[ntp:questions] LAN synch question
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
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
> 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
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