We consider per-user profile replication as a mechanism for faster location lookup of mobile users in a Personal Communications Service system. We present a minimum-cost maximum-flow based algorithm to compute the set of sites at which a user profile should be replicated given known calling and user mobility patterns. We then present schemes for replication plans that gracefully adapt to changes in the calling and mobility patterns. 1 Introduction In a Personal Communications Service (PCS) system, users place and receive calls through a wireless medium. Calls may deliver voice, data, text, facsimile, or video information [JLLM94]. PCS users are located in system-defined cells, which are bounded geographical areas. When a user places a call, the PCS infrastructure must route the call to the base-station located in the same cell as the callee. The base-station then transmits the data in the call to the PCS unit through the wireless medium. We consider the problem of locating users who...

We consider per-user profile replication as a mechanism for faster location lookup of mobile users in a Personal Communications Service system. We present a minimum-cost maximum-flow based algorithm to compute the set of sites at which a user profile should be replicated given known calling and user mobility patterns. We then present schemes for replication plans that gracefully adapt to changes in the calling and mobility patterns. We show the costs and benefits of our replication algorithm against previous location lookup approaches through analysis. We also simulate our algorithm against other location lookup algorithms on a realistic model of a geographical area to evaluate critical system performance measures. A notable aspect of our simulations is that we use well-validated models of user calling and mobility patterns. 1 Introduction In a Personal Communications Service (PCS) system, users place and receive calls through a wireless medium. Calls may deliver voice, data, text, fa...

Transparent data replication has been considered a promising technique for improving system performance for a large distributed network. In this paper, a hybrid transparent replication model is presented. We address the problems of replication proxy placement in the network and data replica placement on the installed proxies given that a maximum of proxies are allowed. Both reads and writes are considered in these problems. The performance objective is to minimize the total data transfer cost. To address the placement problems, we first present the optimal solutions for a single object in a tree network without/with constraint on the number of replicas. Based on that, two schemes, namely, AGGregate Access (AGGA) and Weighted POPularity (WPOP),are proposed for the replication proxy placement problem. An optimal solution is described for the replica placement problem. The performance of the proposed placement schemes is evaluated with a set of carefully designed simulation experiments over a wide range of system parameters. The results give us several helpful intuitions in deploying transparent replication proxies in a practical system.

Traditionally, allocation of data in distributed database management systems has been determined by o�-line analysis and optimization. This technique works well for static database access patterns, but is often inadequate for frequently changing workloads. In this paper we address how to dynamically reallocate data for partionable distributed databases with changing access patterns. Rather than complicated and expensive optimization algorithms, a simple heuristic is presented and shown, via an implementation study, to improve system throughput by 30 � in a local area network based system. Based on arti�cial wide area network delays, we show that dynamic reallocation can improve system throughput by a factor of two and a half for wide area networks. We also show that individual site load must be taken into consideration when reallocating data, and provide a simple policy that incorporates load in the reallocation decision.

This paper addresses the problem of fine-grained data replication in large distributed systems, such as the Internet, so as to minimize the user access delays. With fine-grained data replication, certain data objects, as opposed to a complete site, are duplicated at multiple servers. In this paper, we abstract the distributed system as an agent-based model wherein mobile agents on behalf of their nodes continuously compete for allocation and reallocation of data objects. However, since these agents do not have a global view of the system, the optimization process becomes highly local. This localization may encourage these selfish agents to alter the output of the resource allocation mechanism in their favor by misreporting critical data such as the objects ’ popularity. This paper proposes a game theoretical resource allocation mechanism involving selfish agents. The mechanism ensures that the agents do not misreport, always follow the rules, and that a global optima is achieved. The mechanism is extensively evaluated against some wellknown algorithms, such as: greedy, branch and bound, game theoretical auctions and genetic algorithms. The experimental results reveal that the mechanism provides excellent solution quality, while maintaining fast execution time. 1.

On a Wide Area Network (WAN), fragment allocation is a major issue in distributed database design since it concerns the overall performance of distributed database systems. Here we propose a simple and comprehensive model that reflects transaction behavior in distributed databases. Based on the model and transaction information, two heuristic algorithms are developed to find a near-optimal allocation such that the total communication cost is minimized as much as possible. The results show that the fragment allocation found by the algorithms is close to being an optimal one. Some experiments were also conducted to verify that the cost formulas can truly reflect the communication cost in the real world.

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In this paper, we consider the problem of data allocation in environments of selfmotivated servers, where information servers respond to queries from users. New data items arrive frequently and have to be allocated in the distributed system. The servers have no common interests, and each server is concerned with the exact location of each of the data items. There is also no central controller. We suggest using a negotiation framework which takes into account the passage of time during the negotiation process itself. Using this negotiation mechanism, the servers have simple and stable negotiation strategies that result in efficient agreements without delays. We provide heuristics for finding the details of the strategies which depend on the specific settings of the environment and which cannot be provided to the agents in advance. We demonstrate the quality of the heuristics, using simulations. We consider situations characterized by complete, as well as incomplete, informati...

Self-certified public keys are proposed to eliminate the burden of verifying the public key before using. To alleviate reliance on system authority and strengthen the security of system, Chang et al propose a new digital signature schemes, no redundancy is needed to be embedded in the signed messages in this scheme. Moreover, Chang et al claimed that the schemes are still secure even without the trustworthy system authority, and only the specified recipient can recover the message in his authentication encryption schemes. Unfortunately, In this work, we analyze the security of Chang et al scheme and show that if the system authority is trustless, the scheme is insecure, namely, the system authority can recover the message without the private key of the recipient in Chang ’ authentication encryption schemes. Finally, we propose an improved scheme to overcome the weakness of Chang et al scheme. Povzetek: Predstavljena je tehnika digitalnega certifikata z javnim ključem. 1

This paper presents an algorithm to group correlated objects that are most likely to be requested by a client in a single session. Based on these groups, a centralized algorithm that determines the placements of objects to a cluster of web-servers is proposed to minimize latency. Due to the dynamic nature of the Internet traffic and the rapid changes in the access pattern of the World-Wide Web, we also propose a distributed algorithm where each site relies on some collected information to decide what object should be replicated at that site. The performance of the proposed algorithms is evaluated through a simulation study. Povzetek: Grupiranje objektov na spletu. 1