High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 139

tion . The event specification is the machine-
processable form of the user's description. It
contains the involved sensing features as well as
phenomenon related constraints concerning the
spatial and temporal expansion, detection intervals
and associated handlers that are triggered by upon
event. Here it shall be emphasized that an ap-
propriate event specification is sufficient for
successful configuration of the sensor network.
Every processing and transformation step based
on the event specification and its derivations is
automatically done by the event configuration
system.
It is quite obvious, that general event specifica-
tions cannot be uniformly transferred to different
sensor nodes of heterogeneous hardware. Hence,
event specifications are preprocessed before being
distributed in the sensor network. First, an XML
parser generates the respective tree representa-
tion. Various XML elements of that tree have
to be adapted to the targeted sensor system, i.e.,
conversion of values for sensing, renaming of
identifiers and functions etc. Finally, the adapted
tree is converted and compacted into a deployable
event specification of minimal size, called binary
event specification . Binary event specifications
are distributed in the sensor network for initial
event configuration as well as for updates and
deletions.
On the sensor nodes, the event configuration
environment processes every incoming binary
event specification to generate the respective event
representation as an EDT . According to the sens-
ing features and resources provided by the node,
the EDT is split into local and remote parts. Local
parts can be evaluated by the node itself, whereas
remote parts have to be requested from external
sources, e.g., from neighboring nodes. Thereby,
only those within the collaboration region are
regarded to provide these remote parts. After
further adaptations and configurations of event
related constraints, the final EDT is integrated to
the EDT processing unit. The EDT processing unit

• autonomously collects required sensor

readings,

• frequently evaluates the EDT with respect

to the configured detection interval,

• manages necessary collaboration with oth-

er nodes and

• triggers associated handlers in case of pos-

itive event evaluation.

The EDT processing unit is enabled to admin-
istrate and process several EDTs simultaneously,
too. This is a prerequisite to ensure proper flex-
ibility by allowing the sensor nodes to execute
several tasks concurrently. An integrated update
mechanism enables to replace binary event speci-
fications analogous to code update means. This
feature allows easy reconfiguration or recalibra-
tion of already deployed sensor networks.
The following sections provide insights into
all configuration steps in the user-centric design
by applying an illustrative example aiming at
detecting fires.

Convenient Hardware-
Independent Task Design

To increase the proliferation of WSNs, conve-
nience in creation of applications on WSNs is
highly demanded. It is considered that the deploy-
ment of sensor networks and its applications is
merely configuration rather than programming
and implementation. That is, details on sensor
network hardware or implementation issues,
especially those requiring programming skills,
should be kept away from the user. We consider
a scenario, in which the configuration assistant
knows, which sensors exist in the WSN. Also the
density or the amount of sensors or sensor nodes
per area is known. For designing and deploying
applications on a priori unknown sensor networks
discovery means are required. Given that the
assistant application knows the key data of the
WSN, it supports and guides the user through
the configuration of the WSN. The user may just