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A tale of two synchronizing clocks

Published: 04 November 2009 Publication History

Abstract

A specific application for wastewater monitoring and actuation, called CSOnet, deployed city-wide in a mid-sized US city, South Bend, Indiana, posed some challenges to a time synchronization protocol. The nodes in CSOnet have a low duty cycle (2% in current deployment) and use an external clock, called the Real Time Clock (RTC), for triggering the sleep and the wake-up. The RTC has a very low drift (2 ppm) over the wide range of temperature fluctuations that the CSOnet nodes have, while having a low power consumption (0.66 mW). However, these clocks will still have to be synchronized occasionally during the long lifetime of the CSOnet nodes and this was the problem we confronted with our time synchronization protocol. The RTC to fit within the power and the cost constraints makes the tradeoff of having a coarse time granularity of only 1 second. Therefore, it is not sufficient to synchronize the RTC itself---that would mean a synchronization error of up to 1 second would be possible even with a perfect synchronization protocol. This would be unacceptable for the low duty cycle operation---each node stays awake for only 6 seconds in a 5 minute time window. This was the first of three challenges for time synchronization. The second challenge is that the synchronization has to be extremely fast since ideally the entire network should be synchronized during the 6 second wake-up period. Third, the long range radio used for the metropolitan-scale CSOnet does not make its radio stack software available, as is seen with several other radios for long-range ISM band RF communication. Therefore, a common technique for time synchronization---MAC layer time-stamping---cannot be used. Additionally, MAC layer time-stamping is known to be problematic with high speed radios (even at 250 kbps).
We solve these challenges and design a synchronization protocol called Harmonia. It has three design innovations. First, it uses the finely granular microcontroller clock to achieve synchronization of the RTC, such that the synchronization error, despite the coarse granularity of the RTC, is in the microsecond range. Second, Harmonia pipelines the synchronization messages through the network resulting in fast synchronization of the entire network. Third, Harmonia provides failure handling for transient node and link failures such that the network is not overburdened with synchronization messages and the recovery is done locally. We evaluate Harmonia on CSOnet nodes and compare the two metrics of synchronization error and synchronization speed with FTSP. It performs slightly worse in the former and significantly better in the latter.

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      cover image ACM Conferences
      SenSys '09: Proceedings of the 7th ACM Conference on Embedded Networked Sensor Systems
      November 2009
      438 pages
      ISBN:9781605585192
      DOI:10.1145/1644038
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      Published: 04 November 2009

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      Author Tags

      1. low duty cycle
      2. sensor
      3. synchronization

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      • (2020)Novel Maximum Likelihood Estimation of Clock Skew in One-Way Broadcast Time SynchronizationIEEE Transactions on Industrial Electronics10.1109/TIE.2019.295542767:11(9948-9957)Online publication date: Nov-2020
      • (2019)Dependability in edge computingCommunications of the ACM10.1145/336206863:1(58-66)Online publication date: 20-Dec-2019
      • (2019)REMEDY: Receiver-Initiated MAC Based on Energy-Efficient Duty-Cycling in the IoUTIEEE Access10.1109/ACCESS.2019.29317337(105202-105211)Online publication date: 2019
      • (2018)On sequential frame synchronization with clock misalignment2018 IEEE Wireless Communications and Networking Conference (WCNC)10.1109/WCNC.2018.8377407(1-6)Online publication date: Apr-2018
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      • (2016)Synchronization Protocols and Implementation Issues in Wireless Sensor Networks: A ReviewIEEE Systems Journal10.1109/JSYST.2014.236046010:2(617-627)Online publication date: Jun-2016
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