Networking Reference
In-Depth Information
thus allowing that the services of smart objects are accessed as resources in an ROA
(Resource-Oriented Architecture) approach [ 12 , 17 ]. Besides the standardization
and simplification in the process of applications development, the use of the HTTP
protocol also eliminates compatibility issues between different manufacturers, pro-
prietary protocols and data formats [ 4 ]. Such feature is particularly appealing in the
integration of WSN to the Web, since the devices that compose such networks are
heterogeneous regarding their software and hardware technologies.
To make a WSN Web-enabled is not a trivial task as a consequence of the differ-
ences between Internet applications and WSN applications. WSN applications have
specific Quality of Services (QoS) requirements and data delivery models. They run
on battery operated devices whichmost of the time sleep and wakeup only when there
is data to be exchanged. Furthermore, the WSN protocol stack is very different from
the Internet stack and the multicast and asynchronous communication is the most
frequent style in comparison to the unicast and synchronous approach of standard
Internet applications [ 34 ]. Therefore, there are several issues to be addressed by a
middleware-layer infrastructure to provide WSN as a service in the WoT.
The Web of Things paradigm exposes the functionalities of smart devices in
the Web using two different approaches and both were adopted in the SmartSensor
infrastructure. In the first approach, embedded Web servers are deployed directly
within smart objects, enabling that the features of these devices are available in the
Web as RESTful resources [ 21 , 29 ]. However, whenever a smart object does not
have enough hardware resources to run an embedded server, a different approach
for the WoT integration is required, based on the adoption of a WoT Gateway or
Smart Gateway. Gateways are (more) powerful devices used as a bridge to provide
the functionalities of smart devices through a RESTful interface. Gateways have
two basic functions: to provide a RESTful interface with URIs that identify and
provide access to physical objects (smart devices) and their resources; and to realize
the communication with physical objects by using their specific APIs. Gateways
intercept HTTP request messages issued by client applications and perform any
conversion of data and protocols before forwarding the converted messages to the
WSN devices. In the same way, messages sent by the sensor nodes in response to
application requests are translated by the gateways to the HTTP format. A gateway
can support multiple types of devices through an architecture encompassing a set of
drivers responsible for solving heterogeneity issues.
Smart Gateways can also be used to perform more complex operations with data
obtained from a WSN, and to orchestrate the composition of a highest level Web
application from several lower-level services provided by devices. These Web appli-
cations are the so-called physical mashups created from the composition of informa-
tion provided by devices available as resources through the RESTful API provided
by the gateway [ 10 ]. As a basic example of this type of application lets consider
a set of devices connected to the Web that provide power consumption monitoring
of electronic appliances. The Smart Gateway could provide a service that presents
a map showing the geographical location of several sensor instrumented buildings
(Smart Buildings [ 7 , 13 , 30 ]) and then, whenever a user selects a particular building,
the service returns the consolidate energy consumption monitored by all devices in
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