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

12-2 Industrial Communication Systems
these technologies have made them a viable addition to, or replacement of, traditional wired instrument
networks for factory automation and communication.
12.2 Wireless Sensor Network Basics
In a WSN, a wireless sensor node is an individual device in the network which measures a physical
phenomenon and transmits the sensor reading to the network manager. The data transmission used
by the wireless sensor nodes in a WSN is specified by a network protocol. A WSN stack is traditionally
used as a layered and abstract description of the network protocol design.
12.2.1 Wireless Sensor Node
A typical wireless sensor node consists of several elements as shown in Figure 12.1. The sensing unit is
a sensor element which measures a physical phenomenon (e.g., temperature, pressure, or vibration).
An analog-to-digital converter quantifies and converts the sensor data to the digital representation
needed for further processing and communication. The processing unit analyses and processes the digital
representation of the sensor data and is also responsible for handling the local radio-frequency (RF)
transceiver. The digital sensor data are encapsulated in data packets according to the communication
protocol, before being transmitted to their destination. The communication unit provides the wireless
interface which handles transmission and reception of data packets. It consists of an antenna and an RF
transceiver. The memory and storage are support units for the processing unit, used for both temporary
and permanent storage of firmware, configuration parameters, and sensor data. The power unit, which
normally consists of a battery and some control circuitry, provides power to all the components of the
sensor node. For some applications, a sensor node might also be equipped with a location engine, which
estimates the spatial location of a sensor node based on received signal strengths from neighboring
sensor nodes. The location engine could either be a separate microprocessor or implemented as an
algorithm on the main processing unit.
One of the main challenges for many WSN applications is the desire to have a long battery lifetime.
Low power consumption is therefore a key parameter for a wireless sensor node. As a result, the available
hardware resources on a sensor node are limited. The processing unit is typically a low-power
microprocessor capable of entering a deep-sleep mode with very low power consumption when there is
no scheduled activity.
12.2.2 Wireless Sensor Network Stack
For communication protocols and standards, stacks are used as a layered and abstract description of the
network protocol design. A stack consists of several layers where each layer is a collection of functions
related to the specific task of the layer. A layer is responsible for providing information and services to
Power
Memory/
storage
Sensing Processing Communication
FIGURE 12.1
Wireless sensor node.
© 2011 by Taylor and Francis Group, LLC