Re: [Linuxptp-users] Installation of Linuxptp on Ubuntu PC

[Richard C. <ric...@gm...>]

On Sat, Aug 29, 2015 at 08:35:56PM +0100, Guo Hao wrote:
> But what if there is certain time offset between GPS clock's ToD and host
> PC's ToD, say 3 us when the capture card gets its initial ToD from the PC.
> Then even the capture has 1-PPS from the GPS clock, the timestamp
> difference would be in the range of 3 us?

The ToD is really only telling your endace card the *approximate* date
and time.  Your endace card phase locks (not just frequency locks) to
the PPS.

Endace said:

> Yes, your understanding is correct: the host clock sets the DAG’s initial
> TOD for time stamping and the PPS keeps the oscillator on the DAG from
> drifting. This way once the initial TOD is established if the DAG has
> proper sync the largest possible timing skew would be a few
> nanoseconds.

and I said:

> # so the ToD from the host is irrelevant.  The capture has the GPS's
> # 1-PPS, and so the phase of its time stamps should be correct to a few
> # hundred nanoseconds.

See how you got the same answer twice?

> As far as I know, the GPS clock might already did something about the
> egress and ingress latency.

And just how do you know that?

(It cannot possibly know the ingress latency of the Endace card.)

> Therefore, I guess most of the latency might be introduced by the capture
> card.

Could be.

Cheers,
Richard