The rect specifies the rectan-
Chapter 13 Data Aggregation in Wireless Sensor Networks 333
gular location of the interested area. The attribute naming scheme must be selected
before designing the directed diffusion scheme for the network. When the interest is
propagated through the network, the gradients are constructed. Every sensor that receives
the interest creates a gradient that points to the sensor from which the interest
is received. The gradient specifies the next node to forward packets to the sink, and
also the interval and duration to report/forward packets that match the interest. This
process is illustrated in Figure 2(a) and 2(b).
After the gradients are constructed, sensors can send packets to the sink along
the gradients if they sense the events that the sink is interested in, as shown in Figure
2(c). The data generated by the sensor is still attribute-value pairs, and may look like:
type: vehicle-movement
instance: SUV
location: [150, 180]
timestamp: [13:22:24]
which specifies the instance of the type, the node location, and the time the report is
generated.
The data are forwarded to the sink along the gradient paths that are constructed
during interest dissemination. When a node receives a packet, it checks to see if it
also contains a packet with similar reported data (such as the data with the same
type, located within an area) and a close timestamp. The data in such packets are
supposed to be for the same event and are aggregated.
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