Fault Tolerant

We describe in detail a general strategy for implementing a conditional geometric phase between two spins. Combined with single-spin operations, this simple operation is a universal gate for quantum computation, in that any unitary transformation can be implemented with arbitrary precision using only single-spin operations and conditional phase shifts. Thus quantum geometrical phases can form the basis of any quantum computation. Moreover, as the induced conditional phase depends only on the geometry of the paths executed by the spins it is resilient to certain types of errors and offers the potential of a naturally fault-tolerant way of performing quantum computation.

Responsiveness, the ability to provide real time behavior eveninpresence of faults, is becoming one of the most sought after properties in distributed computing systems. We present a framework for ''High-Performance Responsive Computing''i nn etworked systems whose current implementation works on a network of fiveN eXTSTEP (Mach 2.5)-based HP workstations. Within this paper we discuss the design issues behind the ''Unstoppable Orchestra''a nd showh ow our programming framework supports synchronization and re-configuration of responsive applications.

This tutorial is about design and proof of design of reliable systems from unreliable components. It teaches the concept and techniques of fault-tolerance, at the same time building a formal theory where this property can be specified and verified. The theory eventually supports a range of useful design techniques, especially for multiple faults. We extend CCS, its bisimulation equivalence and modal logic, under the driving principle that any claim about fault-tolerance should be invariant under the removal of faults from the assumptions (faults are unpredictable); this principle rejects the reduction of fault-tolerance to "correctness under all anticipated faults". The theory is applied to the range of examples and eventually extended to include considerations of fault-tolerance and timing, under scheduling on the limited resources. This document describes the motivation and the contents of the tutorial.

Soft error tolerant design becomes more crucial due to exponential increase in the vulnerability of computer systems to soft errors. Accurate estimation of soft error rate (SER), the probability of system failure due to soft errors, is a key factor in design of cost-effective soft error resilient systems. We present a very fast and accurate approach based on enhanced static timing analysis and signal probabilities to estimate the probability of latching an incorrect value in the system bistables (timing derating). Experimental results and comparison with fault injections using timing accurate Monte-Carlo simulations show that the accuracy of our approach is within 1% while orders of magnitude faster.

A method is investigated to directly engineer the voltage swing in SiGe resonant interband tunnel diodes (RITDs). Voltage swing, defined here as the voltage difference between the peak voltage and the projected peak voltage, is independent of series resistance, and thus directly impacts the noise margin in hybrid tunnel diode memory and logic applications. The three components of the total RITD current are analyzed to describe the voltage swing. The dependence of voltage swing on -doping concentrations and post-growth annealing temperatures in SiGe RITDs grown by low-temperature molecular beam epitaxy (LT-MBE) is investigated and the experimental results are compared with a theoretical analysis. Techniques to increase the voltage swing are discussed.

Ultra large scale (ULS) systems are future software intensive systems that have billions of lines of code, composed of heterogeneous, changing, inconsistent and independent elements that are dispersed through worldwide global networks. Huge scale of this heterology in addition to distinctive characteristics like decentralization of operation, management, deployment and evolution, continuous evolving, people participation in system operation and normal failures that dominate in ULS systems face them to new challenges that today software architectures and developers have less or no concerns about them. To face these challenges there must be an appropriate infrastructure with enough or at least extendable features and capabilities to support ULS distinctive characteristics. In this paper we propose computational grid as an appropriate infrastructure for ULS systems that not only supports some of ULS characteristics itself, but also can exploit service oriented architecture, fault tolerant approaches, distributed management algorithms and many other mechanisms in order to extend its fundamental capabilities to adapt itself with ULS systems. We explain what kind of grid is appropriate for ULS and expound how it provides solutions for each of the ULS particular characteristics.

In the application domain of online information services such as online census information, health records and real-time stock quotes, there are at least two fundamental challenges: the protection of users' privacy and the assurance of service availability. We present a fault-tolerant scheme for private information retrieval (FT-PIR) that protects users' privacy and ensures service provision in the presence of malicious server failures. An error detection algorithm is introduced into this scheme to detect the corrupted results from servers. The analytical and experimental results show that the FT-PIR scheme can tolerate malicious server failures effectively and prevent any information of users from being leaked to attackers. This new scheme does not rely on any unproven cryptographic premise and the availability of tamperproof hardware. An implementation of the FT-PIR scheme on a distributed database system suggests just a modest level of performance overhead.

This paper presents a new low-loss modulation technique for the hybrid three-level four-leg converter. The total losses of the converter are reduced by about 18% on average compared to the standard three-leg neutral-point-clamped converter. Furthermore, the low-frequency voltage oscillation in the neutral point is completely cancelled, and the maximum benefit of the dc-link voltage is obtained. All these facts, together with the fault-tolerant ability due to the fourth leg, make this topology very interesting for applications such as wind generation, in which it is important to maximize efficiency and reliability. Some experimental results confirm the good performance of the proposed modulation technique.

The goal of the GUARDS project is to design and develop a generic fault-tolerant computer architecture that can be built from predefined standardised components. The architecture favours the use of commercial off-the-shelf (COTS) hardware and software components. However, the assessment and selection of COTS components is a non-trivial task as it requires balancing a myriad of requirements from end-users and the preliminary architecture design. In this paper, we present the requirements and assessment criteria for a specific COTS software component, the operating system kernel. As an interface specification constitutes a major compatibility criterion for the selection of COTS components in GUARDS, a particular emphasis is placed on operating system conformance to the POSIX 1003.1 standard. We discuss the general lessons learned from the assessment process and raise a number of questions relevant to the assessment of any COTS software component.

In this paper, we present a new deadlock-free fault-tolerant adaptive routing algorithm for the 2D mesh NoC interconnections. The main contribution of this routing algorithm is that it allows both, routing of messages in the networks incorporating the regions not necessarily rectangular, and routing to all nodes which are not completely blocked by faulty nodes. The proposed routing algorithm is based on a modified turn model and well known XY algorithm. We detail the basic principle of this routing algorithm, prove its deadlock freeness, its feasibility and efficiency through the simulation results.

One formidable difficulty in quantum communication and computation is to protect information-carrying quantum states against undesired interactions with the environment. To address this difficulty, many good quantum error-correcting codes have been derived as binary stabilizer codes. Fault-tolerant quantum computation prompted the study of nonbinary quantum codes, but the theory of such codes is not as advanced as that of binary quantum codes. This paper describes the basic theory of stabilizer codes over finite fields. The relation between stabilizer codes and general quantum codes is clarified by introducing a Galois theory for these objects. A characterization of nonbinary stabilizer codes over Fq in terms of classical codes over Fq2 is provided that generalizes the well-known notion of additive codes over F4 of the binary case. This paper also derives lower and upper bounds on the minimum distance of stabilizer codes, gives several code constructions, and derives numerous families of stabilizer codes, including quantum Hamming codes, quadratic residue codes, quantum Melas codes, quantum Bose-Chaudhuri-Hocquenghem (BCH) codes, and quantum character codes. The puncturing theory by Rains is generalized to additive codes that are not necessarily pure. Bounds on the maximal length of maximum distance separable stabilizer codes are given. A discussion of open problems concludes this paper

In this paper, a methodology for the development of fault-tolerant adders based on the radix 2 signed digit (SD) representation is presented. The use of a number representation characterized by a carry propagation confined to neighbor digits implies interesting advantages in terms of error detection, fault localization, and repair. Errors caused by faults belonging to a considered stuck-at fault set can be detected by a parity-based technique. In fact, a carry-free adder preserving the parity of the augends can be implemented allowing fault detection by using a parity checker. Regarding fault localization, the "carry-free" property of the adder ensures the confinement of the error due to a permanent fault to only few digits. The detection of the faulty digit has been obtained by using a recomputation with shifted operands method. Finally, after the fault localization, graceful degradation of the system intended as the reduction of the performances versus a correct output computation can be obtained by using two different procedures. The first one allows obtaining the correct output by recomputing the result performing two different shift operations and using the intersection of the obtained results to recover the correct output, while the second one is based on a reduced dynamic range approach, which allows us to obtain the result in only one step, but with fewer output digits.

In this research work, a survey on Wireless Sensor Networks (WSN) and their technologies, standards and applications was carried out. Wireless sensor networks consist of small nodes with sensing, computation, and wireless communications capabilities. Many routing, power management, and data dissemination protocols have been specifically designed for WSNs where energy awareness is an essential design issue. Routing protocols in WSNs might differ depending on the application and network architecture. A multidisciplinary research area such as wireless sensor networks, where close collaboration between users, application domain experts, hardware designers, and software developers is needed to implement efficient systems. The flexibility, fault tolerance, high sensing fidelity, low cost, and rapid deployment characteristics of sensor networks create many new and exciting application areas for remote sensing. In the future, this wide range of application areas will make sensor networks an integral part of our lives. However, realization of sensor networks needs to satisfy the constraints introduced by factors such as fault tolerance, scalability, cost, hardware, topology change, environment, and power consumption.

A major challenge for quantum computation in ion trap systems is scalable integration of error correction and fault tolerance. We analyze a distributed architecture with rapid high fidelity local control within nodes and entangled links between nodes alleviating long-distance transport. We demonstrate fault-tolerant operator measurements which are used for error correction and non-local gates. This scheme is readily applied to linear ion traps which cannot be scaled up beyond a few ions per individual trap but which have access to a probabilistic entanglement mechanism. A proof-of-concept system is presented which is within the reach of current experiment.

Proactive fault tolerance (FT) in high-performance computing is a concept that prevents compute node failures from impacting running parallel applications by preemptively migrating application parts away from nodes that are about to fail. This paper provides a foundation for proactive FT by defining its architecture and classifying implementation options. This paper further relates prior work to the presented architecture and classification, and discusses the challenges ahead for needed supporting technologies. *

Over the last decade, storage systems have experienced a 10fold increase between their capacity and bandwidth. This gap is predicted to grow faster with exponentially growing concurrency levels, with future exascales delivering millions of nodes and billions of threads of execution. A critical component of future file systems for high-end computing is metadata management. This extended abstract presents ZHT, a zero-hop distributed hash-table, which has been tuned for the specific requirements of high-end computing. The primary goal of ZHT is excellent availability, fault tolerance, high throughput, and low latencies. 1.

Real computer-based systems fail, and hence are often far less dependable than their owners and users need and desire. Individuals, organisations and indeed the world at large are becoming more dependent on such systems, so there has been much work on trying to gain increased understanding of the many and varied types of faults that need to be prevented or tolerated in order to reduce the probability and severity of system failures. In this paper I analyze the concept of system faults and failures, and discuss the assumptions that are often made by computing system designers regarding faults, and a number of continuing research issues related to fault tolerance.

Software architectures are becoming centric to the development of quality software systems, being the first concrete model of the software system and the base to guide the implementation of software systems. When architecting dependable systems, in addition to improving system dependability by means of construction (fault-tolerant and redundant mechanisms, for instance), it is also important to evaluate, and thereby confirm, system dependability. There are many different approaches for evaluating system dependability, and testing has been always an important one, being fault removal one of the means to achieve dependable systems. Previous work on software architecture-based testing has shown it is possible to apply conformance testing techniques to yield some confidence on the implemented system conformance to expected, architecture-level, behaviors. This work explores how regression testing can be systematically applied at the software architecture level in order to reduce the cost of retesting modified systems, and also to assess the regression testability of the evolved system. We consider assessing both ''low-level'' and ''high-level'' evolution, i.e., whether a slightly modified implementation conforms to the initial architecture, and whether the implementation continues to conform to an evolved architecture. A better understanding on how regression testing can be applied at the software architecture level will help us to assess and identify architecture with higher dependability.

In a grid environment, resources and services are distributed with dynamic and heterogeneous characteristics. Efficient service discovery is one challenging issue in a grid environment. In this paper, we propose a new distributed and hierarchical mechanism for improving fault tolerance and speed of service discovery in a grid environment. This approach has five layers that in previous works was presented in four layers. A root layer is added as coverage for the service index of under layers. For improvement fault tolerance in the proposed approach, after the requested service is found, the action of service discovery will not stop. It continues to find several instances of the requested service. These services will send to the cache of the institution layer that will be replaced immediately if necessary. Also for speed-up service discovery, nodes in same level, which have the same parent, send queries to each together directly. Performance evaluation shows that our approach achieves good efficiency, is fault tolerant and consistent. The proposed mechanism for service discovery improves the fault tolerance of services more than 28%.

To address the increasing susceptibility of commodity chip multiprocessors (CMPs) to transient faults, we propose Chiplevel Redundantly Threaded multiprocessor with Recovery (CRTR). CRTR extends the previously-proposed CRT for transient-fault detection in CMPs, and the previously-proposed SRTR for transient-fault recovery in SMT. All these schemes achieve fault tolerance by executing and comparing two copies, called leading and trailing threads, of a given application. Previous recovery schemes for SMT do not perform well on CMPs. In a CMP, the leading and trailing threads execute on different processors to achieve load balancing and reduce the probability of a fault corrupting both threads; whereas in an SMT, both threads execute on the same processor. The inter-processor communication required to compare the threads introduces latency and bandwidth problems not present in an SMT.

Wireless distributed microsensor systems will enable fault tolerant monitoring and control of a variety of applications. Due to the large number of microsensor nodes that may be deployed and the long required system lifetimes, replacing the battery is not an option. Sensor systems must utilize the minimal possible energy while operating over a wide range of operating scenarios. This paper presents an overview of the key technologies required for low-energy distributed microsensors. These include power aware computation/communication component technology, low-energy signaling and networking, system partitioning considering computation and communication trade-offs, and a power aware software infrastructure.

With the increasing complexity of multiprocessor and distributed processing systems, the need to develop efficient and accurate modeling methods is evident. Fault tolerance and degradable performance of such systems has given rise to considerable interest in models for the combined evaluation of performance and reliability [l], [2]. Markov or semi-Markov reward models can be used to evaluate the effectiveness of degradable fault-tolerant systems. Beaudry [l] proposed a simple method for computing the distribution of performability in a Markov reward process. We present two extensions of Beaudry's approach. First, we generalize the method to a semi-Markov reward process. Second, we remove the restriction requiring the association of zero reward to absorbing states only. We illustrate the use of the approach with three interesting applications.

The tremendous advances in wireless networks, mobile computing, and sensor networks, along with the rapid growth of small, portable and powerful computing devices, offers more and more opportunities for pervasive computing and communications. This topic deals with cutting-edge research in various aspects related to the theory and practice of mobile computing or wireless and mobile networking. These aspects include architectures, algorithms, networks, protocols, modeling and performance issues, data management, ...

Interest in the area of pattern recognition has been renewed recently due to emerging applications which are not only challenging but also computationally more demanding. These applications include data mining (identifying a "pattern", e.g., correlation, or an outlier in millions of multidimensional patterns), document classifications (efficiently searching text documents), organization and retrieval of multimedia database, and biometrics (personal identifications based on various physical attributes such as face and fingerprints). The three best known conventional approaches for pattern recognition are: template matching, statistical classification and syntactic or structural matching. The limitations and constraints of these conventional approaches have made researchers to look for alternate techniques based on Artificial Neural Networks. The main characteristics of neural networks are that they have the ability to learn complex nonlinear input-output relationship, use sequential training procedures, and adapt themselves to the data. In this paper, we discuss implementation and fault tolerance analysis of the most commonly used family of neural networks for pattern classification tasks i.e., the feed forward network, which includes a fully interconnected three layered [25-10-1] perceptron. The delta rule weight adjustment is implemented by taking the gradient of error function, which gives the direction in which weights have to be adjusted to get error value within the predefined threshold. Fault tolerance analysis is done for Gaussian and Uniform distribution of weights. The efficacy of neural network based pattern recognition is tested by the computer simulation results. Index terms-Pattern recognition, fault tolerance, neural network, back-propagation

This article describes a self-healing mechanism for statemachine based distributed components. Each component is composed of two layers: a healing (HL) and a service or functional layer (FL). At least, the functional layer must be implemented according to a statemachine specification. The healing layer has the capacity of monitoring the service layer of the component of witch it is a part and responsible of. In the event of a failure of the functional layer, the healing layer acts on it in terms of states. It will be shown that this mechanism, being consistent with classical faulttolerance strategies, allows the HL to act on the FL in a more precise way than employing an entire software module as a replacement unit. It will be shown also that this mechanism is suitable for hot-swapping software modules.

Network-on-Chip (NoC) is used as the communication network in many applications that use multiple cores or Processing Elements (PEs). Routers play a crucial role as connectors since a faulty router can degrade the NoC's performance and cause miscommunication between the network's components. Thus a faulty router may cause the system to fail. To avoid failure in routers in NoCs, a novel self-healing technique is proposed. Self-healing serves to recover hardware faults, and it is defined as the ability of a system to recover from its faults without any external intervention. The proposed self-healing method is to heal faulty routers and their port buffers of faults as they occur. The proposed method uses the neighboring routers of a faulty router for computation. The data packet includes three bits for routing. A neighbor's active router updates these bits according to the destination of the packet. A self-healing block is added inside each router. The proposed method also covers faulty buffers, and it uses active buffers to store the packet of a faulty one. It has been implemented and tested using VHDL and Altera Arria 10 GX FPGA. It has been found to attain improved reliability and Mean Time to Failure (MTTF) at an area overhead of 27%. It has been tested for complex NoC structure, and the results show it is practical, scalable, stable, and robust. INDEX TERMS NoC, self-healing, hardware faults, fault tolerance, neural network, FPGA architecture.

Reliable delivery of messages is an important problem that needs to be addressed in distributed systems. In this paper we present our strategy to enable reliable delivery of messages in the presence of link and node failures. This is facilitated by a specialized repository node. We then present our strategy to make this scheme even more failure resilient, by incorporating support for repository redundancy. Each repository functions autonomously. The scheme enables updates to the redundancy scheme depending on the failure resiliency requirements. If there are N available repositories, reliable delivery guarantees will be met even if N-1 repositories fail.

Artificial intelligence (AI) techniques are becoming useful as alternate approaches to conventional techniques or as components of integrated systems. They have been used to solve complicated practical problems in various areas and are becoming more and more popular nowadays. AItechniques have the following features: can learn from examples; are fault tolerant in the sense that they are able to handle noisy and incomplete data; are able to deal with non-linear problems; and once trained can perform prediction and generalization at high speed. AI-based systems are being developed and deployed worldwide in a myriad of applications, mainly because of their symbolic reasoning, flexibility and explanation capabilities. AI have been used and applied in different sectors, such as engineering, economics, medicine, military, marine, etc. They have also been applied for modeling, identification, optimization, prediction, forecasting, and control of complex systems. The main objective of this paper is to present an overview of the AI-techniques for sizing photovoltaic (PV) systems: stand-alone PVs, grid-connected PV systems, PV-wind hybrid systems, etc. Published literature presented in this paper show the potential of AI as a design tool for the optimal sizing of PV systems. Additionally, the advantage of using an AI-based sizing of PV systems is that it provides good optimization, especially in isolated areas, where the weather data are not always available. #

Peer-to-peer (P2P) overlay networks have recently become one of the hottest topics in OS research. These networks bring with them the promise of harnessing idle storage and network resources from client machines that voluntarily join the system; self-configuration and automatic load balancing; censorship resistance; and extremely good scalability due to the use of symmetric algorithms. However, the use of unreliable client machines leads to two defects of these systems that precludes their use in a number of applications: storage is inherently unreliable, and lookup algorithms have long latencies. In this paper we propose a design of a robust peer-to-peer storage service, composed not of client nodes, but server nodes that are dedicated to running the peer-to-peer application. We argue that our system overcomes the defects of peer-to-peer systems while retaining their nice properties with the exception of utilizing spare resources of client machines. Our system is capable of surviving arbitrary failures of its nodes (Byzantine faults) and we expect it to perform and scale well, even in a wide-area network.

In this paper, we present a new generation of active tactile modules (i.e., HEX-O-SKIN), which are developed in order to approach multimodal whole-body-touch sensation for humanoid robots. To better perform like humans, humanoid robots need the variety of different sensory modalities in order to interact with their environment. This calls for certain robustness and fault tolerance as well as an intelligent solution to connect the different sensory modalities to the robot. Each HEX-O-SKIN is a small hexagonal printed circuit board equipped with multiple discrete sensors for temperature, acceleration, and proximity. With these sensors, we emulate the human sense of temperature, vibration, and light touch. Off-the-shelf sensors were utilized to speed up our development cycle; however, in general, we can easily extend our design with new discrete sensors, thereby making it flexible for further exploration. A local controller on each HEX-O-SKIN preprocesses the sensor signals and actively routes data through a network of modules toward the closest PC connection. Local processing decreases the necessary network and high-level processing bandwidth, while a local analog-to-digital conversion and digital-data transfers are less sensitive to electromagnetic interference. With an active data-routing scheme, it is also possible to reroute the data around broken connections-yielding robustness throughout the global structure while minimizing wirings. To support our approach, multiple HEX-O-SKIN are embedded into a rapid-prototyped elastomer skin material and redundantly connected to neighboring modules by just four ports. The wiring complexity is shifted to each HEX-O-SKIN such that a power and data connection between two modules is reduced to four noncrossing wires. Thus, only a very simple robot-specific base frame is needed to support and wire the HEX-O-SKIN to a robot. The potential of our multimodal sensor modules is demonstrated experimentally on a robot platform.

FPGA based Fault injection and Fault tolerance techniques are used to evaluate and validate the reliability of VLSI circuits. This approach combines the efficiency of hardware based techniques and the flexibility of simulation based techniques. The system efficiency and robustness increases as the reconfiguration of FPGA is not needed for each fault experiment. Fault injection is performed using commercial VHDL simulation tools such as static and dynamic methods. Instrumented 2 bit multiplier circuits were employed to evaluate the fault injection technique. The power analysis results provided for fault free, stuck-at-0 and stuck-at-1 faults in digital circuits validate the point that faulty circuits dissipate more and hence draw more power.

Service Oriented Computing and its most famous implementation technology Web Services (WS) are becoming an important enabler of networked business models. Discovery mechanisms are a critical factor to the overall utility of Web Services. So far, discovery mechanisms based on the UDDI standard rely on many centralized and area-specific directories, which poses information stress problems such as performance bottlenecks and fault tolerance. In this context, decentralized approaches based on Peer to Peer overlay networks have been proposed by many researchers as a solution. In this paper, we propose a new structured P2P overlay network infrastructure designed for Web Services Discovery. We present theoretical analysis backed up by experimental results, showing that the proposed solution outperforms popular decentralized infrastructures for web discovery, Chord (and some of its successors), BATON (and it’s successor) and Skip-Graphs.

This paper describes two different but complementary approaches that can be used to perform SEU-like fault injection sessions in order to predict error rates of digital processors. The Code Emulated Upset (CEU) approach allows fault injection in processor memories (caches and register files), while the FPGA Autonomous Emulation approach allows fault injection in processor flip-flops. Results obtained for a case studied, the LEON processor, illustrate the complementary aspects of proposed strategies.

The emergence of cloud environments has made feasible the delivery of Internet-scale services by addressing a number of challenges such as live migration, fault tolerance and quality of service. However, current approaches do not tackle key issues related to cloud storage, which are of increasing importance given the enormous amount of data being produced in today's rich digital environment (e.g. by smart phones, social networks, sensors, user generated content). In this paper we present the architecture of a scalable and flexible cloud environment addressing the challenge of providing data-intensive storage cloud services through raising the abstraction level of storage, enabling data mobility across providers, allowing computational and content-centric access to storage and deploying new data-oriented mechanisms for QoS and security guarantees. We also demonstrate the added value and effectiveness of the proposed architecture through two real-life application scenarios from the healthcare and media domains.

Software errors are a major cause of outages and they are increasingly exploited in malicious attacks. Byzantine fault tolerance allows replicated systems to mask some software errors but it is expensive to deploy. This paper describes a replication technique, BASE, which uses abstraction to reduce the cost of Byzantine fault tolerance and to improve its ability to mask software errors. BASE reduces cost because it enables reuse of off-the-shelf service implementations. It improves availability because each replica can be repaired periodically using an abstract view of the state stored by correct replicas, and because each replica can run distinct or nondeterministic service implementations, which reduces the probability of common mode failures. We built an NFS service where each replica can run a different off-the-shelf file system implementation, and an object-oriented database where the replicas ran the same, nondeterministic implementation. These examples suggest that our technique can be used in practice-in both cases, the implementation required only a modest amount of new code, and our performance results indicate that the replicated services perform comparably to the implementations that they reuse.

Hybrid systems are at the core of most embedded and many other kinds of systems; formal methods for analysis of hybrid systems have made remarkable progress in the last decade and thus provide a strong foundation for assurance in the system core.

Universal quantum computation on decoherence-free subspaces and subsystems ͑DFSs͒ is examined with particular emphasis on using only physically relevant interactions. A necessary and sufficient condition for the existence of decoherence-free ͑noiseless͒ subsystems in the Markovian regime is derived here for the first time. A stabilizer formalism for DFSs is then developed which allows for the explicit understanding of these in their dual role as quantum error correcting codes. Conditions for the existence of Hamiltonians whose induced evolution always preserves a DFS are derived within this stabilizer formalism. Two possible collective decoherence mechanisms arising from permutation symmetries of the system-bath coupling are examined within this framework. It is shown that in both cases universal quantum computation which always preserves the DFS ͑natural fault-tolerant computation͒ can be performed using only two-body interactions. This is in marked contrast to standard error correcting codes, where all known constructions using one-or two-body interactions must leave the code space during the on-time of the fault-tolerant gates. A further consequence of our universality construction is that a single exchange Hamiltonian can be used to perform universal quantum computation on an encoded space whose asymptotic coding efficiency is unity. The exchange Hamiltonian, which is naturally present in many quantum systems, is thus asymptotically universal.

This research presents an overview to the issue of fault diagnosis in distributed systems and an evaluation study to some of the algorithms proposed in literature for performing distributed fault diagnosis. One algorithm was chosen and adopted for implementation in a simulator for investigation. A strategy for improving the performance of this algorithm and preventing deadlock was proposed in this research. A measure of the improvement in performance was also presented.