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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sun Sep 14 07:48:54 UTC 2008


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?
> 




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