Clock synchronization in distributed real-time systems
IEEE Transactions on Computers - Special Issue on Real-Time Systems
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Communications of the ACM
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OSDI '02 Proceedings of the 5th symposium on Operating systems design and implementationCopyright restrictions prevent ACM from being able to make the PDFs for this conference available for downloading
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Energy harvesting from electromagnetic energy radiating from AC power lines
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MobiSys '11 Proceedings of the 9th international conference on Mobile systems, applications, and services
Demo: a sensor network time synchronization protocol based on fm radio data system
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FLIGHT: clock calibration using fluorescent lighting
Proceedings of the 18th annual international conference on Mobile computing and networking
Clock calibration using fluorescent lighting
Proceedings of the 18th annual international conference on Mobile computing and networking
Energy efficient GPS sensing with cloud offloading
Proceedings of the 10th ACM Conference on Embedded Network Sensor Systems
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Proceedings of the 10th ACM Conference on Embedded Network Sensor Systems
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Clock synchronization is highly desirable in many sensor networking applications. It enables event ordering, coordinated actuation, energy-efficient communication and duty cycling. This paper presents a novel low-power hardware module for achieving global clock synchronization by tuning to the magnetic field radiating from existing AC power lines. This signal can be used as a global clock source for battery-operated sensor nodes to eliminate drift between nodes over time even when they are not passing messages. With this scheme, each receiver is frequency-locked with each other, but there is typically a phase-offset between them. Since these phase offsets tend to be constant, a higher-level compensation protocol can be used to globally synchronize a sensor network. We present the design of an LC tank receiver circuit tuned to the AC 60Hz signal which we call a Syntonistor. The Syntonistor incorporates a low-power microcontroller that filters the signal induced from AC power lines generating a pulse-per-second output for easy interfacing with sensor nodes. The hardware consumes less than 58μW which is 2--3 times lower than the idle state of most sensor networking MAC protocols. Next, we evaluate a software clock-recovery technique running on the local microcontroller that minimizes timing jitter and provides robustness to noise. Finally, we provide a protocol that sets a global notion of time by accounting for phase-offsets. We evaluate the synchronization accuracy and energy performance as compared to in-band message passing schemes. The use of out-of-band signals for clock synchronization has the useful property of decoupling the synchronization scheme from any particular MAC protocol. Our experiments show that over a 11 day period, eight nodes distributed across the floor of the CIC building on Carnegie Mellon's campus remained synchronized on an average to less than 1ms without exchanging any radio messages beyond the initialization phase.