[time-nuts] Using the HP 58503a to correct your PC clock

Attila Kinali attila at kinali.ch
Fri Aug 5 16:41:13 EDT 2016

On Fri, 5 Aug 2016 21:35:05 +0200
Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> > From a "Time Nuts" point of view none of the above are even close to
> > accurate clocks.  A microsecond is a very course and crude measure of
> > time.  Pico and Femto seconds are were it gets interesting.
> Certainly. Look at White Rabbit, which really changes how PTP works.
> It may not be pico second accurate, but you get pretty far with it.

WR achieves sub-ns accuracy. Depending on the environment <200ps offset/skew
are possible.

> > Maybe someday NTP will have a time nuts level of accuracy.  the new up
> > coming version, I hear will be using 64 bits to carry the factional part of
> > a second.  That is truly nuts.
> Well, if NTP takes the main ideas from PTP and White Rabbit, maybe then.

This wont help. The achievable accuracy is dictated by the measurement
and the delay uncertainty. Even with network cards that support time stamping,
you cannot hope to get better than 1/125MHz=8ns. Standard network cards
will just trigger an IRQ at some point after reception and enqueuing of
the packet. The IRQ is measured by the OS, which leads to uncertainties
in the order of several us to a couple of 10s of us. If the card does DMA
the packet directly into main memory, then this value is even more inflated.

The network itself has a relatively high jitter. Assume 10s to 100s of us
on a local network per switch. Once you pass a router, you can assume
jitter in the order of a couple of 100us to a couple of ms per router.

To illustrate this, here a few ping statistics (64byte, 1000 packets each):

Local network, GBit/s, two level1 smart switches:
rtt min/avg/max/mdev = 0.073/0.131/0.362/0.044 ms

Two hosts in colo centers within the same city, same ISP, hence
on the same "network" (ie no conguestion), 4 hops:
rtt min/avg/max/mdev = 0.288/0.437/0.620/0.051 ms

Two hosts in colo centers, within the same city, different ISP
but with direct peering (ie no conguestion), 9 hops:
rtt min/avg/max/mdev = 2.916/3.008/3.505/0.078 ms

Two hosts in colo centers, one in Switzerland, one in Germany, 9 hops
rtt min/avg/max/mdev = 12.636/12.947/28.943/0.609 ms

These are all well connected machines, with "carrier grade" networks
inbetween. No consumer internet connections with their huge delays
and jitter.

So, best you can hope for is an jitter of ~50us rms within the same
city with _good_ network connections. Once the distance increases
and especially if you get routers with conquestion inbetween, then
the delay and its jitter rise quickly.

			Attila Kinali

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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