Cognitive radio has become a promising technology to increase spertrum utilization through spectrum sharing between licensed users (primary users) and unlicensed users (secondary users). Performance evaluation and analysis is of key importance to get more knowledge of this newly emerged technology.
Queueing theory and Markov chain model have been applied to conduct the analysis, where some results are obtained. The existing research sheds light on the performance of cognitive radio networks. However, there are also some limitations. Poisson arrival and exponentially distributed service time are mostly assumed in existing analysis. With these assumptions, existing queueing theory results, particularly M/G/1 priority queue results, can be directly applied, and the Markov chain model can be established. However, these assumptions are too restrictive for modern wireless communication networks, where the traffic can be of different types and the channel capacity can vary over time. In addition, particular focus is made on average values (such as average delay) with little investigation on probabilistic distribution bounds. Read more
Various security measures are ineffective having been designed without adequate usability and economic considerations. The primary objective of this thesis is to add an economic and socio-behavioral perspective to the traditional computer science research in information security. The resulting research is interdisciplinary, and the papers combine different approaches, ranging from analytic modeling to empirical measurements and user studies. Contributing to the fields of usable security and security economics, this thesis fulfills three motivations. Read more
Despite its rapid and tremendous growth, the basic packet delivery service in the Internet has largely remained best-effort and egalitarian. Consequently, the Internet from the outset lacks powerful mechanisms for fair arbitration of its own shared resources among the users it serves. For example, user flows injecting more bytes per time unit can proportionally receive more service, at the expense of others with lower rates. In the extreme case, certain flows or users can completely be denied of service (i.e., access to link and buffer resources) in the network. Read more
The Internet forms a globally distributed network that provides an ubiquitous medium for interaction, the exchange of ideas, and commerce. The web is pervading our everyday lives in ways that were unimaginable even ten years ago. The evolving use of the web requires robust and efficient trust and reputation management mechanisms. During the past decade, online trust and reputation systems have provided cogent answers to emerging challenges in the global computing infrastructures relating to computer and network security, electronic commerce, virtual enterprises, social networks and cloud computing. The goal of these systems in such global computing infrastructures is to allow entities to reason about the trustworthiness of other entities and to make autonomous decisions on the basis of trust. This requires the development of computational trust models that enable entities to reason about trust and to verify the properties of a particular interaction. Read more
The rapid growth of delay sensitive applications, such as VoIP and IP video, requires the performance analysis of systems with quality of service (QoS) support. A fundamental question in this regard is to predict the traffic carrying capacity of a communication network by taking into account the bursty nature and delay requirements of sources which we call the delay constrained throughput in this thesis. The apparently simple problem poses serious challenges due to lack of an efficient tool for the modeling of delay and burstiness. In this work, we make use of network calculus (NetCal) as a tool to model delay and traffic burstiness. In addition, we also obtain the delay performance limit of communication networks. Read more
Hash functions are being used as building blocks in such diverse primitives as commitment schemes, message authentication codes and digital signatures. These primitives have important applications by themselves, and they are also used in the construction of more complex protocols such as electronic voting systems, online auctions, public-key distribution, mutual authentication handshakes and more. Part of the work presented in this thesis has contributed to the âSHA-3 contest'' for developing the new standard for hash functions organized by the NationalI nstitute of Standards and Technology. We constructed the candidate Edon-R which is a hash function based on quasigroup string transformation. Edon-R was designed to be much more efficient than SHA-2 cryptographic hash functions, while at the same time offering same or better security. Most notably Edon-R was the most efficient hash function submitted to the contest.
Anonymous communication is an important building block for privacy enhancing technologies and in addition to this, the deployed anonymity networks have become important tools to protect the usersâ privacy in the Internet where hundreds of thousands of users rely on them. During the last decades, researchers have investigated the security aspects of anonymous communication networks. One important research methodology is the investigation of attacks which helps to understand the security properties of the analyzed systems. Most of the research has been focused on the Tor network, which is the most popular anonymity network. Tor is however not the only deployed anonymity network. Several others have been deployed during the years, e.g., AN.ON, I2P, and MixMaster. Each of them is based on different concepts which entail different security properties as well as attack vectors. Therefore, it is also important to investigate the security aspects of the other networks.
This thesis proposes a hardware implementiation of multivariable quadratic quasi-group (MQQ) public key crypto system that achieves decryption throughput of 399 Mbps on a Xilinx Virtex-5 field programmable gate array (FPGA), which is running on 249.4 MHz. The encryption throughput of MQQ implementation achieves 44.27 Gbps on four Xilinx Virtex-5 chips that are running on 276.7 MHz.
In addition for, the compact hardware implementation of Blue Midnight Wish (BMW) hash function, the thesis introduces a compact processing element on FPGA.
Most real-time voice and video applications are delay/loss sensitive but relaxed in the sense that they can tolerate some packet loss/delay. Using this information, network utilization can be greatly improved by exploiting statistical multiplexing.
To this end, Measurement Based Admission Control (MBAC) has for a long time been recognized as a promising solution. MBAC algorithms, do not require an a priori source characterization which in many cases may be difficult or impossible to attain. Instead, MBAC uses measurements to capture the behavior of existing flows and uses this information together with some coarse knowledge of a new flow when making an admission decision for this requesting flow.
Multimedia presentations of digital media services and devices are meant for human consumption and interaction. Before consumption by the user, the multimedia signal usually goes through several processing stages. Depending on the technologies, including the applied signal processing algorithms, some stages can introduce artefacts that reduce the quality of the multimedia presentation. Quality is a fundamental aspect for the design of any end-to-end multimedia signal processing architecture. A sufficiently high quality level of any multimedia presentations must be provided to the user to ensure her optimal experience. More recently, we have seen a shift of paradigm towards incorporating the user as the most important factor in the quality assessment of multimedia presentations. This shift of paradigm drives the creation of the Quality of Experience (QoE) concept. QoE depends on the user perception making it a qualitative assessment as opposed to a purely quantitative one. The definite way of assessing perception of user is by conducting a perception experiment involving human participants in a controlled environment, and this experiment must be carefully designed. Subjective quality assessment is one example of such experiment. There is another, more practical way of assessing quality from user standpoint; this utilizes perceptual-based metrics that model the human perception as closely as possible. Due to the array of current applications, it is unlikely to have a universal quality metric for assessing QoE of multimedia applications. This thesis will only focus on QoE for Digital Cinema presentations.
It is not economically or technically feasible to make complex computer systems that are completely secure. New attacks are constantly developed by attackers and the security situation can therefore rapidly change. In order to detect and stop attackers before any damage is done, automated tools have to be deployed because there is not enough time for manual intervention. Therefore there is a need for online risk assessment and proactive defense mechanisms like Intrusion Prevention System (IPS). In the area of computer security there have been only a few quantitative security measures until now, and there are few published cases for methods and tools based on such measures.
There will be a need to configure nodes with multiple radios, as ad hoc networks are being deployed. The applications tend to have high bandwidth requirements. At the same time, the deployment scenario may result in range requirements that can not be met by Super High Frequency (SHF) or Very High Frequency (VHF) radios. An alternative is then to use multiple radios, where the radios differentiate in transmission range, bandwidth or by other radio characteristics. The aim of this dissertation is therefore to study various techniques to utilize heterogeneous radio networks, and for improved packet delivery within various configurations of mobile ad-hoc networks. Read more
This thesis deals with the transmission of high quality audio over packet switched networks, such as the internet. Unlike other transmission media, no guarantees are given regarding bandwidth, delay, or loss ratios, and all these factors will typically be time-varying. For interactive, or two-way, applications it is important to keep a low end-to-end delay in order to have a high Quality of Experience. In this scenario, retransmission of lost packets is not possible simply due to the delay restrictions, thus one has to expect and prepare for periods with packet loss.
In this thesis the Differentiated Services (DiffServ) architecture is applied and adapted to Mobile Ad Hoc Networks (MANETs). Using the same QoS architecture will ease the transition between the wireless and wired domain. But the special characteristics of wireless networks require modifications to the original DiffServ architecture. In investigations there was found restrictions on the number of classes to use, and this number was dependent on the type of traffic in the network. A QoS architecture based on the DiffServ framework is proposed, with an admission control based on the concept of shadow classes, and Explicit Congestion Notification (ECN) to avoid congestion.
Abstract Emergent networks like mobile ad hoc networks, sensor networks, opportunistic networks, peer-to-peer networks and social networks are introducing new and exciting opportunities of communication between people and devices. But these dynamic networks also introduce many security- and privacy-related challenges. When dealing with complex and dynamic environments, information about the current level of security or privacy, expressed in a quantified manner, could be of great value in a decision-making process. In order to derive such quantified measures there is a need for mathematical models for security, privacy and trust. The development, application and evaluation of such models are the topics of this thesis.
The interest in watching real-time video content transmitted over packet-based communication networks such as the Internet is growing. When resources are restricted, quality of service support and provisioning of service guarantees are needed in the network to ensure a satisfactory user experience. In addition, the video content, different choices made in the video encoding, the bitrate characteristics of the resulting video stream, and the network performance will have an influence on the perceived quality. Regarding the network performance, the packet loss ratio and packet loss distribution are identified as important performance parameters for real-time video, and are of particular interest. Estimating these parameters is a step towards assessing the perceived quality of service for a video transmission.
In this dissertation, we are focusing on error resilient resource allocation through encoding algorithms in real-time video communications. The thesis is composed of six papers which are grouped into two parts with each part addressing a speciïŹc type of communication scenario.
Dr. Yuan Lin is now working at Nera in Bergen, Norway.
Classical surround sound techniques are based on sweet spot listening while sound field approaches offer higher quality in that they can give reproduction over an extended area. The expenses are higher complexity in the creation and reproduction of the audio content and a high bit rate. The fact that we perceive the direct sound, early reflections and the late part of a room impulse response differently forms the basis for contemporary hybrid formats that can yield a lower bit rate. These formats are, however, based on classical surround sound techniques and are meant for for sweet spot listening. This thesis looks into both sound field reproduction that can be used for direct sound reproduction and the reproduction of the late diffuse part of a room impulse response.
Dr. Audun Solvang is now working at Tandberg in Oslo, Norway.
Unintentional failures affect links and nodes in communication networks. Recovery mechanisms are the key tool for achieving the dependability required by the services using the network. However, high dependability in communication networks comes at a high cost in terms of the capacity needed by these mechanisms. Since the users and services have different requirements, and users have different willingness to pay for a high quality of service, it is desirable to have methods that enable provision of different levels of dependability in the same network, i.e. dependability differentiation. The thesis addresses dependability differentiation in connection-oriented backbone communication networks.
Dr. Anders Mykkeltveit is now working at Telenor Global Services in Oslo, Norway.
Resource limitations lead to a number of challenges in all communication systems. This is also the case for video communication over networks based on the Internet Protocol (IP), where limited rates are shared among competing, heterogeneous users. Packet losses, delays and connectivity losses will be experienced to a varying degree depending on network loads, physical properties and mobility- related issues. These factors, as well as source coding characteristics, inïŹuence the visual quality experienced by the users. One of the main contributions of the work is that performance gains can be attained when considering characteristics of source coding, networks and congestion control jointly. Throughout the presented work, optimization of visual quality is at the centre of attention.
Dr. Stian Johansen is now working at Tandberg in Oslo, Norway.
To be considered trustworthy, a system must be both dependable and secure. However, these two aspects have so far tended to be treated separately. A unified modelling framework for security and dependability evaluation would be advantageous from both points of view. The security community can benefit from the mature dependability modelling techniques, which can provide the operational measures that are so desirable today. On the other hand, by adding hostile actions to the set of possible fault sources, the dependability community will be able to make more realistic models than the ones that are currently in use. The thesis proposes a stochastic modelling approach, which can be used to predict a system's security and dependability behavior.
Dr. Karin Sallhammar is now working at Telenor Research and Innovation (R&I) in Trondheim, Norway.
O. I. Hillestads PhD thesis deals with multimedia communication over unreliable and resource-constrained IP-based packet-switched networks. The focus is on estimating, evaluating and enhancing the quality of streaming media services, and streaming video services in particular.
Dr. Odd Inge Hillestad is now working at T-VIPS.
A. Ă rnes' PhD thesis entitled âRisk, Privacy, and Security in Computer Networksâ introduces a novel approach to real-time network risk assessment based on hidden Markov models to represent the likelihood of transitions between security states. Further, measures for anonymization and pseudonymization are analyzed with respect to the trade-off of performing meaningful data analysis while protecting the identities of the users, and a novel method for performing digital forensic reconstructions in a virtual security testbed is proposed.
From the 1. of January 2007, Dr. Andre Ă rnes is back working for Politiets Datakrimsenter at Nye Kripos in Oslo, Norway.