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ings is limited or cannot further be provided. Of
course, this results in decreased detection accuracy.
Collaboration between sensor nodes exchanging
missing information is a proper means to keep
the functionality of the sensor network and its
configured applications alive. In the context of
EDTs, this requires to exchange values of EDT
nodes. This section analyses the performance of
event detection under random permanent failures
of sensing devices. Therefore, the results of the
standard detection are compared to the detection
results in application of ACK-based and lease-
based collaboration. Here the worst case failure
scenario is simulated. Each available sensing ca-
pability, i.e., the temperature, the carbon monoxide
and the smoke sensing devices, will eventually
fail on each sensor node during the simulation
run. Fortunately, such extreme failure scenario
is far away from real deployments. However, it
is necessary to test the collaboration schemes in
worst case scenarios.
In such failure scenario, the total detection ac-
curacy is the most important issue. Collaboration is
not designed to explicitly enhance the mere detec-
tion of phenomena. It is rather designed to improve
the robustness of the sensor network against failed
sensing devices in the sensor nodes and to keep
the applications running at all nodes. Therefore,
collaboration schemes perform independent of
the final detection results. A successive loss of all
sensing features of each node in the entire WSN
represents a complete operational breakdown for
the application running on the sensor nodes. In
the introduced fire detection scenario, the three
sensing devices measuring carbon monoxide,
temperature and smoke at each sensor node fail
within the 1080 simulated time intervals. To slow
down the decrease of detection performance in our
simulation no more than one sensing device at one
sensor node may fail per interval. The occurrence
of failures is pseudo-randomly distributed. Of
course, this requires to successively adapt (prune)
the local EDT at each node when a sensing capabil-
ity fails. It further triggers collaboration between
neighboring nodes to gather detection results for
substitution of locally missing information. When
all sensing devices at a node failed, this is not
equivalent to a crash. In that case, the respective
EDT degenerated to a tree evaluating the children
of the root node only. The sensor node then acts
as a bridge node.
Table 3 presents a brief summary of the
simulation results in case of permanently failing
sensing capabilities. Unfortunately, the simula-
tion environment was unable to finish the runs
applying ACK-based collaboration. Due to the
successively increasing number of necessary
collaboration messages the process executing
the ACK-based simulation has been killed by the
simulation environment. For the affected runs we
displayed the last known system state and marked
it with an * in the table. This yet indicates that
ACK-based collaboration becomes infeasible with
a growing number of failures.
First, the results of all approaches, i.e., until
the ACK-based simulation has been aborted, are
evaluated. Comparing the average of correct
detection results in the entire network, both col-
laboration schemes clearly improved the total
detection accuracy. The ACK-based scheme per-
formed slightly better than the lease-based one.
This result was expected due to the fact that the
ACK-based scheme always gathers the actual
detection results at each interval. In contrast to
that, the lease-based approach in average reflects
changes slower depending on the leasing time,
even if the lease is optimized to the expected
behavior of the phenomenon. According to the
simulated phenomenon, which moves every six
intervals, the lease factor was also set to six. The
lease-based publish/subscribe approach most
likely will not outperform the ACK-based variant
with regard to the detection accuracy. In fact, the
goal of the lease-based detection is to provide a
detection accuracy that closely meets the result
of the ACK-based scheme, but with a signifi-
cantly reduced message overhead.
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