Database Reference
In-Depth Information
communication-ecient, nodes are organize into groups. Sensors of the
same group are within the broadcast range of each other, so that they
can effectively exchange t and the partial join results. Protocols are
proposed for group formation and join operation. Since connections
in WSNs are unreliable, the protocols also detect sensor failures and
develop recovering mechanism.
W. Xue et al. [33] defines events in a different way and develop cor-
responding detection techniques. Their work is motivated by coal mine
applications. Events are defined by patterns of contour maps and are
detected by matching the contour map of the current sensor readings
with predefined patterns. A contour line of a function is a curve along
which the function has a constant value [35]. In cartography, a contour
line joins points of equal elevation above a given level, such as mean
sea level. A contour map is a map consists of contour lines, for exam-
ple a topographic map, which shows valleys and hills, and the steepness
of slopes.
Figure 3.4(a)
illustrates how a contour map is created while
Figure 3.4(b)
shows a contour map of a hill.
Figure 3.4.
The Contour maps
In the coal mine application, the abnormal distribution of the oxygen
or gas density and the humidity concern the researchers. The distribu-
tion of sensor readings (e.g., gas density) over the monitoring area is
represented by a contour map. The authors observe that when some
events happen, the shapes of the contour maps exhibit certain patterns.
[33] follows the tree topology. During transmission, a sensor generates
its partial contour map by combining all partial maps from its chil-
dren. Since contour maps are typically of large size, the authors propose
compressing techniques to optimize the map transmission. Another ob-
servation is that the contour maps are mostly stable over time. For
continuous event monitoring, instead of transmitting contour maps at
each time stamp, only the updates upon the previous ones are transmit-
