The communication cost is greater than the
computing cost, so the purpose of a sensor network is to reduce transmission
load. Each sensor processes the data produced by itself locally to reduce the
amount of data transmitted. For example, the sensors for measuring tra?±c
compute the number of cars passed by and record the pressures. Each sensor
transforms the pressure to flux, then sends out the number of cars passed by.
Those numbers are aggregated for transmission. Thus, the transmission cost
is greatly reduced. Queries in sensor networks will change depending on di?®erent
times, positions and the requirements of users. For example, commuters
concerned about tra?±c will concentrate on some streets in peak hours, and
not care about other streets. Thus, the control of sampling rate and delivery
rate of sensors is a good method to reduce the communication cost.
The fifth di?®erence is that, there are two kinds of queries in the data
management system of a sensor network. The first kind is a long-running
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Chapter 12 Data Management in Sensor Networks
query. It continuously observes the status of the monitored area during the
period the user specified. Traditional database systems do not have the ability
to process long-running queries. The second kind of query is an ad hoc or
snapshot query, which is for querying the current status of the monitored
area. Given the limited resources in sensor networks, the data management
system should balance the accuracy of the query results and the resources
wasted during the query.
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