High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 155

Figure 9. Pruned EDTs for two different types of sensor nodes monitoring the introduced fire phenomenon.
Nodes of type A provide sensing facilities for carbon monoxide and temperature whereas nodes of type
B provide sensing facilities for carbon monoxide and smoke. Consequently, each type of node prunes
a certain part of the EDT that cannot be evaluated locally. Resulting undecidable nodes are labeled
with question mark. Hence, the Boolean values of these nodes must be obtained from other nodes in the
specified collaboration region.

and hence, the EDT can be reconstructed into its
original form by removing the undecidable mark-
ing from respective EDT nodes.
By pruning, the EDT may degenerate to a
minimal tree consisting of only the root node with
two undecidable children. Such an EDT enables
sensor nodes that possess no suitable sensing
capability for phenomena detection to serve as a
bridge. These nodes are of interest if they are
located between two or more nodes that possess
the required sensing capabilities but cannot com-
municate directly or do not share the same col-
laboration region. The only prerequisite is that
these nodes share the collaboration region with
the bridge node. Figure 10 displays example
deployments for both cases. Here sensor node 2
shares its collaboration region with the nodes 1
and 3 and hence, may perform the bridging func-
tionality for these nodes. In Figure 10 (a), the
nodes 1 and 3 do not share a collaboration region
due to their distance. In Figure 10 (b), these nodes

share their regions indeed but cannot communicate
directly due to an obstacle between them. In such
a scenario, all participating nodes deliver their
parts of EDT evaluation to the bridge node, which
is eventually enabled to decide about the occur-
rence of the observed phenomenon. After having
identified the undecidable parts for local detection
on each sensor node, those have to efficiently
share necessary information. A suitable collabora-
tion scheme maintaining this data exchange is
presented in the next section.

Collaborative Exchange of EDT-Node
Values by Publish/Subscribe

To save energy resources, wireless sensor nodes
should communicate if and only if it is absolutely
necessary. A suitable collaboration mechanism in
sensor networks must further self-adapt to chang-
ing network situations and consider application