Sensor Networks’ Integration - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 90

5.2.2.1 Server-Client

SenseWeb

SenseWeb [ 4 ] provides an infrastructure for sharing information generated by glob-
ally distributed sensor networks. Applications can use SenseWeb to create a variety
of different applications, otherwise not possible due to the lack of sensor network
coverage and diversity of sensor information. Heart of the SenseWeb system archi-
tecture is the coordinator , which is central point of access into the system by all
applications and sensor network contributors. It can be seen as a centralized broker
that coordinates the information access of application to relevant sensor networks.
The coordinator is decomposed into a tasking module and a senseDB . The tasking
module receives application requests and tries to find matching sensor network
information, considering required accuracy, capabilities, and policies of available
sensor networks. The senseDB component of the coordinator tries to optimize data
access across different application queries with overlapping space-time window by
combining requests for common data whenever possible or serving request from
cached data of previous queries. The senseDB also indexes sensor network charac-
teristics and other shared resources in the system and enables their discovery by
applications. Sensor or sensor networks are connected via sensor gateways , which
on one side implement sensor-network-specific access methods, but on the other
side, expose a standardized WS API to allow other SenseWeb components to access
sensor data streams, submit data collection demands, or access sensor characteris-
tics. Sensor gateways typically implement policies defining what sensor informa-
tion is to be shared. Sensors that do not have a gateway can be connected by a
shared gateway referred to as Datahub . In addition, mobile proxies are special GWs
dedicated to one spatial area that allow mobile sensors to opportunistically provide
information, while hiding the temporary availability of different sources to applica-
tions. Senseweb also provides data transformers that convert data semantics by
some processing. Data transformers can be shared across multiple applications and
link themselves between applications and coordinator.
SenseWeb has a variety of features that are able to deal with heterogeneity and
scalability present in the real-world Internet. Heterogeneity in sensor network
access is overcome by providing access through a unified WS interface.
Heterogeneity in terms of sensor information quality and access policies is
addressed by metadata in the sensor descriptions and learning sensor characteristics
(e.g., disruptions of availability) at runtime, while allowing application to explicitly
specify their requirements. Improved scalability is achieved by minimizing data
collection for common data among different application queries and approximating
subset of information, e.g., based on cached information. In addition, SenseWeb
allows data to be collected only when actually required by applications. While these
features certainly contribute to scalability by reducing the amount of generated traf-
fic in the system, the architecture of SenseWeb does not scale well for many appli-
cations and many sensor networks due to its centralized broker nature. While
SenseWeb provides support for inserting data transformers, between the coordinator