[ntp:questions] Re: Time sync over bad modem line
Richard B. Gilbert
rgilbert88 at comcast.net
Thu Oct 28 11:35:04 UTC 2004
Nagy Bela wrote:
>>Not much. Fifteen minutes is a very short interval in which to
>>synchronize a clock. I have watched a late model Compaq/HP PC (D530)
>>running Solaris 8 Intel Platform Edition synchronize to a local server
>>(500 microsecond delay). Fifteen minutes allowed only a crude
>>correction of the clock. After about 12 hours, the clock was as well
>>synchronized as it's ever likely to be and the time was within a 500
>>microseconds or so of the server.
>>
>>
>>
>>>Because most of the time, this computer is connected to the net only for
>>>10-15 minutes and I would like to have my clock as accurate as possible.
>>>
>>>
>>>
>>>
>>How much accuracy do you need? How much are you willing to pay for?
>>
>>
>Well, as usual, the most accuracy possible for the least money.
>(12 hours costs simply too much)
>(cont.d later)
>
>
>
>>>Or a "reverse" question: If my computer is connected to the net for,
>>>say, 15 minutes and we assume nothing about the line (i.e. several second
>>>long delay may occur and this delay may vary widely), then
>>>how accurately can ntp set my clock?
>>>
>>>
>>>
>>>
>>The error is bounded by the delay; it may be less than the delay but it
>>can't be worse.
>>
>>
>Good news. So, roughly speaking, the ping times show the accuracy (?).
>
>
>
>>With the required accuracy known, you can then select the technology
>>required to attain it! Within one second you can set it manually using
>>a broadcast time reference. For within 100 milliseconds you can set
>>your clock once per day with ntpdate or ntpd -g. For time within 10
>>
>>
>Plus-minus 100 ms is OK for me.
>I'll try to combine this solution with the one that Harlan Stenn
>recommended (local clock with high stratum and dyn. ip and dynamic
>reconfiguration + burst, but it's likely that burst wont work because
>I don't permissions for this).
>
>By the way, which ntp servers do you recommend for me?
>(I'm in Hungary and I know only one: time.kfki.hu)
>
>
>
>>milliseconds you will almost certainly need a continuous internet
>>connection and four or five servers, each with a round-trip delay of
>>less than 20 milliseconds. For time within one millisecond, you will
>>probably need a hardware reference clock. Und so weiter. . . .
>>
>>
>Thank you for your help. During this weekend I test these tricks.
>Und danke sehr.
>
>Regards,
>B.N.
>
>
I'm in New Jersey. I've never needed to know which servers in Europe
are good. Go to
http;//www.ntp.org
and follow the link to "Time Servers". Eventually you will get to two
lists of public servers. From the list of stratum 2 servers, select
those servers that offer the smallest values of delay. A single server
is probably adequate for your application. With a permanent internet
connection you would configure not fewer than five servers for the most
reliable operation.
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