Database Reference
In-Depth Information
i
1. In such cases, the time stamps
t
i
become irrelevant, and it is
sucient to use only the index
i
for denoting the time axis.
≥
i
t
i
s
j
x
j
y
j
v
ij
sensor_values
Figure 2.2.
Database table containing the sensor values. The position of the sensor
s
j
is denoted as (
x
j
,y
j
). Since the sensors are assumed to be stationary, the position
can also be stored using a foreign-key relationship between
s
j
and (
x
j
,y
j
). But, for
simplicity, we assume that the
sensor values
table is in a denormalized form.
In this chapter, we assume a scenario where the sensors are used for
environmental monitoring. We assume that all the sensors are monitor-
ing/sensing only one environmental attribute, such as, ambient temper-
ature
2
. As discussed in Section 1, we assume that the environmental
attribute we monitor is suciently smooth and continuous. If necessary
for rendering the discussion complete and convenient, we will introduce
other attributes being monitored by the sensors. But, in most cases, we
restrict ourselves to using only ambient temperature.
Figure 2.2
shows
a conceptual representation of the sensor values in a form of a database
table, denoted as
sensor values
.
2.2 The Sensor Data Acquisition Query
Sensor data acquisition can be defined as the processes of creating
and continuously maintaining the
sensor values
table. In existing lit-
erature, naturally, many techniques have been proposed for creating and
maintaining the
sensor values
table. We shall discuss these techniques
briefly, describing their important characteristics and differences with
other techniques. We use the sensor data acquisition query shown in
Query 2.1 for discussing how different sensor data acquisition approaches
process such a query. Query 2.1 is a query that triggers the acquisition
of ten sensor values
v
ij
from the sensors
s
j
at a sampling interval of
one second. Moreover, Query 2.1, is the typical sensor data acquisition
query that is used by many methods for collecting sensor data.
2
We use
ambient temperature
and
temperature
interchangeably.
