Figure 6 shows two approaches for a user to cause one sensor
to trigger another in a network. In both cases we assume sensors
know their locations and not all nodes can communicate directly.
Part (a) shows a direct way to implement this: the user queries the
initial sensors (small squares), when a sensor is triggered, the user
queries the triggered sensor (the small gray circle). The alternative
shown in part (b) is a nested, two-level approach where the
user queries the triggered sensor which then sub-tasks the initial
sensors. This nested query approach grew out of discussions with
Philippe Bonnet and embedded database query optimization in his
COUGAR database [5].
than the initialsensor, for example as an accelerometer triggering a
GPS receiver) and network traffic (for example, a triggered imager
generates much less traffic than a constant video stream). Alternatively,
in-network processing might choose the best application
of a sparse resource (for example, a motion sensor triggering a
steerable camera).