Slow FTTP. My Zen 115/20 service is delivering 50/20

Hi. As described in heading...
This is a new connection (replacing an FTTC), installed 11th December that has never been right.
I raised a fault with Zen and they talked me through connecting my laptop direct to the ONT, over a couple of different CAT8 cables, showing an erratic 30 to 80mbps download. Upload is a solid 19mbps.
BT were supposed to come out before Christmas, I got the text message, read the safety stuff as requested, watched the video and texted back "viewed yes" and "Rep watched" messages, got the "received" message" and.... they didn't turn up.
They reported to Zen that no fault found.
BT were organised to come out "again" this morning. I got the text message, read the safety stuff as requested, watched the video and texted back the "viewed yes" and "Rep watched" messages, got the "received" message"..... but again they did not arrive.
This time they phoned me to say that they had run diagnostics and the problem was with configuration at Zen.
I got shirty with them and asked how they knew the install was OK when they had not been here. They said they had checked the line to the house and did not need to come inside. I told them that the last 5 poles were all on my land, and nobody had been in the field or in my garden, so how could they have "checked to the house".
They said they would report back to Zen. I phoned Zen and told them I was not happy.
An hour later Openreach phoned again, to say they had "checked the build" and the problem was with my Zen router.
I explained (very slowly....) that we had tested while connected direct to the ONT with the router out of the circuit, (three weeks ago, before even notifying them) and that the problems persisted, so it could NOT be the router. They told me that it would not work plugged directly to the ONT and that a router was required. I explained very slowly that Zen technical support had talked me through the reconfiguration, that it DID "work", but that the speed/reliability problem persisted.
They said that I should get in touch with Zen.
Is it me?
I am new to all this, and the whole performance seems to be out of the 1980's BT playbook....,

The fibre ONT connects at same speed regardless of product bought (2.4 Gbps), and has no rate adaption capabilities.

As with others with similar problems either it is a configuration issue at the handover area, or in the Zen area of control.

If using exactly the same router as you had when on FTTC recommend a reset to factory defaults and set it up again, some routers appear to remember some old settings (usually BT Smart Hub issue).

So should not need anyone to visit and needs Zen and Openreach to rebuild the connection profile in their kit.

Another possibility is that the FTTP and handover has no problems and there is congestion issues slowing you down, and this where a speed test result from https://www.thinkbroadband.com/speedtest can be diagnostic If the issue is a capped download speed you'd expect the download to hit the cap and stay at that speed rather than wander up and down during the test.

Edited by MrSaffron (Mon 04-Jan-21 16:59:47)

The OP described download as "erratic" 30 to 80 Mbps.

I agree that most likely the problem is upstream. However there is one thing to check: when the PC is connected directly to the ONT, check that the interface status on the PC shows 1G

If there were a broken pin in the ONT then it *might* fall back to 100M - although 100M full duplex still ought to give a stable connection, albeit capped at 100M, so I think this is unlikely to be the problem.

In reply to a post by candlerb:

The OP described download as "erratic" 30 to 80 Mbps.

I agree that most likely the problem is upstream. However there is one thing to check: when the PC is connected directly to the ONT, check that the interface status on the PC shows 1G

If there were a broken pin in the ONT then it *might* fall back to 100M - although 100M full duplex still ought to give a stable connection, albeit capped at 100M, so I think this is unlikely to be the problem.

I had a faulty ONT (after a lighting spike). It refused to connect at 1Gbps and defaulted to 100M/FD.

Took a bit of convincing the supplier that the ONT was indeed [censored] (I don't think their MI / dashboard allows them to see the customer side ethernet port speeds), but ultimately they fulfilled their obligations and OR tech was there the next morning with a replacement ONT and service was back to usual.

That's not to say what the issue is here, but these things do break, like anything.

I wonder if OR have a device they can plug in to the ethernet customer end that would be able to test the ONT?

Post deleted by Zarjaz

In reply to a post by MrSaffron:

Another possibility is that the FTTP and handover has no problems and there is congestion issues slowing you down, and this where a speed test result from https://www.thinkbroadband.com/speedtest can be diagnostic If the issue is a capped download speed you'd expect the download to hit the cap and stay at that speed rather than wander up and down during the test.

see speedtest: https://www.thinkbroadband.com/speedtest/16077974533...
(not vey steady...)

If issue was the PON i.e. fibre locally would expect the upload to be affected.

Things to do...

1. Double check that PC is not maxing out CPU before and during the test
2. Make sure your anti virus is not impacting things, i.e. turn off temporarily or test from another device
3. If test is over Wi-Fi make sure to check with Ethernet direct into router
4. Run test again between 1am and 7am i.e. when congestion should be less of an issue

Latency testing does not indicate that the connection is in heavy use by a cloud backup

Looking at another test you seem to have done
http://tbb.st/1609751019536506055
Can see a big delay between clock starting and tests seeing first data, this could indicate a very slow DNS configured.

http://tbb.st/1609751279214699655 has the delayed start issue too.

In reply to a post by MrSaffron:

Can see a big delay between clock starting and tests seeing first data, this could indicate a very slow DNS configured.

Hmm, sounds like a security product interference??

In reply to a post by bowdon:

I wonder if OR have a device they can plug in to the ethernet customer end that would be able to test the ONT?

Yeh. Its called a laptop