Performance of distributed information systems

There is an increasing use of distributed computer systems to provide services in both traditional telephony as well as in the Internet. Two main technologies are Distributed Object Computing (DOC) and Web based services. One common DOC architecture investigated in this thesis is the Common Object Request Broker Architecture (CORBA), specified by the Object Management Group. CORBA applications consist of interacting software components called objects. Two other DOC architectures investigated are the Telecommunications Information Net- working Architecture (TINA) and a CORBA based Intelligent Network (IN/CORBA) system. In a DOC environment, the objects of an application are distributed on mul- tiple nodes. A middleware layer makes the distribution transparent to the application. However, the distributed nature creates a number of potential performance problems. Three problems in DOC systems are examined in this thesis: object distribution, load balancing and overload protection. An object distribution describes how objects are distributed in the network. The objective is to distribute the objects on the physical nodes in such a way that intern-node communication overhead is as small as possible. One way to solve the object distribution problem is to use linear programming. The constraints for the problem are then given by both ease of management of the system and performance concerns. Load balancing is used when there are multiple objects that can be used at a particular time. The objective of load balancing is to distribute the load e±ciently on the available nodes. This thesis investigates a number of de- centralized load balancing mechanisms, including one based on the use of intelligent agents. Finally, overload protection mechanisms for DOC systems are investigated. While overload protection is well-researched for telecom networks, only little work has been performed previously concerning DOC and overload protection. Also, this thesis examines the use of overload protection in e-commerce web servers. Two schemes are compared, one which handles admission to the e-commerce site on request basis, and another which handles admission on session basis. The session based mechanism is shown to be better in terms of user-experienced performance

Quality aspects of Internet telephony

Internet telephony has had a tremendous impact on how people communicate. Many now maintain contact using some form of Internet telephony. Therefore the motivation for this work has been to address the quality aspects of real-world Internet telephony for both fixed and wireless telecommunication. The focus has been on the quality aspects of voice communication, since poor quality leads often to user dissatisfaction. The scope of the work has been broad in order to address the main factors within IP-based voice communication. The first four chapters of this dissertation constitute the background material. The first chapter outlines where Internet telephony is deployed today. It also motivates the topics and techniques used in this research. The second chapter provides the background on Internet telephony including signalling, speech coding and voice Internetworking. The third chapter focuses solely on quality measures for packetised voice systems and finally the fourth chapter is devoted to the history of voice research. The appendix of this dissertation constitutes the research contributions. It includes an examination of the access network, focusing on how calls are multiplexed in wired and wireless systems. Subsequently in the wireless case, we consider how to handover calls from 802.11 networks to the cellular infrastructure. We then consider the Internet backbone where most of our work is devoted to measurements specifically for Internet telephony. The applications of these measurements have been estimating telephony arrival processes, measuring call quality, and quantifying the trend in Internet telephony quality over several years. We also consider the end systems, since they are responsible for reconstructing a voice stream given loss and delay constraints. Finally we estimate voice quality using the ITU proposal PESQ and the packet loss process. The main contribution of this work is a systematic examination of Internet telephony. We describe several methods to enable adaptable solutions for maintaining consistent voice quality. We have also found that relatively small technical changes can lead to substantial user quality improvements. A second contribution of this work is a suite of software tools designed to ascertain voice quality in IP networks. Some of these tools are in use within commercial systems today

Ethernet Networks for Real-Time Use in the ATLAS Experiment

Ethernet became today's de-facto standard technology for local area networks. Defined by the IEEE 802.3 and 802.1 working groups, the Ethernet standards cover technologies deployed at the first two layers of the OSI protocol stack. The architecture of modern Ethernet networks is based on switches. The switches are devices usually built using a store-and-forward concept. At the highest level, they can be seen as a collection of queues and mathematically modelled by means of queuing theory. However, the traffic profiles on modern Ethernet networks are rather different from those assumed in classical queuing theory. The standard recommendations for evaluating the performance of network devices define the values that should be measured but do not specify a way of reconciling these values with the internal architecture of the switches. The introduction of the 10 Gigabit Ethernet standard provided a direct gateway from the LAN to the WAN by the means of the WAN PHY. Certain aspects related to the actual use of WAN PHY technology were vaguely defined by the standard. The ATLAS experiment at CERN is scheduled to start operation at CERN in 2007. The communication infrastructure of the Trigger and Data Acquisition System will be built using Ethernet networks. The real-time operational needs impose a requirement for predictable performance on the network part. In view of the diversity of the architectures of Ethernet devices, testing and modelling is required in order to make sure the full system will operate predictably. This thesis focuses on the testing part of the problem and addresses issues in determining the performance for both LAN and WAN connections. The problem of reconciling results from measurements to architectural details of the switches will also be tackled. We developed a scalable traffic generator system based on commercial-off-the-shelf Gigabit Ethernet network interface cards. The generator was able to transmit traffic at the nominal Gigabit Ethernet line rate for all frame sizes specified in the Ethernet standard. The calculation of latency was performed with accuracy in the range of +/- 200 ns. We indicate how certain features of switch architectures may be identified through accurate throughput and latency values measured for specific traffic distributions. At this stage, we present a detailed analysis of Ethernet broadcast support in modern switches. We use a similar hands-on approach to address the problem of extending Ethernet networks over long distances. Based on the 1 Gbit/s traffic generator used in the LAN, we develop a methodology to characterise point-to-point connections over long distance networks. At higher speeds, a combination of commercial traffic generators and high-end servers is employed to determine the performance of the connection. We demonstrate that the new 10 Gigabit Ethernet technology can interoperate with the installed base of SONET/SDH equipment through a series of experiments on point-to-point circuits deployed over long-distance network infrastructure in a multi-operator domain. In this process, we provide a holistic view of the end-to-end performance of 10 Gigabit Ethernet WAN PHY connections through a sequence of measurements starting at the physical transmission layer and continuing up to the transport layer of the OSI protocol stack

Técnicas de soporte a la flexibilidad funcional en sistemas embarcados distribuidos de tiempo real

Durante la última década, gracias a los avances en diversos campos tecnológicos, se han diversificado el conjunto de entornos en los que es necesario desarrollar sistemas que ofrezcan garantías temporales. Muchos de estos son entornos dinámicos, donde las características de la carga computacional no siempre pueden ser predecibles, y donde ya no es aplicable la aproximación clásica de diseño, habitualmente pesimista, que asegura garantías temporales pero que puede implicar en entornos dinámicos un gasto de recursos prohibitivo. Así, se plantea el reto de adaptar las metodologías empleadas en diferentes niveles arquitecturales a estos nuevos entornos, y explorar nuevas vías y paradigmas que permitan conjugar flexibilidad funcional y dinamismo con predictibilidad temporal. Esta tesis aborda este reto mediante la exploración de la aplicabilidad a sistemas de tiempo real de conceptos propios del paradigma de orientación a servicios, con el fin de ofrecer flexibilidad, y, al mismo tiempo, beneficiarse de algunas de las ventajas que éste ofrece. Para conseguir tal fin, se propone un modelo concreto de sistema basado en una aproximación holística al diseño y configuración, donde las aplicaciones están gobernadas por tiempo. Fijado el modelo de sistema, se propone un modelo de aplicación basada en servicios y se analizan desde diferentes perspectivas las entidades y procesos que estarán presentes en una arquitectura que le dé soporte, diferenciando dos posibles aproximaciones a la composición que influirán en el diseño de dicha arquitectura: estática, que una vez realizada no admite reconfiguraciones, o dinámica, en la cual una aplicación puede reconfigurarse en tiempo de ejecución. Se proponen, además, algoritmos para la composición de aplicaciones, tanto exhaustivos, aplicables en la aproximación estática, como mejorados, con un tiempo de ejecución acotado, apropiados para su empleo en tiempo de ejecución. Finalmente, se realiza la validación del modelo y de las ideas propuestas mediante la implementación de un prototipo sobre un protocolo concreto de comunicaciones de tiempo real, al que se le realizaron pequeñas adaptaciones y sobre el cual se definió una arquitectura adecuada. ____________________________________________During the last decade, due to the advances in several technology fields, the application domains where the development of systems with temporal guarantees is needed has increased. The majority of such domains are dynamic; the characteristics of their computational load cannot always be predicted in advance. Although the classical design approach provides temporal guarantees, it is no longer applicable since it is too pessimistic, and it implies prohibitive resource consumption. Thus, new challenges raise. On one side, it is required to adapt the current methodologies used in different architectural levels to these new environments. Also, it is needed to explore new directions and paradigms that allow combining functional flexibility and dynamism with temporal predictability. The current work addresses these new challenges through the exploration of the applicability of concepts from the service oriented paradigm to distributed real-time systems. The introduction of some of the characteristics of the service oriented paradigm will allow to provide support for dynamic flexibility. Therefore, the current work proposes a concrete system model based on a holistic time-triggered-based approach for design and configuration. Based on this system model, a service-based application model is proposed. Also, it analyses the architectural entities and processes from different points of view, distinguishing between two different architectural design approaches: static and dynamic. The former applies when no reconfigurations at run-time are admitted, and the latter where these reconfigurations are possible. Several application composition algorithms have been proposed: (1) an exhaustive algorithm, applicable to the static approach and (2) an improved algorithm, with bounded execution times, suitable for its usage at run-time. Eventually, to validate the feasibility of the model and the proposed ideas, an architecture has been defined and a prototype of it has been implemented on top of a concrete real-time communications protocol

Actes de l'Ecole d'Eté Temps Réel 2005 - ETR'2005

Pdf des actes disponible à l'URL http://etr05.loria.fr/Le programme de l'Ecole d'été Temps Réel 2005 est construit autour d'exposés de synthèse donnés par des spécialistes du monde industriel et universitaire qui permettront aux participants de l'ETR, et notamment aux doctorants, de se forger une culture scientifique dans le domaine. Cette quatrième édition est centrée autour des grands thèmes d'importance dans la conception des systèmes temps réel : Langages et techniques de description d'architectures, Validation, test et preuve par des approches déterministes et stochastiques, Ordonnancement et systèmes d'exploitation temps réel, Répartition, réseaux temps réel et qualité de service