Effects of mindist setting - NTP

This is a discussion on Effects of mindist setting - NTP ; I have a bunch Endrun Technologies CDMA clock attached to small servers using PPS. To prevent the jitter on the serial port from causing the clock to be marked as falsetick and disabling the PPS, I have added "tos mindist" ...

+ Reply to Thread
Results 1 to 8 of 8

Thread: Effects of mindist setting

  1. Effects of mindist setting

    I have a bunch Endrun Technologies CDMA clock attached to small servers
    using PPS. To prevent the jitter on the serial port from causing the
    clock to be marked as falsetick and disabling the PPS, I have added "tos
    mindist" to my configuration.

    I am unclear on the actual effect of this configuration and that makes
    me a bit unsure of how I should set the value. 0.010 was one
    recommendation and 0.015 was another.

    Any place I can look for an explanation of this? I don't see in in the
    documentation.
    --
    R. Kevin Oberman, Network Engineer
    Energy Sciences Network (ESnet)
    Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
    E-mail: oberman@es.net Phone: +1 510 486-8634
    Key fingerprint:059B 2DDF 031C 9BA3 14A4 EADA 927D EBB3 987B 3751

    _______________________________________________
    questions mailing list
    questions@lists.ntp.isc.org
    https://lists.ntp.isc.org/mailman/listinfo/questions
    -----BEGIN PGP SIGNATURE-----
    Version: GnuPG v1.4.5 (FreeBSD)
    Comment: Exmh version 2.5 06/03/2002

    iD8DBQFFFAebkn3rs5h7N1ERAnX3AJ0VVbe/0JBvnfDXuf5vzJDNcZcxwwCglA8Q
    R88KrFjzHgG9lBrByML9Of8=
    =Kmgb
    -----END PGP SIGNATURE-----


  2. Re: Effects of mindist setting

    Kevin Oberman wrote:
    > I have a bunch Endrun Technologies CDMA clock attached to small servers
    > using PPS. To prevent the jitter on the serial port from causing the
    > clock to be marked as falsetick and disabling the PPS, I have added "tos
    > mindist" to my configuration.
    >
    > I am unclear on the actual effect of this configuration and that makes
    > me a bit unsure of how I should set the value. 0.010 was one
    > recommendation and 0.015 was another.
    >
    > Any place I can look for an explanation of this? I don't see in in the
    > documentation.
    >
    >
    > ------------------------------------------------------------------------
    >
    > _______________________________________________
    > questions mailing list
    > questions@lists.ntp.isc.org
    > https://lists.ntp.isc.org/mailman/listinfo/questions


    Either your posting was delayed many hours or your clock is way off!

  3. Re: Effects of mindist setting

    Kevin,

    See the architecture briefing at the NTP project page and especially the
    slide on the selection/intersection algorithm. The mindist value is
    added to the correctness interval in order to widen it and allow for
    increased jitter.

    Dave

    Kevin Oberman wrote:
    > I have a bunch Endrun Technologies CDMA clock attached to small servers
    > using PPS. To prevent the jitter on the serial port from causing the
    > clock to be marked as falsetick and disabling the PPS, I have added "tos
    > mindist" to my configuration.
    >
    > I am unclear on the actual effect of this configuration and that makes
    > me a bit unsure of how I should set the value. 0.010 was one
    > recommendation and 0.015 was another.
    >
    > Any place I can look for an explanation of this? I don't see in in the
    > documentation.
    >
    >
    > ------------------------------------------------------------------------
    >
    > _______________________________________________
    > questions mailing list
    > questions@lists.ntp.isc.org
    > https://lists.ntp.isc.org/mailman/listinfo/questions


  4. Re: Effects of mindist setting

    Kevin Oberman wrote:
    > I have a bunch Endrun Technologies CDMA clock attached to small servers
    > using PPS. To prevent the jitter on the serial port from causing the
    > clock to be marked as falsetick and disabling the PPS, I have added "tos
    > mindist" to my configuration.
    >
    > I am unclear on the actual effect of this configuration and that makes
    > me a bit unsure of how I should set the value. 0.010 was one
    > recommendation and 0.015 was another.
    >
    > Any place I can look for an explanation of this? I don't see in in the
    > documentation.


    I see Dr Mills pointed you to the mathematical description...

    For the 'code' description - see ntp_proto.c:3448 (the middle of the clock
    intersection algorithm). The root_distance() function uses the mindist value to
    basically state that no clock is 'better' than the given dispersion distance.

    I recommended the 0.015 value for the owamp systems simply because our
    calibration tests of the CDMA serial interface put it between 0.012 and 0.013
    off from the PPS. (fudge time1)

    After thinking this over more - I don't think mindist should be as large as I said.

    First - once we set the time1 value the magnitude of the dispersions should be
    decreased. (Was the 0.010 recommendation in
    http://ntp.isc.org/bin/view/Support/...3801ARefclocks
    based on any tests? Or perhaps assumptions based on Unix scheduling?)

    Second, after more closely examining the code, I realize mindist actually sets
    the root distance for each and every peer to the max of the real distance, and
    this value. (This makes perfect sense - I just did not think through it.)

    Unfortunately, this will effect the error estimate. For the one-way network
    latency measurements - we really want to minimize the error estimates as much as
    we can.

    So, even the 0.010 value is probably larger than we want. We should experiment
    with our hardware/OS combinations and try and use the smallest value we can that
    removes the clock hopping problem.

    jeff

  5. Re: Effects of mindist setting

    >>> In article <12hgom22phb2d2e@corp.supernews.com>, "Jeff W. Boote" writes:
    Jeff> First - once we set the time1 value the magnitude of the dispersions
    Jeff> should be decreased. (Was the 0.010 recommendation in
    Jeff> http://ntp.isc.org/bin/view/Support/...3801ARefclocks based
    Jeff> on any tests? Or perhaps assumptions based on Unix scheduling?)

    No idea - please add your experience and ideas there.

    H

  6. Re: Effects of mindist setting


    >First - once we set the time1 value the magnitude of the dispersions should be
    >decreased. (Was the 0.010 recommendation in
    >http://ntp.isc.org/bin/view/Support/...3801ARefclocks
    >based on any tests? Or perhaps assumptions based on Unix scheduling?)


    I wrote that text.

    I think I pulled that value from a posting here, but that was a long
    time ago. google-groups on mindist might find it. Try the thread
    from Feb 2005, Subject: On jittery clocks and precision PPS signals

    I tried it. It fixed my problem. I haven't done any testing.

    --
    The suespammers.org mail server is located in California. So are all my
    other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited
    commercial e-mail to my suespammers.org address or any of my other addresses.
    These are my opinions, not necessarily my employer's. I hate spam.


  7. Re: Effects of mindist setting

    Guys,

    The tinker mindist given on the cited ISC page is misleading. Each case
    with each refclock, driver and kernel is different. The mindist defaults
    to the minimum dispersion MINDISP (.005 s). This implies a rather close
    agreement between the serial timecode signal and PPS signal. I have
    observed wide variations between various refclocks with these signals,
    due mostly to the FIFO in late model UART chips. The FIFO needs to be
    disabled and, in some systems, the kernel driver software FIFO needs to
    be disabled as well. Means for doing this vary widely between operating
    systems and motherboards.

    Those drivers that use the median filter in the refclock interface
    usually find residual jitter well beneath the noise floor established by
    the measured system precision. Peek at rackety.udel.edu and note the
    jitter is usually only a few microseconds. One reason for the low PPS
    jitter is that the atom driver uses the median filter and kernel PLL
    discipline, but not the kernel PPS discipline. Note also that the
    variation between the timecode and PPS signals is usually only a few
    microseconds. Not too shabby for a very old Intel machine with FreeBSD,
    GPS, parallel-port PPS and many hundreds of clients.

    The experience with rackety suggests an interesting conclusion. The
    residual errors with the timecode signal are almost as low as with the
    PPS signal. So, why use the PPS at all? You need it to calibrate the
    timecode signal with respect to the PPS signal, normally the most
    accurate. The trick is to get the systematic offset (fudge time1) with
    the serial driver) whittled down as far as possible and tinker the
    mindist to something greater than the residual jitter. The calibrate
    command in the configuration file can be used to do this automatically.

    The Spectracom GPS seen by rackety is rather like a rock, but our
    Spectracom WWVB receivers are very much worse due to local RFI generated
    by UPS systems. The only one remaining that works at all is at my
    suburban home, but even there RFI pollution has degraded accuracy and
    consistency between timecode and PPS signals to several milliseconds and
    does require tinker mindist. Your mileage may vary.

    Dave

    Harlan Stenn wrote:

    >>>>In article <12hgom22phb2d2e@corp.supernews.com>, "Jeff W. Boote" writes:

    >
    > Jeff> First - once we set the time1 value the magnitude of the dispersions
    > Jeff> should be decreased. (Was the 0.010 recommendation in
    > Jeff> http://ntp.isc.org/bin/view/Support/...3801ARefclocks based
    > Jeff> on any tests? Or perhaps assumptions based on Unix scheduling?)
    >
    > No idea - please add your experience and ideas there.
    >
    > H


  8. Re: Effects of mindist setting

    On Fri, 22 Sep 2006 15:56:11 GMT, oberman@es.net (Kevin Oberman) wrote
    for the entire planet to see:

    >I have a bunch Endrun Technologies CDMA clock attached to small servers
    >using PPS. To prevent the jitter on the serial port from causing the
    >clock to be marked as falsetick and disabling the PPS, I have added "tos
    >mindist" to my configuration.


    I don't use the PPS but the RTS style of interface to the Endrun CDMA.
    This eliminates any concern about serial latency. I'm not sure it
    offers as good precision as PPS if PPS is working well, but it's has
    much lower potential for jitter and latency problems.

    - Eric


+ Reply to Thread