Image Processing Reference
In-Depth Information
operation not affected if a node goes down). On the other hand, the wireless sensor networks in indus-
trial applications feature a prearranged network topology; have no ability to self-heal; require long
lifetime—both nodes and the network; and tend to be expensive. The prearranged network topol-
ogy is determined by the discrete manufacturing or continuous process equipment arrangement,
or system architecture, which determines where the information is to be collected. A network and
system operation is affected if a node goes down; a physical process parameter such as temperature
or pressure is no longer observable and controllable, for instance. A solution here is node redun-
dancy, though expensive. he nodes are required to have a long lifetime mandated by economics of
the production process and the initial investments. Node or node's components replacement can only
by economically justified during the device-or system-scheduled or preemptive maintenance period.
Fault-tolerance, physical, and operational robustness are some of the requirements imposed on the
nodes. he required long lifetime also applies to batteries, if the node is battery powered. One of the
consequences of the long lifetime requirement is the increased node functionality to provide a cer-
tainlevelofredundancyonchiporonprintedcircuitboardassemblywhichcomesatacost.Another
factor to have an impact on the cost is the IC packaging and node housing to protect the node from
the harmful environmental factors such as temperature, fluids, and to mention some, which have a
potential to shorten the operational lifetime of the node—or to prevent any electrical discharge from
the node in hazardous environments, presence of flammable gases, for instance.
The operation of wireless sensor networks in industrial applications typically imposes some sort
of real-time restrictions, and frequently hard bounds on the maximum delay. In most of “classical”
wireless sensor networks, the time a packet takes to travel to its destination tends not to be an issue.
This latency is typically tens of milliseconds for discrete manufacturing, tens of seconds for process
control, and minutes to hours for the assets management.
The wireless sensor networks in industrial applications tend to have a hybrid wireless-wireline
architecture interconnected via a gateway device, where high capacity and mains powered wireline
is used for data distribution from the collection point []. In this context, the most established net-
work topology is the star topology with wireless links between the sensor nodes and the gateway.
The one-hop communication with the gateway makes this solution suitable for time-bounded com-
munication supporting processes with fast dynamics. he hybrid mesh topology, another commonly
used topology, has at its center the gateway device connected by wireless links to frequently mains
powered router nodes, with the sensor nodes communication in one-hop communication with the
router nodes.
Reliability of a message delivery is an important requirement in wireless sensor networks used in
industrial applications. Lost messages may adversely change dynamics of a process under control,
for instance. One way to improve the reliability is through transmission redundancy. Depending
on the application, a number of options can be considered: space diversity—transmission through
different paths; frequency diversity—on different frequencies; time diversity—several times on the
same frequency; and modulation scheme diversity—different modulation schemes.
With nodes frequently located in inaccessible and/or hazardous places, combined with prohibitive
cost of unscheduled energy source replacement requiring halting operational activities, low power
consumption is of paramount importance in the wireless sensor networks used in industrial applica-
tions. Some of the factors minimizing power consumption include: using low power elements—CPU,
for instance, runs on reduced clock rate with less on-chip functionality; selecting proper opera-
tional regime—adopting sleep/wake-up mode of operation, with transmission activated only if the
value of a measured physical quantity is larger then the predetermined bound; choice of the right
communication protocol; etc.
The communication protocol ultimately dictates the lower bound on the power consumption.
The Wireless Interface to Sensors and Actuators (WISA) protocol [,] is a low power and high
performance protocol based on single-hop transmission to avoid delays in intermediate nodes
and Time Division Multiplexing (TDM) assuring no collisions to occur (a node is alone on the
 
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