Re: [chrony-users] Leveraging PTM

On Tue, Aug 22, 2023 at 5:58 PM Miroslav Lichvar <mlichvar@xxxxxxxxxx> 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