[time-nuts] question on apparent offset between two Endrun CDMA clocks

Hal Murray hmurray at megapathdsl.net
Mon Sep 19 05:23:43 UTC 2011

> I have a minor mystery and if any ready explanation is available I would be
> curious to learn of it.

> Have two Endrun CDMA time sources: 

>     ntpd 4.1.1c-rc2 at 1.866

That's quite old.

>     10-BASE-T HDX network connection
 >    1000-BASE-T FDX network connection

> 'ntpq -pn' and 'ntpdate -q' queries from both systems and from other client
> systems consistently show the Praecis Cntp between 30 and 80 microseconds
> ahead of the Praecis Cf device.  The median offset seems to be about 60
> microseconds.

You might ask Endrun.  (If you get a good answer, please tell us.)

> Round trip time to the Cntp is about 960 microseconds and round-trip time to
> the P4/Cf is about 60 microseconds.  'ntpdate -d' shows that the Cntp takes
> a total of 3600 microseconds to service a request where the P4 takes 65
> microseconds.

[I'm not sure what it means to have 3600 uSec service time with 960 uSec 
round trip time.]

I'm not all that surprised that two similar setups are off by something like 
60 microseconds.  60 out of 3600 is pretty small.

One likely source of quirks is interrupt coalescing which is common on 
Gigabit ethernet chips.  (That's probably not happening if the round trip 
time is only 60 microseconds.)


ntpq -p and ntpdate give you two different sorts of data.

ntpq tells you what the target thinks is going on.
ntpdate tells you the difference between the local clock and the target clock.

If I wanted more info on this type of quirk, I would setup ntpd on a 
monitoring system using both systems as servers and turn on rawstats.  Use 
the noselect keyword if you don't really want to use it as a server.  Set 
minpoll and maxpoll to get the desired amount of data.  Then graph the 
results.  (Poke me off-list if you want more details.)

If you have access to ntp.conf on the servers, turn on loopstats, peerstats, 
and clockstats.  They might show something interesting.

These are my opinions, not necessarily my employer's.  I hate spam.

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