[time-nuts] Making a Raspberry Pi NTP server without soldering

[Chris Albertson]

No,  you are talking microseconds on a system with millisecond performance.
 The data will go through Ethernet switches and routers and then whatever
networking gear is on your clients too.  Today many of the clients will be
WiFi.

The figure of merit here is the timing on the client computers. MAC only
effects the clients.  These clients are typically at the "handful of
milliseconds" level.  NTP measures the round trip delay over the local
network so the things that mess up timing are (1) Asymmetric speed because
NTP assume the one way delay is exactly half the round trip and (2)
Variation in the delay.  So a fixed long latency is just fine but a lot of
variation is not.    On a typical network #2 will be the dominate factor
that you can't control.  It might be effected by things like if someone is
watching a YouTube video or if a file backup is going on.  Yes the USB link
to the MAC would add variation in the timing but other sources of error are
1000 times bigger. (like for example the OS has a queue Ethernet packets.)

You have to look at the purpose of your NTP server.  It is so that all the
computers have a common system time so that file time stamps and log file
entries can be matched up.

How the MAC connects is kind of like worrying if the chrome platting in
that little hook on your tape measure has a well controlled plating
thickness.  Yes of course it matters, just not enough to worry about
because all the other error sources are much larger.


On Tue, Oct 22, 2013 at 8:38 AM, Andy Bardagjy <andy at bardagjy.com> wrote:

> The networking adapter on the Pi is connected to the SoC via USB while on
> the BeagleBone the MAC is native. I suspect this might affect timing.
>
> Andy Bardagjy
> bardagjy.com
>
>
> On Tue, Oct 22, 2013 at 11:36 AM, Chris Albertson <
> albertson.chris at gmail.com
> > wrote:
>
> > On Tue, Oct 22, 2013 at 7:13 AM, Collins, Graham <CollinG at navcanada.ca
> > >wrote:
> >
> > >
> > >
> > > How you done anything with or have compared the PI to the Beaglebone
> > Black?
> >
> >
> > If your only use of the device is to have an NTP server then why pay more
> > for the  Beaglebone?  The Pi based server seems to be better than
> required.
> >  "better" in this case meaning that it keeps time better then it can
> > transfer it over your network.
> >
> > If you need a lower cost Linux server, you can repurpose a PogoPlug.
>  These
> > are roughly the same specs ARM process and a little bit of RAM but come
> > with a case and power supply all for under $20.  You can re-flash them
> with
> > a general purpose Linix-ARM distribution.  But no good place to attach a
> > PPS input except for using a USB-Serial dongle.   Well there is a serial
> > port header inside the box but I've not tried it.
> >
> > Chris Albertson
> > Redondo Beach, California
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-- 

Chris Albertson
Redondo Beach, California