[ntp:questions] LAN synch question

David Woolley david at ex.djwhome.demon.co.uk.invalid
Thu Sep 11 21:33:59 UTC 2008

rochertov at gmail.com wrote:
> I just want to clarify something.  ntpdc can be used to check on the
> offset from the peer, right?  What is the smallest offset in clocks

You cannot find out the offset from the peer; to do that you need to 
synhronise an independent clock to an order of magnitude better, then 
read the two software clocks relative to that alternative time 
synchronisation mechanism.

The value you get for offset is the difference between a smoothed 
estimate of the true time, based on measurements over an extended 
period, and an average of the most recent measured times from the active 
servers.  The latter should contain a lot more measurement noise than 
the former.

> that I can expect on a 1 Gbps LAN?

Depends on interrupt latency, network loading, the total lack of any 
Windows machines in the time synchronisation chain, etc.  100ns should 
be possible for lightly loaded machines on a lightly loaded network, at 
least 90% of the time.

However, my feeling is that the only acceptable solution in your case 
would be to feed a pulse per second signal into a PCI or ISA connected 
serial or parallel port (not USB) from the same line driver.  You will 
need an OS with kernel PPS support.

To use 100ns timing in your applications, you will need an OS and 
hardware that support a timing source with maybe 10ns resolution.  This 
excludes Windows and any OS more then about 3 or 4 years old.

Note that broadcast mode will be critically dependent on the initial 
calibration exchange.

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