It requires 16
messages to send the aggregate result to the client host using the centralized
method.
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Chapter 12 Data Management in Sensor Networks
Fig. 13. Centralized and distributed aggregate.
Fig. 14. Examples of two aggregate approaches [12].
Figure 14(b) presents the distributed aggregate scheme. In this approach,
the leaf nodes of the routing tree only need to send the data to their parent
nodes. Intermediate nodes first accumulate their data with the readings of
their children according to an aggregation function f. Then the partial aggregate
result is propagated along the routing tree. While the partial aggregate
result is being transmitted, it is updated with any required extra data until
it reaches the root node. The amount of the data transmitted in this solution
depends on the aggregation function, e.g., in order to compute an average
function, every intermediate node needs to know the sum and the number of
the nodes in its subtree. In Figure 14(b), it requires five messages to compute
the average function before the root sends the result to the client host.
Pipelined aggregation [10, 12]
During processing aggregation in sensor networks, because of communication
failure or movement of sensor nodes, it is di?±cult to ensure accuracy of the
results. Suppose node P only has one child node C. After node P broadcasts
313
Client host
Sensor node
(a) All nodes send data to client host (b) aggregation is done at intermediate nodes
Jinbao Li, Zhipeng Cai, and Jianzhong Li
a message, node C receives it.
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