[ntp:questions] IEEE 1588 (PTP) at the nanosecond level?

William Unruh unruh at invalid.ca
Tue Mar 18 15:33:56 UTC 2014


On 2014-03-18, Martin Burnicki <martin.burnicki at meinberg.de> wrote:
> Magnus Danielson wrote:
>> On 17/03/14 09:48, Martin Burnicki wrote:
>>> You'd need hardware (FPGA?) which can be clocked at 1 GHz, and even in
>>> the hardware signal processing you'd need to account for a number of
>>> signal propagation delays which you can eventually ignore at lower clock
>>> rates.
>>>
>>> So of course the effort becomes much higher if you want more accuracy,
>>> but this is always the case, even if you compare NTP to the "time"
>>> protocol, or PTP to NTP.
>>
>> You don't need to count at 1 GHz, you can achieve the resolution with
>> *much* lower frequencies. One pair of counters I have achieve 2,7 ps
>> single-shot resolution using 90 MHz clock. Interpolators does the trick.
>> There is many ways to interpolate.
>
> Agreed. I just thought the way to use a higher counter clock is more 
> obvious. All depends on how accurate and precise you can get your 
> timestamps, and this is probably easier with network packet timestampers 
> at both sides of a cable than with a wireless time transfer method like 
> GPS which usually suffers from delays which can't easily be measured, 
> like ionospheric delays. And yes, I know that this can be improved if 
> you receive 2 GPS frequencies instead of only the L1. ;-)

Unless it is a straight wire from one machine to the other, there are
lots of unconstrained delays by wire as well-- all the switches, etc
between you and the other end. Much worse than the ionosphere.



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