wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

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ISO/OSI Model
Gerhard Zucker
Vienna University
of Technology
Dietmar Dietrich
Vienna University
of Technology
1.1 Introduction....................................................................................... 1-1
1.2 Open Standard................................................................................... 1-3
Layer Functionalities
1.3 Vertical and Horizontal Communication..................................... 1-5
1.4 Dynamic Behavior of Services and Protocols............................... 1-6
1.5 Extensions, Benefits, and Discussion............................................. 1-9
References...................................................................................................... 1-9
1.1 Introduction
The ISO/OSI model was developed and standardized in the late 1970s by the International Organization
for Standardization as the standard ISO IS 7498. It supports designers by easing the definition of communication
protocols in a way that they operate correctly and are easy to maintain [Hay 88]. The name
OSI originates from open systems interconnection. The model is not intended as strict implementation
rules, because a real system will always have to adapt to requirements of price, economy, and flexibility.
Instead, the ISO/OSI model represents an abstract definition (independent from hardware or software
implementation) consisting of hierarchical layers. Related functions are grouped together in layers
with strict separation between horizontal and vertical communication. The standard defines services,
protocols, and interfaces. The original intention was to define a model for connecting computers for data
transfer, logging into remote computers, and so on. Requirements like real-time, protocols for embedded
microcontrollers or protocols for field buses were of no concern, which has to be considered when
applying the ISO/OSI model to such systems.
In a first step, all subfunctions that contributed to communications were collected as shown in
Figure 1.1 in the top right cloud, where each subfunction is represented by a circle. These subfunctions
were then assigned to separate layers. The fact that the model consists of seven layers does not have technical
reasons, but represents the common agreement between the participants of the workgroup that
was responsible for the definition of the model. In this way, each layer was assigned to have a dedicated
function, which consists of subfunctions.
The protocol stacks (i.e., the systems that are designed based on this model) shall contain evenly
distributed subfunctions over all layers. Subfunctions that are too different shall not be included in
the same layer and the interfaces shall be designed toward a low amount of information that has to be
exchanged between the layers (in order to keep the overhead low).
The standard defines many more rules that shall encourage a uniform and logical structure. The use
of specific description language (SDL) is required to avoid endless sequences of unstructured code as
well as flow diagram, which in the end lead to the SDL and its corresponding tools [Ols 92] that are used
in telecommunications.
The ISO/OSI model was a great leap forward in the design of protocols. It created orderliness and a
uniform structure that builds a commonly approved base for the standardization of protocols and
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© 2011 by Taylor and Francis Group, LLC