[ntp:questions] Re: Tiime Sync solution
Richard B. Gilbert
rgilbert88 at comcast.net
Tue May 31 21:52:24 UTC 2005
Adrian 'Dagurashibanipal' von Bidder wrote:
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>Clinging to sanity, Richard B. Gilbert mumbled in his beard:
>>Eric Liu wrote:
>[need 1ms accuracy in LAN]
>>For 1 millisecond accuracy you will probably need to install a
>>hardware reference clock on your master server. Getting such accuracy
>>from servers on a WAN is chancy at best.
>While I agree that anything better than 10ms over regular Internet
>connections is pure luck, note that if you only need 1ms accuracy
>*relative* between the machines on the LAN, instead of 1ms accuracy to
>'real' UTC (as determined by whatever reference source you use), you
>may be able to go without a hardware reference.
>When you design your time infrastructure, pay attention that the LAN is
>fast enough and has only little load - if in doubt, use a dedicated
>'time network' in parallel to your regular data network. You didn't
>say what operating system you're using - it is said that Windows is
>horrible at timekeeping, Linux is ok, FreeBSD is best (not sure if the
>latter only applies for hardware reference clock performance or for
>general ntpd use.) I don't know about others - though I remember it
>being discussed in this group.
>My two old 350MHz machines here at home, connected over 10Mb LAN,
>half-duplex, has something in the range 1 to 2ms offset usually, with a
>quite small network load. So I guess faster hardware and especially a
>faster network should easily drive that down to where you want it.
>- -- vbi
> The blessed respite from screaming kids and
> soap operas for which you actually get paid.
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Offset is not quite the same as accuracy. It's the difference between
your clock and the server's clock. When I'm synchronized to GPS, I see
offsets on the internet servers ranging from +/- 2 ms to +/-30ms. A
server is only as good as what it is synchronized with. . . . I have
a server here at home that's technically stratum one because it's
synchronized to a WWV receiver. At night, when I have good, solid,
signal it's within a millisecond or so; during the daylight hours,
reception is poor and synchronization is not good!! During daylight
hours I have seen it be off by more than 30 ms!!!
There are some rock solid servers out there and then there are the
others, with all the stability of a bowl full of Jello! I will name no
names because there, but for the grace of God. . . .
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