A mobile group blog is an example of a Web 2.0 social space, as well as a tool for the instant collection of contextual information, the immediate sharing of information and later reflection. Records in the form of multimedia created through mobile blogging can assist people to keep a versatile representation of artefacts they encounter on the move in everyday life. Overseas students are an example of a large group of people whose cultural learning could be supported by this technology. They could share contextual information and their own stories with other people currently experiencing the host culture, as well as people who do not have the opportunity to experience the host culture first-hand. To examine and evaluate how the mobile group blog could be applied in learning, a case study was conducted on involving overseas students. This research explored the suitability, appropriateness and benefits of a mobile group blog in assisting overseas students to manage their culture shock. It illustrates how the mobility of mobile devices assists the capture of contextual information on the move when overseas students start adapting to the new environment. The group blog site provided a platform to share and exchange their experiences and thoughts, as well as a resource of information on authentic cultural transition for future students. Four sub-studies were conducted around this theme. The first two studies investigated the demands and needs of a mobile group blog application in cultural transition. The third study investigated real and practical mobile blogging activities with a group of twelve Chinese overseas students who had newly arrived in Nottingham. The fourth study was conducted in China. In this study, a number of Chinese students who intended to study abroad were asked to evaluate the contents of the mobile group blog created by the twelve Chinese mobloggers in Study three. Findings from the four studies reveal the possibilities, suitability, strengths and weaknesses of the mobile group blog in assisting cultural transition. The thesis also presents positive feedback from participants as well as feedback on the limitations of this application. Then as added value to this research, it also suggests future educational applications of the mobile group blog.

<p>With the introduction of IP based telephony services, the Internet has started to challenge the traditional PSTN networks as an infrastructure for providing real-time interactive services. This upcoming paradigm shift is not only driven by the desire to provide cost efficient solutions, but by basing the communication on IP we expect that the end-users will experience a greater set of attractive services over a single connection compared to what is provided by a PSTN today. Looking a little further ahead, mobile communication systems will also become IP based. Companies, universities and private persons have started to extend their local area networks to provide wireless access by attaching wireless access points (APs) to their LAN. Wireless ISPs (WISPs) are putting up wireless LAN (WLAN) APs at public hot spots, thereby providing a complement or even a competitive alternative to the wireless WANs (WWANs) being developed and deployed today. As more and more people start to communicate using WLAN access, they will naturally wish to use this infrastructure for interactive real-time applications, such as mobile telephony.</p><p>This thesis concerns mobility and security support for IP telephony in public WLAN environments. The security issues addressed relate both to user requirements such as end-to-end confidentiality, and operator requirements such as network access control. Alternatives for how the voice media stream can be protected and the procedure to establish a secure call using SIP are described. Public WLAN architectures enabling service providers to share access network infrastructure are described and evaluated. To enforce access control the use of either IEEE 802.11i or L2TP/IPSec is suggested, since both meet the proposed security requirements, and both are standardized solutions available on modern systems.</p><p>The case where mobile users perform handovers between APs on the same LAN (layer-2 handover) and across IP subnets (layer-3) is studied. For layer-2 handovers the properties of IEEE 802.11b handover mechanisms and its impact on voice traffic, and the effect of the network access control mechanism on the handover performance are examined. The mechanisms necessary to perform layer-3 handovers and their impact on handover performance are described. The analysis focus on “SIP mobility” and Mobile IPv6, since these mobility management schemes provide optimal routing, thus are well suited for IP telephony</p>

Improvements in fibre optic communication and in VLSI for network switching components have led to the consideration of building digital switched networks capable of providing point to point communication in the gigabit per second range. Provision of bandwidths of this magnitude allows the consideration of a whole new range of telecommunications services, integrating video, voice, image and text. These multi-service networks have a range of requirements not met by traditional network architectures designed for digital telephony or computer applications. This dissertation describes the design, and an implementation, of the Multi-Service Network architecture and protocol family, which is aimed at supporting these services. Asynchronous transfer mode networks provide the basic support required for these integrated services, and the Multi-Service Network architecture is designed primarily for these types of networks. The aim of the Multi-Service protocol family is to provide a complete architecture which allows use of the full facilities of asynchronous transfer mode networks by multi-media applications. To maintain comparable performance with the underlying media, certain elements of the MSN protocol stack are designed with implementation in hardware in mind. The interconnection of heterogeneous networks, and networks belonging to different security and administrative domains, is considered vital, so the MSN architecture takes an internetworking approach.

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This dissertation explores the effects of two-photon absorption and color center induced absorption on all-optical switching devices. The amount of allowable two-photon absorption was quantified by the parameter T = 2βλ/n₂, where λ is the operating wavelength, β is the two-photon absorption coefficient, and n₂ is the nonlinear refractive index coefficient, the latter two being measured at λ. If the value of T exceeds unity, the operation of all-optical switching devices is in general degraded beyond usable regimes. This result was demonstrated by numerical experiments on systems of equations modelling a nonlinear directional coupler, a prototypical all-optical switching device. The value of T was measured in two fibers, one made of lead silicate glass, and one made of TiO₂-doped silica. We find the value of T to be greater than unity at a wavelength of 1.06 μm in both fibers. Significant color center formation was seen in the lead glass fiber. These color centers were created through two-photon absorption and destroyed through one-photon absorption. Color center induced absorption was seen to mimic two-photon absorption in certain regimes. The nonlinear optical response of semiconductor-doped glasses, an example of a one-photon resonant nonlinearity, was studied. A relaxation time which is dependent on the carrier density was found to be important when modelling the response of these glasses.

This dissertation describes three different hierarchical schemes for routing and flow control in communication networks. The basic idea in these schemes is to distribute the decision making capability among different hierarchical levels. A part of the decision is made at the lower level of network nodes and another part is made at the higher level of Coordinators. Specific problems at the lower level of nodes and the higher level of Coordinators are formulated in an optimization framework and solutions to these problems result in the overall control scheme. Two modes of operation for the Coordinators are identified and a step by step procedure to implement these modes of operation is also developed. Performance evaluations of these hierarchical schemes are conducted using stochastic models and simulation experiments. Different models for the nodes are used while developing these hierarchical schemes. In one of these schemes a queueing model is used to design a buffer allocation scheme to implement flow control while in another a state dependent model is used to design a combined routing and flow control scheme. The solution to this combined problem is obtained through the concepts of system stabilization. Major strong points of these hierarchical schemes are as follows. They provide a framework for integrating routing and flow control functions. They provide consideration of multiple objectives such as delay, throughput, and fairness individually. By implementing the Coordinators action at a slower time scale, the trade-off existing in information overheads and optimality can be exploited.

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This thesis investigates terrestrial atmospheric FSO communication systems operating under the influence of turbulence-induced scintillation, beam spreading, optical interchannel crosstalk, amplified spontaneous emission noise and pointing errors. On-off keying-non–return-to-zero (OOK-NRZ) and digital pulse position (DPPM) are the modulation schemes used for the calculations. The possibility of using sophisticated performance evaluation techniques such as moment generating function (MGF)-based Chernoff bound (CB), modified Chernoff bound (MCB) and saddlepoint approximation (SPA) for terrestrial DPPM and OOK-NRZ–based FSO communication systems employing optical amplification are investigated and compared with the conventional Gaussian approximation (GA) method. Relative to the other techniques, the MCB can be considered a safe estimation method for practical systems since it provides an upper bound upon the BER. The turbulent optically preamplified DPPM FSO receiver employing integration over a time slot and comparing the results to choose the largest slot, is seen to give better advantage (about 7 - 9 dB) compared to an equivalent employing OOK-NRZ signalling. The atmospheric turbulence-induced spreading of the beam, ASE noise, and pointing error are seen to combine in a problematic way resulting in high BERs, depending on the size of the receiver and the beam’s jitter standard deviation. Using FSO communication for the distribution links of a passive optical network-like wavelength division multiplexing access network is investigated in the presence of atmospheric turbulence, ASE noise and interchannel crosstalk. The results show that, for clear atmosphere, FSO distribution link length up to 2000 m can be reliably used (depending on turbulence strength) to achieve human eye safety and high capacity access networks. Also, error floors occur due to turbulence accentuated crosstalk effect for the cases of (i) signal turbulent, but crosstalk not and (ii) crosstalk turbulent, but signal not.