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

Unruh unruh-spam at physics.ubc.ca
Tue Apr 1 16:45:12 UTC 2008


Martin Burnicki <martin.burnicki at meinberg.de> writes:

>Hal Murray wrote:
>>>>probably have make use of PTP (precision timing protocol).
>>>
>>>No idea what that is. If you had wanted super precision you would have put
>>>a GPS onto each machine, I hope.
>>>
>>>From the Wikipedia entry on PTP it looks absolutely no different from ntp.
>>>I have no idea what the idea is.
>> 
>> 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
>packets.

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)



>On the other hand, also *every* network node between the PTP endpoints has
>to be PTP-aware and compensate the packet delay it introduces, so you will
>probably only get full PTP accuracy in your local network where you have
>control over all the equipment.

>Switches can very well insert a delay in the range of milliseconds. If there
>are incoming packets at different ports at the same time which shall go out
>on the same port then the packets have to be queued. Unless the network is
>really heavily loaded this may happen only occasionally, but it may happen.

>The switches included in our PTP starter kit
>http://www.meinberg.de/english/ptp-starterkit/
>implement PTP boundary clocks for the ports in order to eliminate the
>queuing delay. Without this special handling PTP would suffer from the same
>latencies as NTP.

>On the other hand, NTP yields quite good results without requiring special
>hardware, even over WAN connections.
> 

>Martin
>-- 
>Martin Burnicki

>Meinberg Funkuhren
>Bad Pyrmont
>Germany




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