As Figure 7 shows, nodes in the sensing range construct the
convey tree, and nodes send data to the sink by forwarding packets to their parent.
Parent nodes wait for their children??™s packets before forwarding their own packets,
therefore packets can be aggregated hop by hop along the path to the root.
In DCTC, nodes are divided into grids just like GAF. Only one node in each grid,
i.e., the grid head, is required to remain awake, and other nodes go to sleep and wake
342 Kai-Wei Fan, Sha Liu, and Prasun Sinha
Fig. 7. Convey Tree rooted at the center of the event.
up periodically to save energy. When a target enters the sensing range, the grid head
will wake up all nodes in its grid and nodes start to track the target and send the
sensing report.
The processes in DCTC can be categorized into three operations: Initial Tree
Construction, Tree Expansion and Pruning, and Tree Reconfiguration. These operations
are described below:
??? Initial Tree Construction: When a target first enters the sensing range of a
sensor network, the nodes that detect the target will construct the initial convey
tree. The first step is to select a root of the tree. Many leader election algorithms,
such as those described in [10] and [11], can be used. To simplify the description
of tree construction, we select the node that is closest to the target as the root
(assume that nodes have the ability to detect the distance between the target and
themselves).
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