[ntp:questions] NTP and SNTP in the end, give me precision of 1ms?

Uwe Klein uwe_klein_habertwedt at t-online.de
Sun Sep 14 09:42:58 UTC 2008


David Woolley wrote:
> Hal Murray wrote:
> 
>>> There is nothing per se that makes this system impossible to deliver 
>>> 1ms. Of course it depends of where those clients are-- if they are at 
>>> the bottom
>>> of the sea communicating with 1bd/sec ultralow frequency radio, you will
>>> not get 1ms precision.
>>
>>
>> What's wrong with a (very) slow link?  As long as there aren't any
>> queueing delays, the delay should be symmetric in both directions
>> and I'd expect ntpd to work OK.
> 
> 
> A one baud link would have an uncertainty of a second in the time, 
> unless the transmit time stamps were synchronised with the signalling 
> units.  It would also have a delay that was on the limits of causing 
> rejection for a normal NTP implementation.
> 
> A 1 bit/second link would definitely have an unacceptable delay.
> 
> A 1 baud/second link would have a continually varying latency, and, 
> unless the bits per signalling unit varied to compensate, a continually 
> varying delay.  I don't think baud/second was the intended unit.  I 
> suspect he was suggesting a 1 bit per signalling unit, 1 signalling unit 
> per second, case.
> 
>>
>> Does 1 bd/sec overflow some of ntpd's assumptions?  Would it need
>> some minpoll tweaking?
>>
>> Or maybe I should ask:  How slow a link is still useful?
>>

DCF77 for example is a "1 bit/s" transmission, a sentence taking 60s.
It seems to not have issues with ms syncing.

/ bits/second and Baud can be a bit mushy: DCF can be
   viewed as a ternary code ( short, long, none ) /

uwe




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