Towards time domain invariant QoS measures for queues with correlated traffic

NoAn investigation is carried out on the nature of QoS measures for queues with correlated traffic in both discrete and continuous time domains. The study focuses on the single server GI(G)/M-[x]/1/N and GI(G)/Geo([x])/1/N queues with finite capacity, N, a general batch renewal arrival process (BRAP), GI(G) and either batch Poisson, M-[x] or batch geometric, Geo([x]) service times with general batch sizes, X. Closed form expressions for QoS measures, such as queue length and waiting time distributions and blocking probabilities are stochastically derived and showed to be, essentially, time domain invariant. Moreover, the sGGeo(sGGo)/Geo/l/N queue with a shifted generalised geometric (sGGeo) distribution is employed to assess the adverse impact of varying degrees of traffic correlations upon basic QoS measures and consequently, illustrative numerical results are presented. Finally, the global balance queue length distribution of the M-Geo/M-Geo/1/N queue is devised and reinterpreted in terms of information theoretic principle of entropy maximisation. (C) 2014 Elsevier Inc. All rights reserved

On the Analytic Assessment of the Impact of Traffic Correlation on Queues in Continuous Time Domain

NoGiven only the traffic correlations of counts and intervals, a Batch Renewal Arrival Process (BRAP) is completely determined, as the least biased choice and thus, it provides the analytic means to construct suitable traffic models for the study of queueing systems independently of any other traffic characteristics. In this context, the BRAP and the Batch Markovian Arrival Process (BMAP) are employed in the continuous time domain towards the analysis of the stable BRAP/GE/1 and BMAP/GE/1 queues with infinite capacity, single servers and generalized exponential (GE) service times. Novel closed form expressions for the steady state probabilities of these queues are obtained, based on the embedded Markov chains (EMCs) technique and the matrix-geometric (M-G) method, respectively. Moreover, the stable GEsGGeo/GE/1 queue with GE-type service times and a GEsGGeo BRAP consisting of bursty GE-type batch interarrival times and a shifted generalized geometric (sGGeo) batch size distribution is adopted to assess analytically the combined adverse effects of varying degrees of correlation of intervals between individual arrivals and the burstiness of service times upon the typical quality of service (QoS) measure of the mean queue length (MQL). Moreover, a comprehensive experimental study is carried out to investigate numerically the relative impact of count and interval traffic correlations as well as other traffic characteristics upon the performance of stable BRAP/GE/1 and BMAP/GE/1 queues. It is suggested via a conjecture that the BRAP/GE/1 queue is likely to yield pessimistic performance metrics in comparison to those of the stable BMAP/GE/1 queues under the worst case scenario (i.e., a worst case scenario) of the same positive count and interval traffic correlations arising from long sojourn in each phase

Improving TCP performance over heterogeneous networks : the investigation and design of End to End techniques for improving TCP performance for transmission errors over heterogeneous data networks

Transmission Control Protocol (TCP) is considered one of the most important protocols in the Internet. An important mechanism in TCP is the congestion control mechanism which controls TCP sending rate and makes TCP react to congestion signals. Nowadays in heterogeneous networks, TCP may work in networks with some links that have lossy nature (wireless networks for example). TCP treats all packet loss as if they were due to congestion. Consequently, when used in networks that have lossy links, TCP reduces sending rate aggressively when there are transmission (non-congestion) errors in an uncongested network. One solution to the problem is to discriminate between errors; to deal with congestion errors by reducing TCP sending rate and use other actions for transmission errors. In this work we investigate the problem and propose a solution using an end-to-end error discriminator. The error discriminator will improve the current congestion window mechanism in TCP and decide when to cut and how much to cut the congestion window. We have identified three areas where TCP interacts with drops: congestion window update mechanism, retransmission mechanism and timeout mechanism. All of these mechanisms are part of the TCP congestion control mechanism. We propose changes to each of these mechanisms in order to allow TCP to cope with transmission errors. We propose a new TCP congestion window action (CWA) for transmission errors by delaying the window cut decision until TCP receives all duplicate acknowledgments for a given window of data (packets in flight). This will give TCP a clear image about the number of drops from this window. The congestion window size is then reduced only by number of dropped packets. Also, we propose a safety mechanism to prevent this algorithm from causing congestion to the network by using an extra congestion window threshold (tthresh) in order to save the safe area where there are no drops of any kind. The second algorithm is a new retransmission action to deal with multiple drops from the same window. This multiple drops action (MDA) will prevent TCP from falling into consecutive timeout events by resending all dropped packets from the same window. A third algorithm is used to calculate a new back-off policy for TCP retransmission timeout based on the network's available bandwidth. This new retransmission timeout action (RTA) helps relating the length of the timeout event with current network conditions, especially with heavy transmission error rates. The three algorithms have been combined and incorporated into a delay based error discriminator. The improvement of the new algorithm is measured along with the impact on the network in terms of congestion drop rate, end-to-end delay, average queue size and fairness of sharing the bottleneck bandwidth. The results show that the proposed error discriminator along with the new actions toward transmission errors has increased the performance of TCP. At the same time it has reduced the load on the network compared to existing error discriminators. Also, the proposed error discriminator has managed to deliver excellent fairness values for sharing the bottleneck bandwidth. Finally improvements to the basic error discriminator have been proposed by using the multiple drops action (MDA) for both transmission and congestion errors. The results showed improvements in the performance as well as decreases in the congestion loss rates when compared to a similar error discriminator.EThOS - Electronic Theses Online ServiceMinistry of Higher Education and King Saud University in Saudi Arabia.GBUnited Kingdo

Integrating student self-assessment and feedback in e-learning applications : a proposed educational model

There is a large demand for the use of e-learning tools to support student learning, in the form of distance or blended learning. The need for e-learning environment that encourages learners to learn independently or in groups in virtual settings is crucial. Some e-learning environments provide repositories of 'resources'. They neither facilitate a strategy for learning or teaching, nor they guide students through the resources, and tutors in constructing their courses. E-learning environments need to incorporate pedagogical practices which support and allow students to learn by removing any barriers that might inhibit their learning. Therefore, one of the most important aspects in developing e-learning environments is defining appropriate models where technology and pedagogy are integrated. This thesis provides such a framework for developing e-learning applications; it aims to make it easier for tutors to implement their lesson content and engage learners to achieve the course objectives. The proposed model incorporates constructive alignment, assessment and feedback and unlike other e-learning environments guides the tutor to construct lessons and help learners to use effective learning environment. Furthermore, the thesis investigates on how supported learning can help students adapt to the different approaches to learning. The empirical work undertaken investigates the role of constructing a well designed self-assessment and feedback unit within a learning environment.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Culturally aligned security in banking : a system for rural banking in Ghana

This thesis is an investigation into the unique rural banking system in Ghana and the role of information systems in fraud control. It presents a robust information security and internal control model to deal with fraud for the banking system. The rural banking industry has been noted for poor internal control leading to fraud. This has resulted in poor performance and even the collapse of some banks. The Focus of the study was on the processes used to deliver banking services. To design a protection system, a number of rural banks were visited. This was to understand the environment, regulatory regimes and the structure and banking processes of the industry and banks. Systemic vulnerabilities within the industry which could be exploited for fraud were found. The lack of structures like an address system and unreliable identification documents makes it difficult to use conventional identification processes. Also the lack of adequate controls, small staff numbers and the cross organisational nature of some transactions among other cultural issues reduces the ability to implement transaction controls. Twenty fraud scenarios were derived to illustrate the manifestation of these vulnerabilities. The rural banking integrity model was developed to deal with these observations. This protection model was developed using existing information security models and banking control mechanisms but incorporating the nature of the rural banking industry and culture of its environment. The fraud protection model was tested against the fraud scenarios and was shown to meet the needs of the rural banking industry in dealing with its systemic vulnerabilities. The proposed community-based identification scheme deals with identification weaknesses as an alternative to conventional identity verification mechanisms. The Transaction Authentication Code uses traditional adinkra symbols. Whilst other mechanisms like the Transaction Verification Code design v internal controls into the banking processes. This deals with various process control weaknesses and avoids human discretion in complying with controls. Object based separation of duties is also introduced as a means of controlling conflicting tasks which could lead to fraud.EThOS - Electronic Theses Online ServiceGBUnited Kingdo