Ph.D. Theses
Permanent URI for this collection
Browse
Browsing Ph.D. Theses by Author "Alagöz, Fatih."
Now showing 1 - 8 of 8
Results Per Page
Sort Options
Item A scheduling model for centralized cognitive radio networks(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2012., 2012.) Gözüpek, Didem.; Alagöz, Fatih.In this thesis, we present a scheduling model for centralized cognitive radio networks. Our model consists of a set of schedulers that focus on the data transmission of the secondary users and determine with which frequency, time slot and data rate each secondary user will transmit to the cognitive base station. Common features of the schedulers are that all of them ensure that the primary users in the service area of the cognitive base station are not disturbed, no collisions occur among the secondary users, and reliable communication of the secondary users with the cognitive base station is maintained. Our schedulers differ from each other mainly in terms of their objectives. We propose schedulers that maximize the overall cognitive radio cell throughput, minimize the average scheduling delay of the secondary users, provide maxmin, weighted max-min and proportional throughput fairness, maximize the number of secondary users that are satisfied in terms of throughput, and take the different delay costs of switching to different frequency bands into account. In addition to heuristic algorithms and simulation based studies, we also present a graph theoretic approach and prove several NP-hardness and inapproximability results, propose polynomial time graph algorithms as well as approximation algorithms.Item Channel selection and assignment schemes for efficient spectrum sharing and energy efficiency in cognitive radio networks(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2012., 2012.) Bayhan, Suzan.; Alagöz, Fatih.In this thesis, we focus on distributed channel selection and centralized channel assignment in cognitive radio networks (CRN). For the former topic, we are concerned with the efficiency of spectrum sharing whereas in the latter, we also aim to improve energy efficiency of the CRN. First, we propose a non-selfish distributed channel selection scheme which improves the efficiency of spectrum sharing by mitigating the spectrum fragmentation. We also present an analytical model for our proposal using Continuous Time Markov Chains. In this thesis, we also devise various centralized channel assignment algorithms that outperform pure opportunistic schedulers in terms of energy efficiency and fairness notion without significantly trading off throughput efficiency. Initially, we consider a CRN which acquires channel occupancy information from a white space database. We develop heuristic algorithms considering transmission, idling and channel switching periods in both contiguous and fragmented spectrum. Finally, we consider a CRN in which CRs apply a listen-before-talk access approach. Different from our previous proposal, this scheduler ensures that interference caused by CRs does not exceed the tolerable limits in any of the primary user (PU) channels. In addition, it considers the differences among the PU channels in terms of probability of being idle as well as the control messaging overhead in downlink and uplink. Considering the tradeoff between the scheduling overhead and PU interference probability, we identify the frame length achieving high throughput. Simulation results show that our proposal achieves high throughput performance comparable to a throughput maximizing scheduler but it consumes lower energy than the latter.Item Content-centric and spectrum sharing heterogeneous wireless networks(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2021., 2021.) Kafıloğlu, Sebahat Sinem.; Alagöz, Fatih.The surging multimedia content demand has put wireless network systems under heavy energy consumption burden in an unprecedented manner. Besides, wireless net works are urged to the limits of service capacity due to the adopted enriched multime dia services. According to these facts, in this thesis we analyze content-centric wireless networks from trade-off natured energy and service capacity perspectives and specif ically elaborate on the spectrum sharing heterogeneous networking paradigm. First, we provide a compound spectrum sharing heterogeneous network model enriched with satellite networking, cognitive radio and device-to-device paradigms and investigate this model rigorously in all aspects. Owing to the fact that multimedia content is the building block of our research, we propose a new content model. We propose caching methods based on our new content model due to the fact that in-network caching is an instrumental method to improve the performance of content transmissions in terms of both system capacity and energy consumption. We enhance our caching evaluation in two direction i) cooperation, ii) optimization. We profoundly analyze all caching verti cals with respect to different system dynamics and compare our proposals to alternative techniques.Item Energy efficiency analysis and modeling of cognitive and heterogeneous wireless networks(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2013., 2013.) Gür, Gürkan.; Alagöz, Fatih.The surging energy costs and the environmental consequences of energy generation and exploitation have put energy e ciency aspect of wireless systems into focus in an unprecedented manner. Moreover, the capacity expectations and requirements for wireless networks have been relentlessly increasing with the adoption of new services and sophisticated wireless terminals. In this thesis, we evaluate cognitive and heterogeneous wireless network paradigms from energy e ciency perspective that has become vital due to the above mentioned phenomena. We speci cally focus on energy e ciency analysis and modeling of these systems for realizing the \green networks'' objective. We rst provide a comprehensive account of energy e ciency of wireless networks. At a cross-sectional level, we consider cognitive radios (CR) paradigm which is a ecting all facets of wireless data communications. The CR concept is evaluated from the \energy-e cient operation" and \energy e ciency enabler" perspectives. At the microscopic level, we focus on small cells, namely femtocells, and propose a new networking paradigm called cognitive femtocell networks (CFN). We analyze them in terms of energy e ciency via our analytical model and compare their performance with that of macrocell-only networks as well as traditional femtocell networks.Item Filtering based defense mechanisms against DDOS attacks for core networks(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2016., 2016.) Kalkan, Kübra.; Alagöz, Fatih.In this thesis, we present filtering based defense mechanisms against Distributed Denial of Service (DDoS) attacks for core networks. Initially, several filtering techniques are analyzed and their advantages and disadvantages are presented. A comparative classification of these methods is provided for security analysts. Classification results suggest that there are a few filtering methods that are both proactive and collaborative. Proactivity provides prevention of attacks before it spreads whereas collaboration enables getting knowledge about di↵erent points of the network and deciding filters together. Thus, we proposed a proactive and collaborative model called ScoreForCore. It is a statistical packet based defense mechanism that selects the most appropriate attributes for current attack traffic. Our results suggest that the success of the system’s behavior on legal and attack packets increased considerably. This strategy is also convenient for current emerging technology for core networks, called Software Defined Networking (SDN). It has several problems related to security that are largely induced by the centralized control paradigm. In that regard, DDoS attacks are specifically valid for SDN environment. Several defense mechanisms in SDN environment are analyzed and comparative classification is provided for rendering the current state of the art in the literature. Then, our defense strategy is applied on SDN environment with capable switches. This mechanism’s(SDNScore) results suggest that it gives perfect results for several known attacks and 84% success for an unknown attack. Since there is a trade-o↵ between SDN paradigm and capable switches in SDNScore, we improved it and proposed another model called Joint Entropy based Scoring for SDN (JESS) that carries all burden to the controller and does not need capable switches. The results suggest that it is an elegant defense method for SDN environment.Item Priority scheduling for heterogeneous workloads in computing clusters(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2015., 2015.) Çavdar, Derya.; Alagöz, Fatih.Nowadays, large computing clusters constantly strive for an optimal tradeo between resource e ciency and performance. In this thesis, we are concerned with the e cient use of system resources and we also aim to improve response time of the tasks. We tackle with the challenges of task scheduling on large heterogeneous clusters executing highly heterogeneous bursty workloads with di erent priorities, resource demands, and performance objectives. Firstly, we propose a scheduling algorithm for tasks with communication needs which improves the response time and resource utilization by controlling communication and computation resources at the same time. Secondly, we propose a novel scheduling framework for exploring various aspects of priority scheduling with heterogeneous workloads while investigating the tradeo between evictions and response time. To better understand the impact of evictions, we rst analyze simple eviction policies and wasted resources associated with evictions by using tracedriven simulations. Furthermore, by exploiting the heterogeneity of the workload, we propose a workload-aware slot con guration and task assignment methodology incorporated with slot-based priority scheduling to improve class-based response time and resource e ciency. Finally, we introduce a task scheduling policy which aims to provide scheduling and execution guarantees for low priorities while preserving the performance bene ts of high priority tasks. The proposed scheduling e ectively handles both prioritization and performance issues of low priorities by utilizing a combination of preemptive and non-preemptive scheduling.Item Routing and network mobility management in next generation satellite networks(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2009.) Korçak, Ömer.; Alagöz, Fatih.Satellite networks are an attractive option to provide broadband telecommunication services to globally scattered users, due to their extensive geographic coverage, high bandwidth availability, inherent broadcast capabilities, etc. Satellites rotating in geostationary orbit (GEO) are very well suited for broadcast services, but they suffer from high free space attenuation and long delays. On the contrary non-geostationary (NGEO) systems consisting of Medium Earth Orbit (MEO) and Low Earth Orbit (LEO) satellites offer smaller latency, lower free space loss, and better re-use of available ground-space communication frequencies, hence they are more suitable for most applications (especially for those running in real-time). However, these advantages come with a price: Footprints of satellites at lower altitudes are smaller, and global coverage can be provided by higher number of satellites that are connected each other with inter-satellite links (ISL). Moreover, lower orbit satellites move with higher speeds relative to the Earth’s surface, resulting in high dynamic in the network topology. Dynamics of the satellite constellation constitute major challenge in providing efficient routing and quality of service (QoS) for rapidly-growing real-time multimedia services. On the other hand, regular NGEO satellite networks has some facilitating features like periodicity, predictability and having highly symmetric and regular topology. For efficient networking in NGEO satellite networks, all these features should be considered. In this thesis, we clarify features of satellite systems that differ them from their terrestrial counterparts and propose novel routing and network mobility management techniques in NGEO satellite networks. Firstly, we make use of geometrical properties of the network topology, and propose a priority-based adaptive routing (PAR) algorithm. Next, we focus on handling the mobility of network by utilizing satellites with Earth-fixed footprints, and extend a well-known mobility handling technique called Virtual Node (VN). We propose Multi-state Virtual Network (MSVN) topology that alleviates deficiencies of VN concept. We clarify potential advantages of MSVN by developing efficient handover mechanisms and beam management techniques in MSVN-based satellite systems. Finally, we investigate efficient integration of NGEO satellites with High Altitude Platforms (HAPs) via highcapacity free-space optical links for carrying dense and real-time multimedia traffic. Considering the mobility and resource limitations of satellites, we propose an efficient solution for the optimal link establishment problem between HAPs and satellites.Item TDMA based wireless sensor network for military monitoring (MILMON)(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Bekmezci, İlker.; Alagöz, Fatih.Wireless sensor network (WSN) is a new network family that enables to create smart environments. Although WSN has many application areas, military applications of WSN are very interesting. In this thesis, a new TDMA based sensor network for military monitoring (MILMON) is proposed. MILMON is developed to operate in large areas for acceptable lifetime periods. Design considerations of MILMON are energy consumption, delay, and fault tolerance. The main problems of TDMA based systems are time synchronization and time slot distribution. In order to realize MILMON, a new time synchronization mechanism (SyncHRT), a new distributed time scheduling mechanism (ft_DTSM) and data indicator slot mechanism (DISM) are proposed. Time synchronization with high range transmitter, SyncHRT is designed to minimize energy consumption and maximize precision. It assumes the existence of high range transmitter, so that many of the nodes can receive the broadcast signal of the high range transmitter. In this way, sensor nodes can be synchronized to a central point. Simulation model shows that SyncHRT can reduce energy consumption and increase precision. Another mechanism proposed for MILMON is delay sensitive, energy efficient and fault tolerant distributed slot assignment algorithm (ft_DTSM). It uses much less energy than the existing slot assignment mechanisms and it reduces delay with the convergecast traffic assumption which is very common traffic type for WSNs. Its fault tolerant structure helps to survive against single point of sensor node failures. Another mechanism proposed for MILMON is data indicator slot mechanism (DISM) which reduces energy consumption especially on low traffic requirements, as in most of the military monitoring systems. Analysis and simulation show that although there are WSN systems that perform better than our system for only energy consumption or for only delay, MILMON realizes an optimization on energy, delay and fault tolerance.