Design Of Dependable Systems Research Articles

Cryogenic system of the high performance 1.3 GHz 9-cell superconducting radio frequency prototype cryomodule

High quality factor 1.3 GHz 9-cell superconducting radio frequency (SRF) cavity cryomodule is the core technology of the international advanced accelerator. China’s first high-quality factor 1.3 GHz 9-cell SRF cavity cryomodule was developed, assembled and tested at the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences for the Dalian Advanced Light Source (DALS) and Circular Electron Positron Collider (CEPC) R&D. The 9-cell cavities in the cryomodule achieved a high average intrinsic quality factor (Q0) of 3.8 × 1010 at 16 MV/m and 3.6 × 1010 at 21 MV/m in the horizontal test. With such high specifications, the SRF cavity system has corresponding requirements for the cryogenic system. Key criteria for stable and reliable operation include a dependable cryogenic refrigeration system and process design, good thermal insulation, pressure control, temperature uniformity, and quench protection, etc. IHEP has successfully completed cryogenic testing development of the first 1.3 GHz 9-cell SRF prototype cryomodule. The whole cooldown period was 74 h, and the automatic cooldown was completed. The successful development and implementation of the 1.3 GHz superconducting cryomodule cryogenic system will provide a stable and reliable test environment for advanced superconducting cavity cryogenic testing, which will be critical in promoting the further development of the Free Electron Laser (FEL) and CEPC projects.

System Design Principles: adaptation to time for long living autonomous systems

The article presents the principles of creating systems with the adaptation to the operation time. In the literature on systems design the adaptation mostly concerned: 1) the unknown object structure; 2) the unknown object parameters; 3) unknown parameters of input signals; 4) the unknown functions of system state dynamics; and 5) unknown environment conditions. It usually assumed that the control process is intended to achieve a certain, usually optimal state of the system, - such way the “adaptive system concept” are close to “optimal system concept”. The other approach for system design with these conditions is robustness, as robust control does not need a priori information about the bounds on these uncertain or time-varying parameters; robust control guarantees that if the changes are within given bounds the control law need not be changed, while adaptive control is concerned with control law changing itself. In the last 10 – 15 years there was introduced the new approach as “resilient systems”. “Resilience engineering” may look like “repairing engineering” – it is assumed that errors or malfunctions occur “for sure” and the system should respond appropriately to this.
 The system operation time, as the cause of adaptation, was very rare considered, mostly when reliability issues are discussing. The proposed approach is new. The proposed principles should be used with known approaches of dependable system design, – these are engineering and information theory redundancy. Both approaches must be used in the design phase and are unchanged structural parameters of the system during operations. There were concerned mostly “long-living systems” and the same task of reliability.
 Proposed principles can be used in the development of the systems designed for continuous operation with absence of the possibility of external human intervention to restore system performance or some maintenance procedures. By «system» in this article are meant «Complex Adaptive Systems» (CAS). Currently, the proposed approach can be attributed to the development of "Artificial General Intelligence" (AGI). Examples of such systems include space-based and underwater-based robotic systems. By “Adaptability of the system to time” – in the sense of control process – it is meant a certain structural reconfiguration of the system, considering the non-stationary nature of stochastic processes of errors, damages, and system failures.
 The formulation of principles is of a general declarative nature – at this stage the author gives preference to the essence of the proposal, rather than its formalization. The article does not provide specific design guidelines, but contains some examples of possible applications, mainly to highlight the essence of the proposals.

Cyber LOPA: An Integrated Approach for the Design of Dependable and Secure Cyber-Physical Systems

Safety risk assessment is an essential process to ensure a dependable cyber-physical system (CPS) design. Traditional risk assessment considers only physical failures. For modern CPSs, failures caused by cyber attacks are on the rise. The focus of latest research effort is on safety–security lifecycle integration and the expansion of modeling formalisms for risk assessment to incorporate security failures. The interaction between safety and security lifecycles and its impact on the overall system design, as well as the reliability loss resulting from ignoring security failures, are some of the overlooked research questions. This article addresses these research questions by presenting a new safety design method named cyber layer of protection analysis (CLOPA) that extends the existing layer of protection analysis (LOPA) framework to include failures caused by cyber attacks. The proposed method provides a rigorous mathematical formulation that expresses quantitatively the tradeoff between designing a highly reliable and a highly secure CPS. We further propose a co-design lifecycle process that integrates the safety and security risk assessment processes. We evaluate the proposed CLOPA approach and the integrated lifecycle on a practical case study of a process reactor controlled by an industrial control testbed and provide a comparison between the proposed CLOPA and current LOPA risk assessment practice.

CONCEPTION AND APPLICATION OF DEPENDABLE INTERNET OF THINGS BASED SYSTEMS

Context. The problem is in the design, development, maintenance and commisioning of interoperable dependable systems using on the Internet of Things based on von Neumann paradigm of “building reliable systems from unreliable components” for dependable service-oriented systems and infrastructures.Objective. The goals of the paper are in the development of concepts and principles and assessment technologies for creation and maintenance of complex critical systems based on Internet of Things (IoT) as well as implementation of research in various domains.Method. In the paper the concept of development of dependable systems on the basis of the Internet of things is described. The multisectoral analysis of methods and models of reliability and cybersecurity (dependability) evaluation of information and control systems of critical applications using the Internet of things has been performed for different domains: power, healthcare, industrial, etc. The analysis has shown that some software failures and malfunctions, cyberattacks and consequences of influence of attacks are identical for all domains, but there are specific features for each domain, which are necessary to consider at working out of methodology of maintenance of denepdability of reliability of systems of critical applications using the Internet of things.Results. The developed conception, methods, tools and technologies for the creation and implementation of dependable information & control systems for critical applications based on the Internet of Things.Conclusions. The paper proposes a conception that includes a set of scientific and applied tasks for the development of methods, tools and technologies for the creation and implementation of dependable information & analytical and information & control systems for critical applications based on the Internet of Things. The prospects for further research may include the detailing of the developed models, methods and technologies to ensure the dependability of complex information & control systems for critical applications based on the Internet of Things.

Design and Analysis of Secure and Dependable Automotive CPS: A Steer-by-Wire Case Study

The next generation of automobiles (also known as cybercars) will increasingly incorporate electronic control units (ECUs) in novel automotive control applications. Recent work has demonstrated the vulnerability of modern car control systems to security attacks that directly impacts the cybercar's physical safety and dependability. In this paper, we provide an integrated approach for the design of secure and dependable automotive cyber-physical systems (CPS) using a case study: a steer-by-wire (SBW) application over controller area network (CAN). The challenge is to embed both security and dependability over CAN while ensuring that the real-time constraints of the automotive CPS are not violated. Our approach enables early design feasibility analysis of automotive CPS by embedding essential security primitives (i.e., confidentiality, integrity, and authentication) over CAN subject to the real-time constraints imposed by the desired quality of service and behavioral reliability. Our method leverages multicore ECUs for providing fault tolerance by redundant multi-threading (RMT) and also further enhances RMT for quick error detection and correction. We quantify the error resilience of our approach and evaluate the interplay of performance, fault tolerance, security, and scalability for our SBW case study.

Dependability of Directional Millimeter Wave Vehicle-to-Infrastructure Communications

Vehicle-to-infrastructure (V2I) communication is an important enabler for intelligent transportation and rail traffic management systems, which are expected to provide significant road traffic safety and efficiency enhancements. Such systems require a level of dependability of the wireless communication link that can be hard to support by state-of-the-art technologies. Transmissions in the millimeter wave (mmWave) band have the potential to provide sufficient bandwidth to support not only traffic management services, but additionally also web and entertainment applications for car and train passengers. mmWave transmissions require directional antennas at the transmitter and/or receiver to compensate for the significant pathloss and to achieve a practically acceptable maximum coupling loss. Such directional communication is feasible in V2I scenarios, because the mobility of users is confined to streets and roads. In this article, we discuss the dependability of directional mmWave V2I communications under two-ray fading conditions. We review methods for enhancing the dependability of the communication link by exploiting macroscopic spatial diversity and by mitigating interference from other transmitters. We furthermore provide guidelines for a dependable system design under two-ray fading and identify open research questions in this context.

A review of applications of fuzzy sets to safety and reliability engineering

Safety and reliability are rigorously assessed during the design of dependable systems. Probabilistic risk assessment (PRA) processes are comprehensive, structured and logical methods widely used for this purpose. PRA approaches include, but not limited to Fault Tree Analysis (FTA), Failure Mode and Effects Analysis (FMEA), and Event Tree Analysis (ETA). In conventional PRA, failure data about components is required for the purposes of quantitative analysis. In practice, it is not always possible to fully obtain this data due to unavailability of primary observations and consequent scarcity of statistical data about the failure of components. To handle such situations, fuzzy set theory has been successfully used in novel PRA approaches for safety and reliability evaluation under conditions of uncertainty. This paper presents a review of fuzzy set theory based methodologies applied to safety and reliability engineering, which include fuzzy FTA, fuzzy FMEA, fuzzy ETA, fuzzy Bayesian networks, fuzzy Markov chains, and fuzzy Petri nets. Firstly, we describe relevant fundamentals of fuzzy set theory and then we review applications of fuzzy set theory to system safety and reliability analysis. The review shows the context in which each technique may be more appropriate and highlights the overall potential usefulness of fuzzy set theory in addressing uncertainty in safety and reliability engineering.

Verification of the European Rail Traffic Management System in Real-Time Maude

The European Rail Traffic Management System (ERTMS) is a state-of-the-art train control system designed as a standard for railways across Europe. It generalises traditional discrete interlocking systems to a world in which trains hold on-board equipment for signalling, and trains and interlockings communicate via radio block processors. The ERTMS aims at improving performance and capacity of rail traffic systems without compromising their safety. The ERTMS system is of hybrid nature, in contrast to classical railway signalling systems which deal with discrete data only. Consequently, the switch to ERTMS poses a number of research questions to the formal methods community, most prominently: How can safety be guaranteed? In this paper we present the first formal modelling of ERTMS comprising all subsystems participating in its control cycle. We capture what safety means in physical and in logical terms, and we demonstrate that it is feasible to prove safety of ERTMS systems utilising Real-Time Maude model-checking by considering a number of bi-directional track layouts. ERTMS is currently being installed in many countries. It will be the main train control standard for the foreseeable future. The concepts presented in this paper offer applicable methods supporting the design of dependable ERTMS systems. We demonstrate model-checking to be a viable option in the analysis of large and complex real-time systems. Furthermore, we establish Real-Time Maude as a modelling and verification tool applicable to the railway domain. The approach given in this paper is a rigorous one. In order to avoid modelling errors, we follow a systematic approach: First, as a requirement specification, we identify the event-response structures present in the ERTMS. Then, we model these structures in Real-Time Maude in a traceable way, i.e., specification text in Real-Time Maude can be directly mapped to requirements. We explore our models by checking if they have the desired behaviour, and apply systematic model-exploration through error injection – both these steps are carried out using the formal method Real-Time Maude. Finally, we analyse ERTMS by model-checking, thus applying a formal method to the railway domain, and we mathematically prove that our analysis of ERTMS by model-checking is complete, i.e., that it guarantees safety at all times. • First full model of ERTMS, i.e., comprising all subsystems required for the full control cycle. • Systematic modelling, validation, and verification process on ERTMS. • Completeness argument for the verification of a hybrid system through finitely many time samples. • Modelling technique for time-robustness and tick-stability in case of numerical rounding errors. • Successful application of Real-Time Maude to a new industrially relevant domain.

PARTICULAR CHARACTERISTICS OF TODAY’S MICROELECTRONICS AND MATTERS OF HIGHLY DEPENDABLE AND SECURE CONTROL SYSTEMS DESIGN

Aim. Drawing the readers’ attention to the growing number of industrial disasters, associated damage, increasing human casualties and the connection of this phenomenon with computer-based automation systems. The authors produce arguments regarding the requirement for design technology with extended security features in view of the multifold growth of abnormal natural and industrial effects. The paper describes and analyzes distinctive features of control systems of critical application facilities and consequences of disregarding additional inspection of circuitry and software. Of special note is the growing risk caused by the introduction of unmanned technologies and their mass application in railway and automotive transportation. The paper examines the problems of control systems resilience to faults and external actions depending on the used components. Statistics of industrial disasters are provided, their connection with the indicators of control systems instability is examined. A special emphasis is put on the distinctive features of today’s microelectronic components and the effects of technological progress on the systems’ interference immunity and fault rate. Of note is the growing number of hazardous failures in systems based on 0.13-μm and lower microcontrollers. A significant attention is given to the research of the distinctive features of modern chips, their layout, particularly of the main element of a control system, i.e. the microcontroller and digital signal processor, the influence of the external effects on the chip. The matters related to CMOS layout in microprocessor-based units are considered, the dependance is shown between the rising noise influence and migration to new CMOS technology. Attention is drawn to the requirement to train an appropriate class of specialists able to work with such systems who have not only software engineering skills, but also profound knowledge of physics, fundamentals of control systems design and their stability. Results. A comparative evaluation of stability of 0.5 μm and 130 nm CMOS stability has been conducted. The resultant difference in threshold power of interference is over 4000 times. It is noted that most developers who design software for such systems are mislead by the non-availability of any public information on the fault rate of processing elements from the manufacturing companies. By taking the dependability figures as the main parameter they misjudge the safety integrity level, as instead of the fault rate parameters they erroneously use the microchip’s dependability figures provided by the manufacturer. Additionally, standard methods of improving the safety level used by developers (e.g. redundancy) often prove to be inefficient. Conclusions. Designing highly dependable and safe control systems must take into consideration the distinctive features of today’s computer components given the fact that new generations of modern microchips due to their fault rate characteristics are often unusable in highly dependable system design. It appears to be of relevance improving existing standards and developing new ways of increasing the stability and safety of systems. Also noted is the requirement of maintaining the level of education and awareness of a wide community of developers who work with control systems in transportation, energy, industrial automation, weapon systems, etc. as regards the importance of ensuring the required level of functional safety.

A synthesis of logic and bio-inspired techniques in the design of dependable systems

Much of the development of model-based design and dependability analysis in the design of dependable systems, including software intensive systems, can be attributed to the application of advances in formal logic and its application to fault forecasting and verification of systems. In parallel, work on bio-inspired technologies has shown potential for the evolutionary design of engineering systems via automated exploration of potentially large design spaces. We have not yet seen the emergence of a design paradigm that effectively combines these two techniques, schematically founded on the two pillars of formal logic and biology, from the early stages of, and throughout, the design lifecycle. Such a design paradigm would apply these techniques synergistically and systematically to enable optimal refinement of new designs which can be driven effectively by dependability requirements. The paper sketches such a model-centric paradigm for the design of dependable systems, presented in the scope of the HiP-HOPS tool and technique, that brings these technologies together to realise their combined potential benefits. The paper begins by identifying current challenges in model-based safety assessment and then overviews the use of meta-heuristics at various stages of the design lifecycle covering topics that span from allocation of dependability requirements, through dependability analysis, to multi-objective optimisation of system architectures and maintenance schedules.

A Synthesis of Logic and Biology in the Design of Dependable Systems

Abstract: The technologies of model-based design and dependability analysis in the design of dependable systems, including software intensive systems, have advanced in recent years. Much of this development can be attributed to the application of advances in formal logic and its application to fault forecasting and verification of systems. In parallel, work on bio-inspired technologies has shown potential for the evolutionary design of engineering systems via automated exploration of potentially large design spaces. We have not yet seen the emergence of a design paradigm that combines effectively and throughout the design lifecycle these two techniques which are schematically founded on the two pillars of formal logic and biology. Such a design paradigm would apply these techniques synergistically and systematically from the early stages of design to enable optimal refinement of new designs which can be driven effectively by dependability requirements. The paper sketches such a model-centric paradigm for the design of dependable systems that brings these technologies together to realise their combined potential benefits.

CTL Model Repair for Bounded and Deadlock Free Petri Nets

Abstract: In dependable system design, formal techniques such as model checking are useful to perform property analysis of system models. Computation tree logic (CTL) model repair is an approach that complements model checking for obtaining new admissible models that can represent the correct behaviour of systems. In this paper a CTL model repair method for bounded and deadlock free Petri nets (PN) is presented. First, CTL semantics is defined for dealing with PN and a set of basic PN structure transformation operations are introduced. Then an algorithm to repair PN models for fulfilling CTL formulae is presented. The method is illustrated through an example regarding a system which must hold a safety property.

Multi-objective Architecture Optimisation Modelling for Dependable Systems

The design of dependable systems must address both cost and dependability (i.e. safety, reliability, availability and maintainability) concerns. For large systems, the design space of alternatives with respect to both dependability and cost is very large and automation is essential to explore this space. The model-based approach to the development and analysis of complex dependable systems is increasingly popular and recently, the Architecture Analysis and Design Language (AADL) has emerged as a potential future standard for model-based development of dependability-critical systems. The paper tackles the problem of describing, within an AADL model, the design space of alternative designs. A new AADL property set is proposed for modelling component and system variability for cost and dependability optimisation. The proposed method is illustrated with an example of an AADL model of a safety critical embedded system.

A methodology for the improvement of dependability of self-optimizing systems

The conceivable development of communication and information technology opens up fascinating perspectives which move far beyond current standards of mechatronics: mechatronic systems having inherent partial intelligence. We call such systems self-optimizing systems. Self-optimizing systems react autonomously and flexibly on changing environmental conditions. The design of dependable self-optimizing systems is challenging. The main reasons are the involvement of different domains and the integration of partial intelligence which leads to non-deterministic behavior. In particular, it has to be ensured that the self-optimization works dependable itself. In order to accomplish this, dependability engineering methods have to be used which are suitable to the underlying development task. In such cases the developers face a great number of methods, from which they have to manually select the appropriate ones. This selection is tedious and error-prone. In this contribution we introduce a methodology for the improvement of dependability of self-optimizing systems. It consists of a method database, a guide for selection and planning of dependability engineering methods and a software tool. The methodology supports the developers by search, selection and planning of dependability engineering methods (e.g. Fault Tree Analysis), which are suitable for their particular development task.

Conditional fault diagnosis of hierarchical hypercubes

The design of large dependable multiprocessor systems requires quick and precise mechanisms for detecting the faulty nodes. The system-level fault diagnosis is the process of identifying faulty processors in a system through testing. This paper shows that the largest connected component of the survival graph contains almost all remaining vertices in the hierarchical hypercube HHC n when the number of faulty vertices is up to two or three times of the traditional connectivity. Based on this fault resiliency, we establish that the conditional diagnosability of HHC n (n=2 m +m, m≥2) under the comparison model is 3m−2, which is about three times of the traditional diagnosability.

Electro-thermal simulation in the time domain of GaN HEMT for RF switch-mode amplifier

Accurate transient temperature prediction in GaN devices is an increasingly important process in the design of dependable radio frequency systems. In this paper, a methodology for dynamic electro-thermal simulation of GaN microwave and power devices is presented, which bases on the extraction of compact thermal models by three-dimensional finite element simulation. The obtained compact thermal model is coupled to an electric compact model, where the temperature dependence of the lumped elements is described analytically. The proposed methodology is applied to the case of GaN HEMTs used in a voltage mode D-class radio frequency amplifier operated in the frequency range from 300MHz up to 3GHz.

The handbook of human-machine interaction: a human-centered design approach

Contents: Introduction: a human-centered design approach, Guy A. Boy Part I Analysis: Analysis, modeling and simulation of human operator's mental activities, Thierry Bellet Psychophysiology and performance: considerations for human-centered design, Anil K. Raj, Margery J. Doyle and Joshua D. Cameron Automation and situation awareness, Anke Popken and Josef F. Krems Human error, interaction and the development of safety-critical systems, Christopher Johnson Operating documents that change in real-time: dynamic documents and user performance support, Barbara K. Burian and Lynne Martin The authority issue in organizational automation, Guy A. Boy and Gudela Grote. Part II Design: Scenario-based design, John M. Carroll and Stephen R. Haynes Socio-cognitive issues in human-centred design for the real world, Saadi Lahlou Cognitive function analysis in the design of human and machine multi-agent systems, Guy A. Boy Authority and cooperation between humans and machines, Patrick Millot, Serge Debernard and FrA(c)dA(c)ric Vanderhaegen Formal description techniques for human-machine interfaces: model-based approaches for the design and evaluation of dependable usable interactive systems, David Navarre, Philippe Palanque, CA(c)lia Martinie, Marco A.A. Winkler and Sandra Steere Designing human-automation interaction, Amy Pritchett and Michael Feary Human-agent interaction, Jeffrey M. Bradshaw, Paul J. Feltovich and Matthew Johnson. Part III Evaluation: From usability to user experience with interactive systems, Jean-Marc Robert and Annemarie Lesage Designing and evaluating user experience, Jean-Marc Robert and Annmarie Lesage Eye tracking from a human factors perspective, Alexandre Lucas Stephane Operator fatigue: implications for human-machine interaction, Philippa Gander, Curt Graeber and Gregory Belenky Transversal perspectives on human-machine interaction: the effect of age in human-machine systems, Anabela dos Santos SimA es, Marta Pereira and Maria Panou Error on the flight deck: interfaces, organizations and culture, Don Harris and Wen-Chin Li The diminishing relevance of human-machine interaction, Erik Hollnagel Conclusion and perspectives: from automation to interaction design, Guy A. Boy Index.

An approach to optimization of fault tolerant architectures using HiP‐HOPS

AbstractNew processes for the design of dependable systems must address both cost and dependability concerns. They should also maximize the potential for automation to address the problem of increasing technological complexity and the potentially immense design spaces that need to be explored. In this paper we show a design process that integrates system modelling, automated dependability analysis and evolutionary optimization techniques to achieve the optimization of designs with respect to dependability and cost from the early stages. Computerized support is provided for difficult aspects of fault tolerant design, such as decision making on the type and location of fault detection and fault tolerant strategies. The process is supported by HiP‐HOPS, a scalable automated dependability analysis and optimization tool. The process was applied to a Pre‐collision system for vehicles at an early stage of its design. The study shows that HiP‐HOPS can overcome the limitations of earlier work based on Reliability Block Diagrams by enabling dependability analysis and optimization of architectures that may have a network topology and exhibit multiple failure modes. Copyright © 2011 John Wiley & Sons, Ltd.

The body size, age structure, and growth of Bosc's fringe-toed lizard, Acanthodactylus boskianus (Daudin, 1802)

Balance is one of the most significant and commonly used measurement instruments in wind tunnels. It is thus critical that this instrument is fault-tolerant and reliable. To help in the design of dependable balance systems, one must model and analyze the system from this perspective. We achieved this by considering the structure of a typical balance system and the types of errors it may encounter during operation. We estimated the failure rates of the components of the balance system, such as the balance structure, strain gages, and power supply, using manufacturer technical documents and fatigue tests, and identified the effect of each component on the reliability of the balance system. On this basis, we presented a Markov model for the reliability of the balance system, assuming that the data acquisition of a typical flying model should be complete in less than 15 min in order for the balance system to be considered reliable. Simulation of the proposed model with SHARPE software showed that if we reduced the time of the experiments from 30 min to 15 min, the reliability of the balance system improved by about 30%. In addition, the cost of the experiments in the wind tunnel was reduced by 20%-30%.

Editorial: Special issue on dependable peer-to-peer systems

As an emerging technology, peer-to-peer (P2P) systems attract attention worldwide, ranging from casual Internet users to venture capitalists. At the same time, innovations of P2P systems also offer many interesting avenues of research for scientific communities. As a major design pattern for future systems opposite to the traditional clientserver paradigm, research on P2P networks is extremely important and could significantly change the way of use of computer networks. However, P2P is sometimes regarded as a destructive technology rather than a constructive technology by Internet Service Providers (ISPs). “How to harness the power of P2P?” and “how to design dependable P2P systems for sustainable provision of services and integration of services?” remain fundamental problems. There are strong needs for improving the dependability of P2P systems to make them more secure and robust for service sharing and delivery. However, developing dependable systems is difficult because various types of properties can influence the dependability of system. Achieving dependability in a P2P environment is even more complicated due to the lack of centralized servers to monitor and control systems. The purpose of this special issue is to collate efforts and main achievements that contribute to scalability, availability, reliability, security and maintainability of P2P systems. This special issue focuses on fundamental issues in modelling, simulation and design of dependable P2P systems. This special issue contains six high quality papers which were selected from 17 submissions after a peer review process. Load balancing is a well-known crucial problem in P2P systems. Vinod et al. present a mechanism to optimise server-node ratio for load balancing in P2P networks. This mechanism builds upon a load-balancing scheme, called Enhanced-Reconfigurable-Gnutella-Overlay (ERGO). In this paper, the influence of various parameters such as network load, node capacity, server capacity and the corresponding effects of server-node ratio on metrics such as success of queries, balanced nodes and the cost of achieving the balance have been systematically investigated and discussed. An optimum server-node ratio is ascertained, optimum implying the maximum balance at minimum cost and high success of queries. Previous measurement studies have shown that some nodes observed in existing P2P networks tend to download a large number of files, but share few files or none at all. L. Liu (*) Department of Computer Communications, School of Engineering and Information Sciences, Middlesex University, London NW4 4BT, UK e-mail: l.liu@mdx.ac.uk