Each node broadcasts its (id, d) where id is node id and d is the distance
to the target. If a node does not receive packets with smaller d, it becomes
the root candidate, otherwise, it selects the node with the smallest d among its
neighbors as its parent in the convey tree. There may be multiple root candidates
in the network after the election. These root candidates flood their (id, d) to the
nodes sensing the event. The node with the smallest d becomes the root, and
other candidates create a route to the node flooding the packet with smallest d.
Therefore, the initial convey tree is constructed.
??? Tree Expansion and Pruning: When the target is moving, there will be new
nodes joining the convey tree while detecting the target. Also there will be some
nodes in the tree being removed because the target is moving out of those nodes??™
sensing range. A conservative scheme or a prediction-based scheme can be used.
In a conservative scheme, the root calculates the possible nodes that may detect
the target within time t, where t is the duration to execute the tree expansion and
pruning process. As shown in Figure 8(a), based on the current target moving
velocity vt and the sensing radius ds, the root knows which nodes should be
included in the tree, and which nodes should be removed. The dc in Figure 8(a)
Chapter 13 Data Aggregation in Wireless Sensor Networks 343
is the maximum range of nodes that may sense the target after time t, and dc =
(vt + ??) ?— t + ds where vt + ?? is expected maximum velocity of the target.
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