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

Martin Burnicki martin.burnicki at meinberg.de
Wed Feb 11 16:42:05 UTC 2009

Chris Adams wrote:
> 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.
> There may be asymmetric routing going on (very often the case when you
> talk to a host across the Internet, especially if either your ISP or the
> remote host's ISP are multihomed), but it is highly unlikely it is
> happening between the end user and his own ISP.

Of course it makes a difference whether the link speeds in both directions
are the same, or not.

If my ntpd polls an upstream server across my ADSL link there is an offset
of a few milliseconds. If the route goes via a SDSL link then the offset is
below 1 ms.

We have observed it even makes a difference of a few microseconds if one NTP
server uses a 10 MBit link and the other one a 100 MBit link, and both are
connected via a dual speed switch.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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