Openreach GPON - Dropouts, often clustered

We've got Sky FTTP (1GBPS), delivered to an openreach ONT, which goes straight into a UDM-SE.

The setup has been rock solid for about 4 years now, not an issue. I've got continuous ping monitoring on a stack of hosts, and its always sat around 2-2.5ms, no loss whatsoever. Bulletproof.

We've seen in the past few weeks periods of regular packet loss, and the UDM reporting that the link had gone down (i.e. that the actual ethernet link between it and the ONT had gone down). Replaced the cable between them, no dice. Some days it wouldn't happen at all, some days a couple of times.

Today, around midday, we saw it going up and down about 4-5 times in an hour. Packets will start dropping (noticed as was on a zoom call for 2 hours and it freezes... left ping running alongside, and the UDM will report that the link interface has gone down.

I've just tried going up into the loft and pulling the fibre from the ONT while its running a ping and tailing the logs, and exactly the same thing happens, which goes against my theory that it was something internal.

Is this likely to be something happening fibre side? I'd wondered given the tight clustering earlier if it was an engineer up a pole or similar, but couldn't see anyone vaguely near.

Is there any better way to track this down? I'm somewhat fearful of going round the houses with Sky, as I think I'd rather stick needles in my eyes as opposed to answer questions about using a mac or a PC, being in the same room as the router and using a cable 😳. Is there typically a good approach to such issues?

I wonder if you have a poor splice somewhere ? Giving a high light loss reading.

Trouble is, the only way you’re going to find out if that IS the issue, you’ll have to take some anti depressants, bite the bullet, and speak to Sky.

Other thought …. ONT in the loft … not sure whereabouts you are in the country, but it’s been as hot as hell round this way. Electrical equipment doesn’t like extremes of temperature. Is the ONT wall mounted ? I’ve seen ONT’s laid flat, and they don’t cool themselves well.

Can you set up a time lapse camera pointing at the ONT to see if you get a LOS light come on, or if the ONT is just rebooting itself?

In theory if you report to Sky that you lose connection and it coincides with the angry red light that should be in simple enough terms for them to deal with it.

In reply to a post by Zarjaz:

ONT in the loft … not sure whereabouts you are in the country, but it’s been as hot as hell round this way. Electrical equipment doesn’t like extremes of temperature. Is the ONT wall mounted ? I’ve seen ONT’s laid flat, and they don’t cool themselves well.

Pointing a small fan at the ONT may make all the difference.

Yeah the op needs to do a combination of jpn and Zarjaz's suggestion and if it showing los, try a fan.

Thanks all - Yes I think the idea of a camera sounds sensible. I have a Nest cam and the recording plan (not sure if it needs motion, or if it would be enough "motion") which could be sacrificed to it for a couple of weeks. Failing that I'm sure I could work out something with a Raspberry Pi and an old webcam. Will give it a go!

Loft temperatures have been pretty warm, but around the time it was going mad was significantly lower than later in the day (when it had become stable again).

could we see a pic of where the ont is?

Only asking as it may give some of us a better understanding of issues. I do think you will end up having to raise this as a fault........ But you should have a better understanding of what the fault is!

If you do go the route of the pi webcam, add a thermonitor unit to plot the temps

Edited by Taras (Sat 30-May-26 21:18:23)

Is there a good place to drop images now imgur has gone?

Its on a rafter in the loft at floor level - Have a thermometer up there, seems fairly comfortable (19degC at the moment).

Connected a webcam up to the frigate box that lives up there driving CCTV - works entirely locally which is handy given the issue. I've got it open next to me while working, and I see quite regularly the PON light drop out, and a bit less regularly the PON light drop out, and then the LOS light flash. I've got it recording on a rolling day so hopefully thats enough.

Have logged the fault with Sky, now just need to wait a day or two (!!) and hope! Regretting not buying a Starlink Mini when they were on offer for £99 a couple of months ago...

That does sound like a bum splice somewhere, it ought to show itself by a high light loss reading when tested at the ONT.