Despite the prevalence of GPS services, they still suffer from intermittent positioning with poor accuracy in partially shadowed regions like urban canyons, flyover shadows, and factories' indoor areas. Existing wisdom relies on hardware modifications of GPS receivers or power-hungry infrastructures requiring continuous plug-in power supply which is hard to provide in outdoor regions and some factories. This paper fills the gap with GPSMirror, the first GPS-strengthening system that works for unmodified smartphones with the assistance of newly-designed GPS backscatter tags. The key enabling techniques in GPSMirror include: (i) a meticulous hardware design with microwatt-level power consumption that pushes the limit of backscatter sensitivity to re-radiate extremely weak GPS signals with enough coverage approaching the regulation limit; and (ii) a novel GPS positioning algorithm achieving meter-level accuracy in shadowed regions as well as expanding locatable regions under inadequate satellites where conventional algorithms fail. We build a prototype of the GPSMirror tags and conduct comprehensive experiments to evaluate them. Our results show that a GPSMirror tag can provide coverage up to 27.7 m. GPSMirror achieves median positioning accuracy of 3.7 m indoors and 4.6 m in urban canyon environments, respectively.

Replication is a technique commonly used to increase the availability of services in distributed systems, including grid and Web services. While replication is relatively easy for services with fully deterministic behavior, grid and Web services often include nondeterministic operations. The traditional way to replicate such nondeterministic services is to use the primary-backup approach. While this is straightforward in synchronous systems with perfect failure detection, typical grid environments are not usually considered to be synchronous systems. This paper addresses the problem of replicating nondeterministic services by designing a protocol based on Paxos and proposing two performance optimizations suitable for replicated grid services. The first improves the performance in the case where some service operations do not change the service state, while the second optimizes grid service requests that use transactions. Evaluations done both on a local cluster and on Planet-Lab demonstrate that these optimizations significantly reduce the service response time and increase the throughput of replicated services

The combination of grid technology and Web services has produced an attractive platform for deploying distributed applications: grid services, as represented by the Open Grid Services Infrastructure (OGSI) and its Globus toolkit implementation. As the use of grid services grows in popularity, tolerating failures becomes increasingly important. This work addresses the problem of building a reliable and highly-available grid service by replicating the service on two or more hosts using the primary-backup approach. The primary goal is to evaluate the ease and efficiency with which this can be done, by first designing a primary-backup protocol using OGSI, and then implementing it using Globus to evaluate performance implications and tradeoffs. We compared three implementations: one that makes heavy use of the notification interface defined in OGSI, one that uses standard grid service requests instead of notification, and one that uses low-level socket primitives. The overall conclusion is that, while the performance penalty of using Globus primitives - especially notification - for replica coordination can be significant, the OGSI model is suitable for building highly-available services and it makes the task of engineering such services easier.

Group communications systems have been designed to provide an infrastructure for fault-tolerance in distributed systems, including wide-area systems. In our work on master-worker computation for GriPhyN, which is a large project in the area of the computational grid, we asked the question should we build our wide-area master-worker computation using wide-area group communications? This paper explains why we decided doing so was not a good idea.