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I hope I can explain this coherently... we have a copper wire connection to the building from a FTTC cabinet and a typical UK VDSL modem plugged in via traditional UK phone plug, providing internet connection for a small business. On 'my' side of the modem I have various hard-wired CAT6 cabling/hubs as well as some WiFi access points.

We have increasingly been having intermittent internet service outages and serious speed drops which the infrastructure provider (OpenReach in UK) is able to see logged (as suspected HR faults if that means anything to anyone) but not trace to a specific cause. Some of the infrastructure to the property is quite old and the distance to the FTTC cabinet is further than ideal so somewhat 'flaky' service is reasonable but it is worsening.

The engineer is suggesting that having tested everything on 'their side' up to the DSL modem, something on our side is triggering the faults. i.e. that if there is faulty wiring in my CAT6 cabling this could somehow propagate a fault up the phone line.

I am not asking for help diagnosing what is wrong - clearly that's far too vague. My question is whether this is plausible. It seems a bit strange to me that packet loss or something on the network would 'break' the ISP connection, they are fundamentally different technology surely so what happens in my network shouldn't be able to cause a line fault? I suspect the engineer is not familiar with networking (that's not his job) since he essentially said "well it's all the same wire" and is perhaps keen to push the fault to me. We see no issues on the internal network either wired or wireless even when the DSL connection is dropped and DSL load is typically not anywhere near our bandwidth limits when faults occur.

Can either a fault in my DSL modem or our internal network realistically cause this sort of up-stream fault, before I start doing a lot of diagnostic work on our end?

Mr. Boy
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    In my experience it's less likely to be related to your internal network and if anything, it could be down to something like REIN (repetitive electrical impulse noise) which can be caused by a number of factors. Just to clarify something though; when you say it's FTTC but also uses a standard DSL modem; do you mean a VDSL modem (e.g. one of the OR supplied Huawei or ECI ones)? – BE77Y Jun 07 '22 at 11:21
  • @BE77Y this really isn't my area of expertise but I am pretty sure you are right, the modem plugs into a typical UK phone socket and I think VDSL is the standard for the copper-wire to the exchange from my premises. If you had anything to expand on REIN in an answer I'd be appreciative. – Mr. Boy Jun 08 '22 at 10:24

2 Answers2

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In short no it (your internal equipment) can't cause a problem further up the line but it can cause the same symptom as a line fault. You need to do some fault finding internally.

Start by stripping down your network as much as possible.

1 PC, connected directly into the router/modem (I'm assuming this is a single device). Do you get the same symptoms? If so try a different vdsl modem and repeat the test.
If you don't go through the process of adding devices bit by bit until you do get the problem again and go from there.<br<

You'll either prove it isn't internal and need to go back to your isp or prove it is and hopefully find the source of the issue

--In response to your comment--

The only time the ISP would see any kind of "fault" from the internal equipment would be if the fault was with the vdsl and it was causing the connection to be dropped. This can cause them to downgrade the connection to try and get greater stability. Anything else is the LAN side of the vdsl modem and the ISP won't see it.

Drifter104
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  • _"it can cause the same symptom as a line fault"_ - do you mean to me or to the ISP? If the latter then this _could_ trigger them to downgrade the service/cause outages or whatever, trying to correct for the perceived fault? – Mr. Boy Jun 08 '22 at 10:54
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    Updated the answer – Drifter104 Jun 08 '22 at 11:02
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I agree with many of the points made by Drifter104 in their answer, but just to expand on my own comment:

Ideally you need to track down what exactly the problem is before trying to figure out what is causing it.

There are a couple of key points here in my opinion:

suspected HR faults

the distance to the FTTC cabinet is further than ideal

intermittent internet service outages and serious speed drops which the infrastructure provider (OpenReach in UK) is able to see logged

To me, these points narrow things down somewhat.

  • HR faults are "high resistance" faults, normally caused by a physical problem with the copper cable running from the local cabinet up to the modem at your end (it could be anywhere in between and for example could be caused by a wet joint or a partially broken wire, etc). They can be tricky to track down but would be OpenReach's responsibility. The fact that they're querying your side of the modem could be interpreted as either a) they have eliminated the possibility of a HR fault on their side or b) they're trying to pass blame, as you have suggested. I'd keep pushing them to verify that they've eliminated line/HR faults on their side of the modem. I recommend you perform a Quiet Line Test (QLT) using an analogue phone. If there's a line fault you'll probably be able to hear crackling/noise on the line. Here's a guide.
  • Intermittence in service can have a number of causes but speed drops are typically only really caused by a few things; desync of your modem with the one at the ISP's end in their DSLAM; local congestion of the ISP network; local congestion of your network; poor performance of your router; poor performance of your modem. In my opinion given what you're describing it sounds like you either have a line fault or your modem is dying. I'd suggest asking OpenReach (or your ISP) to provide you a new VSDL modem, or you can try to find one on eBay - they're pretty commonly found second hand (search for ECI or Huawei OpenReach VDSL modems).
  • finally to your question on whether your internal network can be causing these issues - there are a number of possible causes of what you're describing with the most likely culprits outlined above. However an outside possibility IMO is Repetitive Electrical Impulse Noise (REIN) inducing noise for your modem, causing it to be less than stable and possibly even losing sync with the ISP end. kitz has a good overview of REIN with some possible causes which you could eliminate if you want.

Just an extra bit of context here; if your modem (or the one at the ISP end) detect instability on their sync, they'll resync at a more stable SNR, which to you would look like a temporary drop of service and then resyncing with lower available bandwidth.

I would reiterate what Drifter104 has said in that there's value in simplifying the network at your end from a troubleshooting perspective. I'd also add that you could, for example, add monitoring of packet loss to your troubleshooting. You can monitor packet loss from inside your network by pinging an external address, and you can do the same from outside your network by pinging your router IP (I use this service personally, run by an old friend)

So to me the tl;dr would be that I think either you have a modem fault or a line fault, but it could be something else which you should now have some more info to track down.

BE77Y
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