As a result of evergrowing data and recent interest in storing and analyzing it, distributed storage systems (DSS), which is also known as cloud storage, have become one of the most important research areas in the literature. Not only such networks are being used as backbone systems for companies like Google, Microsoft and Facebook but also they have accelerated the growth of cloud computing, which is an essential business line for institutions such as IBM, Amazon and Salesforce. In this dissertation, the focus is on the storage side of cloud in order to address the important questions in designing such systems. First, coding theoretic approach is taken to handle correlated failures of multiple storage nodes. In particular, this dissertation studies distributed storage systems that can provide resilience against correlated failure patterns that affect the availability of multiple storage nodes, i.e., power loss that may affect multiple disks. Maximum file size that can be stored in such DSS is studied and then optimal code constructions are provided. This dissertation also studies cloud storage systems that prevent data loss from mixed failure patterns of disks and sectors in disk drives. Specifically, a general code construction is proposed to overcome such failures for any given parameter set. Due to its large field size requirement of proposed construction, a relaxation on the efficiency of storage system is considered to provide codes with smaller field sizes. Maintenance of cloud storage systems is also studied. To that end, this dissertation first studies the maintenance of DSS that include a backup node, which is called hierarchical DSS. Hierarchical DSS can model cellular networks such as femtocell as well as caching in wireless networks. In particular, we present an upper bound on the file size that can be stored over hierarchical DSS and propose optimal code constructions. Then, maintenance cost and data access cost for users of such DSS are studied. Lastly, mobility effects of cloud storage over wireless devices are studied. Specifically, an analysis on the mobile cloud storage system that initiates the maintenance process after certain number of devices remains in the network is performed and different maintenance strategies are proposed that are optimal with respect to average cost in certain mobility regimes.

M.Ing. (Electrical and Electronic Engineering) / Long transmission lines in power system require high line loading in order to lower voltage limits due to line losses. For relatively long lines, line charging is high and thus higher voltage limits reached at low loading. It follows then that it is a challenge to maintaining the voltages between the acceptable limits for relatively long lines. This dissertation highlights the problems experienced when load varying from very low to very high is supplied by very long parallel lines of different impedance characteristic. When the load is extremely high, there are low voltages experienced which are solved by use of shunt capacitors and/or adding more lines. When the load is extremely low, there are high voltages experienced which are solved by use of shunt reactors and/or switching some of the lines off. The type of solutions to this two loading extremes as indicated above, can be problematic, in that; new lines requires servitudes which can take too long, shunt capacitors and reactors in this type of the network is not desirable since the introduction of too many of these devices have maintenance implications and they would require continuous switching to maintain acceptable voltages, resulting in complicated operation of the network. This research proposes the use of a phase shifting transformer located on one of two parallel corridors supplying power to a load located remotely from the rest of the system. The transformer is able to rearrange the active power flows to vary loadings of the corridors and the improvements in voltage regulation can be realised during both low and high load conditions.

The thesis reviews recent and current literature on leadership, and in particular on distributed leadership and complexity leadership theories. It describes my research into the factors affecting the success of transition towards a more distributed approach to leadership in two cases: one is a large UK private company and the other is a large UK university. The longitudinal research was conducted over the period from 2011 to 2013, using repeated interviews at senior and middle management levels, document analysis and observation to collect a rich set of data about both cases. I used a template to help analyse the data from each case. Through subsequent cross-case analysis the thesis identifies certain factors that influence the degree of success in making the transition to a distributed form of leadership, which involves not only devolved decision making but also increased levels of collaboration and organisational agility, which are key concerns of leaders of large organisations according to recent research across top 250 companies in the UK (Ipsos MORI, 2015: 5). The conclusion from my research is a framework called connected leadership, which describes the critical success factors and how they inter-relate. The first factor is having senior leadership committed to being role models, which is a pre-requisite for successful transition. There are then two factors that lay a strong foundation for the transition, namely having a shared organisational purpose and vison and values-based approach to leadership behaviour. Finally there are factors that then make distributed leadership work in practice: consistently devolved decision making, an emphasis on collaborative achievement, and agility and learning. The thesis provides practitioners with insight at both the organisational and leadership role levels, based on the connected leadership model. At the organisational level, I have derived from the research certain indicators for each factor that help diagnose and plan for the introduction of a distributed leadership approach. At the leadership role level the framework provides a helpful guide to developing leadership capability and role definition. The connected leadership model represents a coherent guide for leaders to use as a template for successful transition to a more distributed, collaborative and agile organisation, which is able to compete effectively in the 21st century networked society. Academically, this thesis provides a synthesis of distributed and complexity leadership theories, as well as drawing on authentic leadership theory, in order to understand the organisational and human dynamics that influence the transition to a more distributed leadership approach. Both cases are large organisations, which means that the factor framework provides relevant insight into how distributed leadership can be effective in large and relatively complex organisations.

This thesis includes studies of developing distributed optical fiber vibration sensor based on Rayleigh backscattering with broad frequency response range and high spatial resolution. Distributed vibration sensor based on all-polarization-maintaining configurations of the phase-sensitive optical time domain reflectometry (OTDR) is developed to achieve high frequency response and spatial resolution. Signal fading and noise induced by polarization change can be mitigated via polarization-maintaining components. Pencil-break event is tested as a vibration source and the layout of the sensing fiber part is designed for real applications. The spatial resolution is 1m and the maximum distance between sensing fiber and vibration event is 18cm. Wavelet denoising method is introduced to improve the performance of the distributed vibration sensor based on phase-sensitive OTDR in standard single-mode fiber. Noise can be reduced more effectively by thresholding the wavelet coefficient. Sub-meter spatial resolution is obtained with a detectable frequency up to 8 kHz. A new distributed vibration sensor based on time-division multiplexing (TDM) scheme is also studied. A special probe waveform including a narrow pules and a quasi-continuous wave can combine the conventional phase-sensitive OTDR system and polarization diversity scheme together in one single-mode fiber without crosstalk. Position and frequency of the vibration can be determined by these two detection systems consecutively in different time slots. Vibration event up to 0.6 MHz is detected with 1m spatial resolution along a 680m single-mode sensing fiber. Continuous wavelet transform (CWT) is investigated to study the non-stationary vibration events measured by our phase OTDR system. The CWT approach can access both frequency and time information of the vibration event simultaneously. Distributed vibration measurements of 500Hz and 500Hz to 1 kHz sweep events over 20 cm fiber length are demonstrated using a single-mode fiber. Optical frequency-domain reflectometry (OFDR) for vibration sensing is proposed for the first time. The local Rayleigh backscatter spectrum shift in time sequence could be used to determine dynamic strain information at a specific position of the vibrated state with respect to that of the non-vibrated state. Measurable frequency range of 0-32 Hz with the spatial resolution of 10 cm is demonstrated along a 17 m fiber.

Web crawlers visit internet applications, collect data, and learn about new web pages from visited pages. Web crawlers have a long and interesting history. Quick expansion of the web, and the complexity added to web applications have made the process of crawling a very challenging one. Different solutions have been proposed to reduce the time and cost of crawling. New generation of web applications, known as Rich Internet Applications (RIAs), pose major challenges to the web crawlers. RIAs shift a portion of the computation to the client side. Shifting a portion of the application to the client browser influences the web crawler in two ways: First, the one-to-one correlation between the URL and the state of the application, that exists in traditional web applications, is broken. Second, reaching a state of the application is no longer a simple operation of navigating to the target URL, but often means navigating to a seed URL and executing a chain of events from it. Due to these challenges, crawling a RIA can take a prohibitively long time. This thesis studies applying distributed computing and parallel processing principles to the field of RIA crawling to reduce the time. We propose different algorithms to concurrently crawl a RIA over several nodes. The proposed algorithms are used as a building block to construct a distributed crawler of RIAs. The different algorithms proposed represent different trade-offs between communication and computation. This thesis explores the effect of making different trade-offs and their effect on the time it takes to crawl RIAs. We study the cost of running a distributed RIA crawl with client-server architecture and compare it with a peer-to-peer architecture. We further study distribution of different crawling strategies, namely: Breath-First search, Depth-First search, Greedy algorithm, and Probabilistic algorithm. To measure the effect of different design decisions in practice, a prototype of each algorithm is implemented. The implemented prototypes are used to obtain empirical performance measurements and to refine the algorithms. The ultimate refined algorithm is used for experimentation with a wide range of applications under different circumstances. This thesis finally includes two theoretical studies of load balancing algorithms and distributed component-based crawling and sets the stage for future studies.

In order to improve simulation performance and integrate simulation resources among geographically distributed locations, the concept of distributed simulation is proposed. Several types of distributed simulation standards, such as DIS and HLA are established to formalize simulations and achieve reusability and interoperability of simulation components. In order to implement these distributed simulation standards and manage the underlying system of distributed simulation applications, Grid Computing and Cloud Computing technologies are employed to tackle the details of operation, configuration, and maintenance of the simulation platforms in which simulation applications are deployed. However, for modelers who may not be familiar with the management of distributed systems, it is challenging to create a simulation-run-ready environment that incorporates different types of computing resources and network environments. In this thesis, we propose a new multi-layered cloud-based scheme for enabling modeling and simulation based on different distributed simulation standards. The scheme is designed to ease the management of underlying resources and achieve rapid elasticity, providing unlimited computing capability to end users; energy consumption, security, multi-user availability, scalability and deployment issues are all considered. We describe a mechanism for handling diverse network environments. With its adoption, idle public resources can easily be configured as additional computing capabilities for the local resource pool. A fast deployment model is built to relieve the migration and installation process of this platform. An energy conservation strategy is utilized to reduce the energy consumption of computing resources. Security components are also implemented to protect sensitive information and block malicious attacks in the cloud. In the experiments, the proposed scheme is compared with its corresponding grid computing platform; the cloud computing platform achieves a similar performance, but incorporates many of the cloud's advantages.

Semiconductor corrugated ridge waveguide (CRW) distributed feedback (DFB) lasers offer compelling advantages over standard DFB lasers. Indeed, the use of surface gratings etched on the ridge waveguide sidewalls in CRW-DFB devices avoids any epitaxial overgrowth. This provides a considerable simplification in the fabrication process, reducing cost and time of manufacturing, and ultimately increasing yield. It offers also the potential for monolithic integration with other devices, paving the way towards low-cost and mass-production of photonics integrated circuits. In recent years, the re-consideration of growth-free DFB lasers has drawn considerable attention, particularly with the current state-of-the-art photolithography machines. In this work, we present an experimental investigation on two generations of InGaAsP/InP multiple-quantum-well (MQW) CRW-DFB lasers that have been fabricated using stepper lithography. An early developed 1310 nm CRW-DFB laser showed stable single mode with high side-mode suppression ratios (SMSR) (>50 dB), albeit with thresholds higher than anticipated. A subsequent single-mode 1550 nm CRW-DFB laser showed stable operation with SMSR (>50 dB) and narrow spectral linewidths (≤250 kHz), observed for a wide range of current injection. Besides, novel multi-electrode CRW-DFB lasers have been tested. The experimental investigation showed that narrower linewidth (<150 kHz) and wide wavelength tunability (>3 nm) have been recorded using different multi-electrode current injection configurations. The application of a time-domain modeling approach for semiconductor CRW-DFB lasers is then described for the first time. We numerically studied the effect of the radiation modes on CRW-DFB laser properties by using time-domain coupled wave equations. High-order corrugated gratings with λ/4 phase-shit were analyzed, where the degree of longitudinal spatial hole burning (LSHB) can be effectively reduced by means of fine tuning of the grating duty cycle. Additionally, we showed how the side-mode suppression ratio can be predicted depending on the device geometry.

Distributed simulation over Cloud environment is still a new subject. Cloud computing is expected to bring new benefits to conventional distributed simulation, including elasticity on computation resource, cost saving on investment and convenience on service accessibility. A few researches have been done on applying Cloud computing on distributed simulation. However, there are various drawbacks and limitations on those works. Lack of interoperability across Cloud platforms is one of critical drawbacks among them. It can greatly limit the usability and flexibility of distributed simulation over Cloud environment. Based on the investigation on Cloud computing and existing distributed simulation systems over Cloud environment, a novel interoperable HLA-based (High Level Architecture) simulation system over a Cloud environment, ISSC (Interoperable Simulation System over a Cloud Environment), is proposed in this thesis. ISSC aims to address the interoperability issue of simulation system across various Cloud platforms. It employs OCCI and a set of technologies, including Ruby on Rails, OpenVPN and RESTful web services, to build the interoperability across Cloud platforms. It adopts a distributed architecture to construct flexibility and expansibility of the system. The prototype and related experiments performed provides an excellent demonstration that ISSC is a reliable and effective solution on interoperable simulation system over a diverse Cloud environment.

Ethernet is a common practice to reconstruct a networked telemetry system. However, Ethernet switch cannot best meet the requirement of data transportation in a telemetry system because of its asynchronous mode and the uncertainty of latency time. In addition, the temporal order of the telemetry data will be disrupted when using an Ethernet switch. A device similar to Ethernet switch is presented in this paper, which can transport data synchronously without losing the original temporal order of the telemetry data. Meantime, a special condition is arranged by the device that the timing signal could be transmit from the device to the receiver in a certain delay time, so a timing mechanism derived from IEEE 1588 PTP protocol could be adopted for high accuracy of timing and synchronous sampling control.

This work describes the design and implementation details of a reliable group communication mechanism. The mechanism guarantees that messages will be received by all the operational members of the group or by none of them (atomicity). In addition, the sequence of messages will be the same at each of the recipients (order). The message ordering property can be used to simplify distributed database systems and distributed processing algorithms. The proposed mechanism continues to operate despite process, host and communication link failures (survivability). Survivability is essential in fault-tolerant applications. / Science, Faculty of / Computer Science, Department of / Graduate