The Statistics of Computer Clocks and the Design of Synchronization Algorithms

Abstract

In this study, I used standard statistical tools (such as the various forms of the two-sample Allan variance) to characterize the clocks in computers, and I show how the results of this study are used to design algorithms to synchronize the computer clocks. These synchronization algorithms can be used to synchronize the time of a computer to a local reference clock or to a remote server. The algorithms by themselves are not intended to be a simple replacement for software that implements the Network Time Protocol (NTP) or any other similar application. Instead, they describe the statistical principles that should be used to design an algorithm to synchronize any computer clock by using data from any external reference received in any format. These algorithms have been used to synchronize the clocks of the computers that support the Internet Time Service operated by the National Institute of Standards and Technology (NIST), and I illustrate the performance of the algorithm with real-time data from these servers. In addition to presenting the design principles of the algorithm, I illustrate the principles with two specific examples: synchronizing a computer clock to a local reference signal, and the design of a synchronization process that is based on time-difference data received from a remote server over the public Internet. The message exchange between the local system and the remote server in this configuration is realized in NTP format, but that is not a fundamental requirement.

A resilient architecture for the realization and distribution of coordinated universal time to critical infrastructure systems in the United States :

“…The various forms of the Allan deviation [8,33,34] are often used to describe the stability of clocks, oscillators, and channels that transport time and frequency information [32]. These metrics are generally useful, but are not adequate by themselves for characterizing the performance of the Special Calibration Test for these reasons:…”

“…This delay is internal to the hardware that applies a time stamp and is in addition to all the other delays discussed in the previous sections. The magnitude of this delay is a crucial issue because it can be difficult to maintain a timing accuracy of ±1 µs if this time stamping process is realized on general-purpose hardware [32]. Although this aspect is important, it will not be considered here, and is mentioned here because it is often both more important and more difficult to address when the reference time signals do not originate from a locally-connected GNSS receiver, which is the configuration described in this text.…”