Accuracy of syncing with Internet time servers?? - NTP

This is a discussion on Accuracy of syncing with Internet time servers?? - NTP ; What is the accuracy of setting the time using WinXP time sync utility? It lists two servers: time.windows.com time.nist.gov I assume it is based on NTP. Does NTP take into account the transmission/network delay between the server and my PC? ...

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Thread: Accuracy of syncing with Internet time servers??

  1. Accuracy of syncing with Internet time servers??

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

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

    I assume it is based on NTP. Does NTP take into account the
    transmission/network delay between
    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
    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?


  2. Re: Accuracy of syncing with Internet time servers??

    maruk2@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
    >
    > I assume it is based on NTP. Does NTP take into account the
    > transmission/network delay between


    You assume wrongly! It uses SNTP rather than the full NTP implementation.
    > 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
    > 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?
    >


    If you want millisecond accuracy you need to run the Windows version of
    ntpd. You will also need at least four internet servers that are near
    you in "net space". If you are very lucky you can get within five or
    ten milliseconds of the correct time. Better still would be a hardware
    reference clock such as a GPS timing receiver.

    I run a ten year old Sun Ultra 10 workstation with a 440 MHz processor,
    ntpd V4.2, and a Motorola GPS Timing receiver and I get microsecond
    accuracy. I could do the same with a PC though I wouldn't choose
    Windows as the operating system (Solaris X86 or Free BSD).

  3. Re: Accuracy of syncing with Internet time servers??

    In article <1164939931.742775.54520@16g2000cwy.googlegroups.co m>,
    maruk2@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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