[ntp:questions] Garmin 18 LVC: whether to fudge

Kevin Oberman oberman at es.net
Wed Feb 11 17:19:01 UTC 2009


> From: cmadams at hiwaay.net (Chris Adams)
> Date: Wed, 11 Feb 2009 08:26:39 -0600
> Sender: questions-bounces+oberman=es.net at lists.ntp.org
> 
> Once upon a time, Richard B. Gilbert <rgilbert88 at comcast.net> said:
> >My bet would be that there is an asymmetry in your ADSL link!   If I'm 
> >not mistaken, the "A" in ADSL stands for asymmetric!
> 
> The asymmetry in ADSL is in bandwidth, not path or latency.  More
> frequency space is used for downstream (ISP->end user) communication
> than for upstream, but both travel the same path.

Depending on the difference in bandwidth, that does impact latency. ntp
assumes that the time from message transmission to message receipt is
the same in both directions. If the packet takes longer on the wire,
that is an asymmetric latency.

Also, the jitter on your time is extremely high for PPS which should be
< 1 us. I suspect that this is a system problem, but I can't be sure. I
never see this much jitter on any of our PPS synced reference clocks (of
which we have about 25 scattered across the US). If the jitter on any of
my reference clocks using PPS exceeds .005, I consider this a big
problem. I use FreeBSD which has excellent PPS support.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751



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