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certain monitoring context only. The context
description is defined by a propositional logic,
which evaluates to true as long as a specified
context is given. Combinations of primitive events
may form global complex events that may even
be distributed. Those are observed by a compos-
ite event detection engine. That engine seems to
adapt automatically to current network situations
but the general question of how composite events
are distributed and processed on several devices
is left open. Unfortunately, the author does not
provide implementation details.
(Wang, Han, Varshney, & Chen, 2005) prevents
from having potential SPoFs.
Transparency:
Dealing with heterogeneous
nodes and network structures, sudden changes in
the environment or failures during collaboration
etc., consequently requires continuous adaptation
and device configuration. These processes must
be hidden to remain fully transparent to the user.
Especially pervasive WSNs are expected to make
use of various sensors with similar or complemen-
tary capabilities. An automatic hard- and software
abstraction can cover such heterogeneity.
Energy efficiency:
Small devices, like wireless
sensor nodes, usually are subject to strict energy
constraints, e.g., by battery packs providing lim-
ited power only. Transmission is the most power-
hungry operational mode of WSNs consuming
orders of magnitudes more energy than local
processing. Since collaboration simultaneously
requires communication between sensor nodes,
it significantly increases the energy consumption
and hence, decreases the maximum reachable
node lifetime. To cope with that, enhancing the
cost-efficiency of collaboration by reducing the
number of transmissions and the amount of ex-
changed data is of primary concern. In addition,
all parameters regarding sensing intervals, duty
and sleep cycles, adaptation rate etc., should ide-
ally be configurable to best customize the energy
consumption to application requirements.
Convenience:
To gain a broad consumer ac-
ceptance of WSNs, it is required to provide means
that enable the non-professional users to make use
of a WSN. These non-professionals are usually
short on experience of programming languages
and sensor networks, Therefore a straightforward
method to define tasks and configure sensor
nodes without requiring knowledge about hard-
or software or node deployment is in demand.
Convenient design of WSN applications not only
requires the task definition part to provide a high
abstraction level. It additionally implies to support
fully automatic WSN configuration regarding the
aforementioned criteria.
Criteria for Reliable
Autonomous Execution of
Event-Based Applications
User-centric application design of course requires
a process of automatic WSN configuration to
support reliable applications in WSNs. In order
to enable an appropriate comparison of existing
approaches and to set the objectives of user-centric
application design, this section introduces design
criteria for development of reliable event-based
applications. A suitable approach for reliable event
detection in WSNs must consider the following
design criteria:
Robustness:
Sensor nodes and its applications
must continue event detection even if the context
changes, sensors fail or nodes move. The sensor
nodes need to (re-)adapt their on-node as well as
in-network processing for automatic resource-
oriented event configuration. This regards both,
on-node adjustments in case of missing or failed
sensing facilities and adaptation of distributed
detection if connections to collaborating nodes
are interrupted due to failed or moved nodes.
Autonomy:
In addition to the autonomous
nature of sensor nodes, every node in the network
must be enabled to perform all necessary tasks for
event detection. A fully decentralized approach
avoiding assignment of superior devices such as
super nodes (Cardei, Yang, & Wu, 2008), event
gateways (Vu, Beyah, & Li, 2007) or fusion centers
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