Re: Leveraging PTM

On Tue, Aug 22, 2023 at 5:58 PM Miroslav Lichvar <[email protected]>
wrote:

>
> > In the last year, FS.com has released some cheap switches with good
> > PTP support. I would recommend the IES3110-8TF-R:
> >
> > https://www.fs.com/products/148180.html
>
> That looks great. How much asymmetry and jitter in PTP measurements do
> you see when you compare a direct connection and different pairs of
> ports of the switch?
>

The only measurements I've done (and it was a while ago) was end-to-end and
used the ES3110-8TF model not the -R model: GPS connected to a CM4
connected to the switch connected to another CM4 with PPS output compared
to PPS output of a GPSDO.

https://github.com/jclark/rpi-cm4-ptp-guide/tree/main#results

I only compared with using the switch without PTP support enabled and it
was a lot worse, but I don't remember the exact figures. I need to get
myself setup again to do some more tests.

Currently it doesn't take advantage of PTP support in switches (i.e.
> the correction field). It would have to be a unicast one-step E2E
> transparent clock. Do you know any switches that can do that?
>

The  IES3110-8TF-R web UI has some configuration options that suggest it
supports unicast. See attached screenshot. I haven't experimented with it.
It also has option to select one-step vs two-step, but I haven't
experimented with that either.

The main point of NTP-over-PTP is to enable HW timestamping of NTP
> messages on PTP-only NICs. NTP doesn't need HW support in switches to
> perform well when the network is (moderately) loaded. You can still
> get sub-microsecond accuracy with good (non-PTP) switches if there
> are not too many of them between the server and client.


AS you say, it seems to perform well as is. But if it supported transparent
switches, could the accuracy with NTP-over-PTP be as good as PTP?

James