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24.2 Wireless Industrial Communications and Wireless
Fieldbus: Challenges and Problems
In this section we survey some of the problem areas arising in wireless fieldbus systems.
24.2.1 System Aspects
First of all, wireless fieldbus systems will operate in similar environments as wired ones. Typically, a
small to moderate number of stations are distributed over geographically small areas with no more
than  m between any pair of stations []. Wired fieldbus systems offer bitrates ranging from
hundreds of kilobits to (tens of) megabits per second, and wireless fieldbus systems should have
comparable bitrates. The wireless transceivers have to meet electromagnetic compatibility require-
ments, meaning that they not only have to restrict their radiated power and frequencies, but also
should be properly shielded from strong magnetic fields and electromagnetic noise emanated by
strong motors, high voltage electrical discharges, and so on. his may pose a serious problem when
off-the-shelf wireless transceivers are used (e.g., commercial IEEE . hardware), since these are
typicallydesignedforoiceenvironmentsandhavenoindustrial-strengthshielding.
Another problem is that many small fieldbus devices get their energy supply from the same wire
as used for data transmission. If the cabling is to be removed from these devices, there is not only the
problem of wireless data transmission but also the issue of wireless power transmission [], which
requires substantial effort.
For battery-driven devices the need to conserve energy arises. This has important consequences
for the design of protocols [,] but is not discussed anymore in this chapter.
24.2.2 Real-Time Transmission over Error-Prone Channels
In industrial applications often hard real-time requirements play a key role. In accordance with []
we assume the following important characteristics of hard real-time communications: (a) safety-
critical messages must be transmitted reliably within an application-dependent deadline, (b) there
should be support for message priorities to distinguish between important and unimportant mes-
sages, (c) messages with stringent timing constraints typically have a small size, and (d) both periodic
andaperiodic/asynchronoustraicarepresent.hequaliier“hard”stemsfromthefactthatlosses
or deadline misses of safety-critical packets can cost life or damage equipment. Both periodic and
aperiodic messages in fieldbus systems can be subject to hard real-time constraints.
Wireless media tend to exhibit time-variable and sometimes high error rates, which creates a
problem for fulfilling the hard real-time requirements. As an example, the measurements presented
in[]haveshownthatinacertainindustrialenvironmentforseveralsecondsnopacketgets
through the channel. Therefore, seeking deterministic guarantees regarding timing and reliability
is not appropriate. Instead, stochastic guarantees become important. An example formulation might
be the percentage of safety-critical messages which can be transmitted reliably within a prespeci-
iedtime-boundshouldbeatleast. x %. Of course, the error behavior limits the application areas
of wireless industrial LANs—when deterministic guarantees in the range of - ms are essential,
wireless transmission is ruled out (at least at the current state of art). However, if occasional emer-
gency stop conditions due to message loss or missing deadlines are tolerable, wireless technologies
can offer their potential. he goal is to reduce the frequency of losses and deadline misses.
It depends on the communication model how transmission reliability can be implemented. In
many fieldbus systems (e.g., PROFIBUS) packets are transmitted from a sender to an explicitly
addressed receiver station without involving other stations. Reliability can be ensured by several
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