... important.¹

Note that as sensor devices become more integrated, it is expected that the ratio of communication to computation costs will get more important over time as silicon efficiency increases while the physical costs of pushing radio waves over the air remain constant.

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... network.²

Note that, as an optimization, it may be useful for the routing layer to limit the extent to which queries are propagated based on properties of the query - for example, a short-lived query over constrained geographic area need not be sent to motes far away from that area. We reserve such optimizations for future work.

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... below.³

We omit a detailed discussion of how new aggregate functions are registered with motes. For now, aggregates are pre-compiled into motes. Virtual-machine languages recently proposed for TinyOS-style (16) motes could also be used for this purpose.

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... DISTINCT⁴

The HISTOGRAM aggregate sorts sensor readings into fixed-width buckets and returns the size of each bucket; it is content-sensitive because the number of buckets varies depending on how widely spaced sensor readings are.

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... HISTOGRAM⁵

COUNT DISTINCT returns the number of distinct values reported across all motes.

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.... ⁶

Although independent failures are not always a valid assumption, they will occur when local interference is the cause of loss. For example, a hidden node may garble communication to P₁ but not P₂, or one parent may be in the process of using the radio when the message arrives.

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