[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 10:26:12 UTC 2008


Uwe Klein wrote:

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

That's because it transmits rather a large number of bits per signalling
unit.  log2(resolution of timing of edge within second) bits of time
information are sent on any long or short signalling unit.  (In
practice, you won't recover every bit on every measurement, so you need
several seconds to improve the signal to noise ratio.  Once one is
synchronised, the number of useful bits drops greatly, because there is
a very high level of redundancy in the coding.)

There is nothing in NTP protoco that guarantees that signalling units
are second aligned.  However, one could get high resolution on very slow
links by making the transmit time represent the start of signalling unit
time, rather than the time the transmit request was queued. That would
require special, low level, drivers.




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