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Wireless sensor networks can enable numerous advanced monitoring and control applications. They are characterized by a need to maximize their efficiency, especially with respect to energy consumption, since the lifetime of a sensor node has a huge impact on the usability of a wireless sensor network. In this thesis, an energy efficient MAC protocol for wireless sensor networks is proposed. The protocol is based on IEEE 802.11, with some modifications to accommodate power efficiency and scalability which are the two main design factors for wireless sensor networks. The proposed Multi-Phase MAC (Φ-MAC) protocol uses periodic listen and sleep modes to save energy. Sensor nodes in Φ-MAC protocol are allocated into different phases so that the listen periods of the different phases are non-overlapping. Distributing the nodes among the phases allows the nodes to sleep longer and conserve battery power. This distribution also effectively reduces the traffic seen by the nodes in each phase, which reduces the probability of collisions and retransmissions. The results of the numerical simulations show that Φ-MAC outperforms other proposed MAC protocols in its efficiency of consuming power.

Networks are ubiquitous, underlying systems as diverse as the Internet, food webs, societal interactions, the cell, and the brain. Of crucial importance is the coupling of network structure with system dynamics, and much recent attention has focused on how information, such as pathogens, mutations, or ideas, ow through networks. In this dissertation, we advance the understanding of how network structure a ects information ow in two important classes of models. The rst is an independent interaction model, which is used to investigate the propagation of advantageous alleles in evolutionary algorithms. The second is a threshold model, which is used to study the dissemination of ideas, fads, and innovations throughout populations. This journal-format dissertation comprises three interrelated studies, in which we investigate the in uence of network structure on the dynamical properties of information ow. In the rst study, we develop an analytical technique to approximate system dynamics in arbitrarily structured regular interaction topologies. In the second study, we investigate the ow of advantageous alleles in degree-correlated scale-free population structures, and provide a simple topological metric for assessing the selective pressures induced by these networks. In the third study, we characterize the conditions in which global information cascades occur in threshold models of binary decisions with externalities, structured on degree-correlated Poisson-distributed random networks.

ABSTRACT. The failure of the 1990 political transition in the Democratic Republic of Congo to deliver on promises of better living conditions for all Congolese and the diminishing job opportunities available to young Congolese have resulted in many looking for economic opportunities outside their country of nationality. With most of the European and North American countries effectively off limits due to restrictive measures on migration (Bauman, 1998; Soguk 1999). The post-apartheid South Africa is relatively prosperous. This fact couple with a corrupt immigration and asylum system make the country very attractive for an increasing number of Congolese migrants who desire a better standard of living. Although the borders are relatively porous, the expense and hazards of moving require resources that are not available to all. This project explores how migrant networks can provide those resources through information and access to documentation, housing, and opportunities for income-generation. In particular, this thesis explores the role of social networks in structuring the movements of Congolese into Johannesburg and their integration into its social fabric. It intersects with a part of a growing body of literature demonstrating numerous new ways in which contemporary global migrants remain closely connected to their places of origin, to co-nationals or co-ethnics across nation-state borders, and indeed across the world 1(Transnational Communities Programme; 1999). While this analytical perspective has been applied fairly extensively to other groups of migrants, few scholars have sought to examine the extent to which refugees and asylum seekers maintain such a worldwide web of relationships (Crisp, 1999). Indeed, academic discourses on refugees, and also the practical efforts made on their behalf by United Nations High Commissioner of Refugees (UNHCR) and other humanitarian organizations, continue to be informed by the assumption of a rigid separation between the exile country of origin and country of asylum (Crisp, 1999). 1 Programme of a workshop on ‘Policy challenge of the new migrants diasporas’, Chatham House, London, 22-23 April 1999. Quoted by Jeff Crisp in: Policy challenges of the new diasporas: migrant networks and their impact on asylum flows and regimes. WPTC-99-05 Policy Research Unit, UNHCR, CP 2500, CH-1211 Geneva Switzerland. www.transcomm.ox.ac.uk/working%20papers/riia3.pdf Jean Didier Losango Nzinga 5 Against this background, social networks play an important role in facilitating migration, whether across borders or across regions (Guzman, Haslag and Orrenius 2004). These networks are likely to act as an important source of information to prospective migrants, refugees and asylum seekers, providing them with details on matters such as transport arrangements, entry requirements, asylum procedures and social welfare benefits, as well as the detention and deportation policies of different destination states (Crisp, 1999). Consequently, refugees and other migrants who have access to such data are better placed to negotiate entry into developed countries than those who do not2. 2 Given the increasingly important role these networks play in facilitating movement of people around the world today, the emergence of the internet plays a major role in making these networks possible. See report prepared for the Knowledge for Development Program of the World Bank: Role of Diaspora in Facilitating Participation in Global Knowledge Networks: Lessons of Red Caldas in Colombia (Bogota, December 2004).

Resolving the trade-offs between suspension travel, ride comfort, road holding, vehicle handling and power consumptions is the primary challenge in designing Active-Vehicle-Suspension-Systems (AVSS). Controller tuning with global optimization techniques is proposed to realise the best compromise between these con icting criteria. Optimization methods adapted include Controlled-Random-Search (CRS), Differential-Evolution (DE), Genetic-Algorithm (GA), Particle-Swarm-Optimization (PSO) and Pattern-Search (PS). Quarter-car and full-car nonlinear AVSS models that incorporate electrohydraulic actuator dynamics are designed. Two control schemes are proposed for this investigation. The first is the conventional Proportional-Integral-Derivative (PID) control, which is applied in a multi-loop architecture to stabilise the actuator and manipulate the primary control variables. Global optimization-based tuning achieved enhanced responses in each aspect of PID-based AVSS performance and a better resolve in con icting criteria, with DE performing the best. The full-car PID-based AVSS was analysed for DE as well as modi ed variants of the PSO and CRS. These modified methods surpassed its predecessors with a better performance index and this was anticipated as they were augmented to permit for e cient exploration of the search space with enhanced exibility in the algorithms. However, DE still maintained the best outcome in this aspect. The second method is indirect adaptive dynamic-neural-network-based-feedback-linearization (DNNFBL), where neural networks were trained with optimization algorithms and later feedback linearization control was applied to it. PSO generated the most desirable results, followed by DE. The remaining approaches exhibited signi cantly weaker results for this control method. Such outcomes were accredited to the nature of the DE and PSO algorithms and their superior search characteristics as well as the nature of the problem, which now had more variables. The adaptive nature and ability to cancel system nonlinearities saw the full-car PSO-based DNNFBL controller outperform its PID counterpart. It achieved a better resolve between performance criteria, minimal chatter, superior parameter sensitivity, and improved suspension travel, roll acceleration and control force response.

Xiao Lurong. / Thesis submitted in: June 2005. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references (leaves [74]-76). / Abstracts in English and Chinese. / ABSTRACT --- p.I / 摘要 --- p.III / ACKNOWLEDGMENTS --- p.IV / CONTENT --- p.V / LIST OF FIGURES --- p.VII / LIST OF TABLES --- p.IX 1 / Chapter CHAPTER 1 --- INTRODUCTION / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Related Works --- p.3 / Chapter 1.3 --- Active Service Network --- p.12 / Chapter 1.4 --- Motivations and Contributions --- p.13 / Chapter 1.5 --- Outline --- p.14 / Chapter CHAPTER 2 --- ACTIVE SERVICE NETWORKS (ASN) ARCHITECTURE --- p.16 / Chapter 2.1 --- Internet Architecture --- p.16 / Chapter 2.2 --- ASN Enterprise Model --- p.19 / Chapter 2.3 --- ASN network Architecture --- p.20 / Chapter 2.4 --- ASN System Architecture --- p.24 / Chapter 2.5 --- Summary --- p.26 / Chapter CHAPTER 3 --- ASN VS. CLIENT SERVER APPROACH --- p.27 / Chapter 3.1 --- Application Scenario --- p.21 / Chapter 3.2 --- Application Models --- p.28 / Chapter 3.3 --- Analytical Models --- p.29 / Chapter 3.4 --- Performance Results --- p.34 / Chapter 3.5 --- Chapter Summary --- p.37 / Chapter CHAPTER 4 --- DESIGN PROBLEMS --- p.38 / Chapter 4.1 --- Problem Definitions --- p.38 / Chapter 4.2 --- Related works --- p.40 / Chapter 4.3 --- Problem Formulation --- p.43 / Chapter 4.4 --- Heuristic Algorithms --- p.49 / Chapter 4.5 --- Simulation Results --- p.51 / Chapter 4.6 --- Chapter Summary --- p.57 / Chapter CHAPTER 5 --- PERFORMANCE EVALUATION --- p.58 / Chapter 5.1 --- Simulation Model --- p.58 / Chapter 5.2 --- Blocking Performance --- p.60 / Chapter 5.3 --- Advanced Capabilities --- p.64 / Chapter 5.4 --- Chapter Summary --- p.69 / Chapter CHAPTER 6 --- CONCLUSION --- p.71 / Chapter 6.1 --- Contributions --- p.71 / Chapter 6.2 --- Future works --- p.72 / REFERENCES --- p.74

Jiang Wenjie. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references (leaves 89-96). / Abstracts in English and Chinese. / Acknowledgement --- p.iii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Challenges --- p.2 / Chapter 1.3 --- Our Contribution --- p.4 / Chapter 1.4 --- Structure of the thesis --- p.5 / Chapter 2 --- Background Study --- p.7 / Chapter 2.1 --- An Introduction to Overlay Networks --- p.8 / Chapter 2.1.1 --- What is an Overlay Network? --- p.8 / Chapter 2.1.2 --- Benefits of Overlay Networks --- p.13 / Chapter 2.2 --- Taxonomy of Overlay Networks --- p.16 / Chapter 2.2.1 --- Routing Overlay Networks --- p.16 / Chapter 2.2.2 --- Content Delivery Networks (CDNs) --- p.25 / Chapter 2.2.3 --- Security Overlay Networks --- p.28 / Chapter 3 --- Mathematical Models for Overlay Routing --- p.32 / Chapter 3.1 --- Formulation of Routing in Overlay Networks --- p.32 / Chapter 3.2 --- Optimal Overlay Routing Policy --- p.34 / Chapter 3.3 --- Illustration of Overlay Routing Policy --- p.37 / Chapter 4 --- Overlay Routing Game --- p.40 / Chapter 4.1 --- Strategic Nash Routing Game --- p.40 / Chapter 4.2 --- Stable Property of Overlay Optimal Routing --- p.43 / Chapter 4.3 --- Routing Game in Other Forms --- p.44 / Chapter 5 --- Comparison of Routing Strategies: A Spectrum of Efficiency --- p.46 / Chapter 5.1 --- Global Optimal Routing --- p.47 / Chapter 5.2 --- Selfish User Routing --- p.49 / Chapter 5.3 --- Optimal Overlay Routing --- p.51 / Chapter 5.4 --- Performance Comparison --- p.54 / Chapter 6 --- Simulations on Routing Game --- p.56 / Chapter 6.1 --- Fluid Level Simulation --- p.56 / Chapter 6.2 --- Packet Level Simulation --- p.59 / Chapter 7 --- Understanding Various Issues & Implications of Overlay Interaction --- p.65 / Chapter 7.1 --- Sub-optimality of Nash Equilibrium --- p.66 / Chapter 7.2 --- Slow convergence to Nash equilibrium --- p.67 / Chapter 7.3 --- Fairness Paradox --- p.68 / Chapter 8 --- Overlay Pricing --- p.71 / Chapter 8.1 --- Pricing mechanism to improve end-to-end delay --- p.71 / Chapter 8.1.1 --- Fluid-level Simulation --- p.74 / Chapter 8.1.2 --- Packet-level Simulation --- p.77 / Chapter 8.2 --- Pricing mechanism to improve fairness --- p.77 / Chapter 9 --- Related Work --- p.83 / Chapter 10 --- Conclusion --- p.86 / Chapter 10.1 --- Summary of the Contribution --- p.86 / Chapter 10.2 --- Future Directions --- p.87 / Bibliography --- p.89 / Chapter A --- Proof of Existence of Nash Equilibrium --- p.97