George C Polyzos | Athens University of Economics and Business (original) (raw)
Papers by George C Polyzos
Proceedings of the Workshop on Mobile Edge Communications
Abstract. This paper explores the problems associated with the multicasting of continuous media t... more Abstract. This paper explores the problems associated with the multicasting of continuous media to support multimedia group applications. The interaction between multicasting and the delivery of multiple time-correlated continuous-media streams with real-time delay requirements poses various new and interesting problems in research on communication pro-tocols and architectures. We describe these problems, and identify where the opportunities are for effective solutions, all in the context of providing an overview of the current state of research in multimedia multicasting. The issues we discuss include quality of service, resource reservations, routing, error and traffic control, heterogeneity, and the use of hierarchical coding and open-loop control techniques.
This report presents a comprehensive set of measurements made on a commercial 2.4 GHz DSSS Wirele... more This report presents a comprehensive set of measurements made on a commercial 2.4 GHz DSSS Wireless LAN and analyzes its behavior in detail. We examine the impact on performance of issues such as system heterogeneity, bidirectional (TCP) communications and error modeling, that have not been discussed elsewhere. We describe the testing environment, justify the design of our test suite, and explain the post processing steps in detail. Our experiments gathered enough data to uncover many problems with TCP and UDP performance in this system. We examine the causes of these problems and discuss the effectiveness of alternative solutions based on the data gathered.
The General Packet Radio Service extends the existing GSM mobile communications technology by pro... more The General Packet Radio Service extends the existing GSM mobile communications technology by providing packet switching and higher data rates in order to e#ciently access IP-based services in the Internet. We adapt the Di#erentiated Services Quality-of-Service support framework and apply it over the GPRS air interface in order to provide various levels of service di#erentiation. We also focus on applying a charging technique so as to publish a unit price for each service class. These prices are designed to lead to the maximization of Social Welfare and the users' net benefit.
This article discusses the problems arising when the TCP/IP protocol suite is used to provide Int... more This article discusses the problems arising when the TCP/IP protocol suite is used to provide Internet connectivity over existing and emerging wireless links. Due to the strong drive toward wireless Internet access through mobile terminals, these problems must be carefully studied in order to build improved systems. We review wireless link characteristics using wireless LANs and cellular communications systems as examples. We then outline the performance problems of the TCP/IP protocol suite when employed over those links, such as degraded TCP performance due to mistaking wireless errors for congestion. We present various proposals for solving these problems and examine their benefits and limitations. Finally, we consider the future evolution of wireless systems and the challenges that emerging systems will impose on the Internet protocol suite.
Abstract—Future Internet technologies are offering new capabilities for service and data delivery... more Abstract—Future Internet technologies are offering new capabilities for service and data delivery. We present the potential for enabling the context-aware content adaptation and specialized delivery of healthcare information of the Publish-Subscribe Internetworking (PSI) Future Internet architecture. The Information-Centric Networking paradigm of PSI brings information at the center of the approach, rather than muddying it with the ‘who ’ and ‘where ’ of the information, or data. In addition, it enables seamless information morphing, effective access control policies, and other security features. Keywords; Future Internet; Information-Centric Networking; content adaptation; healthcare; security
Proceedings 2018 Workshop on Decentralized IoT Security and Standards
According to some studies, there are well over 300 different Internet of Things (IoT) platforms a... more According to some studies, there are well over 300 different Internet of Things (IoT) platforms and several dozens of (so-called) standards. In practice, this has led to a situation where most of the deployed IoT systems are closed and largely incapable of communicating with other IoT systems. Furthermore, it is likely that at least a few different basic IoT communication protocols will co-exist, including the Constrained Application Protocol (CoAP), Message Queue Telemetry Transport (MQTT), and HTTP. In this paper, we introduce SOFIE, Secure Open Federation for Internet Everywhere. SOFIE is a proposal for applying distributed ledger technology (DLT) to securely and openly federate IoT platforms. SOFIE's approach is based on the idea of using interconnected distributed ledgers as a cornerstone to build decentralised business platforms that support the interconnection of diverse IoT systems. Among other things, the ledgers are used to provide openly accessible metadata about the individual IoT platforms, to define business and other rules on how to connect to the platforms, and to securely record audit trails that can be used to resolve disputes. The SOFIE concept will be prototyped and studied in an EU Horizon 2020 funded project, commencing in early 2018 and running for three years. The SOFIE concept will be prototyped and studied in an EU Horizon 2020 funded project, commencing in early 2018 and running for three years.
Proceedings of the 3rd ACM Conference on Information-Centric Networking
Named Data Networking (NDN) of Things has gathered interest among the Internet-of-Things (IoT) re... more Named Data Networking (NDN) of Things has gathered interest among the Internet-of-Things (IoT) research community due to its hassle-free deployment and configurability. This approach to interconnecting devices provides the network with additional information about the data transported, which can be used for improved efficiency. This is one of the targets for mass IoT or Machine-to-Machine (M2M) communications modes of 5G. In this paper, we propose a context based forwarding strategy that makes forwarding decisions based on the tolerance to inaccuracies in data defined for the application requesting it. We also propose a modification to the hierarchical naming scheme by appending the name of the application generating the interest. In addition to faster context based forwarding, this modification simplifies the naming as it is easier to understand the context under which the interest was generated even with generic names. We demonstrate an improvement in energy efficiency of the system using the proposed technique compared to an IP based implementation. In addition, the proposed technique offers a platform to implement application level access control and Quality-of-Service support.
We introduce an object-based multimedia stream representation model using XML and present a syste... more We introduce an object-based multimedia stream representation model using XML and present a system that supports conformance to device capabilities and user control in multimedia stream transformation and presentation.
This paper shows how the multicast nature of proposed future publish/subscribe network architectu... more This paper shows how the multicast nature of proposed future publish/subscribe network architectures can assist mobility. Publish/subscribe is an information-centric net- working paradigm. Unlike the current send/receive based In- ternet architecture which favours the sender of information, the publish/subscribe paradigm leads to a more balanced re- lationship between entities. Publish/subscribe is considered a promising architecture for the future Internet as it can potentially address various current Internet problems, such as spam and (Distributed) Denial of Service attacks. In a pub/sub architecture all data are transmitted via multicast and end host identification, which has a less critical role to begin with, is decoupled from location identification. In such an environment, fast mobility can be supported effectively, particularly for continuous media distribution (such as mobile TV) and other types of real-time multimedia applications. In this paper, a prototype overlay multi...
In metropolitan areas, public infrastructures for high-speed wireless networking can be built thr... more In metropolitan areas, public infrastructures for high-speed wireless networking can be built through the private contributions of individual “micro-operators” who use their Internet-connected Wireless LANs (WLANs) to forward foreign traffic from and to nearby low-mobility clients. We have designed a practical WLAN aggregation scheme that (1) assumes that micro-operators are selfish and do not trust each other and uses a secure incentive technique to encourage their contribution; (2) protects the real-world identities of micro-operators and clients by relying only on disposable opaque identifiers (public keys); (3) is fully distributed, open to all, and does not rely on any authority to resolve disputes or to control membership; (4) is automated, using standard hardware and software we developed for some of the main available platforms (Linux-based WLAN access points and Windows Mobile-based cell phones).
2017 14th IEEE Annual Consumer Communications & Networking Conference (CCNC), 2017
We analyze the design space for implementing the Constrained Application Protocol (CoAP) within C... more We analyze the design space for implementing the Constrained Application Protocol (CoAP) within Content-Centric Networks (CCN), identifying several CoAP specific scenarios and seeing how they map to CCNs. We present an evaluation, recommendations for implementations and extensions, and directions for future work. Our key result is that while several protocol features and flows map naturally, some communication patterns are more difficult to capture without modifications or additions to the CCN model. In addition to many conceptual and also non-performance related advantages, our experimental evaluation demonstrates that it is feasible and useful to implement CoAP over CCN, with the performance similar to that of CoAP over UDP over IP, and in several cases outperforming it significantly.
Proceedings of the 3rd ACM Conference on Information-Centric Networking, 2016
Named Data Networking (NDN) of Things has gathered interest among the Internet-of-Things (IoT) re... more Named Data Networking (NDN) of Things has gathered interest among the Internet-of-Things (IoT) research community due to its hassle-free deployment and configurability. This approach to interconnecting devices provides the network with additional information about the data transported, which can be used for improved efficiency. This is one of the targets for mass IoT or Machine-to-Machine (M2M) communications modes of 5G. In this paper, we propose a context based forwarding strategy that makes forwarding decisions based on the tolerance to inaccuracies in data defined for the application requesting it. We also propose a modification to the hierarchical naming scheme by appending the name of the application generating the interest. In addition to faster context based forwarding, this modification simplifies the naming as it is easier to understand the context under which the interest was generated, even with generic names. We demonstrate an improvement in energy efficiency of the sys...
George Xylomenos, Department of Informatics, Athens University of Economics and Business, Greece,... more George Xylomenos, Department of Informatics, Athens University of Economics and Business, Greece, xgeorge@aueb.gr George C. Polyzos, Department of Informatics, Athens University of Economics and Business, Greece, polyzos@aueb.gr Petri Mähönen, Center for Wireless Communications, University of Oulu, Finland, Petri.Mahonen@oulu.fi Mika Saaranen, Center for Wireless Communications, University of Oulu, Finland, Mika.Saaranen@ieee.org
The main problem of the today’s IEEE 802.11 WLANs is the small number of available channels. In t... more The main problem of the today’s IEEE 802.11 WLANs is the small number of available channels. In this paper we focus on a novel way of maximization of the network throughput and the provision of fairness which are key challenges in IEEE WLANs, using Game Theory. We examine two types of power control games, namely the non-cooperative and the cooperative power control game. In the case of non-cooperative power control game we find the Nash equilibrium in a distributed way. In the case of cooperative power control game we assume that there exists a central entity called coordinator which announces the calculated Nash bargaining solution to the access points. Finally, we present the results of simulations implemented in MATLAB through a series of plots and tables.
Hierarchical coding techniques split signals into components of varying importance. The aggregati... more Hierarchical coding techniques split signals into components of varying importance. The aggregation of these components reconstructs the original data, but subsets of them can also provide various degrees of approximation to the original signal. Hierarchical coding is very important for the efficient use of high-speed packet-switching networks. The main issue for such network architectures is the significant congestion control problems that can arise due to the statistical multiplexing of many very high burstiness signals. A key property of any congestion control approach, which is not based on resource reservation at the peak rate of the sources, is the ability to shed load quickly without causing an avalanche of retransmissions of dropped traffic. With hierarchical coding of continuous media, when network congestion arises it is possible to drop the less important signal components without causing service interruption, and without the need for retransmissions. Since it is expected...
2010 Future Network & Mobile Summit, 2010
Content-centric networking is often regarded as a promising paradigm for future networks. Neverth... more Content-centric networking is often regarded as a promising paradigm for future networks. Nevertheless current content-based networks-such as file sharing peer-to-peer (P2P) networks-have been proved to be vulnerable to content pollution attacks. A significant amount of research efforts has been launched in order to mitigate this kind of attacks. The majority of these efforts focuses on users' ranking, based on their behavior, while little work has been done in ranking information itself. We show in this paper that solutions based on users' ranking can be by-passed by malicious users. Furthermore we propose inforanking, a light-weight solution for ranking information, based exclusively on positive votes. We compare our solution to Credence object-based reputation system. Our solution demonstrates significant less burden to the network and outperforms Credence in terms of polluted content isolation.
2008 International Symposium on Performance Evaluation of Computer and Telecommunication Systems, 2008
Recent advances in mobile communications and computing and strong interest of the scientific comm... more Recent advances in mobile communications and computing and strong interest of the scientific community in the grid have led to research into the mobile grid. Based on a realistic mobile grid architecture we formulate the problem of job scheduling in a mobile environment and investigate key design decisions with respect to fundamental performance metrics. We extend previous work by introducing a new scheduling policy based on the notion of installments and continue the evaluation of the expanded set of scheduling strategies in an effort to overcome the intermittent character of connectivity in a mobile environment. Our findings, based on real wireless traces, demonstrate the superiority of the proposed policy, show the feasibility of a mobile grid system and further provide valuable input for the design of efficient scheduling policies subject to the underlying mobility characteristics of the mobile resources.
Past research on high performance computers for scientific applications has concentrated on CPU s... more Past research on high performance computers for scientific applications has concentrated on CPU speed and exploitation of parallelism, but has, until very recently, neglected I/O considerations. This paper presents a study of the production workload at the San Diego Supercomputer Center from an I/O requirements and characteristics perspective. Results of our analyses support our hypothesis that a significant proportion of scientific applications with intensive I/O demands have predictable I/O requirements.
Proceedings of the Workshop on Mobile Edge Communications
Abstract. This paper explores the problems associated with the multicasting of continuous media t... more Abstract. This paper explores the problems associated with the multicasting of continuous media to support multimedia group applications. The interaction between multicasting and the delivery of multiple time-correlated continuous-media streams with real-time delay requirements poses various new and interesting problems in research on communication pro-tocols and architectures. We describe these problems, and identify where the opportunities are for effective solutions, all in the context of providing an overview of the current state of research in multimedia multicasting. The issues we discuss include quality of service, resource reservations, routing, error and traffic control, heterogeneity, and the use of hierarchical coding and open-loop control techniques.
This report presents a comprehensive set of measurements made on a commercial 2.4 GHz DSSS Wirele... more This report presents a comprehensive set of measurements made on a commercial 2.4 GHz DSSS Wireless LAN and analyzes its behavior in detail. We examine the impact on performance of issues such as system heterogeneity, bidirectional (TCP) communications and error modeling, that have not been discussed elsewhere. We describe the testing environment, justify the design of our test suite, and explain the post processing steps in detail. Our experiments gathered enough data to uncover many problems with TCP and UDP performance in this system. We examine the causes of these problems and discuss the effectiveness of alternative solutions based on the data gathered.
The General Packet Radio Service extends the existing GSM mobile communications technology by pro... more The General Packet Radio Service extends the existing GSM mobile communications technology by providing packet switching and higher data rates in order to e#ciently access IP-based services in the Internet. We adapt the Di#erentiated Services Quality-of-Service support framework and apply it over the GPRS air interface in order to provide various levels of service di#erentiation. We also focus on applying a charging technique so as to publish a unit price for each service class. These prices are designed to lead to the maximization of Social Welfare and the users' net benefit.
This article discusses the problems arising when the TCP/IP protocol suite is used to provide Int... more This article discusses the problems arising when the TCP/IP protocol suite is used to provide Internet connectivity over existing and emerging wireless links. Due to the strong drive toward wireless Internet access through mobile terminals, these problems must be carefully studied in order to build improved systems. We review wireless link characteristics using wireless LANs and cellular communications systems as examples. We then outline the performance problems of the TCP/IP protocol suite when employed over those links, such as degraded TCP performance due to mistaking wireless errors for congestion. We present various proposals for solving these problems and examine their benefits and limitations. Finally, we consider the future evolution of wireless systems and the challenges that emerging systems will impose on the Internet protocol suite.
Abstract—Future Internet technologies are offering new capabilities for service and data delivery... more Abstract—Future Internet technologies are offering new capabilities for service and data delivery. We present the potential for enabling the context-aware content adaptation and specialized delivery of healthcare information of the Publish-Subscribe Internetworking (PSI) Future Internet architecture. The Information-Centric Networking paradigm of PSI brings information at the center of the approach, rather than muddying it with the ‘who ’ and ‘where ’ of the information, or data. In addition, it enables seamless information morphing, effective access control policies, and other security features. Keywords; Future Internet; Information-Centric Networking; content adaptation; healthcare; security
Proceedings 2018 Workshop on Decentralized IoT Security and Standards
According to some studies, there are well over 300 different Internet of Things (IoT) platforms a... more According to some studies, there are well over 300 different Internet of Things (IoT) platforms and several dozens of (so-called) standards. In practice, this has led to a situation where most of the deployed IoT systems are closed and largely incapable of communicating with other IoT systems. Furthermore, it is likely that at least a few different basic IoT communication protocols will co-exist, including the Constrained Application Protocol (CoAP), Message Queue Telemetry Transport (MQTT), and HTTP. In this paper, we introduce SOFIE, Secure Open Federation for Internet Everywhere. SOFIE is a proposal for applying distributed ledger technology (DLT) to securely and openly federate IoT platforms. SOFIE's approach is based on the idea of using interconnected distributed ledgers as a cornerstone to build decentralised business platforms that support the interconnection of diverse IoT systems. Among other things, the ledgers are used to provide openly accessible metadata about the individual IoT platforms, to define business and other rules on how to connect to the platforms, and to securely record audit trails that can be used to resolve disputes. The SOFIE concept will be prototyped and studied in an EU Horizon 2020 funded project, commencing in early 2018 and running for three years. The SOFIE concept will be prototyped and studied in an EU Horizon 2020 funded project, commencing in early 2018 and running for three years.
Proceedings of the 3rd ACM Conference on Information-Centric Networking
Named Data Networking (NDN) of Things has gathered interest among the Internet-of-Things (IoT) re... more Named Data Networking (NDN) of Things has gathered interest among the Internet-of-Things (IoT) research community due to its hassle-free deployment and configurability. This approach to interconnecting devices provides the network with additional information about the data transported, which can be used for improved efficiency. This is one of the targets for mass IoT or Machine-to-Machine (M2M) communications modes of 5G. In this paper, we propose a context based forwarding strategy that makes forwarding decisions based on the tolerance to inaccuracies in data defined for the application requesting it. We also propose a modification to the hierarchical naming scheme by appending the name of the application generating the interest. In addition to faster context based forwarding, this modification simplifies the naming as it is easier to understand the context under which the interest was generated even with generic names. We demonstrate an improvement in energy efficiency of the system using the proposed technique compared to an IP based implementation. In addition, the proposed technique offers a platform to implement application level access control and Quality-of-Service support.
We introduce an object-based multimedia stream representation model using XML and present a syste... more We introduce an object-based multimedia stream representation model using XML and present a system that supports conformance to device capabilities and user control in multimedia stream transformation and presentation.
This paper shows how the multicast nature of proposed future publish/subscribe network architectu... more This paper shows how the multicast nature of proposed future publish/subscribe network architectures can assist mobility. Publish/subscribe is an information-centric net- working paradigm. Unlike the current send/receive based In- ternet architecture which favours the sender of information, the publish/subscribe paradigm leads to a more balanced re- lationship between entities. Publish/subscribe is considered a promising architecture for the future Internet as it can potentially address various current Internet problems, such as spam and (Distributed) Denial of Service attacks. In a pub/sub architecture all data are transmitted via multicast and end host identification, which has a less critical role to begin with, is decoupled from location identification. In such an environment, fast mobility can be supported effectively, particularly for continuous media distribution (such as mobile TV) and other types of real-time multimedia applications. In this paper, a prototype overlay multi...
In metropolitan areas, public infrastructures for high-speed wireless networking can be built thr... more In metropolitan areas, public infrastructures for high-speed wireless networking can be built through the private contributions of individual “micro-operators” who use their Internet-connected Wireless LANs (WLANs) to forward foreign traffic from and to nearby low-mobility clients. We have designed a practical WLAN aggregation scheme that (1) assumes that micro-operators are selfish and do not trust each other and uses a secure incentive technique to encourage their contribution; (2) protects the real-world identities of micro-operators and clients by relying only on disposable opaque identifiers (public keys); (3) is fully distributed, open to all, and does not rely on any authority to resolve disputes or to control membership; (4) is automated, using standard hardware and software we developed for some of the main available platforms (Linux-based WLAN access points and Windows Mobile-based cell phones).
2017 14th IEEE Annual Consumer Communications & Networking Conference (CCNC), 2017
We analyze the design space for implementing the Constrained Application Protocol (CoAP) within C... more We analyze the design space for implementing the Constrained Application Protocol (CoAP) within Content-Centric Networks (CCN), identifying several CoAP specific scenarios and seeing how they map to CCNs. We present an evaluation, recommendations for implementations and extensions, and directions for future work. Our key result is that while several protocol features and flows map naturally, some communication patterns are more difficult to capture without modifications or additions to the CCN model. In addition to many conceptual and also non-performance related advantages, our experimental evaluation demonstrates that it is feasible and useful to implement CoAP over CCN, with the performance similar to that of CoAP over UDP over IP, and in several cases outperforming it significantly.
Proceedings of the 3rd ACM Conference on Information-Centric Networking, 2016
Named Data Networking (NDN) of Things has gathered interest among the Internet-of-Things (IoT) re... more Named Data Networking (NDN) of Things has gathered interest among the Internet-of-Things (IoT) research community due to its hassle-free deployment and configurability. This approach to interconnecting devices provides the network with additional information about the data transported, which can be used for improved efficiency. This is one of the targets for mass IoT or Machine-to-Machine (M2M) communications modes of 5G. In this paper, we propose a context based forwarding strategy that makes forwarding decisions based on the tolerance to inaccuracies in data defined for the application requesting it. We also propose a modification to the hierarchical naming scheme by appending the name of the application generating the interest. In addition to faster context based forwarding, this modification simplifies the naming as it is easier to understand the context under which the interest was generated, even with generic names. We demonstrate an improvement in energy efficiency of the sys...
George Xylomenos, Department of Informatics, Athens University of Economics and Business, Greece,... more George Xylomenos, Department of Informatics, Athens University of Economics and Business, Greece, xgeorge@aueb.gr George C. Polyzos, Department of Informatics, Athens University of Economics and Business, Greece, polyzos@aueb.gr Petri Mähönen, Center for Wireless Communications, University of Oulu, Finland, Petri.Mahonen@oulu.fi Mika Saaranen, Center for Wireless Communications, University of Oulu, Finland, Mika.Saaranen@ieee.org
The main problem of the today’s IEEE 802.11 WLANs is the small number of available channels. In t... more The main problem of the today’s IEEE 802.11 WLANs is the small number of available channels. In this paper we focus on a novel way of maximization of the network throughput and the provision of fairness which are key challenges in IEEE WLANs, using Game Theory. We examine two types of power control games, namely the non-cooperative and the cooperative power control game. In the case of non-cooperative power control game we find the Nash equilibrium in a distributed way. In the case of cooperative power control game we assume that there exists a central entity called coordinator which announces the calculated Nash bargaining solution to the access points. Finally, we present the results of simulations implemented in MATLAB through a series of plots and tables.
Hierarchical coding techniques split signals into components of varying importance. The aggregati... more Hierarchical coding techniques split signals into components of varying importance. The aggregation of these components reconstructs the original data, but subsets of them can also provide various degrees of approximation to the original signal. Hierarchical coding is very important for the efficient use of high-speed packet-switching networks. The main issue for such network architectures is the significant congestion control problems that can arise due to the statistical multiplexing of many very high burstiness signals. A key property of any congestion control approach, which is not based on resource reservation at the peak rate of the sources, is the ability to shed load quickly without causing an avalanche of retransmissions of dropped traffic. With hierarchical coding of continuous media, when network congestion arises it is possible to drop the less important signal components without causing service interruption, and without the need for retransmissions. Since it is expected...
2010 Future Network & Mobile Summit, 2010
Content-centric networking is often regarded as a promising paradigm for future networks. Neverth... more Content-centric networking is often regarded as a promising paradigm for future networks. Nevertheless current content-based networks-such as file sharing peer-to-peer (P2P) networks-have been proved to be vulnerable to content pollution attacks. A significant amount of research efforts has been launched in order to mitigate this kind of attacks. The majority of these efforts focuses on users' ranking, based on their behavior, while little work has been done in ranking information itself. We show in this paper that solutions based on users' ranking can be by-passed by malicious users. Furthermore we propose inforanking, a light-weight solution for ranking information, based exclusively on positive votes. We compare our solution to Credence object-based reputation system. Our solution demonstrates significant less burden to the network and outperforms Credence in terms of polluted content isolation.
2008 International Symposium on Performance Evaluation of Computer and Telecommunication Systems, 2008
Recent advances in mobile communications and computing and strong interest of the scientific comm... more Recent advances in mobile communications and computing and strong interest of the scientific community in the grid have led to research into the mobile grid. Based on a realistic mobile grid architecture we formulate the problem of job scheduling in a mobile environment and investigate key design decisions with respect to fundamental performance metrics. We extend previous work by introducing a new scheduling policy based on the notion of installments and continue the evaluation of the expanded set of scheduling strategies in an effort to overcome the intermittent character of connectivity in a mobile environment. Our findings, based on real wireless traces, demonstrate the superiority of the proposed policy, show the feasibility of a mobile grid system and further provide valuable input for the design of efficient scheduling policies subject to the underlying mobility characteristics of the mobile resources.
Past research on high performance computers for scientific applications has concentrated on CPU s... more Past research on high performance computers for scientific applications has concentrated on CPU speed and exploitation of parallelism, but has, until very recently, neglected I/O considerations. This paper presents a study of the production workload at the San Diego Supercomputer Center from an I/O requirements and characteristics perspective. Results of our analyses support our hypothesis that a significant proportion of scientific applications with intensive I/O demands have predictable I/O requirements.