[ntp:questions] high precision tracking: trying to understand sudden jumps

Martin Burnicki martin.burnicki at meinberg.de
Wed Apr 2 09:50:11 UTC 2008


Unruh wrote:
> hal-usenet at ip-64-139-1-69.sjc.megapath.net (Hal Murray) writes:
>>You can build a clock into the network adapter and sync it up to the
>>system clock.
> 
> And how do you sync it up to the system clock without going through the
> kernel, etc? 

Maybe it's better to do it the other way round, i.e. sync the system clock
to the NIC's timestamp counter.

> Ie, I have a clock on my gps receiver that is good to 100ns. 
> It links to the system clock via interrupts and ntp. You have to do
> something like that if you are going to sync the clock on your nic to the
> system clock as well. Ie, I see no advantage to this procedure over
> putting in a cheap gps clock on each of the computers and just using that
> ( or running a buffered PPS line from one gps receiver to each of the
> machines (using some of the spare lines in a Cat5e cable if need be).Sure
> sounds cheaper than special nic cards with high accuracy on board clocks!

Just like an NTP server a PTP/IEEE1588 grandmaster can synchronize a huge
number of clients. Depending on the application this can either be the
"official" UTC time, or just the "same" time for all devices.

The target for PTP is more in industrial applications, where you have a
dedicated network environment and more and more embedded devices which can
be synchronized with high accuracy.

For example, the new LXI standard (Lan EXtensions for Instrumentation, see
http://en.wikipedia.org/wiki/LXI) which is a LAN-based successor to the old
GPIB bus, explicitely uses PTP/IEEE1588 to time-trigger measurements
accurately in spite of network latencies.

There are now NIC chips available which support PTP timestamping, and in
many measurement instruments there is a good oscillator which can be used
both for time stamping and implementation of the system clock. Those
devices normally have special printed circuit boards and dedicated software
which supports that on-board hardware.

So this is different from using PTP to synchronize a standard PC where you
don't know which OS or version of an OS is running, and which types of
hardware (e.g. NICs) are installed, and how to synchronize the system time
to the counter chain on a PCI card or whatever.


Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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