Subject: [TICTOC] Re: TICTOC NTP ensembling

From: [email-masked]
Date: Mon, 19 Mar 2007 16:47:54 +0000

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

Semtech Ltd
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From: "STUART VENTERS" <[email-masked]>
Date: Mon, 19 Mar 2007 13:39:27 -0500

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: [email-masked] [mailto:[email-masked]]
Sent: Monday, March 19, 2007 11:48 AM
To: [email-masked]
Subject: [TICTOC] Re: TICTOC NTP ensembling

Dave

Semtech Ltd

---

From: [email-masked]
Date: Tue, 20 Mar 2007 11:50:50 +0000

Hi Stuart,

Running a network with just a single master clock would not be robust
enough for carriers, so I see the problem as being one of keeping the slave
clock output phase within the tight bounds required by the applications as
the set of master clocks in the ensemble changes. I think it does mean that
the master clocks need to be accurately aligned to each other (time, not
just frequency). If there are then alternative paths back to any particular
master clock then the PDV-filtering should be capable of sorting that out
before it goes into the ensemble.

Regarding PRCs, these are useful for supporting applications like CES
(sorry, PWE3), and the requirement is that the output phase of the clock
remain under the MTIE mask as the network delays and master ensemble
changes. As for applications which need better phase alignment, such as 3G
node Bs running TDD (ie, in LTE), I believe this is going to need
time-alignment. PRCs on their own are not good enough to deliver time -
G.811 allows them to drift by more than 1 microsecond per day, so they soon
violate the 3G TDD requirement (generally, they are much better than this,
but it's just a question of time (sorry!) before they drift too far). The
way I see it, it's going to be necessary for a slave clock to be able to
identify whether a timing source can deliver time or just frequency, and to
what accuracy.

Cheers

Dave

Semtech Ltd

                                                                           
             "STUART VENTERS"                                              
             <stuart.venters@a                                             
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                                                                   Subject 
                                       RE: [TICTOC] Re: TICTOC NTP         
                                       ensembling                          

                                                                           

Dave,

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

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

Regards,

Stuart

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]
Sent: Monday, March 19, 2007 11:48 AM
To: [email-masked]
Subject: [TICTOC] Re: TICTOC NTP ensembling

Dave

Semtech Ltd