The provision of pervasive computing services is a challenging research area. This paper elaborates on the notion of Personal Smart Spaces (PSSs) that allows the delivery of pervasive services to mobile users. PSSs aim to couple the facilities offered by next generation mobile communications with the features provided by the static smart spaces to support a more ubiquitous, context-aware and personalised smart space that is able to follow the user wherever he/she goes. Moreover, PSSs provide interfaces between the user and the various services and sensors that are available via the Internet. One of the core PSS features is the management of static and dynamic context information of users. This paper elaborates on the context management approach that has been designed and implemented in order to address the advanced requirements of PSSs regarding context awareness.

Learning and maintaining ontologies play critically important roles in the Semantic Web. Ontologies provide formal and explicit semantics to available knowledge and make possible shared understanding of a domain. A great deal of knowledge in the Sema

The alignment of the context information with the service objectives facilitates the successful provisioning of context-aware services. Moreover, the fact that context may come from transient and/or unreliable sources needs consideration. False or stale context can dramatically affect the decisions and the actions taken by context-aware services. This paper presents an approach for evaluating and selecting the information to be used by context-aware services that takes into account the quality of information, and automatically adapts to any source changes and failures. The proposed quality-aware discovery of context information sources allows services to be ported easily to environments with different sets of sources.

Recently, Web services (WSs) have evolved to a quite popular research field. Nevertheless, there are obstacles that prevent the introduction of WS provision in the wide market. Among these is the inability to represent the non-functional features of WSs, i.e. their quality-of-service. Integrating QoS features in WS profiles is to the advantage of both users and providers, as it enables QoS-aware WS selection and composition addressing the user's QoS requirements and objectives, while giving WS providers a significant competitive advantage in the e-business domain, also maximizing their resources' utilization. This paper focuses on the formulation of a QoS ontology framework that is used to support QoS-aware WS provision. It is based on work carried out in the IST-Amigo Integrated Project for Ambient Intelligence (AmI) homes, which, exploiting heterogeneous technologies and infrastructures, aims to develop an open, standardized, interoperable middleware for the provision of QoS-aware AmI WSs to domestic users.

Pervasive computing systems that strive to be minimally intrusive and exhibit inherent proactiveness and dynamic adaptability to the user preferences and environment need to be context-aware. Context-awareness provides computing environments with the ability to adapt the services or content they provide, by implicitly sensing and automatically deriving the users' needs from the context that surrounds them, making services more attentive, responsive and predictive. This paper is concerned with the development of an ambient context management system that aims to maximize the automation of the complete context knowledge lifecycle via the establishment of flexible prototype context ontology.

In order to improve system's efficiency and scalability for the provision of time-critical context-aware services, we propose a set of decentralised usage-aware search mechanisms. These mechanisms exploit the locality of reference properties exhibited by context usage patterns with efficient means provided by active networks. Simulation results show that the proposed mechanisms reduce the network traffic while they maintain the good search time of flood broadcasting methods.

The virtual home environment (VHE) is an important portability concept of the 3rd generation mobile systems as it promises personalized services for roaming users across network and terminal boundaries. This paper examines the roaming issues involved in the provision of VHE services. It aims to present an overview of the service portability mechanisms proposed and tested within 3G systems, while it introduces an efficient VHE roaming approach and exposes the features of the core entities that realize it.

Code mobility and mobile agents have received a lot of attention as a paradigm based on which distributed applications can be built. There has been little work however on the mathematical modeling of component mobility. In this direction, we present and analyze three optimization models for component mobility. We are seeking the optimal allocation of components to network nodes and the optimal component routes that will minimize the network traffic that is incurred in the course of component communication. Optimal solutions are found either through a mixed binary integer programming formulation or by employing max flow computations. Complexity results are also reported.

The success of service providers in the competitive communications market of the future will be determined by the efficiency with which the services will be developed, and the quality levels at which the services will be provided. A service creation environment (SCE) should provide the service developer with solutions to these problems. In this paper we present an SCE focusing on the service creation practice, the software technologies applied for the development of the service logic, the support of standardised service architectures, through libraries of service components, and the provision of QoS handling related facilities.

Efficient spectrum utilization in third-generation cellular systems has motivated the study of the traffic adaptive channel allocation problem: given the set of channels, the cell structure and the load to be accommodated in each cell find the optimal allocation of channels to cells, subject to the reuse distance constraints. In this paper, we extend this scheme in order to include time fluctuating traffic loads. The extended problem is formulated and solved by reduction to 0-1 (linear and quadratic) programming.

We present new results in the area of reconfiguration of stateful interactive processes in the presence of faults. More precisely, we consider a set of servers/processes that have the same functionality, i.e., are able to perform the same tasks and provide the same set of services to their clients. In the case when several of them turn out to be faulty, we want to reconfigure the system so that the clients of the faulty servers/processes are served by some other, fault-free, servers of the system in a way that is transparent to all the system clients. We propose a novel method for reconfiguring in the presence of faults: compensation paths. Compensation paths are an efficient way of shifting spare resources from where they are available to where they are needed. We also present optimal and suboptimal simple reconfiguration algorithms of low polynomial time complexity O(nmlog(n/sup 2//m)) for the optimal and O(m) for the suboptimal algorithms, where n is the number of processes and m is the number of primary-backup relationships. The optimal algorithms compute the way to reconfigure the system whenever the reconfiguration is possible. The suboptimal algorithms may sometimes fail to reconfigure the system, although reconfiguration would be possible by using the optimal centralized algorithms. However, suboptimal algorithms have other competitive advantages over the centralized optimal algorithms with regard to time complexity and communication overhead.

Frequency reuse is a key property of cellular mobile communication systems. The frequency spectrum can be more effectively exploited if the allocation of the available channels to cells takes into account the traffic distribution. Dynamic channel allocation (DCA) algorithms adapt continuously to the traffic changes. In this paper both static and dynamic algorithms are proposed and studied. The optimization criterion used is the maximization of the total traffic volume, which is accommodated by the mobile system.

Important factors that will determine the success of service providers in the competitive communications market of the future are the efficiency with which the services will be developed, the "quality" of the service deployment, and the quality levels at which the services will be provided. A service creation environment (SCE) should provide the service developer with solutions to these problems. In this paper we present an SCE focusing on some essential characteristics, namely, the service creation practice underlying the operation of the SCE, and realisation in terms of existing software technologies and tools, the support of standardised service architectures.

Traditionally the performance of a distributed system or a telecommunications network is taken into account only in the last steps of its design and it is seen as a final improvement. Recent attempts to incorporate performance considerations in the mainstream design rely on the development of a functional model consisting of entities that must be optimally distributed over a network of physical nodes. The optimal allocation is environment sensitive and probable different environments must be taken into account. Functional entities that are likely to be grouped together in different environments compose the so-called network entities. In this paper the problem of optimally composing network entities is examined. Different variations of the problem have been studied.

Despite the business risks, the vigorous investments by the technology and telecommunications industries clearly demonstrate the belief that digital video and interactive television are going to happen. The complete scenario for the provision of interactive multimedia services from the residential user point of view is considered focusing on the software components. Initially, based on the DAVIC paradigm, a reference architecture configuration that is able to provide residential interactive broadband services is presented. However, besides the hardware capabilities, the system will succeed only if it is useable and worth the time and money. Therefore, a set of service requirements is introduced, focusing on three key issues: navigation, access validation and mobility. Solutions dealing with the above issues are proposed.

One of the most central issues in third generation telecommunication systems is efficient spectrum utilisation. This objective has motivated extensive research in the area of channel allocation techniques. The basic version of the channel allocation problem is the following: given a set of channels, the cell structure and the load to be accommodated in each cell, find the optimum allocation of channels to cells, subject to the restrictions concerning the distance of cells, where the same channel can be used. This problem handles a traffic load that is fixed with time. We extend this formulation so as to include time variant traffic loads and, hence, to obtain efficient spectrum utilisation. We consider two approaches to this extension, which are differentiated by the fact that the first involves a total reconfiguration of the channel allocation, while the second only a partial reconfiguration. Both approaches are theoretically formulated and solved. Finally, we present results and draw subsequent conclusions.

A fundamental notion in third generation mobile systems is efficient spectrum utilisation. A first approach of the traffic adaptive spectrum allocation problem is the following: given the set of channels, the cell structure and the load to be accommodated in each cell, find the optimal allocation of channels to cells, subject to the restrictions arising from the minimum permissible distance of cells where the same channel can be used at the same time. This problem presupposes a traffic load that is fixed with time. In this paper we extend this scheme in order to include fluctuating traffic loads. This extended scheme is theoretically formulated, and found to be a difficult problem. Finally, a set of results is presented.

The asynchronous transfer mode (ATM)-based broadband integrated services digital network (B-ISDN) dominates the research interest in telecommunications. Traffic and network modeling are the techniques used for performance evaluation and assessment of B-ISDN design alternatives and planning. The specific nature of the ATM technique requires new approaches for traffic modeling. Selecting appropriate traffic source models is an important issue, since it is closely related to the successful design and efficient performance of the networks to be built in the future. The source (multilevel, multiservice structured Markovian) model presented provides a simple way for characterizing the behavior of a telecommunication user recognizing the multilevel discipline of a telecommunication session. The traffic model is general enough to fit a variety of different services by appropriately varying the values of the parameters defining it, but also it is simple enough because it is described by a small number of parameters and the defining parameters make a good intuitive sense by being closely related to actual physical quantities.

Handover is a key feature of mobile communication networks which mainly resides in the base station subsystem (BSS). Therefore the BSS design is heavily affected by handover process and vice versa. In the first part of this paper handover related performance evaluation issues are outlined. Different types of handover are taken into account. In the second part the design methodology of the base station system architecture is discussed together with some concluding remarks.>

In future mobile telecommunication systems the stepwise user penetration rate increase will demand "proper" cell planning (towards microcells and layered cell structures) to carry the offered traffic. As the system evolves small size cells will increase the number of handovers per call affecting thus the offered QoS (e.g., call dropping probability) and the network performance (e.g., signalling load on the radio link and processing power requirements). In this paper we consider three cell layout scenarios and we estimate the user penetration rate that the system can withstand so as to meet the QoS criteria (i.e., call blocking, handover blocking and call dropping probabilities) for various handover handling schemes. To confront the apparent complexity a simulation tool has been developed.>

A queued TDM system with a general arrival process is analyzed. Each call consists of a geometrically distributed number of cells. The interarrival times between successive calls are generally distributed independent random variables. Call applications, which cannot be immediately satisfied, are queued in a finite capacity buffer, and are forwarded according to a FIFO discipline. Waiting times, queue lengths, and throughput are the performance measures considered in this paper.>

The economic evaluation of a mature ATM network is presented. The authors discuss the issues involved and present two examples. Four customer categories are taken into account, namely residential, small business, medium business, and large business customers. The aim is to introduce a simple meaningful model, which incorporates the most important factors that influence the economic viability of an asynchronous transfer mode (ATM) network, and to present the limits and limitations of the area of economic evaluation. The revenue, cost, and net income (of the network operator) are calculated for two different scenarios, that is with and without residential customers.>

Delay, queue length, and throughput are the main performance characteristics of a data transmission system with ARQ (automaticrepeat-request) error control. Various protocols which have been proposed use, as a part or as a whole, the basic selective-repeat ARQ protocol. Their performance analysis has, however, been restricted to the throughput characteristics. An easily applicable method to analyze this basic protocol has not appeared in the literature so far. In this paper, two different methods for the queue length and delay analysis are presented. The system is modeled as a discrete time queue with infinite buffer storage. Transmission errors are considered to be independent, and block arrivals may follow an arbitrary interarrival time distribution. The first method uses an exact Markov state model, on which the theory of absorbing and ergodic Markov chains is applied, and leads to a computational algorithm. The second method, which is based on a specific assumption, uses a substantially simpler stochastic model and results in equations which are easily solved by means of iterative computation. In the case of geometrically distributed interarrival times, simple analytical formulas are extracted. The results are compared to the exact ones (that is, those obtained by the first method or by extended simulation runs) and surprising agreement is observed.