Hello,
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:
Praecis Cntp stand-alone time server
internal time module is Ce or variant thereof
kernel 2.2.13-DOC
ntpd 4.1.1c-rc2@1.866
10-BASE-T HDX network connection
Praecis Cf timing module
single 2.4 GHz P4 system
old Dell Dimension 2400 (incredibly reliable)
CentOS 4 kernel 2.6.9-100.EL
ntpd 4.2.4p4@1.1520-o
Palisade mode 127.127.29.0
1000-BASE-T FDX network connection
Intel 82546GB e1000, IRQ coalescing disabled
Both are attached to the same SMC GS24 gigabit
switch with short-run cables.
Both are synced to the same CDMA source (Verizon
cell, primary B) with the antennas a few feet
apart. 'cdmastat' on the Cntp reports SNR of
about 5 and zero errors. SPSTAT query to Cf
reports SNR of about 8 and zero errors.
Both 'ntpd' daemons consistently claim to track
their Praecis device to +/- five microseconds or
less. Jitter on the Cntp is 8 microseconds, on
the P4 is 4 microseconds. Polling interval is
16 seconds, everywhere.
'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.
Is this just some minor network timing asymmetry?
Or is it possible that the two devices have
slightly different takes on the CDMA time?
Network jitter is generally reported to be between
10 and 50 microseconds. 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.
Thanks!
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