A Statistical Time Synchronization Method for Frequency-Synchronized Network Clocks in Distributed Systems

Takao YAMASHITA  Satoshi ONO  

Publication
IEICE TRANSACTIONS on Information and Systems   Vol.E87-D   No.7   pp.1878-1886
Publication Date: 2004/07/01
Online ISSN: 
DOI: 
Print ISSN: 0916-8532
Type of Manuscript: PAPER
Category: Computer Systems
Keyword: 
frequency synchronization,  Allan variance,  frequency stability,  confidence interval,  hypothesis test,  real-time communication,  

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Summary: 
In this paper, we propose a statistical method of time synchronization for computer clocks that have precisely frequency-synchronized oscillators. This method not only improves the accuracy of time synchronization but also prevents degradation in the frequency stability of the precise oscillators when the errors in the measured time offsets between computer clocks caused by network traffic possess a Gaussian distribution. Improved accuracy of time synchronization is achieved by estimating the confidence interval of the measured time offsets between the clocks. Degradation in frequency stability is prevented by eliminating unnecessary time correction for the computer clock, because time correction generally causes changes in the frequency accuracy and stability of the precise oscillators. To eliminate unnecessary time correction, our method uses an extended hypothesis test of the difference between the current mean and the mean at the last time adjustment to determine whether time correction is needed. Evaluation by simulating changes in the time offset of the existing ISDN clock synchronization system showed that this method achieves accurate time and stable frequency synchronization.