Web caching has become an important problem when addressing the performance issues in Web applications. The expiration time of the Web data item is useful a piece of information for performance enhancement in Web caching. In this paper, we introduce the notion of the effective reference probability that incorporates the effect of expiration time for Web caching. For a formal approach, we propose the continuous independent reference model extending the existing independent reference model. Based on this model, we define formally the effective reference probability and derive it theoretically. By simply replacing the reference probability in the existing cache replacement algorithms with the effective reference probability, we can take the effect of expiration time into account. The results of performance experiments show that the replacement algorithms using the effective reference probability always outperform existing ones. In particular, when the cache fraction is 0.05 and data update is comparatively frequent (i.e., the update frequency is more than 1/10 of the reference frequency), the performance is enhanced by more than 30% in LRU-2 and 13% in Aggarwal's method. The results show that the effective reference probability significantly enhances the performance of Web caching when the expiration time is given.

We propose a two-level hierarchical method for dependability evaluation of distributed systems with replicated programs and data files. Since Markov modeling is limited only to each component in this method, state explosion can be circumvented successfully. Simulation results show that the method can accomplish evaluation even for large systems for which Markov modeling is not feasible.

In this paper, we propose a new location tracking method for pointer forwarding with distributed HLRs schemes to decrease the location tracking cost. Similar to the distributed HLR schemes, the location information is replicated on all HLRs in our scheme. However, unlike the traditional distributed HLR schemes, we propagate the most recent location changes from an HLR to reset the forwarding pointers in other HLRs, and thus reduce the cost of location tracking for call deliveries. The signaling cost of location propagation may be too costly to be practical. Therefore, we allow some inconsistency to exist among the location data of HLRs and defer the propagation of location changes to reduce the propagation cost. Three delay propagation policies are also proposed and analyzed using simulation. The performance result shows that the delay propagation policies reduce the propagation cost and the propagation resetting process degrades the location tracking cost. Our method preserves a low total cost for call deliveries, registrations and propagations in distributed HLRs.

In this paper, we propose an effective SOI yield engineering methodology by practical usage of 2D simulations. Process design for systematic yield of Fully-Depleted SOI MOSFET requires specific consideration of floating-body effects and parasitic channel leakage currents. The influence of varied SOI layer thickness to such phenomena is also complicated and substantial. Instead of time-consuming 3D simulators, 2D simulators are used to optimize the process considering these effects in acceptable turn around time. Our methodology is more effective in future scaled-down process with decreased SOI layer thickness.

In a client-server system, when LRU or its variant buffer replacement strategy is used on both the client and the server, the cache performance on the server side is very poor mainly because of pages duplicated in both systems. This paper introduces a server buffer replacement strategy which uses a replaced page-id than a request page-id, for the primary information for its operations. The importance of the corresponding pages in the server cache is decided according to the replaced page-ids that are delivered from clients to the server, so that locations of the pages are altered. Consequently, if a client uses LRU as its buffer replacement strategy, then the server cache is seen by the client as a long virtual client LRU cache extended to the server. Since the replaced page-id is only sent to the server by piggybacking whenever a new page fetch request is sent, the operation to deliver the replaced page-id is simple and induces a minimal overhead. We show that the proposed strategy reveals good performance characteristics in diverse situations, such as single and multiple clients, as well as with various access patterns.

A lattice system in this paper is a system whose components are ordered like the elements of (m, n) matrix. A representative example of a lattice system is a connected-(r, s)-out-of-(m, n):F lattice system which is treated as a model of supervision system. It fails if and only if all components in an (r, s) sub lattice fail. We modify the lattice system so as to include a maintenance action and a restriction on the number of failed components. Then, this paper presents availability and MTBF of the repairable system, and reliability when the system stocks spare parts on hand to ensure the specified reliability level.

We present an architecture of a VOD system employing proxy servers. The proposed VOD system provides efficient and reliable VOD services and solves the problems caused by traditional VOD systems of centralized, hierarchical or distributed architecture. The proxy servers are placed between video servers and user systems. The proxy server is a small size video server that has not only caching function but also intelligence such as VCR-like video stream control or navigation of other proxy/video servers to search for a selected video program. Using a VOD system of the proposed architecture, the VOD services can be provided to more users because it reduces the workload of video servers and network traffic. We provide the performance model of the system. Service availability is also analyzed. The proposed architecture shows better performance and availability than the traditional VOD architectures.

In this paper, we show two process allocation schemes to tolerate multiple faults when the primary-backup replication method is used. The first scheme, called multiple backup scheme, is running multiple backup processes for each process to tolerate multiple faults. The second scheme, called regenerative backup scheme, is running only one backup process for each process, but re-generates backup processes for processes that do not have a backup process after a fault occurrence to keep the primary-backup process pair available. In both schemes, we propose heuristic process allocation methods for balancing loads in spite of the occurrence of faults. Then we evaluate and compare the performance of the proposed heuristic process allocation methods using simulation. Next, we analyze the reliability of two schemes based on their fault-tolerance capability. For the analysis of fault-tolerance capability, we find the degree of fault tolerance for each scheme. Then we find the reliability of each scheme using Markov chains. The comparison results of two schemes indicate that the regenerative single backup process allocation scheme is more suitable than the multiple backup allocation scheme.

Authentication protocols are necessary for the receiver of a message to ascertain its origin in a distributed environment. Since they exchange cryptographic messages at the beginning of communication, their security is an essential requirement. However, most of the protocols have suffered from several kinds of attacks. A replay attack is one kind of those attacks. Attackers could launch it easily by replaying an eavesdropped message. Moreover, there are many types of replay attacks while most of the formal methods are not capable of detecting them. [3] classified various kinds of replay attacks and proposed a taxonomy. Therefore, it is necessary to verify authentication protocols deliberately with such a taxonomy for a basis. In this paper, at first, we give a clear definition and several remarks on replay attacks. Secondly we review the taxonomy of replay attacks presented in [3], and comment on its minor mistake. Finally we examine on the basis of the taxonomy the password-based authentication protocol, K1P, which was proposed in our earlier papers for protecting weak secrets efficiently. As a result of the examination, we have found that three way mutual K1P shown in [2] was vulnerable to one of replay attacks. Therefore, we improve three way K1P on security against the replay attack. Improved three way K1P is secure against replay attacks as well as guessing attacks and therefore it may be useful for security services of various communication networks.

The optimistic consistency scheme has been established with respect to data consistency and availability in distributed systems. The nomadic data consistency model using version vectors to support data versioning for data synchronization and concurrent conflict detection is suitable for an optimistic replication system that supports large-scale wireless networks. This paper describes the architecture and its data consistency model using data versioning and its access domain control targeted for nomadic data sharing systems, such as collaborative works using database and messaging, and the data transfer optimizations of the model. We evaluate our data versioning scheme comparing with a traditional data versioning and the data transfer optimization by estimation and measurement assuming a mobile worker's job. We generate arithmetic formulas for data transfer estimation using the optimizing techniques and apply them to large-scale data sharing configurations in which collaboration groups are dynamically formed and data is exchanged in each group. The data versioning with an access domain increases flexibility in data sharing configurations, such as mobile collaboration systems and client/server type mobile systems. We confirmed that the combination of the general optimizations and the access domain configurations based on our data consistency model is applicable for large-scale mobile data sharing systems.

Recently, mobile computing has received much attention from database community. Sharing information among mobile users is one of the most challenging issues in mobile computing due to user mobility. Replication is a promising technique to this issue. However, adopting replication into mobile computing is a non-trivial task, since we are still facing other problems such as the lack in disk capacity and wireless network bandwidth used by mobile users. We have proposed a dynamic replica allocation strategy called User Majority Replica Allocation (UMRA) that is well suited to the modern architecture of mobile computing environment while avoiding such problems mentioned above. In this paper, we propose two relocation decision policies for UMRA and we provide a cost analysis for them. We also provide a cost analysis for another replica allocation strategy called Static Replica Allocation (SRA) for a comparison purpose.

Mobile databases will play an important role in mobile computing environment, to provide data storing and data retrieval functionalities which are needed by most applications. In mobile computing environment, the wireless communication poses some problems, which require us to minimize its use. Replication is a database technique that is commonly used to fulfill the requirement in minimizing network usage. In this paper, we propose two replica allocation strategies, called primary-copy tracking replica allocation (PTRA) and user majority replica allocation (UMRA), which are better suited to the mobile computing environment. Their proposals are intended to cope with cost performance issues in data replication due to user mobility in mobile computing environment. To investigate their effectiveness, we provide access cost analysis and comparison on these strategies and the static replica allocation (SRA) strategy. We show that our proposed strategies outperform the SRA strategy when user mobility (inter-cell movement) is relatively low as compared with data access rate.

In this paper, we extend the circuit partitioning algorithm which we have proposed for multi-FPGA systems and present a new algorithm in which the delay of each critical signal path is within a specified upper bound imposed on it. The core of the presented algorithm is recursive bipartitioning of a circuit. The bipartitioning procedure consists of three stages: 0) detection of critical paths; 1) bipartitioning of a set of primary inputs and outputs; and 2) bipartitioning of a set of logic-blocks. In 0), the algorithm computes the lower bounds of delays for paths with path delay constraints and detects the critical paths based on the difference between the lower and upper bound dynamically in every bipartitioning procedure. The delays of the critical paths are reduced with higher priority. In 1), the algorithm attempts to assign the primary inputs and outputs on each critical path to one chip so that the critical path does not cross between chips. Finally in 2), the algorithm not only decreases the number of crossings between chips but also assigns the logic-blocks on each critical path to one chip by exploiting a network flow technique. The algorithm has been implemented and applied to MCNC PARTITIONING 93 benchmark circuits. The experimental results demonstrate that it resolves almost all path delay constraints with maintaining the maximum number of required I/O blocks per chip small compared with conventional alogorithms.

In circuit partitioning for FPGAs, partitioned signal nets are connected using I/O blocks, through which signals are coming from or going to external pins. However, the number of I/O blocks per chip is relatively small compared with the number of logic-blocks, which realize logic functions, accommodated in the FPGA chip. Because of the I/O block limitation, the size of a circuit implemented on each FPGA chip is usually small, which leads to a serious decrease of logic-block utilization. It is required to utilize unused logic-blocks in terms of reducing the number of I/O blocks and realize circuits on given FPGA chips. In this paper, we propose an algorithm which partitions an initial circuit into multi-FPGA chips. The algorithm is based on recursive bi-partitioning of a circuit. In each bi-partitioning, it searches a partitioning position of a circuit such that each of partitioned subcircuits is accommodated in each FPGA chip with making the number of signal nets between chips as small as possible. Such bi-partitioning is achieved by computing a minimum cut repeatedly applying a network flow technique, and replicating logic-blocks appropriately. Since a set of logic-blocks assigned to each chip is computed separately, logic-blocks to be replicated are naturally determined. This means that the algorithm makes good use of unused logic-blocks from the viewpoint of reducing the number of signal nets between chips, i.e. the number of required I/O blocks. The algorithm has been implemented and applied to MCNC PARTITIONING 93 benchmark circuits. The experimental results demonstrate that it decreases the maximum number of I/O blocks per chip by a maximum of 49% compared with conventional algorithms.

In this paper we propose a co-design method for control systems using combination of models. By co-design," we mean a cooperative design method in which the behavior of the entire system is simulated as a single model while parameters of the system are being optimized. Our co-design method enables the various subsystems in the system, which have been designed independently as tasks assigned to different designers in the traditional design method, to be designed simultaneously in a unified cooperative way from the system-wide perspective of a system designer. Our proposed method combines models of controlling and controlled subsystems into a single model for the behavior of the entire control system. After the optimum control conditions are determined through simulation of the combined models, based on the corresponding algorithms and parameters, ASIC design proceeds quickly with accurate verification using iterative replacements of the behavior model by the electronic circuit model. To evaluate the proposed method, we implemented a design environment. We then applied our method to the design of ASICs in three test cases (in a control system and in audio-visual systems) to investigate its effectiveness. This paper introduces the concepts of the proposed co-design method, the design environment and the experimental results, and points out the new issues for system design.

This letter deals with the reliability function in the case of periodic preventive replacement of items in order to increase MTBF, that is, two replacement policies; strictly periodic replacement (SPR) and randomly periodic replacement (RPR). We stress on simple introduction of the reliability theory under preventive replacement policies using the Laplace transform and obtain the theoretical results of SPR and RPR. Then these results are applied to the Weibull distribution and finally in order to show useful information of preventive replacement, the numerical results of SPR are provided.

In this paper we present and analyze multi-level quorum schemes for maintaining the consistency of replicated data in the presence of concurrency and failures in a large distributed environment. The multi-level quorum method operates on a logical hierarchy of the nodes in the network and applies well known flat voting algorithms for replicated data concurrency control in a layered fashion. We show how the number of hierarchy levels, the number of logical entities per level and the voting algorithms used on each level affect the costs and the degree of availability associated with a wide range of multi-level quorum schemes. The results of the analysis are used to provide guidelines for designing the most suitable multi-level quorum strategy for a given application scenario. Comparative performance measurements in a simulated network are presented to illustrate the properties of multi-level approaches when some of the assumptions of the analytical investigation do not hold.