Information Technology Reference
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Some of the standards are at draft stage and still undergoing specification and
further harmonization between the different standards brought into the framework
from outside, such as SensorML and TML is required. The framework does not
explicitly address aspects of security, privacy, trust, accounting, and resource arbi-
tration. In addition, the SWE representation formats allow application only to
express simplistic queries and are not suitable for the high-level declarative service
interface as well as the complex processing of sensor information inside of the
system based on ontological models.
Hourglass
Hourglass [ 13 ] aims at creating an Internet-based framework for connecting hetero-
geneous geographically distributed sensor networks with applications that require
sensor information. Hourglass provides an infrastructure for data collection referred
to as data collection network, which handles service naming, service discovery,
route setup from sensor networks to applications, and provides support for integrat-
ing internal services along the data dissemination path to perform aggregation or
buffering of sensor information. Hourglass primarily addresses stream-based aggre-
gation and processing of sensor information that is required by applications over a
longer period of time. Hourglass treats the sensing and processing capabilities sensor
networks offer as services, and extends the service concept to also encompass any
intermediate processing service on sensor data. Typically services can act as data
consumers, data producers, or both. Services in Hourglass are organized into service
providers. Each service provider comprises more Hourglass nodes forming a single
administrative domain, entering or leaving the Hourglass system as a unit. Each
service provider needs to support minimum functionality in terms of a circuit manager
and a registry . In addition a service provider can provide several generic or
application-specific services. A service registry is a (distributed) repository of
information about various services and active circuits in the Hourglass system. It is
a lookup service that allows the resolution of service endpoints. Each service pro-
vider typically maintains an own local registry, with which active services of a
service provider register via service announcements. Such service announcements
typically contain communication endpoint identifiers, topic name, predicates, and
expiration time as entries are kept as soft state. An application that aims to establish
a “streaming session” with one or more sensor information sources and intermediate
processing first queries the service registry for available services in the Hourglass
system. It then specifies its query requirements as so-called circuit descriptions that
link one or more data producers and a data consumer with possible intermediary
in-network services into a logical data flow. The circuit manager instantiates the
described logical flow as network data flow by establishing connections between
the different physical nodes offering the respective services. The hourglass service
layer manages the invocations to the service interface and the multiplexing of data
to and from the connected circuits. Sensor data are routed along the established
path and possible processed at intermediate nodes.
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