Sensor Networks’ Integration - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 91

and applications, it does not seem to provide means to insert such mechanisms
closer to the source between coordinator and sensor networks, where they often
make more sense. Continuous queries are static in a sense that once the coordinator
decides on a way to serve a sensor request, they cannot adapt to changing avail-
ability of services. Only mobile proxies provide a limited support to deal with such
changes at sensor network level. While mentioning incentives, cost-sharing, secu-
rity, privacy and trust, SenseWeb does not provide explicit support for such func-
tions in its infrastructure.
Janus
Janus [ 6 ] is an attempt to break up the tight coupling between sensor networks and
their application, by inserting an intermediate broker into the interaction path. Janus
makes use of extensible resolution protocol (XRP) and introduces two entities as part
of its architecture, namely an XRP agent and an XRP engine . Instead of directly inter-
acting with a sensor network, applications interact with an intermediate broker real-
ized by the XRP agent , typically located somewhere in the access network. The XRP
agent then interacts with XRP engine ( s ) located in the gateway of to the sensor
network(s), via an RPC-style interface. The XRP agent can discover available ser-
vices at a sensor network and gain access to the services by receiving a locator bound
to local functions calls at the corresponding XRP engine. These locators can be used
as selectors to identify services in subsequent RPC function calls at the remote sensor
network, realizing both query-reply-based as well as event-based interactions.
Different applications can be interfaced to the XRP agents via the implementation of
application specific proxies.
Janus achieves a decoupling of sensor networks and applications, by intro-
ducing itself as level of indirection between the two systems. Thus, applications
and sensor networks can evolve independently, while relying on the Janus
framework to remain an invariant achieving compatibility via XRP. Janus is
able to integrate different heterogeneous networks, as long as they implement
an XRP engine that exports the available services via new RPC selectors and
that implements a translation of the function RPC calls to the sensor network
native mechanisms. Heterogeneous applications can be supported; however, for
each application a specific proxy needs to be implemented that interacts with
the XRP agent. Clear interfaces to the XRP agent seem not to be specified,
which makes it difficult to write application proxies. The reliance on an XRP
agent as centralized broker makes Janus not scale well for large number of
applications and sensor networks. Janus does not provide any support for com-
position of context information of different sensor networks or functions to aid
the automatic selection of appropriate sensor networks for interaction. It shifts
the onus to perform this task to the application. Janus does not provide any
mechanisms that are able to optimize the delivery of same sensor information
to multiple applications, nor does it address functions for security, privacy and
trust, and accounting.