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

1-2 Industrial Communication Systems
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FIGURE 1.1
Development of the model.
communication interfaces. The model defines functions, which are expected by the components that
contribute to communication. It explains how these functions can be based on each other and how they
cooperate. Before the ISO/OSI model was created, different experts from different fields of communication
(e.g., telecommunications, computer communication, automation, process engineering, or equipment
technology) created completely different solutions. They defined proprietary sets of functions and often
forgot other important functions, they aligned the different functions individually to meet the requirements
at hand, and they based the design on different communication principles. The ISO/OSI model
defined a layer for each subfunction and specified when which layer should be accessed. Thus, the model
helps to simplify the decision, which subfunctions are relevant for a certain kind of communication and
how they correlate. It builds the base for new protocol standards (e.g., the various fieldbus standards in
[Zur 05]) and eases linking different types of networks.
Some literature states that the ISO/OSI model has finished the language confusion that existed in
technical communication. This is a bit of exaggeration, since the model does not claim to define a uniform
language for all communication tasks. If that would have been the goal, the model would only
contain descriptions for these tasks; instead, it describes the principles of communication. Considering
the vastly different communication tasks in, e.g., an airplane and in a washing machine, this appears to
be the only feasible approach.
Where shall the communication system be located within a system? There is no common opinion to
this question, especially since at the time the ISO/OSI model was defined, the only separation that was
done was between application and communication system; an operating system was not considered
at all.* Today, the operating system is well established as being located underneath the application,
and sometimes the communication system is included into the operating system, sometimes it is not
included. Automation prefers a design according to Figure 1.2, where the communication system is a
separate unit underneath the operating system and establishes the connection to different networks.
Consistent with the hierarchical model, the interface of the communication unit provides its services
to the module above it—the operating system (given that it exists in the system), which again provides
its services to the applications. According to the top-down design, the developer shall specify this interface
only after the application and the operating system have been specified. However, reality shows
that this is often not the case. Communication has to follow a standard, since it generally connects
systems of different vendors. The dilemma can be resolved partly by allowing the definition of different
* The fact that the operating system did not play an important role is also shown in the name “application layer,” which
would more accurately be named “operating system layer.”
© 2011 by Taylor and Francis Group, LLC