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Thesis (M.Eng.)(Computer Engineering)--University of Pretoria, 2005. / Title from opening screen (viewed March 20, 2006). Summaries in English and Afrikaans. Includes bibliographical references.

International Telemetering Conference Proceedings / October 26-29, 1992 / Town and Country Hotel and Convention Center, San Diego, California / The Air Force Flight Test Center (AFFTC) at Edwards, California has installed the Edwards Digital Switch (EDS). The EDS is a DS-1 switch capable of switching voice, encrypted voice, and Time Space Position Information (TSPI) data. Communicating encrypted voice and TSPI data reliably from the EDS to any other communication system requires the two systems operate using the same frequency reference. The need to communicate with other test ranges and over commercially leased lines using the EDS requires synchronization to a standard frequency source. The current synchronization method used at the AFFTC is unacceptable for the EDS. A hierarchal system of synchronization networking will be implemented to provide a common reference to each of the systems interfacing the EDS. The Master Timing Station (MTS), which provides synchronization reference to most of the AFFTC resources, is aging and must be upgraded. The upgrade of the MTS and the restructuring of the synchronization signal distribution will be completed simultaneously. Traceability to the United States Naval Observatory (USNO) and the National Bureau of Standards (NBS) is required to provide the necessary synchronization to allow the EDS to interface reliably with other systems. The Global Positioning System (GPS) is being considered as the primary source for frequency calibration traceable to the USNO and NBS.

This thesis presents some new techniques in frame oriented transmission systems with particular emphasis on HDLC (High Level Data Link Control) links. The advent of multimedia and applications requiring high bandwidth has prompted the development of high performance networks. In addition to that, accessibility is also an important issue. Satellite links have become a major contributor in providing access to larger networks where terrestrial connections are not possible due to geographical constraints. The development of VSATs (Very Small Aperture Terminal) has further increased the interest in providing extra-terrestrial links as a means of accessing a network. Because VSAT links are less reliable than terrestrial connections, bandwidth management becomes an important issue. The major problems are identified in this thesis. Frame oriented transmission systems have provided a very efficient method for data transmission. Data from several sources may be multiplexed into a common link. Recently, data networks have been successfully used for the transmission of various types of traffic. This diverse traffic has different QOS requirements such as throughput and delay. Certain types of traffic have an inherent priority. This traffic is referred to as time-critical or temporal. Temporal data has a short lifetime after which the data become useless regardless of whether the data has been corrupted or not. Some data sources such as voice and video, are able to tolerate small amounts of errors provided they are received within their lifetime period. From here we see that there are two very contrasting features of the data types. Conventional packet protocols are designed to ensure data integrity in return for packet delays due to retransmission and congestion control.

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<p>Despite the rapid development of adversarial attacks on machine learning models, many types of new adversarial examples remain unknown. Undiscovered types of adversarial attacks pose a</p><p>serious concern for the safety of the models, which raises the issue about the effectiveness of current adversarial robustness evaluation. Image semantic segmentation is a practical computer</p><p>vision task. However, segmentation networks’ robustness under adversarial attacks receives insufficient attention. Recently, machine learning researchers started to focus on generating</p><p>adversarial examples beyond the norm-bound restriction for segmentation neural networks. In this thesis, a simple and efficient method: AdvDRIT is proposed to synthesize unconstrained controllable adversarial images leveraging conditional-GAN. Simple CGAN yields poor image quality and low attack effectiveness. Instead, the DRIT (Disentangled Representation Image Translation) structure is leveraged with a well-designed loss function, which can generate valid adversarial images in one step. AdvDRIT is evaluated on two large image datasets: ADE20K and Cityscapes. Experiment results show that AdvDRIT can improve the quality of adversarial examples by decreasing the FID score down to 40% compared to state-of-the-art generative models such as Pix2Pix, and also improve the attack success rate 38% compared to other adversarial attack methods including PGD.</p>

Energy usage has become an important issue in wireless communication systems. The energy-intensive nature of wireless communication has spurred concern over how best systems can make the most use of this non-renewable resource. Research in energy-efficient design of wireless communication systems show that one of its challenges is that the overall performance of the system depends, in a coupled way, on the different submodules of the system i.e. antenna, power amplifier, modulation, error control coding, and network architecture. Network architecture implementation strategies offer protocol software implementors an opportunity of incorporating low-power strategies into the design of the network protocols used for data communication. This dissertation proposes a methodology that would allow a software protocol implementor to analyze the power consumption of a wireless communication system. The foundation of this methodology lies in the understanding of the formal specification of the wireless interface network architecture which can be used to predict the performance of the system. By extending this hypothesis, a protocol implementor can use the formal specification to derive the power consumption behaviour of the wireless system during a normal operation (transmission or reception of data). A high-level formalism like state-transition graphs, can be used to track the protocol processing behaviour and to derive the associated continuous-time Markov chains. Because of their diversity, Markov reward models(MRM) are used to model the power consumption associated with the different states of a specified protocol layer. The models are solved analytically using the Mobius performance and dependability tool. Using the MRM accumulation and utilization measures, a profile of the power consumption is generated. Results from the experiments on the protocol layers show the individual power consumption and utilization of the different states as well as the accumulated power consumption of different protocol layers when compared. Ultimately, the results from the reward model solution can be used in the energy-efficient design of wireless communication systems. Lastly, in order to get an idea of how wireless communication device companies handle issues of power consumption, we consulted with the wireless module engineers at Siemens Communication South Africa and present our findings on current practices in energy efficient protocol implementation.

Evaluating the “success” of an information visualization (InfoVis) where its main purpose is communication or presentation is challenging. Within metrics that go beyond traditional analysis- and performance-oriented approaches, one construct that has received attention in recent years is “user engagement”. In this research, I propose Affective Engagement (AE)-- user's engagement in emotional aspects as a metric for InfoVis evaluation. I developed and evaluated a self-report measurement tool named AEVis that can quantify a user's level of AE while using an InfoVis. Following a systematic process of evidence-centered design, each activity during instrument development contributed specific evidence to support the validity of interpretations of scores from the instrument. Four stages were established for the development: In stage 1, I examined the role and characteristics of AE in evaluating information visualization through an exploratory qualitative study, from which 11 indicators of AE were proposed: Fluidity, Enthusiasm, Curiosity, Discovery, Clarity, Storytelling, Creativity, Entertainment, Untroubling, Captivation, and Pleasing; In stage 2, I developed an item bank comprising various candidate items for assessing a user's level of AE, and assembled the first version of survey instrument through target population and domain experts' feedback; In stage 3, I conducted three field tests for instrument revisions. Three analytical methods were applied during this process: Item Analysis, Factor Analysis (FA), and Item Response Theory (IRT); In stage 4, a follow-up field test study was conducted to investigate the external relations between constructs in AEVis and other existing instruments. The results of the four stages support the validity and reliability of the developed instrument, including: In stage 1, user's AE characteristics elicited from the observations support the theoretical background of the test content; In stage 2, the feedback and review from target users and domain experts provides validity evidence for the test content of the instrument in the context of InfoVis; In stage 3, results from Exploratory and Confirmatory FA, as well as IRT methods reveal evidence for the internal structure of the instrument; In stage 4, the correlations between total scores and sub-scores of AEVis and other existing instruments provide external relation evidence of score interpretations. Using this instrument, visualization researchers and designers can evaluate non-performance-related aspects of their work efficiently and without specific domain knowledge. The utilities and implications of AE can be investigated as well. In the future, this research may provide foundation for expanding the theoretical basis of engagement in the fields of human-computer interaction and information visualization.

Optimal decision directed equalization techniques for time dispersive communication channels are often too complex to implement. This thesis considers reduced complexity decision directed equalization that lowers complexity demands yet retains close to optimal performance. The first part of this dissertation consists of three reduced complexity algorithms based on the Viterbi Algorithm (VA) which are: the Parallel Trellis VA (PTVA); Time Reverse Reduced State Sequence Estimation (TR-RSSE); and Forward-Backward State Sequence Detection (FBSSD). The second part of the thesis considers structural modifications of the Decision Feedback Equalizer (DFE), which is a special derivative of the VA, specifically, optimal vector quantization for fractionally spaced DFEs, and extended stability regions for baud spaced DFEs using passivity analysis are investigated.¶ For a special class of sparse channels the VA can be decomposed over a number of independent parallel trellises. This decomposition will be called the Parallel Trellis Viterbi Algorithm and can have lower complexity than the VA yet it retains optimal performance. By relaxing strict sparseness constraints on the channel a sub-optimal approach is proposed which keeps complexity low and obtains good performance.¶ Reduced State Sequence Estimation (RSSE) is a popular technique to reduce complexity. However, its deficiency can be the inability to adequately equalize non-minimum phase channels. For channels that have energy peaks in the tail of the impulse response (post-cursor dominant) RSSE's complexity must be close to the VA or performance will be poor. Using a property of the VA which makes it invariant to channel reversal, TR-RSSE is proposed to extend application of RSSE to post-cursor dominant channels.¶ To further extend the class of channels suitable for RSSE type processing, FBSSD is suggested. This uses a two pass processing method, and is suited to channels that have low energy pre and post-cursor. The first pass generates preliminary estimates used in the second pass to aid the decision process. FBSSD can range from RSSE to TR-RSSE depending on parameter settings.¶ The DFE is obtained when the complexity of RSSE is minimized. Two characterizing properties of the DFE, which are addressed in this thesis, are feedback and quantization. A novel fractionally spaced (FS) DFE structure is presented which allows the quantizer to be generalized relative to the quantizer used in conventional FS-DFEs. The quantizer can be designed according to a maximum a posteriori criterion which takes into account a priori statistical knowledge of error occurrences. A radically different quantizer can be obtained using this technique which can result in significant performance improvements.¶ Due to the feedback nature of the DFE a form of stability can be considered. After a decision error occurs, a stable DFE will, after some finite time and in the absence of noise, operate error free. Passivity analysis provides sufficient conditions to determine a class of channels which insures a DFE will be stable. Under conditions of short channels and small modulation alphabets, it is proposed that conventional passivity analysis can be extended to account for varying operator gains, leading to weaker sufficient conditions for stability (larger class of channels).