RE: [TICTOC] Re: TICTOC NTP ensembling

Dave,

If an ensemble is a collection of separate reference clocks, then I can see your logic.

If an ensemble is a collection of paths back to the same PRC,
  then it seems like if the noise goes up as you add paths, 
  then there is something wrong with the combining algorithm, 
   not the basic plan to combine clocks.

Which sort of ensemble do you feel is more suitable for "carrier class" timing?

(Also I wonder if the tradeoffs between providing 
   a wander free frequency reference and 
   a correct time of day reference are mixed into this.)

Regards,

Stuart 

-----Original Message-----
From: DTonks@semtech.com [mailto:DTonks@semtech.com]
Sent: Monday, March 19, 2007 11:48 AM
To: tictoc@ietf.org
Subject: [TICTOC] Re: TICTOC NTP ensembling

Attending the WSTS07 last week, I was left with the impression that people
considered one of the good things about NTP is the ability to form a clock
from an ensemble of master clocks. However, I see this as another
potentially serious source of noise which may swamp the output noise budget
for many telecom applications. I can see that having an ensemble could
offer good performance, but I think that would only be acheived if the
ensemble was a stable collection. If master clocks can appear and
disappear, then their individual biasses will pull the output phase around
a little. For the generally-accepted accuracy of 1-10ms, this effect is
probably not visible, but when the application requires +/-500ns accuracy
then I think this problem will dominate. I am not an NTP expert, but I can
imagine this problem is more difficult to solve than PDV-filtering. I would
also like to know what the carriers think of this - do they consider this
approach to be "carrier-class"?

Dave

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