@book {4065, title = {Discrete Mathematics: A) The Mathematics of Computer Science, B) Problems and Solutions}, year = {2008}, author = {Christos Bouras and Lefteris Kirousis and Paul Spirakis and George Voutsadakis} } @article {3048, title = {Competitive Video on Demand Schedulers for Popular Movies}, journal = {Discrete Applied Mathematics, Special Issue Algorithmic Aspects of Communication, Elsevier Science, Vol. 129, Issue 1}, year = {2003}, pages = {PP. 49 - 61}, abstract = {

In this paper we investigate the online video on demand problem, namely having to accept or reject a request for a movie without knowing the future requests. We present online movie-scheduling schemes that implement the principles of refusal by choice and delayed noti- 4cation. A novel way to schedule movies that exploits the knowledge of the distribution of the preference of requests for movies, is shown to have a competitive ratio that outperforms all the previously knownschemes inpractical situations. Infact, our scheduler has a competitive ratio bounded above by a constant, independent of the number of the users, channels, or movies, in the case that a large fraction of the requests tends to concentrate in a small number of movies. We extend our approach by presenting an {\textquotedblleft}adaptive{\textquotedblright} randomized scheduler which initially is not aware of the movie popularities but it adapts to it, and achieves the same asymptotic competitive ratio.

}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and G Pantziou} } @article {3053, title = {An Architecture for Interactive Distributed Multimedia Information Services}, journal = {Telecommunication Information Management Journal, Volume 2, Issue 4}, year = {2002}, pages = { 3 - 10}, abstract = {In this work, we present an architecture for interactive distributed multimedia information services. The service consists of three basic parts, the clients, the access servers, and the content servers. Clients manage the presentation of the requested multimedia documents, while also ensure the synchronization among various involved media. Special policies for the synchronized presentation of multimedia documents and user interactions management are discussed. Access servers authorize the users for legitimate use of the service, while also provide various searching capabilities Content servers manage the transportation of multimedia documents over a broadband network and the user interactions. The proposed architecture preserves scalability, flexibility and modularity due to the distributed nature of the architecture, while also results in good performance due to the dynamic allocation of resources.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and D Moiras and V Ouzounis} } @article {3054, title = {STEPS: Supporting Traditional Education ProcedureS-A TCP/IP Multimedia Networks-Based Model}, journal = {Multimedia Tools And Applications, Kluwer Academic Publishers, Vol. 16, No 3}, year = {2002}, month = {March}, pages = { 251 - 276}, abstract = {This paper describes an integrated model for the realization of an Open and Distance Learning (ODL) environment supporting traditional learning procedures, through collaborative learning and active participation of the involved group of students. Internet technologies are used as a vehicle for distribution of interactive multimedia at the user{\textquoteright}s desktop. We also present off-the-shelf solutions for the architecture and implementation of an Educational Network necessary to support the ODL environment. An Educational Entity (specially shaped part of a legal educational authority) is also prescribed as the central point for the Educational Network, being the content provider and the learning procedure evaluator. It is mainly intended to be used in a higher education setting, aspiring to be an efficient solution for the common educational needs of a group of students and teachers or researchers.}, author = {Christos Bouras and Paul Spirakis and P Lampsas} } @conference {2797, title = {An efficient algorithm to improve handoff in mobility for IP v6 protocol}, booktitle = {8th International Conference on Software, Telecommunications and Computer Networks-SoftCOM 2000, Split, Rijeka, Dubrovnik(Croatia), Trieste, Venice(Italy), Vol I}, year = {2000}, month = {11 - 14 October}, pages = { 505 - 511}, abstract = {This paper describes an extension of the mobility support for the IPv6 protocol, which is currently an Internet Draft. We argue that by processing geographical data information a mobile node can efficiently switch between different LANs without disturbance to ongoing data sessions. Furthermore the number of location update messages diminishes as the mobile node transmits them only when switching to a new LAN. This extension requires that each router of the foreign networks the mobile node visits, advertises the GPS coordinates of x points that describe its region. It also requires that the mobile node is capable of receiving GPS signals. Once having collected this information the mobile node can safely figure whether it will continue to be served by the same LAN or it has to switch to a different one because soon it will be out of the old LAN{\textquoteright}s range. Furthermore using this information the mobile node can also decide to which LAN it is preferable to switch. Lower-layer information, when it is available, such as signal strength or signal quality can be used as auxiliary indications for the switch decision.}, author = {Christos Bouras and Paul Spirakis and Y Siahos} } @book {3151, title = {Introduction to graphs: Theory, problems and solutions}, series = {Gutenberg}, year = {1999}, author = {Christos Bouras and Lefteris Kirousis and Paul Spirakis and G Stamatiou} } @article {3062, title = {On-Demand Hypermedia/Mutimedia Service using Pre-Orcestrated Scenarios over the Internet}, journal = {Networking and Information Systems Journal, Hermes Science Publishing, Volume 2, No. 5-6}, year = {1999}, pages = { 741 - 762}, abstract = {In this paper we present a unified approach for delivering hypermedia/multimedia objects over broadband networks. The described service consists of several multimedia servers and a set of functions that intend to present to the end user interactive information in real-time. The hypermedia documents are structured using a hypermedia markup language that keeps information of the spatiotemporal relationships among document{\textquoteright}s media components. In order to deal with the variant network behavior, buffering manipulation mechanisms and grading of the transmitted media quality techniques are proposed to smooth presentation and synchronization anomalies.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and D Moiras and V Ouzounis and A Tatakis} } @article {3064, title = {Analytical Performance Models for Multistage Interconnection Networks with finite, infinite and zero length buffers}, journal = {PERFORMANCE EVALUATION JOURNAL, Elsevier Science, Vol. 34}, year = {1998}, pages = { 169 - 182}, abstract = {Multistage Interconnection Networks (MINs) with crossbar switches have been used to interconnect processors and memory modules in parallel multiprocessor systems. They also play an increasingly important role in the development of ATM networks. In this paper we analyze the general case of MINs, made of kxk switches with finite, infinite or zero length buffers (unbuffered). The exact solution of the steady state distribution of the first stage is derived for all cases. We use this to get an approximation for the steady state distributions in the second stage and beyond. In the case of unbuffered switches we reach the known exact solution for all the stages of the MIN. Our results are validated by extensive simulations.}, author = {Christos Bouras and Paul Spirakis and Vasilis Triantafillou and J Garofalakis} } @conference {2834, title = {On Demand Delivery of Multimedia Documents using Distributed Objects}, booktitle = {2nd International Conference on Parallel and Distributed Computing Networks-PDCN 98, Brisbane, Australia}, year = {1998}, month = {14 - 16 December}, pages = { 357 - 360}, abstract = {In this paper, we propose an architecture for the ondemand delivery of multimedia documents over broadband networks using distributed objects. The basic components of our model are the network agents and the presentation agents. Our model preserves flexibility, transparency and modularity and value added opportunity due to the usage of distributed objects.}, author = {Christos Bouras and Paul Spirakis and V Ouzounis} } @conference {2827, title = {Implementing spatio-temporal relations for hypermedia presentations using an HTML-like language}, booktitle = {Workshop on Internet Programming Languages(in conjunction with 1998 IEEE International Conference on Programming Languages), Chicago, Illinois, USA}, year = {1998}, month = {13 May}, abstract = {Hypermedia is currently the trend in delivering information composed of various forms of media. Up to now most systems lack the ability to present the user with hypermedia information containing various media that have specific spatio-temporal relations. In this paper we propose and describe a model that can describe such kinds of hypermedia presentations. The core of the model is an HTML-like language that supports tags for the description of timing relations between the information objects that compose a hypermedia presentation.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and D Moiras and V Ouzounis and A Tatakis} } @inbook {3125, title = {MRP II-based Production Management Using Intelligent Decision Making}, booktitle = {Advanced Models and Methods for Production Planning, Editors. A. Drexl, A. Kimms}, year = {1998}, pages = { 379 - 412}, publisher = {Springer Verlag}, organization = {Springer Verlag}, abstract = {

This paper presents an extended MRP II-based production management system (PMS), which improves the traditional MRP II paradigm by attaching an intelligent decision supporting system (IDSS) to it. The IDSS is attached to the lowest level of the PMS, namely the production activity control (PAC) subsystem, and includes a simulator, that imitates real system behaviour, a knowledge-based component, that imitates expert reasoning, and a real-time database manager, that acts as the data pool and the communication gate between them. The IDSS is capable of performing off-line and on-line rescheduling, thus resulting in more realistic short-term plans. Analysis of the related case problem and implementation of the system are also discussed.

}, author = {D Sofotassios and Paul Spirakis and Vasilis Triantafillou and Ioannis Hatzilygeroudis and N Dendris and A Tsakalidis} } @conference {2825, title = {Multimedia Architecture offering Open Distance Learning Services over Internet}, booktitle = {5th IEEE International Conference on Telecommunications - ICT 98, Chalkidiki, Greece, Vol. 4}, year = {1998}, month = {22 - 25 June}, pages = { 145 - 149}, abstract = {The increase in the use of the Internet not only as a repository of resources for learning but also as a means for the delivery of courses and specially prepared teaching material is a particularly significant innovation in the range of education. The main concept here is a Virtual class, which is based on the principles of CSCL (Computer Supported Collaborative Learning) systems. This paper describes an integrated, WWW based platform for the development of web enabled environments over TCP/IP networks, such as Internet. This platform is designated through a Teletraining and Collaborative Learning Application. The proposed platform aspires to be an efficient solution for a group of students and teachers that need a shared workspace, supporting remote interactive lectures, flexible exchange of teaching material and various communication utilities, with focus on transparent and secure interaction.}, author = {Christos Bouras and Paul Spirakis and P Lampsas and P Zarafidis and A Bazaios and A Zoura} } @article {3065, title = {Results and experience from the application of a common methodology for Users Requirements Specification in Distance Education using Telematics}, journal = {Journal of Technology and Teacher Education -JTATE, AACE, Vol. 6, Issue 1}, year = {1998}, pages = { 61 - 74}, abstract = {The cooperation of the Information and Communications Technologies (ICT) and the Educational communities, is recognised as a necessity that must result in the adaptation and tuning of the education processes in terms of pedagogical methodologies, school infrastructure, human resources development and organisational restructuring, in order to efficiently absorb and utilize technological developments. The ICT community has to realize the significance of the users{\textquoteright} needs, the difficulties of the intervention to an old and traditional process. And the need to view their technology applications not only as a technical exercise. The initial phases of the TRENDS project, which are described in this paper, resulted in an agreed common methodology for carrying out the necessary investigation of the user needs in secondary education. This methodology, which was followed more or less in all participating countries, produced answers to questions concerning the role of the educators and their basic training, their familiarity with technology, education policies from a trans-European perspective, the content, forms and aims of further training, the distance training of teachers in the use of new technologies, the motives, incentives and organisational schemes for enhancing re-education, the needs of the educational environment and technology-based training, the needs of the educators in practical didactic methods, etc.}, author = {Christos Bouras and Paul Spirakis and P Lampsas and G Tsakarissianos and J Garofalakis and K Antonis and N Kastis and S Kontogiannis} } @conference {2833, title = {Training Centres: An Architecture for the Realisation of Open and Distance Learning Environments using Telematics}, booktitle = {9th International Conference Society for Information Technology and Teacher Education-SITE 98, Washington, DC, USA, Vol. 1}, year = {1998}, month = {10 - 14 March}, pages = { 118 - 120}, abstract = {In this paper, we are describing an architecture for the realisation of Open and Distance Learning (ODL) Environments using Telematics. The approach of using Training Centres as dedicated education services providers, located at the premises of an educational authority is discussed. These Training Centres may be Points of Presence of an international educational network (as is the case in the TRENDS Project in the context of which Training Centres were studied). Crucial aspects of the feasibility study, aiming to investigate the operational and organisational conditions of the Centres, that must precede the implementation of a Training Centre, are cited.}, author = {Christos Bouras and Paul Spirakis and P Lampsas} } @conference {2847, title = {Competitive Video on Demand Schedulers for Popular Movies}, booktitle = {Workshop on Algorithmic Aspects of Communications (ICALP satellite workshop), Bologna, Italy}, year = {1997}, month = {11 - 12 July}, abstract = {

In this paper we investigate the online video on demand problem, namely having to accept or reject a request for a movie without knowing the future requests. We present online movie-scheduling schemes that implement the principles of refusal by choice and delayed noti cation. A novel way to schedule movies that exploits the knowledge of the distribution of the preference of requests for movies, is shown to have a competitive ratio that outperforms all the previously known schemes in practical situations. In fact, our scheduler has a competitive ratio bounded above by a constant, independent of the number of the users, channels, or movies, in the case that a large fraction of the requests tends to concentrate in a small number of movies. We extend our approach by presenting an \adaptive" randomized scheduler which initially is not aware of the movie popularities but it adapts to it, and achieves a similar asymptotic competitive ratio.

}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and G Pantziou} } @conference {2837, title = {Conclusions on the Analysis of User Requirements concerning the use of ICT in the Didactic Process}, booktitle = {ED-MEDIA/ED-TELECOM 97, Calgary, Canada}, year = {1997}, month = {14 - 19 June}, pages = { 13 - 18}, abstract = {The co-operation of the Information and Communications Technologies (ICT) and the Educational Society, is deemed necessary for the adaptation and tuning of the didactic process in terms of pedagogical methodologies, school infrastructure, human resources utilisation and organisational restructuring. The use of ICT in the classroom should be viewed as a highly influencing tool that will support the educators{\textquoteright} effort for interaction with their students, rather than as a technical exercise. The initial phases of the TRENDS project, which are described in this paper, resulted in a widely accepted common methodology, that complies with the international standards for determining the User Requirements in the Secondary Education. The major results emerged from this survey are presented here, as well as some common axes that deal with the application of ICT in the didactic process.}, author = {Christos Bouras and Paul Spirakis and P Lampsas and G Tsakarissianos and J Garofalakis and K Antonis and N Kastis and S Kontogiannis} } @conference {2841, title = {The Design and Development of Advanced Network Services over the Patras University Campus Network}, booktitle = {5th International Conference on Computers in Education-ICCE 97, Sarawak, Malaysia}, year = {1997}, month = {2 - 6 December}, pages = { 420 - 427}, abstract = {This paper presents some results of the initial phases of a big internal project running at the University of Patras. In this project - which is co-funded by the European Union - our team is responsible for the development of the Advanced Network Services over the Campus Network, which is being established during the project. In the following chapters we first present the general perspective and the architecture of the integrated system which offers the set of advanced services to the University community. We then describe in detail the different services (WWW and Intranet, On-line teletraining, Computer supported cooperative work, and Off-line teletraining). A key characteristic of the system is the integration of existing commercial tools, with tools that we are developing for the specific needs of the foreseen users. It is evident that this system can be considered as a pilot project for large and demanding user communities, as is the case with the University of Patras.}, author = {Christos Bouras and Paul Spirakis and Vasilis Triantafillou and J Garofalakis and J Tsaknakis and J Tzimas and A Tsakalidis and I Antoniou and D Karageorgopoylos and B Basiliadis and P Kappos} } @conference {2842, title = {Enabling Distance Education over the World Wide Web}, booktitle = {World Conference of the WWW, Internet and Intranet- WebNet 97,Torodo,Canada}, year = {1997}, month = {31 Octiber - 5 November}, abstract = {The recent advancements in the field of the information and communication technology resulted in a revolution comparable to the industrial revolution. The basis of this revolution is the information and the value it has as the pure expression of human knowledge. The technological advancements offer us the ability to process, store, retrieve and transmit information in multiple formats (text, sound, image, video) independently of time, volume and distance. In this paper we present the technology that is required and an architecture for the realization of distance education over the World Wide Web. We also address some basic aspects of the users{\textquoteright} needs that every software tool for distance education should meet.}, author = {Christos Bouras and Paul Spirakis and P Lampsas and I Antoniou} } @conference {2845, title = {A General Performance Model for Multistage Interconnection Networks}, booktitle = {Euro-Par {\textquoteright}97, Passau, Germany}, year = {1997}, month = {25 - 29 August}, pages = { 993 - 1000}, abstract = {Multistage Interconnection Networks (MINs) with crossbar switches are used to interconnect processors and memory modules in parallel multiprocessor systems. In this paper we analyze the general case of MINs, made of kk switches with nite, in nite or zero length bu ers (unbu ered). The exact solution of the steady state distribution of the rst stage is derived for all cases. We use this to get an approximation for the steady state distributions in the second stage and beyond. In the case of unbu ered switches we reach the known exact solution for all the stages of the MIN. Our results are validated by extensive simulations. Keywords: analytical models, queueing theory models, evaluation.}, author = {Christos Bouras and Paul Spirakis and Vasilis Triantafillou and J Garofalakis} } @conference {2840, title = {Hermes Service: Distributed Hypermedia Educational Services on Demand}, booktitle = {ED-MEDIA/ED-TELECOM 97, Calgary, Canada}, year = {1997}, month = {14 - 19 Jne}, pages = { 19 - 24}, abstract = {In this paper we present a distributed HypERMEdia SERVICE (HERMES SERVICE) which supports integrated capabilities for distance education and learning purposes. Hermes service consists of hermes servers and hermes browsers. The users are connected with the service using hermes browsers and retrieve hypermedia lessons organised in thematical units. The lessons are stored in various distributed Hermes servers which retrieve the requested lesson and transmit it to the corresponding browser on on-demand basis. The requested lesson is presented to the user according to a well defined presentation scenario. The media objects are rendered in a synchronized manner according to the spatio-temporal relationships existing among them. Finally, special monitoring mechanism controls the state of the network and adjusts the presentation process so as to improve presentation quality.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and D Moiras and V Ouzounis and I Antoniou and D Karageorgopoylos} } @conference {2844, title = {An HTML like language supporting time-depended transmission of hypermedia}, booktitle = {The Eighth International ACM Hypertext Conference-Hypertext 97, Southampton, UK}, year = {1997}, month = {6 - 11 April}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and A Tatakis} } @conference {2846, title = {HY-MOST: HYpermedia MOdel for Synchronised presenTations}, booktitle = {3rd Workshop on Open Hypermedia Systems, Southampton, UK}, year = {1997}, month = {6 - 11 April}, pages = { 18 - 23}, abstract = {A brief abstract. In this paper we present a new model for hypermedia presentations. This model is based on an HTML-like language which provides the necessary modules for the intermedia synchronisation of the inline media that comprise the hypermedia document. The model also enables a distributed approach in the storage of the hypermedia documents. Additionally, it is capable of incorporating the necessary protocols needed for the real-time delivery of continuous media such as audio and video.}, author = {Christos Bouras and Paul Spirakis and D Moiras and V Ouzounis and I Antoniou} } @conference {2838, title = {In-Service Training through ODL Environments:From User Needs To Functional Specifications}, booktitle = {8th International Conference Society for Information Technology and Teacher Education-SITE 97, Orlando, USA}, year = {1997}, month = {1 - 5 April}, pages = { 519 - 522}, author = {Christos Bouras and Paul Spirakis and P Lampsas} } @conference {2836, title = {ODYSSEAS: The Greek Educational Network for Secondary Education}, booktitle = {EUROAMERITEL{\textquoteright}97 Conference, San Jose, Costa Rica}, year = {1997}, month = {5 - 7 November}, abstract = {The purpose of this paper is the description of the project ODYSSEAS. The aim of the project is the design, development, and pilot operation of an integrated network of schools in secondary education in the Greek areas of Achaia, Thrace and Aegean Islands. A special network infrastructure will be created and network software and services (containing a WWW server, network software for teaching the Greek language, applications of administrative support and so on) will be developed. Training of the teachers and pupils and evaluation of the project results will be the final project actions.}, author = {Christos Bouras and Paul Spirakis and A Hatzilakos and M Koutlis and N Panopoulos} } @article {3067, title = {Pure greedy Hot-Potato routing in the 2-d mesh with random destinations}, journal = {Parallel Processing Letters Vol. 7 No. 3}, year = {1997}, pages = { 249 - 258}, abstract = {We analyze here a pure greedy hot-potato routing strategy on a two-dimensional mesh of n^2 nodes. We specifically study the case of n^ 2 packets, originating one per node, to be delivered at random uniform destinations. Each packet attempts to follow the shortest path leading first to the destination row/column (whichever is closest) and then to the actual destination node. A de ection policy is adopted to solve con icts. We prove that all packets are delivered to the destinations in average time O(nlogn). The average is taken over all possible destination functions. No average case analysis of pure greedy hot-potato routing was known up to now.}, author = {Paul Spirakis and Vasilis Triantafillou} } @conference {2843, title = {Superhighways for Open and Distance Learning}, booktitle = {1997 EDEN Conference, Budapest, Hungary}, year = {1997}, month = {23 - 25 June}, pages = { 32 - 36}, abstract = {An educational information network, is a multimedia network with a centralized architecture for the provision of educational services. The educational services comprise basic network applications (such as multimedia e-mail, fora for debates on educational subjects, access to already existing information), video distribution of multimedia lessons and a custom developed teletraining tool for the conduction of lessons over a broadband networking infrastructure. The suitability of the widely available broadband networking technologies (ATM, ISDN), as opposed to the alternative of using already existing networking infrastructure (Internet, CATV), for the implementation of educational networks for the realization of an Open and Distance Learning Environment will be examined. More specifically the types of educational services that can be best hosted by each technology and the costs as opposed to the maturity for the deployment of each networking technology will be cited. Finally, a proposal for an implementation scheme of an educational information network, concerning the appropriate (and cost effective) networking infrastructure that can best accommodate the aforementioned educational services will be elaborated.}, author = {Christos Bouras and Paul Spirakis and P Lampsas} } @article {3066, title = {Using Multimedia/Hypermedia Tools over Networks for Distance Education and Training}, journal = {Journal of Educational Technology Review - ETR, AACE, No. 7}, year = {1997}, month = {Summer}, pages = { 20 - 26}, abstract = {The rapid developments in the Information Technology sector as well as the deployment of faster networks brings us to the era of the New Information Technology (NIT). The citizen of the near future must be able to use them else he will be seriously handicapped in his everyday life. Thus NIT becomes part of the educational process while on the other hand offers the tools and the infrastructure for the establishment of the distance education process. The educators are one of the first professions that must learn to use NIT and its tools. Multimedia/hypermedia tools offer solutions to the problems posed by the distance education process. They can be user-friendly and enticing and they can cover most, if not all, the needs of their users. A lot of prototype systems (tools) for distance education have been developed, each offering a number of functions that aid the process of distance education. The rapid growth of World Wide Web is adding another factor that can lead to the wider deployment of distance education systems. Although the value of distance education is unquestionable there is still a lot of work to be done before a wide spread implementation platform emerges.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and P Lampsas and A Tatakis and S Kontogiannis and D Fotakis} } @conference {2850, title = {FINE:Implementation of a Distributed Telematics Environment for Clinical Engineers}, booktitle = {1996 Concurrent Engineering Europe, Cambridge, UK}, year = {1996}, month = {10 -12 April}, pages = { 56 - 60}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and A Tatakis and D Fotakis and P Balaouras and L Chadelis and N Palikarakis and R Sandalatzopoulos} } @conference {2852, title = {An Interactive Cooperative Teleworking Environment-Tηλε{\textmu}άθεια *}, booktitle = {ED-TELECOM 96, Boston, USA}, year = {1996}, month = {17 - 22 June}, pages = { 37 - 42}, abstract = {

In this paper we present the design and the implementation of an interactive environment for cooperative teleworking. Telemathea offers tools that support the communication and the teleworking between tutor and trainee, who may reside in remote geographical regions. This environment offers audio communication over the network as well as support for joint editing of text documents and still images. It supports different audio coding techniques so as to adjust to the available bandwidth, and makes use of the RTP protocol in order to facilitate the exchange of audio and, in the future, video packets.

}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and P Lampsas and A Tatakis and S Kontogiannis and D Fotakis and K Kyriakou} } @conference {2848, title = {On-Demand Hypermedia/Multimedia Service over Broadband Networks}, booktitle = {5th IEEE International Symposium On High Performance Distributed Computing-HPDC-5, Syracuse, New York, USA}, year = {1996}, month = {6 - 9 August}, pages = { 224 - 231}, abstract = {In this paper we present a uni ed approach for de- livering hypermedia/multimedia objects over broadband networks. Documents are stored in various multimedia servers, while the inline data may reside in their own media servers, attached to the multimedia servers. The described service consists of several multimedia servers and a set of functions that intend to present to the end user interactive information in real-time. Users interact with the service requesting multimedia docu- ments on demand. Various media streams are trans- mitted over di erent parallel connections according to their transmission requirements. The hypermedia doc- uments are structured using a hypermedia markup lan- guage that keeps information of the spatiotemporal re- lationships among document{\textquoteright}s media components. In order to deal with the variant network behavior, bu ering manipulation mechanisms and grading of the transmitted media quality techniques are proposed to smooth presentation and synchronization anomalies.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and D Moiras and V Ouzounis and A Tatakis} } @article {3068, title = {Performance Modeling of Distributed Timestamp Ordering: Perfect and Imperfect Clocks}, journal = {PERFORMANCE EVALUATION JOURNAL, Elsevier Science, Vol. 25, No. 2}, year = {1996}, month = {April}, pages = { 105 - 130}, abstract = {This work presents a model of a distributed database system which provides the framework to study the performance of timestamp ordering concurrency control. Locking and timestamping are two popular approaches to concurrency control in database systems. Timestamp-based algorithms have been proposed to protect distributed databases from inconsistencies during concurrent access. In these algorithms, transactions may reach a particular site in different order than their timestamps, due to unexpected network delays. This causes conflicts which the distributed concurrency control mechanism has to cope with. We exhibit an analytical solution, which has been tested with extensive simulation. The accuracy seems to be very high. We assume perfect and also imperfect clocks for synchronization and quantify the way in which local clock inaccuracies affect the phenomenon of transaction conflicts. In particular, we derive a lot of interesting performance measures such as probability of abort, mean waiting time, throughput, mean queue length and others.}, author = {Christos Bouras and Paul Spirakis} } @conference {2849, title = {Randomized adaptive video on demand}, booktitle = {15th ACM-PODC, Philadelphia PA, USA}, year = {1996}, pages = {179}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and G Pantziou} } @conference {2851, title = {TRENDS: Training Educators through Networks and Distributed Systems}, booktitle = {EDEN Conference 1996, Futuroscope Poitiers, France}, year = {1996}, month = {8 - 10 July}, pages = { 195 - 199}, abstract = {The TRENDS project aims at the in-service, distance training of 2,400 school teachers in Secondary Education, and the use of Information Technology and Telematics in the learning process, by six countries (Greece, Italy, Spain, Portugal, France, and United Kingdom). The training process will be implemented by flexible and distance learning methods, through the development and use of an in-service school-based training system, which will be based on multimedia telematics and existing mature network technologies, and the establishment and operation of a European Teacher{\textquoteright}s Training Network, consisted of six interconnected National Sites (Training Centre, schools and teachers per country). Each Training Centre in the Network will act as service provider to the schools and the teachers. The major aspect of the project is to provide the educator and their trainers, with various services including : e-mail contact with other educators and trainers, access to multimedia information, forums for the discussion and debate on educational projects, access to curriculum-related information for the educators, and multimedia teletraining for the conduction of lessons over the network. Due to the nature of the planned services and applications, the network infrastructure that will be used for the realization of the TRENDS, must offer high speed, multicasting, trans-European interoperability and certified quality of services.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and G Tsakarissianos and N Kastis} } @conference {2853, title = {HIPPOCRATES: A multimedia tool for distance education}, booktitle = {ED-MEDIA 95, Gratz, Austria}, year = {1995}, month = {17 - 21 June}, pages = { 103 - 108}, abstract = {In this paper we present a software tool for the realisation of the distance education service, that is the conduction of classes through the use of computers. The classes are directed towards personnel in various areas and aim at the retraining and update of the personnel in subjects of special interest in their line of work. The conduction of classes takes place, from and towards, remote regions with the aid of a computer network. Hippocrates is a flexible and easy-to-use application, which permits students, who sit in front of remotely situated workstations, to attend classes or seminars given by a teacher or expert, who can also be located in some remote geographical area. Hippocrates can be thought of as a distributed transparency projector. The teacher can project transparencies on the screens of the students{\textquoteright} workstations and can emphasize or indicate regions of interest on them by using the graphical tools offered by Hippocrates. Immediate results of the use of Hippocrates are the fast updating of the personnel in remote areas and the upgrading of the quality of the services they offer.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and P Lampsas and A Tatakis and D Fotakis and G Papoutsopoulos} } @conference {2854, title = {Multilingual Implementations of OSI Applications}, booktitle = {10th International Symposium on Computers and Information Sciences-ISCIS X, Izmir, Turkey}, year = {1995}, month = {30 October - 1 November}, pages = { 367 - 374}, abstract = {In this work the problem of supporting multilingual environments over OSI applications is addressed. Initially a solution based on the idea of using multilingual structures and transcription techniques is presented. Another solution based on the major principles of the former follows, which exploits the coding of all of character sets uniformally.}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and P Lampsas and A Tatakis and S Kontogiannis and D Fotakis} } @conference {2855, title = {Randomized Competitive Algorithms for Admission Control in General Networks}, booktitle = {14th ACM-PODC, Ottawa, Canada}, year = {1995}, author = {Vaggelis Kapoulas and Paul Spirakis} } @book {3152, title = {Discrete Mathematics: Problems and Solutions}, series = {Gutenberg}, year = {1994}, author = {Christos Bouras and Lefteris Kirousis and Paul Spirakis and G Voutsadakis} } @conference {2857, title = {The perfect and imperfect clocks approach to performance analysis of basic timestamp ordering in distributed databases}, booktitle = {ICCI{\textquoteright}93, Sudbury, Ontario, Canada}, year = {1993}, month = {27 - 29 May}, pages = { 403 - 407}, abstract = {Locking and timestamping are two popular approaches to concurrency control in databases systems. Although more than a dozen analytic performance studies of locking techniques have recently appeared in the literature, analytic performance study of timestamp{\textendash}based concurrency algorithms largely remains an unexplored area. This work presents a model of a distributed database system which provides the framework to study the performance of timestamp ordering concurrency control. We exhibit an analytical solution, which has been tested with extensive simulation. The accuracy seems to be very high. We assume perfect and also imperfect clocks for synchronization and quantify the way in which local clock inaccuracies affect the phenomenon of transaction conflicts. In particular, we derive a lot of interesting performance measures such as probability of abort, throughput and others.}, author = {Christos Bouras and Paul Spirakis} } @conference {2856, title = {Performance Models for Perfect and Imperfect Clocks on Timestamp Ordering in Distributed Databases}, booktitle = {MASCOTS{\textquoteright}93, San Diego, California, USA}, year = {1993}, month = {17 - 20 January}, pages = { 243 - 246}, abstract = {This work presents a model of a distributed database system which provides the framework to study the performance of timestamp ordering concurrency control. We exhibit an analytical solution, which has been tested with extensive simulation. The accuracy seems to be very high. We assume perfect and also imperfect clocks for synchronization and quantify the way in which local clock inaccuracies affect the phenomenon of transaction conflicts.}, author = {Christos Bouras and Paul Spirakis} } @book {3153, title = {Discrete Mathematics}, series = {Gutenberg}, year = {1992}, author = {Christos Bouras and Lefteris Kirousis and Paul Spirakis} } @conference {2858, title = {MIPL:An Image Processing Library for Medical Applications}, booktitle = {S/CAR{\textquoteright}92, Baltimore, Maryland, USA}, year = {1992}, month = {14 - 17 June}, pages = { 519 - 525}, author = {Christos Bouras and Vaggelis Kapoulas and Paul Spirakis and T Georgantas and K Goutis} } @conference {2860, title = {The effect of clock drifts on the performance of distributed timestamp ordering}, booktitle = {3rd COMAD 91, Bombay, India}, year = {1991}, month = {12 - 14 December}, pages = { 69 - 85}, abstract = {Timestamp{\textendash}based algorithms have been proposed to protect distributed database from inconsistencies during concurrent access. In such algorithms, transactions may reach a paricular site out of the order of their timestamps, due to unexpected network delays. This causes conflicts which the distributed concurrency control mechanism has to cope with. In this paper we analyze the essential features of the conflict phenomenon under the realistic assumption of the local site clocks out of total synchrony. We quantify the way in which local clock inaccuracies affect the phenomenon of transaction conflicts. In particular, we express the probability of order{\textendash}reverse as a particular increasing function of the maximum clock drift.}, author = {Christos Bouras and Paul Spirakis} } @conference {2859, title = {RECPHONE: A new environment for medical remote expert consultation}, booktitle = {EuroPACS{\textquoteright}91, 9th International Meeting, Berlin, Germany}, year = {1991}, month = {2 -3 July}, pages = { 22 - 23}, author = {Christos Bouras and Paul Spirakis and J Garofalakis and G Papoutsopoulos and P Basiroglou} } @conference {2861, title = {Simplified performance models of the reordering issue in timestamp ordering concurrency control in distributed databases}, booktitle = {5th International Symposium on Computers and Information Sciences- ISCIS V, Volume II, Cappadocia, Nevsehir, Turkey}, year = {1990}, month = {30 October - 2 November}, pages = { 719 - 727}, author = {Christos Bouras and Paul Spirakis} } @conference {2862, title = {Queueing Delays in Buffered Multistage Interconnection Networks}, booktitle = {ACM SIGMETRICS Conference 1987, Banff, Alberta, Canada}, year = {1987}, month = {11 - 14 May}, pages = { 111 - 121}, author = {Christos Bouras and Paul Spirakis and Vasilis Triantafillou and J Garofalakis} }