Chapter 6
Conclusions
This thesis identified missing features of reliable event detection in Wireless Sen-sor Networks (WSNs). To remedy these shortcomings, it introduces objectives for reliable event-based applications in WSNs in terms of design criteria. These areFault tolerance,Adaptivity,Autonomy,Transparency,Energy efficiency, Con-venience.
Existing work mostly provides fault tolerance and adaptivity but disregards sufficient energy efficiency, autonomy and convenience. It has further been shown that existing fault tolerant solutions lack of means to achieve an acceptable cost efficiency. The envisioned pervasiveness of WSNs faces two major problems.
These are high fault probability and configuration complexity. First, pervasive WSNs consisting of large numbers of devices demand to use low cost sensor nodes with limited resources, which feature a high fault probability. Sensing devices at-tached to the sensor nodes possess certain errors of measurement. Further,WSNs are subject to sudden changes in operational conditions, varying deployments and hazardous environments that again increase the fault probability. Moreover, strict energy constraints on used devices require fault tolerant methods to achieve a high cost efficiency. Second, an ease of use for task definition and configuration of WSNs is the most important issue to make them widely accepted. Means that provide a high abstraction of WSNs are in demand. These must enable also non-professional users, which are usually short on experience of programming languages and sensor networks, to make use of WSNs.
6.1 Contributions
This thesis introduced a novel event concept for definition and configuration of reliable event detection in WSNs. It tackles all design criteria and features cost efficient fault tolerance and a proper usability. So it combines a flexible event
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inition language with a self-adapting event detection scheme. The Event Specifi-cation Language (ESL) provides ease of use for appliSpecifi-cation programming allowing the user to ignore low-level details of the sensor network and to concentrate on a high abstraction level. Namely this is the event itself and its related constraints.
To cope with the fault probability in WSNs, cost efficient means for collabo-rative event detection and evaluation of detection results by voting have been introduced and proven to be functional. In detail, the following contributions are made:
High abstraction for ease of use of event definition. The ESLhides low level details of WSNs to focus on pure event definition, which allows manual or auto-matic event configuration. TheXML-styled language enables to combine sensing features defining the complex phenomena to be sensed. Further, it enhances an event specification by assignment of customised application requirements regard-ing the spatial and temporal resolution and parameters for votregard-ing and collabo-ration schemes. Especially with regard to voting, the ESL allows to fine tune the voting procedure by determining the voting region, the preferred number of voters and a time limit. Finally, the event description generator transparently processes and adapts event specifications to the target sensor platform.
The ESL addresses the following design criteria: Fault tolerance, Adaptivity, Transparency,Energy efficiency,Convenience
A novel decentralised mechanism to autonomously set up event detection and in-network processing on sensor nodes, called Event Decision Tree (EDT).
Specified event descriptions are deployed on the sensor nodes asEDTs, which are directly generated on the nodes by a tiny GFSMrequiring eight states only. An EDT enables the sensor nodes to self-divide event queries according to its own resources into local and remote parts by pruning. Local parts can be evaluated by the node itself whereas values of remote parts must be requested from EDTs at other nodes. Sensor nodes are enabled to maintain several EDTs at the same time. UsingEDTs, every node in the network can execute the complete evaluation process without a Single Point of Failure (SPoF).
EDTs address the following design criteria: Adaptivity,Autonomy,Transparency, Convenience
Cost efficient means to maintain EDTs in case of missing or failing sensing devices. The EDTs are enabled to continue event detection with a high accu-racy even in case of missing resources or failed sensing devices. For those cases, EDTs provide efficient collaborative event detection between neighboring nodes using a lease-based publish/subscribe approach. Appropriate on-node processing of sensed data allows to efficiently share event information by a few bytes only.
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The simulations clearly announced that the cost-efficiency of the lease-based ap-proach is very high in contrast to theACK-based variant, which provides the best detection accuracy. By choosing a proper leasing time, the lease-based approach closely meets the detection results of ACK-based collaboration but reduces the number of collaboration messages by a factor of 20 or higher. The leasing time is set by the user in the event specification. As learnt from simulations, a proper leasing time meets both of the following restrictions:
1. The maximum leasing time is less or equal to the mean time of exposure to the phenomenon to be sensed.
2. The maximum leasing time is less than the mean time to failure.
The lease-based publish/subscribe approach addresses the following design cri-teria: Fault tolerance, Adaptivity, Autonomy, Transparency, Energy efficiency, Convenience
Reactive Majority Voting (RMV) to cope with uncertainty in sensor readings.
Voting is a proper means to enhance the reliability of event detection and to cope with deviating sensor readings but introduces an overhead in time and communi-cation. To reduce this overhead, RMV locally triggers a voting only to evaluate detected events. By that, RMV rejects False positives and isolates the nodes that most probably correctly detected an event. This is sufficient for monitoring scenarios such as fire detection. Voting only in case of events reduces the number of voting messages and hence, significantly improves the cost efficiency. To avoid a fixed node collecting all votes as a SPoF, RMV applies unfixed local voting regions around the nodes to independently apply voting. The accuracy of voting highly depends on the size of that voting region. The size of the voting region can be customised in the event specification. To ease the configuration of voting for non-professional users, they should apply the following principles:
1. The minimum size of the voting region is the mean distance between neigh-bouring sensor nodes, which is determined by the density of the sensor network, and transmission technology.
2. The maximum size of the voting region is the estimated size of the phe-nomenon.
RMV addresses the following design criteria: Fault tolerance,Autonomy,Energy efficiency
Indicator for the significance of sensor readings The significance indicatoriS upgrades the detection facilities of WSNs applyingEDTs. It statistically exam-ines behavioural trends in sensor readings to indicate the significance of actual
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measurements in relation to the configured event description. The iS is indepen-dent from the kind of sensor readings and is efficient in calculation and memory effort. The significance indicatoriS can automatically be attached to each event to support users or overlaying systems in decision-making. In the example sce-nario based on data of real test cases,iS indicates a flaming fire 88 seconds and a smouldering fire 48 seconds before the threshold-based method triggers the alarm.
This concept addresses the following design criteria: Fault tolerance,Energy ef-ficiency,Convenience
To summarise, this thesis presented and evaluated means to enhance the fault tolerance and reliability of event-based application in WSNs. As a final result, criteria for proper event definition were deduced from the simulation results.
These criteria ease the configuration of a proper voting region and leasing time by definition of lower and upper bounds.