were not always so restrictive. If one takes
a closer look at industrial Ethernet as it is today, it turns
out that the appraised uniqueness is more wishful thinking
than reality. In fact, not even the use of standard Ethernet is
really a common denominator, and above the data-link layer,
the approaches are completely different. Some use standard
Transmission Control Protocol (TCP)/User Diagram Protocol
(UDP)/IP mechanisms for transmitting data, which maybe
enhanced by additional software layers to support both realand
nonreal-time communication, whereas others use dedicated
communication stacks that bypass the entire IP suite. Some
employ well-known fieldbus application protocols to maintain
some backward compatibility with the fieldbus world, and some
are entirely new developments. Fig. 4 sketches the various
appearances of the protocol stack.