OVERVIEW OF WIRELESS SENSOR NETWORKS

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SCOPE

The scope of this research thesis is to improve message routing between sensors and sinks in order to increase the network lifetime, and to coordinate which mobile sinks (actors) will respond to an event in real-time. For routing of messages, sensor nodes are assumed to be stationary and the sensor node locations do not change after deployment. Sink nodes can be stationary or mobile. Areas of related research such as node deployment, network topology and media access protocols are not covered in depth. It is assumed that adequate coverage of the application area has been achieved and that all nodes spaced within the specified range of one another are able to communicate with neighbouring nodes. Security concerns such as eavesdropping from the wireless medium are not addressed. The primary focus is on routing at the network layer. Related issues such as flow control and quality of service are not considered as influencing the energy usage effect of routing messages across the network.

RESEARCH CONTRIBUTION

There are three contributions from this research, namely:
1. A WSN is modelled as a small world network by placing sink nodes at specific points within the application area. The sink nodes create long edges within the network, resulting in the total number of messages sent and received being significantly less. It is shown in chapter 3 of this thesis that modelling a WSN as a small world network is possible as long as the number of sinks are placed a specified distance from each other and irrespective of whether the sensor nodes are randomly scattered or carefully placed within the application area. The effect of modelling a WSN as a small world network on total number of messages sent and received within an application area is analysed in chapter 4. A comparison of the small world routing model and routing using flooding and gossiping indicates that the small world routing model reduces the total number of messages transmitted within a network when routing a message from a sensor node to a sink. The effect of using an initialisation message in a small world model to determine a route path versus routing using flooding at the individual node level in terms of the total number of messages sent and received by individual nodes is discussed and analysed in chapter 5. The results of the analysis in chapter 5 indicate that using an initialisation message does not negatively impact the energy resources of any node.
The results of chapters 3, 4 and 5 indicate that routing using the small world model and initialisation message results in increased node longevity and hence increased WSN lifetime.
2. Actor-actor coordination needs to be able to choose an actor(s) to respond to an event as quickly as possible, even when there are uncertainties about an actor(s)’s resources. IGDT can be used where robust solutions are required in an uncertain environment. Uncertainty about an actor’s energy and location is modelled as an information gap, and the optimum set of actor(s) that should respond to the event is calculated. In chapter 7, an IGDT model is used to coordinate which actor(s) should respond to an event in real-time, while ensuring that the number of messages transmitted in the network to reach a decision is kept to the bare minimum required to inform the relevant actor(s) of an event and the decision of which actor should respond. An analysis of the results from simulations in chapter 7 show that IGDT can be used as an effective method to select a few optimum actors, which will adequately respond to en event, even when there is uncertainty about other actors’ energy availability and location. The robustness of the decision ensures that even if the optimal set of actors is not chosen to respond to an event, then those actor(s) chosen have sufficient resources to respond to the event.
3. The calculation of an optimum path for a mobile sink or actor to follow in an application area is described in chapter 8. The use of a mobile sink or actor ensures that those nodes that are located close to a static sink are not unduly burdened with the responsibility of re-transmitting messages from other nodes to the sink. An analysis of the number of intermediate re-transmissions of messages required when routing a message to a mobile sink or actor is discussed in chapter 8.

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CHAPTER 1 RESEARCH OVERVIEW
1.1 Introduction
1.2 Wireless Sensor Network Applications
1.3 Relevance of Wireless Sensor Networks
1.4 Scope
1.5 Problem Statement
1.6 Research Objective
1.7 Research Contribution
1.8 Outline of the thesis
1.9 Flow Chart: Thesis’ Themes and Publications
CHAPTER 2 OVERVIEW OF WIRELESS SENSOR NETWORKS
2.1 Introduction
2.2 Routing in WSNs
2.3 Routing Design Challenges
2.4 LocaliSation
2.5 Wireless Sensor Actor Networks
2.6 Mobile Sinks and Mobile Relays
2.7 Standards
CHAPTER 3 ARCHITECTURE: SINK PLACEMENT
3.1 Introduction
3.2 Small World Networks
3.3 Algorithm Design
3.4 Examples of the use of small world networks in wireless sensor networks
3.5 Results and Analysis
3.6 Conclusion
3.7 Declaration
CHAPTER 4 ENERGY-EFFICIENT MESSAGE ROUTING
4.1 Introduction
4.2 Algorithm Design
4.3 Examples of relevant routing algorithms
4.4 Experimental Simulation
4.5 Results and Analysis
4.6 Conclusion
4.7 Declaration
CHAPTER 5 EFFECT OF SMALL WORLD ROUTING ON NODE LONGEVITY
5.1 Introduction
5.2 Algorithm Design
5.3 Alternative Approaches to improving network lifetime
5.4 Results and Analysis
5.5 Conclusion
CHAPTER 6 LOCALISATION IN A WSN
6.1 Introduction
6.2 Algorithm Design
6.3 Related Work
6.4 Experimental Simulation
6.5 Results and Analysis
6.6 Conclusion
6.7 Declaration
CHAPTER 7 ACTOR-ACTOR COORDINATION IN A WSN
7.1 Introduction
7.2 Background: Info-gap Decision Theory
7.3 Algorithm Design
7.4 Related Work
7.5 Experimental Simulation
7.6 Results and Analysis
7.7 Conclusion
7.8 Declaration
CHAPTER 8 OPTIMUM PATHS FOR MOBILE SINKS/ACTORS
8.1 Introduction
8.2 Algorithm Design
8.3 Related Work
8.4 Experimental Simulation
8.5 Results and Analysis
8.6 Conclusion
CHAPTER 9 FINAL CONCLUSIONS AND FUTURE WORK
REFERENCES

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