1 Reference
within ISO. The OS1 Reference Model was also recognized by
Model.

I . INTRODUCTION

CCITT Rapporteur’s Group on Public Data Network Services.
Thepresentpaperdescribes
the OSI Architecture Model
as it has been transmitted t o TC97. Sections 11-V i ntroduce
concepts of a layered architecture, along with the associated
vocabulary defined by SC16. Specific use of those concepts
inthe OS1 seven layers architecture are then presented in
Section VI. Finally, some indications on t he likelydevelopment of OS1 standard protocols aregiven i n SectionVII.

N 1 977, the International Organization for Standardization
(ISO) recognized the special and urgent need for standards
for heterogeneous informatic networks and decided t o create
a
new subcommittee (SC16) for “Open SystemsInterconnection.”
The initial development of computer networks had been
fostered by experimentalnetworkssuch
as ARPANET [ 11 N ote onan ‘YnterconnectionArchitecture”
or CYCLADES [2],
immediately
followed
by
computer
T he basic objective of SC16 is t o standardize the rules of
manufacturers[3] , [ 4]. While experimentalnetworks were interactionbetweeninterconnected
systems. Thus, only the
conceived as heterogeneousfromthe
very
beginning, each external behavior ofOpenSystemsmustconformto
OS1
manufacturer developed his own set of conventions...