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

Unruh unruh-spam at physics.ubc.ca
Tue Apr 1 19:45:46 UTC 2008

hal-usenet at ip-64-139-1-69.sjc.megapath.net (Hal Murray) writes:

>>>> The basic idea is to do the time stamping in hardware deep in
>>>> the network adaper.  That avoids lots and lots of jitter.
>>>Yes, PTP can yield an accuracy better than 100 ns if both the NICs at the
>>>clients and the server support hardware timestamping of sent/received PTP
>>I am still confused. To timestamp you have to read the computer's clock.
>>That is a software operation-- reading the counter in the cpu, translating
>>to time, returning the result through the kernel, etc. That has all kinds
>>of variable latencies,etc. I am having trouble seeing 100ns. Also seeing
>>the PPS from the hardware clock and its interrupts. Or are you replacing
>>all of the hardware and software of the system? (new kernel, new interrupt
>>system, new nics, etc)

>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? 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!


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