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The cooperative nature of WSN still adds value though, as setup and installation of the sensors
becomes easier and the overall operation more reliable. Since the position of the measurement point
mightnotbeoptimalfromaradiofrequency(RF)perspective,theself-organizingand-healing
nature of WSN help alleviate communication disturbances related to radio propagation in industrial
environments. In stead of investing lots of time in frequency planning and network design, perfor-
mance of the network can be improved by adding router nodes that provide alternate/redundant
paths for the wireless sensors.
27.2.2.2 Time Is of Essence
Time is money, also in the industrial automation world. Whereas a data packet in a standard WSN
may spend an unknown time from its source to its destination, an industrial application will fre-
quently require hard bounds on the maximum delay allowed. This can put some strict bounds on
network topology, e.g., prescribing a star configuration rather than a multi-hop meshed network,
and requires a careful trade-off between power conservation and response time. Although there is
usually a strict bound on the maximum delay, the requirements can differ quite dramatically between
applications. his makes a generic design of the WSN quite difficult and the “one size fits all” prob-
lem quite prevalent in industrial standards groups who seek to minimize the number of overlapping
wireless communication standards.
27.2.2.3 Wireless in a Wired World
Although WSNs have made their entry on the industrial automation scene, the majority of sen-
sors and devices have a wired infrastructure. This means that we need to cope with a mix of
wireless and wired devices over a foreseeable future. Applications that are built upon a wired infras-
tructure can afford to “waste” bandwidth and make use of high performance networks to solve
their mission critical goals. Simply adding a wireless sensor to the equation is often not straight
forward.
A quite common mixed scenario on the other hand is to add wireless connectivity to already wired
devices. This is done to provide access to diagnostic or asset data in the device in a nonintrusive
manner, i.e., without modifying or interrupting the wired network.
Since wireless solutions in industry will tend to also have a wired infrastructure, the data will
emanatefromthesensors,andripplethroughthenetworktosomewiredaggregationpoint.From
here, it will in general, be transported over a high-speed bus to a controller. Apart from the classical
mesh networking topology of WSN, we have two more common topologies in industrial settings
(Figure .).
In the star topology, the most prevalent topology today, the wireless nodes communicate with a
gateway device that bridges the communication to a wired network. his network configuration will
have greater chances of minimizing delays in the network and can thus be used in applications that
have strict bounds on the communication delay. Another common intermediate solution of WSN is
to have router devices (often mains powered) that communicate with the gateway. he sensors only
need to perform point-to-point communication with the routers and can therefore remain simple
andlowpower,whiletherangeandredundancyofthenetworkareimproved.
27.2.3 Industrial Automation Applications
27.2.3.1 Industrial Plants, an Unfriendly Setting for WSN
The environmental conditions of industrial automation plants pose some heavy challenges on the
development of embedded field devices in general, and specifically for WSN devices.
