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streams of different characteristics using SQL-like operations and conditionally
produce output streams (event-like). Virtual sensors can be dynamically deployed
on the GSN containers and the production of its output stream is dynamically trig-
gered by the arrival of input streams. Besides running instances of deployed virtual
sensors, GSN containers provide additional functionality supporting the manage-
ment of the virtual sensor instances and their required resources, function to man-
age streams and resources required for stream processing, query management
(request input data from other virtual sensors and keep track of other virtual sensors
requiring their output), and a storage layer for the management of persistent storage
of data streams. Access to GSN-container internal functions is provided by an
interface layer, which is used to communicate between GSN containers or can be
accessed directly via web interfaces. Besides providing access control at different
levels of granularity down to the virtual sensor level, the interface layer provides
integrity and confidentially functions for the exchange of data streams.
GSN provides many interesting features. The ability to create aggregate virtual
sensors from different heterogeneous sensor information sources suits well the
context information processing demands. Unlike other surveyed frameworks GSN
offers access control to sensor information and integrity protection. In addition it
provides a plug-and-play-like feature for integration of new sensors which allows
upon detection of a new sensor the dynamic download of a IEEE1451 transducer
electronic data sheet and automatic generation of a virtual sensor (given the wrap-
per code for the WSN technology is known). A current limitation represents the
SQL-like stream processing operations, which do not allow complex processing
data stream and data fusion to be performed in the system. The decentralized peer-
to-peer nature of the system seems to indicate good scalability properties. It is
unclear however, how data streams between GSN containers can be optimized, e.g.,
by selecting the same virtual sensor streams for different independent queries.
Point-to-point transmission of streams between GSN containers may be another
feature potentially reducing its scalability. Although meta-information is provided
for virtual sensors, which can be used for service discovery, it is still unclear what
mechanisms would be used to discover virtual sensors. The current descriptions
seem to “hard code” required input streams into the virtual sensor descriptions,
which does not make dynamic composition or late binding of virtual sensors pos-
sible at runtime. GSN does not seem to provide any infrastructure support for
accounting nor does it provide a little information on an infrastructure that can be
used for dynamically composing or modifying active virtual sensor services.
Sensor Web Enablement
Sensor Web Enablement (SWE) [ 19 ] is an initiative by the Open Geospatial
Consortium aiming at the development of a set of standards to enable the discovery,
exchange, and processing of sensor information and tasking of sensor systems over
existing Internet. SWE strives for plug-and-play-like integration of sensor networks
and enabling protocols to make those accessible and controllable by web-based
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