If a sensor node missed the initial aggregate request, it can still use the
shared channel to monitor the communication of the nodes nearby and participate
in the aggregate if the nodes near it propagate their aggregates.
This not only increases the number of sensor nodes engaged in aggregation,
the shared channel technique also reduces transmitted messages. Once a sensor
node finds that others propagate aggregate messages, it can join the procedure
without any extra information. In Figure 15, those messages denoted by black
arrows are not obliged to transmit. Consequently, the transmitted messages
are reduced from 22 to 17. Listening techniques can also be used to reduce
the number of the transmitted messages.
Hypnosis testing during aggregation [12]
Although pipelined aggregation can conserve the bandwidth and improve the
accuracy of results, it still requires all sensor nodes to participate in the aggregate
computation. Hypnosis testing can be applied to reduce the number of
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Jinbao Li, Zhipeng Cai, and Jianzhong Li
the nodes involved in aggregating. By monitoring the transmission of nearby
nodes, this technique permits only the nodes that can greatly a?®ect the final
aggregation to propagate messages.
For example, suppose the aggregate function is MAX or MIN and every
sensor node can hear the values transmitted by others. If local reading cannot
a?®ect the final result, i.e. it is lower (for MAX) or greater (for MIN) than
the broadcasted value, a sensor will not send out its data.
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