High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 153

the thresholds regarding the carbon monoxide
and the combination of smoke and temperature.
The equivalent EDT is depicted in Figure 8. For
further processing, the tree nodes are pre-order
numbered during their creation from the binary
event specification. That assures the same initial
tree labeling on every device in the network,
which is necessary for efficient exchange of event
information later.
An EDT evaluation procedure is either trig-
gered by internal EDT related timing constraints,
which are specified in the execution element, or
by collaboration requests from other devices. The
EDT is evaluated automatically in a bottom-up
manner starting from the leaf nodes in order to
determine a Boolean value at the root node, i.e.,
the final EDT evaluation result. All EDT nodes
representing sensing capabilities are assigned with
actual sensor readings. In the example, these are
the node CO for carbon monoxide numbered as
3, the node T for temperature numbered as 7 and
the node S for smoke readings numbered as 10.
Afterwards, the EDT is evaluated by execution

of the operations stated at the parent nodes with
the values of its children as operands. As a result,
Boolean values are assigned to relational and
logic nodes. If the value of the EDT root node
evaluates to true, i.e., the phenomenon was de-
tected, all specified event handlers are triggered
for further processing. A new EDT evaluation is
triggered when the next assigned evaluation in-
terval has elapsed.

Local Adaptation of EDTs by Pruning

Up to here, it was assumed that sensor nodes
possess all sensing capabilities to evaluate the
complete EDT itself. If that assumption cannot be
assured, local detection of phenomena becomes
impossible without collaboration with other sensor
nodes possessing the locally missed capabilities.
The EDT engine additionally enables sensor nodes
to evaluate the EDT even if they provide only a
subset or even no sensing capability. Such lack of
capabilities could either be intended by design or
due to failed sensing units. Hence, certain branches

Figure 8. Pre-order numbered EDT of the fire detection example