How do you know what time it is?

By on 28 Apr 2020

Category: Tech matters

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Louis Essen and J. V. L. Parry standing next to the world's first caesium atomic clock, developed at the UK National Physical Laboratory in 1955. (Image: Wikimedia Commons)

At Netnod, we are really interested in time.

We provide one of the first Network Time Protocol (NTP) services to be enabled with Network Time Security (NTS). We also provide Precision Time Protocol (PTP) services for organizations that need to ensure the highest level of time accuracy.

Read: Network Time Security: new NTP authentication mechanism

In this post, we will take a look at some of the fundamentals in providing accurate time. These include looking at what makes a clock, how to ensure accuracy down to the level of nanoseconds and what we are doing to ensure accurate time throughout Sweden. 

What makes a clock?

At the basic level, to measure time you need something that ‘ticks’ (for example, sand in an hourglass or the oscillations of a caesium atom) and something that enables you to count the frequency of these ticks. If the frequency is stable, it makes it easier to measure time accurately. 

For thousands of years, humans have used the rotation of the earth and its movement around the sun to measure time but the frequency is not stable enough to allow highly accurate time measurements. Today we use the oscillations of a caesium atom to create what is known as International Atomic Time (TAI). This is defined as the weighted average of the time kept by about 200 atomic clocks in over 50 national laboratories worldwide. The other two main timescales in use are:

  • Universal time (UT1) — defined by the Earth’s rotation, with adjustment for polar wandering.
  • Coordinated Universal Time (UTC) — uses TAI and adds leap seconds when needed to keep the difference between UTC and UT1 less than 0.9s.

How to ensure accuracy?

High precision time measurements depend on using multiple clocks to ensure redundancy and stability. But when using two or more clocks you need to account for what are known as frequency and phase errors (see Figure 1).

Figure 1 — Example of frequency and phase errors.
Figure 1 — Example of frequency and phase errors.

The difference between A and B shows when two clocks are out of frequency and C shows when the frequencies don’t peak at the same time (known as a phase error). 

Frequency and phase errors can cause significant problems especially in sectors where the highest level of time accuracy is needed (for example, when running power networks). Minimizing these errors is an essential part of running a time service but can pose an interesting engineering problem.

If you have two clocks and need to establish accuracy to the level of single-digit nanoseconds, you can face a situation where you are effectively racing against the speed of light. Given that the speed of light is about one foot per nanosecond, if the clocks are far enough apart, the propagation time between the two clocks might be larger than the phase error you are trying to correct.

Netnod’s time services

Netnod provides NTP, NTS and PTP services offering a robust, reliable and highly accurate source for time and frequency. 

Netnod’s NTP service, funded by the Swedish Post and Telecom Authority, uses a distributed timescale on autonomous nodes throughout Sweden to provide a time service available over IPv4 or IPv6 and traceable to within 250 nanoseconds of official Swedish time UTC (SP). Each site has redundant servers, two caesium clocks, and two FPGA boards providing an extremely fast hardware implementation of NTP.

Figure 2 — Netnod’s redundant time and frequency node, with caesium atomic clocks.
Figure 2 — Netnod’s redundant time and frequency node, with caesium atomic clocks.

Netnod staff have been instrumental in the creation of NTS. Apart from editing the IETF proposed standard, Netnod staff have also developed both server and client-side implementations of NTS.

Netnod’s NTP service is one of the first anywhere in the world to offer NTS. Netnod’s PTP service provides traceable time over a dedicated fibre, which enables organizations to get time and frequency to the highest degree of accuracy. 

Next steps

Netnod will add a time and frequency node in Luleå, Sweden, and will work to make all nodes more robust against jamming and replay attacks against the Global Navigation Satellite System. We will continue to develop and deploy the NTS protocol.

This post is based on two previous talks I gave: one at APNIC, where Netnod has a long-standing relationship involving, among other things, the deployment of I-root servers in the Asia Pacific region; and the second at the FOSS North 2020 conference (see video below).

Adapted from original post which appeared on Netnod’s Blog.

Patrik Fältström is Technical Director and Head of Security at Netnod.

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The views expressed by the authors of this blog are their own and do not necessarily reflect the views of APNIC. Please note a Code of Conduct applies to this blog.

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