I am running a software on a Linux machine, I am synchronizing time via default ntpd configurations available in Linux (/etc/ntp.conf).
I am connecting to the customer NTP server.
Now i was requested to tell which synchronization type is used on my machine (Frequency Synchronization/ Time Synchronization or Time-of-day Synchronization)

Now after some googling it seems there is two types for NTP synchronization:
1- Phase Synchronization.
2- Frequency Synchronization.

But how can I decide which of them is used by Linux? I am already connected the customer NTP server and they told that their NTP doesn't support Time Synchronization, so should it be Frequency Synchronization which is used by Linux machines?

The customer is refering to RFC5905 in his questions but I couldn't find anything useful about my question in the RFC.

Thanks in advance for assistance.

2 Months
Discussion Span
Last Post by rproffitt

Why does this matter to you? How accurately you can sync your clock? Anything within one second is not discernable to humans.


Since NTP can reduce synchronization offsets to times of the order of a few milliseconds over the public Internet, the ONLY time this question came up is when cellural comms as in base stations were concerned. For all practical considerations NTP is orders of magnitude better than what a human needs.

I can write that is is not phase sync...


Hi Rubberman,
I really dont care about this but its a question I got from the customer, the NTP vendor wants to know the type of sync used by all clients connected to their NTP server.

Hello Rproffitt,
Thank you for your answer, yes this question came up only when talking abour radio network element in mobile operators usually, but for some reason they wanted to know which exact sync algorithm is used by us and I tried to check online for an answer but couldn't find anything useful about if Linux is using frequency or phase sync, however yes I came up that the algorithm used by Linux is more near frequency but can't be phase sync.

Thank you!


@fo2sh. The code is open source which means open to inspection. I neglected to note the algorithm is on the web too but frequency? Nope, it's just setting the clock from NTP sources to within a few milliseconds. If that is not good enough then you need one of those cellular equipment time sources. My background includes cellular base station design.

http://www.cisco.com/c/en/us/about/press/internet-protocol-journal/back-issues/table-contents-58/154-ntp.html has the basics and references on NTP.

I've yet to find anyone ask for more since the goal is the time be set to withing a second. Since system response time can vary under load, it would be silly to think the clock being off by 100 milliseconds to matter. Base stations and the DSP system there would need better than this so they use a station clock (those dedicated highly accurate clocks.)

Votes + Comments
Well said. Cell phone clocks are usually synced to the local cell tower.
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