Please read the abstract in the front matter of this document. / Dissertation (M Eng (Electrical Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

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The emerging behavior of a mobile system is determined by its software architecture (structure, dynamics, deployment), the underlying communication networks (topology, properties like bandwidth etc.) and interactions undertaken by the users of the system. In order to assess whether a mobile system fulfills its non-functional requirements like response times or availability already at design time, the emergent behavior of such a system can be simulated by using an architectural model of the system and applying an simulation approach where a network model and a user interaction model are used for providing the contextual information. In this paper we show how such an architectural model can expressed in our ADL Con Moto, how functional and non-functional properties of an architecture can be modeled and how simulation of the mobile system can be used to yield the desired properties.

Network Coding is a critical technique when designing next-generation network systems, since the use of network coding can significantly improve the throughput and performance (delay/reliability) of the system. In the traditional design paradigm without network coding, different information flows are transported in a similar way like commodity flows such that the flows are kept separated while being forwarded in the network. However, network coding allows nodes in the network to not only forward the packet but also process the incoming information messages with the goal of either improving the throughput, reducing delay, or increasing the reliability. Specifically, network coding is a critical tool when designing absolute Shannon-capacity-achieving schemes for various broadcasting and multi-casting applications. In this thesis, we study the optimal network schemes for some applications with less restrictive network models. A common component of the models/approaches is how to use network coding to take advantage of a broadcast communication channel.<div><br></div><div>In the first part of the thesis, we consider the system of one server transmitting K information flows, one for each of K users (destinations), through a broadcast packet erasure channels with ACK/NACK. The capacity region of 1-to-K broadcast packet erasure channels with ACK/NACK is known for some scenarios, e.g., K<=3, etc. However, existing achievability schemes with network coding either require knowing the target rate in advance, and/or have a complicated description of the achievable rate region that is difficult to prove whether it matches the capacity or not. In this part, we propose a new network coding protocol with the following features: (i) Its achievable rate region is identical to the capacity region for all the scenarios in which the capacity is known; (ii) Its achievable rate region is much more tractable and has been used to derive new capacity rate vectors; (iii) It employs sequential encoding that naturally handles dynamic packet arrivals; (iv) It automatically adapts to unknown packet arrival rates; (v) It is based on GF(q) with q>=K. Numerically, for K=4, it admits an average control overhead 1.1% (assuming each packet has 1000 bytes), average encoding memory usage 48.5 packets, and average per-packet delay 513.6 time slots, when operating at 95% of the capacity.</div><div><br></div><div>In the second part, we focus on the coded caching system of one server and K users, each user k has cache memory size M_k and demand a file among the N files currently stored at server. The coded caching system consists of two phases: Phase 1, the placement phase: Each user accesses the N files and fills its cache memory during off-peak hours; and Phase 2, the delivery phase: During the peak hours, each user submits his/her own file request and the server broadcasts a set of packet simultaneously to K users with the goal of successfully delivering the desired packets to each user. Due to the high complexity of coded caching problem with heterogeneous file size and heterogeneous cache memory size for arbitrary N and K, prior works focus on solving the optimal worst-case rate with homogeneous file size and mostly focus on designing order-optimal coded caching schemes with user-homogeneous file popularity that attain the lower bound within a constant factor. In this part, we derive the average rate capacity for microscopic 2-user/2-file (N=K=2) coded caching problem with heterogeneous files size, cache memory size, and user-dependent heterogeneous file popularity. The study will shed some further insights on the complexity and optimal scheme design of general coded caching problem with full heterogeneity.<br></div><div><br></div><div>In the third part, we further study the coded caching system of one server, K= 2 users, and N>=2 files and focus on the user-dependent file popularity of the two users. In order to approach the exactly optimal uniform average rate of the system, we simplify the file demand popularity to binary outputs, i.e., each user either has no interest (with probability 0) or positive uniform interest (with a constant probability) to each of the N file. Under this model, the file popularity of each user is characterized by his/her file demand set of positive interest in the N files. Specifically, we analyze the case of two user (K=2). We show the exact capacity results of one overlapped file of the two file demand sets for arbitrary N and two overlapped files of the two file demand sets for N = 3. To investigate the performance of large overlapped files we also present the average rate capacity under the constraint of selfish and uncoded prefetching with explicit prefetching schemes that achieve those capacities. All the results allow for arbitrary (and not necessarily identical) users' cache capacities and number of files in each file demand set.<br></div>

Interference limits the throughput of modern wireless communication networks, and thus the successful mitigation of interference can have a significant impact on network performance. Successive interference cancellation (SIC) has emerged as a promising physical layer method, where multiple packets received simultaneously need not be treated as a ``collision'' requiring retransmission; rather, under certain conditions, all of the packets can be decoded. Obviously, using SIC can thus serve as an important design element that can provide higher performance for the network. However, it also requires a rethinking of the way that traditional routing and scheduling algorithms, which are designed for a traditional physical layer, are developed. In order to consider routing and scheduling over a physical layer employing SIC, some tools such as the oft-employed conflict graph need to be modified. In particular, a notion of links interfering with other links ``indirectly'' is required, and this issue has been ignored in many past works. Therefore, considering the dependencies and interferences between links, a joint routing and scheduling algorithm that employs an understanding of the SIC that will be employed at the physical layer is presented and shown to surpass previous algorithms. We know that the maximum throughput scheduling problem is NP-hard. On the other hand, even if we can reach maximum throughput scheduling, while being throughput efficient, it can result in highly unfair rates among the users. Hence, proportional fairness is developed in the proposed algorithm.

Quality of Service (QoS) routing considered as one of the major components of the QoS framework in communication networks. The concept of QoS routing has emerged from the fact that routers direct traffic from source to destination, depending on data types, network constraints and requirements to achieve network performance efficiency. It has been introduced to administer, monitor and improve the performance of computer networks. Many QoS routing algorithms are used to maximize network performance by balancing traffic distributed over multiple paths. Its major components include bandwidth, delay, jitter, cost, and loss probability in order to measure the end users¿ requirements, optimize network resource usage and balance traffic load. The majority of existing QoS algorithms require the maintenance of the global network state information and use it to make routing decisions. The global QoS network state needs to be exchanged periodically among routers since the efficiency of a routing algorithm depends on the accuracy of link-state information. However, most of QoS routing algorithms suffer from scalability problems, because of the high communication overhead and the high computation effort associated with marinating and distributing the global state information to each node in the network.The goal of this thesis is to contribute to enhancing the scalability of QoS routing algorithms. Motivated by this, the thesis is focused on localized QoS routing that is proposed to achieve QoS guarantees and overcome the problems of using global network state information such as high communication overhead caused by frequent state information updates, inaccuracy of link-state information for large QoS state update intervals and the route oscillating due to the view of state information. Using such an approach, the source node makes its own routing decisions based on the information that is local to each node in the path. Localized QoS routing does not need the global network state to be exchanged among network nodes because it infers the network state and avoids all the problems associated with it, like high communication and processing overheads and oscillating behaviour. In localized QoS routing each source node is required to first determine a set of candidate paths to each possible destination. In this thesis we have developed localized QoS routing algorithms that select a path based on its quality to satisfy the connection requirements. In the first part of the thesis a localized routing algorithm has been developed that relies on the average residual bandwidth that each path can support to make routing decisions. In the second part of the thesis, we have developed a localized delay-based QoS routing (DBR) algorithm which relies on a delay constraint that each path satisfies to make routing decisions. We also modify credit-based routing (CBR) so that this uses delay instead of bandwidth. Finally, we have developed a localized QoS routing algorithm for routing in two levels of a hierarchal network and this relies on residual bandwidth to make routing decisions in a hierarchical network like the internet. We have compared the performance of the proposed localized routing algorithms with other localized and global QoS routing algorithms under different ranges of workloads, system parameters and network topologies. Simulation results have indicated that the proposed algorithms indeed outperform algorithms that use the basics of schemes that currently operate on the internet, even for a small update interval of link state. The proposed algorithms have also reduced the routing overhead significantly and utilize network resources efficiently.

Analytical modelling and characterization of Weighted Fair Queueing (WFQ) have recently received considerable attention by several researches since WFQ offers the minimum delay and optimal fairness guarantee. However, all previous work on WFQ has focused on developing approximations of the scheduler with an infinite buffer because of supposed scalability problems in the WFQ computation. The main aims of this thesis are to study WFQ system, by providing an analytical WFQ model which is a theoretical construct based on a form of processor sharing for finite capacity. Furthermore, the solutions for classes with Poisson arrivals and exponential service are derived and verified against global balance solution. This thesis shows that the analytical models proposed can give very good results under particular conditions which are very close to WFQ algorithms, where accuracy of the models is verified by simulations of WFQ model. Simulations were performed with QNAP-2 simulator. In addition, the thesis presents several performance studies signifying the power of the proposed analytical model in providing an accurate delay bounds to a large number of classes. These results are not able to cover all unsolved issues in the WFQ system. They represent a starting point for the research activities that the Author will conduct in the future. The author believes that the most promising research activities exist in the scheduler method to provide statistical guarantees to multi-class services. The author is convinced that alternative software, for example, on the three class model buffer case, is able to satisfy the large number of buffer because of the software limitation in this thesis. While they can be a good topic for long-term research, the short-medium term will show an increasing interest in the modification of the WFQ models to provide differentiated services. / Ministry of Higher Education

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Balises are transponders installed in railways. These devices are nowadays powered by means of a radiofrequency signal emitted by each running train that passes above them. It is only during this moment that the health state of the balises is checked. Hence, there is currently no way to check whether the balises are properly working before the train passes by them. With the aim of executing regular health checks to the balises, an additional source of energy to monitor the balises should be contemplated. Energy harvesting is observed as a suitable solution for this issue. However, a lack of suitability studies is contemplated which englobes the available energy harvesting solutions in railway environments. Therefore, this thesis presents an exploratory work that uses the health monitoring of the balises as a test case for the study of the compatibility of different energy harvesters in diverse railway environments. Hazardous and remote areas are identified as locations of interest for the implementation of the technology, as cabling in those areas is costly and health checks to balises that are not constantly active are of outmost interest. Thus, the addition of wireless communication networks is also studied, due to the need of sending the information obtained in the health checks to monitoring control units. After an initial research study is performed, requirements in railway environments are defined, and three railway scenarios are selected for a suitability study. Then, the investigated energy harvesters and wireless communication networks are compared analytically, and possible technologies for the storage of the harvested energy are presented. It is found that no energy harvester exists that suits all the environments and shows a sufficient power output to make constant checks in remote areas. Nonetheless, piezoelectric and wind harvesters are proposed, because of the commercial availability of the former and the potential of the latter. In terms of wireless communication networks, LoRaWAN shows a low power consumption, while it offers a wide communication range and global coverage. It is, therefore, proposed as the best framework for the wireless communication networks. / Baliser är transpondrar installerade i järnvägar. Dessa enheter drivs numera med hjälp av en radiofrekvenssignal som sänds ut av varje tåg som passerar ovanför baliserna. Det är först i detta ögonblick som balises hälsotillstånd kontrolleras. Därför finns det för närvarande inget sätt att kontrollera om baliserna fungerar korrekt innan tåget passerar dem. I syfte att utföra regelbundna hälsokontroller på baliserna bör en ytterligare kraftkälla för att övervaka baliserna övervägas. Energy harvesting observeras som en lämplig lösning för denna fråga. Det råder dock brist på lämplighetsstudier som förenar de tillgängliga energy harvesting lösningarna i järnvägsmiljöer. Därför presenterar denna avhandling ett undersökande arbete som använder hälsoövervakningen av baliserna som ett testfall för att studera kompatibiliteten hos olika energiskördare i olika järnvägsmiljöer. Farliga och avlägsna områden identifieras som platser av intresse för genomförandet av tekniken, eftersom kablar i dessa områden är kostsamma och hälsokontroller till baliser som inte ständigt är aktiva är av yttersta intresse. Således studeras också tillägget av trådlösa kommunikationsnätverk på grund av behovet av att skicka den information som erhållits vid hälsokontrollerna till övervakningskontrollområdena. Efter att en inledande forskningsstudie genomförts definieras krav i järnvägsmiljöer och tre järnvägsscenarier väljs ut för en lämplighetsstudie. Sedan jämförs de undersökta energiskördarna och trådlösa kommunikationsnätverk analytiskt, och eventuell teknik för lagring av den skördade energin presenteras. Det konstateras att det inte finns någon energiskördare som passar alla miljöer och visar en tillräckligt effekt för att göra konstanta kontroller i avlägsna områden. Ändå föreslås piezoelektriska och vindskördare på grund av den förstnämnda kommersiella tillgänglighet och den senare potentialen. När det gäller trådlösa kommunikationsnätverk visar LoRaWAN en låg strömförbrukning, medan det erbjuder ett brett kommunikationssortiment och global täckning. Det föreslås därför som den bästa ramen för de trådlösa kommunikationsnäten.