Browsing by Author "Johnson, David"

Now showing 1 - 4 of 4
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    Open Access
    A new connectivity strategy for wireless mesh networks using dynamic spectrum access
    (2021) Maliwatu, Richard; Johnson, David; Densmore, Melissa
    The introduction of Dynamic Spectrum Access (DSA) marked an important juncture in the evolution of wireless networks. DSA is a spectrum assignment paradigm where devices are able to make real-time adjustment to their spectrum usage and adapt to changes in their spectral environment to meet performance objectives. DSA allows spectrum to be used more efficiently and may be considered as a viable approach to the ever increasing demand for spectrum in urban areas and the need for coverage extension to unconnected communities. While DSA can be applied to any spectrum band, the initial focus has been in the Ultra-High Frequency (UHF) band traditionally used for television broadcast because the band is lightly occupied and also happens to be ideal spectrum for sparsely populated rural areas. Wireless access in general is said to offer the most hope in extending connectivity to rural and unconnected peri-urban communities. Wireless Mesh Networks (WMN) in particular offer several attractive characteristics such as multi-hopping, ad-hoc networking, capabilities of self-organising and self-healing, hence the focus on WMNs. Motivated by the desire to leverage DSA for mesh networking, this research revisits the aspect of connectivity in WMNs with DSA. The advantages of DSA when combined with mesh networking not only build on the benefits, but also creates additional challenges. The study seeks to address the connectivity challenge across three key dimensions, namely network formation, link metric and multi-link utilisation. To start with, one of the conundrums faced in WMNs with DSA is that the current 802.11s mesh standard provides limited support for DSA, while DSA related standards such as 802.22 provide limited support for mesh networking. This gap in standardisation complicates the integration of DSA in WMNs as several issues are left outside the scope of the applicable standard. This dissertation highlights the inadequacy of the current MAC protocol in ensuring TVWS regulation compliance in multi-hop environments and proposes a logical link MAC sub-layer procedure to fill the gap. A network is considered compliant in this context if each node operates on a channel that it is allowed to use as determined for example, by the spectrum database. Using a combination of prototypical experiments, simulation and numerical analysis, it is shown that the proposed protocol ensures network formation is accomplished in a manner that is compliant with TVWS regulation. Having tackled the compliance problem at the mesh formation level, the next logical step was to explore performance improvement avenues. Considering the importance of routing in WMNs, the study evaluates link characterisation to determine suitable metric for routing purposes. Along this dimension, the research makes two main contributions. Firstly, A-link-metric (Augmented Link Metric) approach for WMN with DSA is proposed. A-link-metric reinforces existing metrics to factor in characteristics of a DSA channel, which is essential to improve the routing protocol's ranking of links for optimal path selection. Secondly, in response to the question of “which one is the suitable metric?”, the Dynamic Path Metric Selection (DPMeS) concept is introduced. The principal idea is to mechanise the routing protocol such that it assesses the network via a distributed probing mechanism and dynamically binds the routing metric. Using DPMeS, a routing metric is selected to match the network type and prevailing conditions, which is vital as each routing metric thrives or recedes in performance depending on the scenario. DPMeS is aimed at unifying the years worth of prior studies on routing metrics in WMNs. Simulation results indicate that A-link-metric achieves up to 83.4 % and 34.6 % performance improvement in terms of throughput and end-to-end delay respectively compared to the corresponding base metric (i.e. non-augmented variant). With DPMeS, the routing protocol is expected to yield better performance consistently compared to the fixed metric approach whose performance fluctuates amid changes in network setup and conditions. By and large, DSA-enabled WMN nodes will require access to some fixed spectrum to fall back on when opportunistic spectrum is unavailable. In the absence of fully functional integrated-chip cognitive radios to enable DSA, the immediate feasible solution for the interim is single hardware platforms fitted with multiple transceivers. This configuration results in multi-band multi-radio node capability that lends itself to a variety of link options in terms of transmit/receive radio functionality. The dissertation reports on the experimental performance evaluation of radios operating in the 5 GHz and UHF-TVWS bands for hybrid back-haul links. It is found that individual radios perform differently depending on the operating parameter settings, namely channel, channel-width and transmission power subject to prevailing environmental (both spectral and topographical) conditions. When aggregated, if the radios' data-rates are approximately equal, there is a throughput and round-trip time performance improvement of 44.5 - 61.8 % and 7.5 - 41.9 % respectively. For hybrid links comprising radios with significantly unequal data-rates, this study proposes an adaptive round-robin (ARR) based algorithm for efficient multilink utilisation. Numerical analysis indicate that ARR provides 75 % throughput improvement. These results indicate that network optimisation overall requires both time and frequency division duplexing. Based on the experimental test results, this dissertation presents a three-layered routing framework for multi-link utilisation. The top layer represents the nodes' logical interface to the WMN while the bottom layer corresponds to the underlying physical wireless network interface cards (WNIC). The middle layer is an abstract and reductive representation of the possible and available transmission, and reception options between node pairs, which depends on the number and type of WNICs. Drawing on the experimental results and insight gained, the study builds criteria towards a mechanism for auto selection of the optimal link option. Overall, this study is anticipated to serve as a springboard to stimulate the adoption and integration of DSA in WMNs, and further development in multi-link utilisation strategies to increase capacity. Ultimately, it is hoped that this contribution will collectively contribute effort towards attaining the global goal of extending connectivity to the unconnected.
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    Open Access
    Design considerations and implementation of an electromechanical battery system
    (2007) Johnson, David; Pillay, Pragasen
    This thesis covers the design and implementation of a low cost electromechanical battery using an outside rotor permanent magent Halbach array with hybrid magnetic bearings and heat pipe cooling. The theoretical calculations include finite element analysis. A dipole Halback array produces a uniform flux distribution inside the cylindrical statoe, where straight windings on an iron-less stator are placed near the inner boundary.
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    Open Access
    Improving content delivery in low-resource networks: a case study of the African Internet Ecosystem
    (2021) Phokeer, Amreesh Dev; Johnson, David; Densmore, Melissa
    The Internet is one of the most successful inventions of recent times. The COVID-19 pandemic has, once again, proven the importance of the Internet to society, and this has further demonstrated how critical it is to build networks that are resilient, accessible and inclusive. However, the Internet in many African countries is still limited by both technical and regulatory challenges. It is along these lines that this thesis proposes a series of studies, supported by empirical evidence, to better understand the challenges of content delivery in African networks. The thesis starts by providing an understanding how the Internet is being used and consumed by low-income mobile Internet users in South African townships. By means of a mixed-methods study, combining quantitative network measurements with qualitative survey data, the thesis provides some useful insights about Internet usage patterns and the underlying reasons for specific user behaviour with regards to mobile data management. The research revealed how Internet usage patterns of users in low-resource settings are restrained by the lack of access, availability of services and data cost.It also reinforced the concept of locality of interest and at the same time showing that the mainstream Internet services remain very popular. Next, the thesis investigates the impediments faced by African users to access local content and cloud-based services. This is achieved through a study on web content hosting, focusing specifically on African local news and public sector websites. It was found that 85% of local news websites are hosted outside their respective countries by foreign companies, mostly in Europe and in the US. This section revealed how a majority of Africa's local content is still hosted remotely and this has a major incident on the Quality of Experience (QoE) of users in Africa. Considering the set of challenges of content delivery in Africa, companies such as Facebook and Google have introduced alternative mechanisms to deliver content to the end-user - purportedly using bandwidth-friendly and cost-effective technologies. This thesis performs an in-depth Quality of Service (QoS) analysis of: (1) Free Basics, a “zero-rated” service from Facebook and (2) Accelerated Mobile Pages (AMP), a mobile optimisation technology by Google. The aim of both Free Basics and AMP is to reduce the cost of access and improve the QoE on mobile devices through different techniques - albeit with some caveats pertaining to net neutrality and data privacy. However, the thesis reveals that Free Basics services provide weaker network performance than their paid counterparts, which contributes to a disjointed user experience. On the other hand, Google AMP pages can reduce traditional page sizes by a factor of 8 and the results show that Page Load Time (PLT) on African networks can significantly be improved. However, both Free Basics and Google AMP introduce serious concerns with regards to net neutrality and data privacy. One way to minimise the effects of cross-continental path is to host the content as close as possible to the end-users. For this, there must exist a robust interconnection ecosystem between African networks. The thesis performs a deep-dive in both intra-country and inter-country connectivity in Africa, looking into both delays and network path by means of a longitudinal active measurement study. The latter exposes interesting topological characteristics of cross-border connectivity and provides evidence on the existence of circuitous routing and a lack of peering within African networks. The thesis reveals a series of “communities”, in which countries have built up low-delay interconnectivity, dispelling the myth that intra-delays in Africa are universally poor. Finally and taking into account the above, the thesis studies how the development of localised Internet infrastructure such as Internet Exchange Points (IXPs) and Data Centres (DCs) can help democratise access to local content. Using a simple multi-level maturity model, the thesis categorises the readiness of African countries to provide a localised Internet infrastructure. The thesis further explores the above hypothesis by estimating the effect of increasing the number of participants of an IXP on the local content activity of the country. The latter was achieved by using a fixed-effects econometric model and a positive correlation was found between the scale of an IXP and the local content activity. The thesis finally provides some key policy points on how to improve content delivery in African networks.
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    Open Access
    Understanding the biomechanics of standing-up and sitting-down
    (1989) Johnson, David; Mr. B. Davis, Professor G. Jaros and Dr. C. Warton
    Functional Neuromuscular Stimulation (FNS) has been shown to be effective in restoring certain upper and lower limb movements of spinal cord injured patients. The successful application of FNS to achieve standing-up and sitting-down, in persons with lower limb paralysis, depends largely on two factors: firstly, the torques generated at the hip, knee and ankle joints during these movements, and secondly, the extent and distribution of the accompanying muscular activity. With this knowledge it is possible to determine the electrical stimulation parameters and sequence required to restore the above movement in spinal cord injured patients. This thesis has a two-fold aim to study these two factors. A mathematical model to predict the torques generated at the hip, knee and ankle joints of individual participants was developed to achieve the first aim. This model is presented in the form of a computer program which requires anthropometrical and experimental data from the subject. The model was developed both from kinematic and kinetic aspects. The kinematic approach requires the varying angles at the hip, knee and ankle joints as inputs, whereas the kinetic approach requires the position of the ground reaction force in relation to the lateral malleolus, together with the ankle angles, as inputs. The various experiments performed to obtain the above mentioned variables are described. The second aim was achieved using electromyography to determine the sequential muscle activity, of the lower limb, during standing-up and sitting-down procedures. A description of the method used to obtain these electromyograms, together with a diagrammatic representation of the results, is presented. i1 As a result of the above mentioned theoretical modelling as well as simulation and validation experiments, the following conclusions can be made: (1) The mathematical model does predict reasonable torque values compared to that obtained in the literature. (2) The diagrammatic representation of the muscular activity is an acceptable reflection of the actual muscle activity. (3) In the light of the various problems encountered, a number of recommendations relating to both the mathematical model and the sequence of muscular activity are made