[ntp:questions] Asymmetric Delay and NTP

Joe Gwinn joegwinn at comcast.net
Mon Mar 31 13:34:59 UTC 2014


In article <lh9fe1$plc$2 at dont-email.me>, William Unruh
<unruh at invalid.ca> wrote:

> On 2014-03-30, Joe Gwinn <joegwinn at comcast.net> wrote:
> > Magnus,
> >
> > In article <53375ABA.5070006 at rubidium.dyndns.org>, Magnus Danielson
> ><magnus at rubidium.dyndns.org> wrote:
> >
> >> On 24/03/14 14:38, Joe Gwinn wrote:

[snip]
> >> [MD] The *one* thing you can figure out with more measurements is how 
> >> non-zero-mean noise such as network traffic contribute to asymmetry.
> >> You can do pretty good approximations of that contribution. However, if 
> >> there is an underlying asymmetry in static delay sources, they won't 
> >> disclose themselves with more measurements of the set measurements.
> >
> > [JG] Yes.  One way to think of it is to describe the delay asymmetry as a
> > random process having a mean and a standard deviation.  One can
> > estimate the standard deviation, but not the mean, from received
> > timestamp packets.
> 
> IF there is another source to compare it with. If you only have one
> source of time, you cannot differentiate the fluctuations in the
> assymetry from the fluctuations in the remote or local clock. With more
> than one source you can, but as you say, you cannot discover the mean.
> While for short term fluctuations in the asymmetery you could probably
> assume that they are path, not clocks, for longer term ones you cannot.

We generally assume that the clocks are good, which is to say that they
don't jump around nearly as rapidly the the delay asymmetry.  So one
can tease clock offset from standard deviation of delay asymmetry by
their spectra.

Joe Gwinn



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