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

ISO/OSI Model 1-3
Application
Operating system
Communication
FIGURE 1.2
Location of the communication system in a device.
communication protocols. While on one hand, there should be only a few different protocols to meet
the requirements of the standard (i.e., little variation, lots of common properties), on the other hand, we
need a lot of different protocols to meet the different requirements.
Today, we see that most fieldbus protocols in industrial automation consist of at most three layers;
building automation protocols usually have more layers. The LonTalk protocol [Loy 05] (ANSI/CEA
709.1 and ISO/IEC 14908-1) even implements all seven layers.
1.2 Open Standard
The term “open system” is well defined in ISO: a “system” is in this context a complete facility (unit), that
is, computers (like process computers) and data processing machines with peripheral devices like storage,
front-end computers, data stations, or application software. Such a system contains communication
partners and (parts of) a communication system.
The term “open” has a stricter meaning than as it is used in common language, and cannot be used
freely in the area of communications (even if marketing wants to interpret it differently). A system is
“open,” if it meets certain requirements. The first authoritative requirement is that the protocol is officially
standardized (nationally or internationally). A quasi-standard or a special standard defined by a
company is not valid. In such a way, the distribution (publication) is guaranteed—which shall ensure
equal opportunities between companies.
In 1997, the different European committees struggled hard to define the requirement how the term
may be interpreted. CEN and CENELEC agree that—in accordance with the reference model—a system is
open, if the protocols have been opened (i.e., standardized) and are not protected by a patent that prevents
competitors to design the same protocol. Licenses have to be available for everyone at “reasonable” prices.
The ISO/OSI reference model describes functions, not hardware or software. It does not dictate
manufacturers which technology to use in their products. It only prescribes how the technology has to
behave, seen from the outside.
Unfortunately, the standard for a protocol cannot be compared with a physical unit like a screw.
A standard is complex and thus never completely error free. This was considered regarding different
terms. Two systems are called interconnectable, if they are subject to the same standard. This does,
however, not mean that they cooperate. If cooperation shall be guaranteed, interworkability is required,
which means that the system can in principle exchange data according to the protocol. However, the
© 2011 by Taylor and Francis Group, LLC