NTP question

Keith Medcalf kmedcalf at dessus.com
Thu May 2 00:48:15 UTC 2019


On Wednesday, 1 May, 2019 15:36, Harlan Stenn <stenn at nwtime.org> wrote:

>So I gotta ask, just as a reality check:

>- Why do folks want to have one or more NTP server masters that have
>at least 1 refclock on them in a data center, instead of having their
>data center NTP server masters that only get time over the internet?

That entirely depends on what you need the time for.  

For example, in a Continuous Control environment you really do not care about the accuracy of the time -- just like a printer will not suddenly fail to print documents with dates in them because of Y2K, the printer neither cares nor knows what time it is.

What you may care about, however, is that all your Distributed Control and Outboard Systems have the SAME TIME and that that time, relative to each other, is closely synchronized.  This has a huge impact when comparing log events from one system to another.  What is important is that they all have the same time, and that they all drift together.

If you have one such installation, then you really do not care about the "accuracy" of the time.  However if you have multiple such installations then you want them all to have the same time (if you will be comparing logs between them, for example).  At some point it becomes "cheaper" to spend thousands of dollars per site to have a single Stratum 0 timesource (for example, the GPS system) at each site (and thus comparable time stamps) than it is to pay someone to go though the rigamarole of computing offsets and slew rates between sites to be able to do accurate comparison.  And if you communicate any of that info to outsiders then being able to say "my log timestamps are accurate to +/- 10 nanoseconds so it must be you who is farked up" (and be able to prove it) has immense value.

---
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