Networking Reference
In-Depth Information
way for devices that interact with the respective driver. These messages are generated
by classes of the Publish and Discovery component and should be sent to the sen-
sors, which respond with a message (AdvertiseMessage) advertising their services,
residual energy, among other relevant information. The getData operation is used
to task the sensor nodes to collect the sensing data as requested by the client appli-
cation, according to the desired data delivered model. The publishData operation is
responsible for receiving data messages sent by the sensors (containing the collected
sensing data).
4.2.1.3 Publish and Discovery
There are two levels of service discovery in a WSN: internal and external (phase (i)
of the networks operation, as described in the Chap. 3 ) , and both are implemented
by classes from the Publish and Discovery component .
The internal discovery enables that sink/gateway nodes know the capabilities of
all sensor nodes that compose a given WSN connected to the gateway. In order to
implement this feature, in the SmarSensor infrastructure a special message, called
AdvertiseMessage, was defined to allow sensor nodes to advertise their capabilities.
Such message include the node (local) identifier, a timestamp, the types of sensing
units available in the node, geographic location, residual energy, maximum data
accuracy/precision provided; supported data rates, supported aggregation functions
and supported duty cycles. Advertising messages are sent by a sensor node (i) at the
node initialization (upon the network deployment in the target area), (ii) when a new
sensor joins a pre-deployed network, and (iii) from time to time, either as a keep
alive message sent with a predefined periodicity or upon request by the gateway
(via a RequestAdvertiseMessage). Such periodic sent of advertising messages is
required given the dynamic nature of the WSN environment, where sensors may
be damaged, moved, have their energy depleted, thus no longer participating from
the network infrastructure. In SmartSensor, if a connected device does not respond
to three consecutive RequestAdvertiseMessage sent by a gateway, such device is
considered unreachable and should be removed from the list of devices maintained in
the database. AdvertiseMessage messages are disseminated throughout the network
by using the communication/routing protocols available at the nodes, until they reach
the gateway node. In the gateway, the content of suchmessages is extracted and stored
in a database containing data for the respective WSN connected to the gateway.
Gateways are organized in a logical hierarchy and interact among themselves in
order to exchange data from their respective WSNs. PEM components execute in
the gateways positioned in the highest level of the hierarchy. While gateways at
the lower levels only keep information on their respective WSNs, the higher level
gateways keep a database with updated information on all networks connected to the
SmartSensor infrastructure.
The external service discovery is used by client applications to discover which
WSNs provide the services they require, and how to access such services. This is a
traditional phase of service discovery according to the Web Services technologies.
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