Mobile computing allows ubiquitous and continuous access to computing resources while the users travel or work at a client’s site. The flexibility introduced by mobile computing brings new challenges to the area of fault tolerance. Failures that were rare with fixed hosts become common, and host disconnection makes fault detection and message coordination difficult. This paper describes a new checkpoint protocol that is well adapted to mobile environments. The protocol uses time to indirectly coordinate the creation of new global states, avoiding all message exchanges. The protocol uses two different types of checkpoints to adapt to the current network characteristics, and to trade off performance with recovery time. 1

Emerging communication-intensive applications require significant levels of networking resources for efficient operation. In the context of mobile computing environments, limited and dynamically varying available resources, stringent application requirements, and user mobility make it difficult to provide sustained quality of service to applications. This paper addresses the above issues, and proposes a new service model in cellular mobile computing environments. The major results of this paper are the following: ffl An Adaptive Service model in mobile computing environments, which enables the network and applications to adaptively (re)negotiate QoS depending on dynamic network conditions ffl An algorithmic framework that provides cost-effective resource adaptation in the presence of resource and network dynamics ffl A unified implementation architecture for adaptive service support A brief overview of an ongoing testbed implementation is provided. 1 INTRODUCTION Recent years have ...

Mobile computing environments that seek to support communication-intensive applications need to provide sustained end-to-end networking resources to static and mobile flows in the presence of scarce and variable wireless bandwidth, bursty wireless channel error, and user mobility. In order to achieve this goal, we present the TIMELY adaptive resource management architecture and algorithms for resource reservation, advance reservation, and network layer and end-to-end adaptation in mobile computing environments. The key novelty of our approach is the coordination of adaptation between the different layers of the network in order to solve the problems introduced by scarce and dynamic network resources. 1 Introduction Mobile computing is becoming increasingly popular because of the availability of indoor and outdoor wireless packet networks such as Wavelan, Rangelan, RAM and CDPD. In order to effectively support communication-intensive applications such as web browsing and multime...

The advent of affordable off-the-shelf widearea wireless networking solutions for portable computers will result in partial (or intermittent) connectivity becoming the common networking mode for mobile users. This paper presents the design of PFS, a mobility-aware file system specially designed for partially connected operation. PFS supports the extreme modes of full connection and disconnection gracefully, but, unlike other mobile file systems, it also provides an interface for mobility-aware applications to direct the file system in its caching and consistency decisions. Using PFS, it is possible for an application to maintain consistency on only the critical portions of its data files. Since PFS provides adaptation at the file system level, even unaware applications can `act' mobile-aware as a result of the transparent adaptation provided by PFS.

Mobile computing environments that seek to support communication-intensive applications need to provide sustained end-to-end networking resources to static and mobile flows in the presence of scarce and variable wireless bandwidth, bursty wireless channel error, and user mobility. In order to achieve this goal, we present the design and implementation of a new resource management architecture and algorithms for resource reservation, advance reservation and resource adaptation in mobile computing environments. In this technical report, we present a service model that captures the requirements of both stationary and mobile multimedia flows in a mobile computing environment, and propose a resource adaptation approach that maximizes the resource allocation for stationary flows and minimizes the variance in resource allocation for mobile flows in such environments. We introduce the notion of `network revenue' for resource adaptation, and present a rate adaptation algorithm that allocates ra...

Distributed systems are being used to support the execution of applications ranging from long-running scientific simulators to e-commerce on the Internet. In this type of environment, the failure of one of its components, either a computer or the network, may prevent other components from completing their tasks. Since the probability of failure increases with the number of computers and execution time, it is likely that these applications will be interrupted unless provision is made for failure handling. In this thesis we address the problem of fault recovery in distributed systems. The thesis describes two variations of a coordinated checkpoint protocol that uses time to re-move most causes of overhead, and to avoid all types of direct coordination. The time-based pro-tocol does not have to transmit extra messages, does not need to tag the application messages, and only accesses the stable storage when the checkpoints are saved. The thesis also describes a new coordinated checkpoint protocol that is well adapted to mobile environments. It uses time to indi-rectly coordinate the creation of new global states, and it saves two different types of checkpoints to adapt its behavior to the current network characteristics. Traditional techniques for fault diagnosis in distributed systems, either based on watch-dogs or polling, exchange performance with detection latency. The thesis introduces a complementary

Current distributed multimedia applications demand Quality of Service (QoS) from the supporting system to facilitate effective services to the end users. However, within the range of QoS demands specified by the application, lower level transport facilities may not be able to constantly provide guaranteed QoS without perturbations, especially in the case of wireless communication. In this scenario, we introduce facilities in the middleware level of the end systems to perform QoS adaptations on a specific QoS metric that is measured from the raw QoS provided by the underlying layers. We refer to the facility performing this task as adaptors. We are able to analyze the behavior of QoS adaptations using developed theories from digital control systems, based on a simplified model for QoS metrics. In particular, we are able to configure these adaptors according to the desired adaptation agility of their adaptation behavior, while still preserving their stability. We show the viability of th...

Mobile computing systems have non-functional properties that differ from fixed distributed systems due to the radio and mobile terminal characteristics. Traditional middleware does not manage these non-functional properties. Hence, the middleware must be designed specifically for mobile computing. Existing mobile middleware platforms and state-of-the-art research focus on handling network quality of service (QoS). There is limited attention on a general approach to QoS that takes into consideration the QoS characteristics of both end-systems and the mobile communication system. This article presents a general QoS framework with QoS specific elements for mobile middleware, which combines and extends existing QoS frameworks. Following this, we use the QoS framework to systematically and qualitatively compare and discuss state-of-the-art QoS-aware mobile middleware platforms. In general, existing work sets a firm baseline for future research. A potential area within mobile middleware that needs more research is QoS architectures that can incorporate new and existing QoS mechanisms to adapt application and middleware to context changes and resource fluctuations. Keywords: Mobile middleware; Reflective middleware; QoS mechanisms; QoS-aware adaptation; Dynamic service reconfiguration;

Current distributed multimedia applications demand Quality of Service from the system to facilitate effective services to the end users. However, within the range of QoS demands appropriately specified by the application, lower level transport facilities may not be able to constantly provide Quality of Service without perturbations. We introduce facilities in the middleware level to perform Quality of Service adaptations on a specific Quality of Service metric that is measured from the raw Quality of Service provided by the underlying layers, and refer to the component performing the task as adaptors. We are able to approach Quality of Service adaptations from a different perspective with a simplified but precise model for Quality of Service metrics, adaptations, and the adaptation behavior. Utilizing theories and techniques from the digital control theories and digital signal processing, we are able to model adaptation stability and agility of the performed adaptation behavior, and to introduce methods to analyze and configure the transformations according to the demands specified by the distributed multimedia applications. iii TO MY PARENTS iv Acknowledgments First and foremost, I would like to thank my thesis advisor, Professor Klara Nahrstedt, for her invaluable directions and support throughout my research efforts towards this thesis. Her insights and suggestions to the problems presented in this thesis enlightened me in various detailed aspects through the work. I would also like to acknowledge the constant assistance and encouragements from my best friends. Among them, special thanks goes to Dongyan Xu, who was also conducting research in the area of Quality of Service, and who has been generously taking time to discuss research possibilities with me, as well...

The need for "information anywhere anytime" has been a driving force for the increasing growth in Web and Internet technology, wireless communication, and portable computing devices. The field of mobile computing is the merger of these advances in computing and communication with the aim of providing seamless and ubiquitous computing environment for mobile users. Mobile computing environments are characterized by severe resource constraints and frequent changes in operating conditions. This has motivated research in many challenging problems which span several areas of computer science, computer engineering and electrical engineering, such as network protocols to support mobility, efficient and adaptive resource management techniques for wireless bandwidth and battery power, predicting mobility patterns, performance modeling and simulation of mobile applications, and supporting mobile real-time multimedia applications.