[ntp:questions] how to have offset < 1ms

David Lord snews at lordynet.org
Mon Apr 12 21:19:37 UTC 2010


lhommedumatch wrote:
> I have my PC clock synchronised with a ntp server (ntpgmtaceb).
> 
> With ntpq -p, I see an offset of 6ms.
> How can I be sure to have an offset < 1ms

Even if you select five internet sources I don't believe
you could guarantee better than 1ms with delays in 10's
of milliseconds.

For < 1ms a refclock is best option and a GPS receiver will
likely do the job if you have a sufficiently clear view of
sky from your location and should give < 100us without much
difficulty and < 1us at best. Otherwise a radioclock from
whatever is your nearest LF radio time signal should manage
< 10ms and very much better than that if close to transmitter.
Here a cheap MSF module can manage mostly to be within 300us
but adverse atmospheric conditions can cause occasional signal
cancellation so < 1ms couldn't be guaranteed, and that's about
160 km from transmitter location. DCF around 900 km distance
can almost be guaranteed to suffer signal cancellation early
morning and evening otherwise is ok for < 1ms and if close to
transmitter < 1 us if your receiver can phase lock to the
carrier.

Suitable GPS receivers can be obtained for around $60
DIY radioclock modules around $10


David
> 
> ntpq -p
>      remote           refid      st t when poll reach   delay
> offset  jitter
> ==============================================================================
>  LOCAL(0)        .LOCL.          10 l   61   64  377    0.000
> 0.000   0.001
> *ntpgmtaceb      .1PPS.           1 u  709 1024  377   29.431
> -6.450   0.917
> 
> Here is my ntp.conf:
> # Permit time synchronization with our time source, but do not
> # permit the source to query or modify the service on this system.
> restrict default kod nomodify notrap nopeer noquery
> 
> # Permit all access over the loopback interface.  This could
> # be tightened as well, but to do so would effect some of
> # the administrative functions.
> restrict 127.0.0.1
> restrict -6 ::1
> 
> # Hosts on local network are less restricted.
> #restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
> 
> # Use public servers from the pool.ntp.org project.
> # Please consider joining the pool (http://www.pool.ntp.org/
> join.html).
> 
> #broadcast 192.168.1.255 key 42         # broadcast server
> #broadcastclient                        # broadcast client
> #broadcast 224.0.1.1 key 42             # multicast server
> #multicastclient 224.0.1.1              # multicast client
> #manycastserver 239.255.254.254         # manycast server
> #manycastclient 239.255.254.254 key 42  # manycast client
> 
> # Undisciplined Local Clock. This is a fake driver intended for backup
> # and when no outside source of synchronized time is available.
> server 127.127.1.0
> fudge   127.127.1.0 stratum 10
> 
> # Drift file.  Put this in a directory which the daemon can write to.
> # No symbolic links allowed, either, since the daemon updates the file
> # by creating a temporary in the same directory and then rename()'ing
> # it to the file.
> driftfile /var/lib/ntp/drift
> 
> # Key file containing the keys and key identifiers used when operating
> # with symmetric key cryptography.
> keys /etc/ntp/keys
> 
> # Specify the key identifiers which are trusted.
> #trustedkey 4 8 42
> 
> # Specify the key identifier to use with the ntpdc utility.
> #requestkey 8
> 
> # Specify the key identifier to use with the ntpq utility.
> #controlkey 8
> server ntpgmtaceb
> restrict ntpgmtaceb mask 255.255.255.255 nomodify notrap noquery
> 
> Regards,
> Ludovic




More information about the questions mailing list