[ntp:questions] Onboard Local Oscillator Change Improvements

elio.gi at gmail.com elio.gi at gmail.com
Wed Oct 10 09:14:59 UTC 2007


Hi ,

On 8 Ott, 22:06, "David L. Mills" <mi... at udel.edu> wrote:
> The performance with ntpd running in a rusting Pentium hulk with several
> hundred clients and PPS is surprisingly good. Our rackety.udel.edu runs
> FreeBSD 6.1 with PPS via the parallel port. It usually shows residual
> offset and jitter in the order of a few microseconds. It is good enough
> that the daily variations in the air-conditioned machine room
> temperature can be measured accurately.
>
> Measuring the PPS offset within a few hundred nanoseconds is brilliant;
> however, the measured latency to read the system clock from an
> application is in the order of a microsecond and this polluted by
> context switches, cache misses, timer interrupts, etc.

I work for a big international telecom Company (Number 1 in Mobile)
and I think that I have to give you more details of our Company
project that could use NTP technology.
In this moment we're making our tests with NTP on a test pc, but our
aim is to run NTP on our telecom blades which have onboard their own
processor and an embedded linux OS.
These blades will get a PPS signal derived from a GPS and, thanks to
ntpd daemon, will synchronize their local time.
These "Stratum 1" blades will send to other (Stratum 2) blades NTP
messages, so the question is:

- is the precision of these "messages" (or timestemps) affected by the
stability of the local oscillator ?

The application will run in a MAN corporate network where packet
delays should be small compared to the Internet delays. So it could be
worth to enhance the accuracy of the timestemps provided by the ntp
stratum1 server; than the second question is:

- which kind of accuracy can we reach enhancing the stability of the
oscillator ? (nanoseconds? )
(note that we already use high stability oscillator inside the
equipment, that could also feed the processor clock)

The same question at client side is:

- which accuracy of local clock can we expect using high stability
oscillators? (we have measured around 60 microseconds  peak-to-peak,
using standard crystal in *environment test* configuration- see
below )

****************************************
* Environment Test Configuration *
****************************************

GPS
   ||
   \/
Derived PPS on a cable conneted to the serial port of Stratum 1 pc
   ||
   \/
Stratum 1 PC with ntpd running
   ||
   \/
Ethernet Cable
   ||
   \/
Stratum 2 PC with ntpd running
  ||
  \/
Output PPS signal passing by parallel port pin
  ||
  \/
Measure Instrument with GPS Ref. Clock
**************************************************************

Thanks in advance,
Best Regards,
Elio




More information about the questions mailing list