[ntp:questions] Accuracy of syncing with Internet time servers??

David Woolley david at djwhome.demon.co.uk
Fri Dec 1 07:16:33 UTC 2006

In article <1164939931.742775.54520 at 16g2000cwy.googlegroups.com>,
maruk2 at hotmail.com wrote:

> What is the accuracy of setting the time using WinXP time sync utility?

> It lists two servers:
> time.windows.com
> time.nist.gov

For high accuracy, you should use servers that are close in network
terms, not overloaded ones halfway across the world.

> I assume it is based on NTP. Does NTP take into account the

Prior to the lastest service pack of Windows 2003, it is a broken 
implementation of SNTP, which, in unbroken form, uses NTP wire formats,
but none of the tactics used by NTP for high accuracy.  The Windows 2003
version appears not to have the clock interpolation that is present
in the reference implementation.

> transmission/network delay between

NTP assumes transmission delays are symmetrical and corrects by half the
round trip delay.  It also uses the delay as a measure of server
quality.  I can't say for certain what the Microsoft implementations do.

> the server and my PC? Assuming the server itself is very accurate, is
> it possible to set the time of my PC to one millisecond?  What is the

On Unix and similar yes.  On Windows, you may succeed, when using
the reference implementation of NTP, but not with W32Time, but ordinary
programs will only be able to read the time to an order of magnitude 
worse accuracy, because they will only see the clock tick resolution.

In my experience, even the reference implementation only works well when
the Windows machine is unloaded, however I haven't used a recent version.

Windows is also prone to losing clock interrupts during disk activity,
causing abrupt steps of over 10ms.

> most accurate way to set the PC time? Are there any GPS devices that
> connect to a PC and can set the time to a single millisecond?

The reference implementation can do this with any GPS time transfer 
receiver with a pulse per second output, although I'm not sure if
PPS is supported on Windows.

Note that true NTP systems take some time to correct any initial error.

Generally, do not use Windows if you need 1ms time resolution; it is
simply not designed for this sort of use.  This is regardless of how
you synchronise its time.

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