Image Processing Reference
In-Depth Information
Application layer
Transport layer
Network layer
Data-link layer
Physical layer
FIGURE .
Protocol stack representation of the architecture. (From Akyildiz, I., Su, W., Sankarasubramaniam, Y.,
and Cayirci, E.,
IEEE Commun. Mag.
, , , .)
the number of nodes and their placement (in very densely deployed sensor networks, energy might
besavedbyturningcertainsensorsoftoreducetheamountofredundantinformationsensed).
In the following we give a description of the main building blocks needed to setup a sensor network.
The description follows the OSI model. his should not imply that this is the right structure for these
networks, but is offered merely as a reference point:
•
Physical layer
: It is responsible for the management of the wireless interface(s). For a
given communication task, it defines a series of characteristics as operating frequency,
modulation type, data coding, interface between hardware/software, etc.
The large majority of already built sensor networks prototypes and most of the envi-
sioned application scenarios assume the use of a radio transceiver as the means for
communication. The unlicensed industrial, scientific, and medical (ISM) band is pre-
ferred because it is a free band designed for short-range devices using low-power radios
and requiring low data-transmission rates. The modulation scheme used is another
important parameter to decide upon. Complex modulation schemes are not preferred
because they require important resources (in the form of energy, memory, and com-
putation power). On the other hand, recent technological developments and efforts in
standardization make the free . GHz communication band in combination with Zigbee
technology [] an attractive possibility for sensor networks.
In the future, the advancements of the integrating circuits technology (e.g., ASIC,
FPGA) will allow extended use of modulation techniques such as ultrawide band (UWB)
or impulse radio (IR). Building general purpose sensor nodes using off-the-shelf compo-
nents keeps (at least at the moment of writing this document) modulation techniques at
a quite modest level.
Basedonthemodulationtypeandonthehardwareused,aspeciicdataencoding
scheme is chosen to assure both the synchronization required by the hardware compo-
nent and a first level of error correction. At the same time, the data frame includes some
carefully chosen preambles as well (series of initial bytes needed for the conditioning of
thereceivercircuitryandclockrecovery).
