Ah, nice ELI5, thank you.

Looks like an engineer visit is needed then...

That's how the BT Wholesale DLM works on ADSLx, but the Openreach one on FTTC/VDSL2 does not manipulate the noise margin.

It uses higher levels of interleaving, and often G.INP on the downstream which brings the interleaving down to a very low level, then if things get really serious it uses banding, where it set maximum and minimum sync speeds.

Kitz has a useful article, but note that at the top of the page she says it is "a work in progress". This section looks pretty good to me.

Well, I've contacted them via chat and a very nice lady called me back.

She told me that's she'd "reset the line" and to wait 4 hours and reboot the router. Obviously this isn't going to do anything, and is pretty muich the same thing I've been told 3 days in a row now, so I requested an escalation.

I'm apparently going to get a call from an engineer today.

Ah interesting, will have a read

Seriously doubt the 'reset of the line' was an actual DLM reset.

Those are done by Openreach and usually only happen as part of an engineer visit

BT SIN 498 Section 2.2.1 introduces the idea but is badly written. It basically describes the banding, with no useful information, and doesn't say that it is a kind of last resort. Interleaving and G.INP are part of the normal day to day management and not mentioned there.

From the information available it seems it wouldn't do any good even if they did reset the DLM.

Quick update as I've been investigating this all day lol

I think the OpenReach engineer has faceplated the wrong socket, or used the wrong set of wires; hear me out...

In my living room there are two phone sockets, one on an outside wall and one on an internal wall. I've just moved in and we're told one is a wired link to the socket in the bedroom, as in you use a splitter from the main socket into the other one and you then have a working socket in the bedroom too. The engineer has fitted a faceplate on the internal one.

Trouble is, even when using the master socket on the new faceplate the other one is still active, i can even connect to Infinity on it; so they're linked even via the master socket. Each socket has two sets of wires, twisted together into the same connectors (pins 2 and 5) on the non faceplated one and also the bell wire (pin 3) is connected on both, if that matters.

The engineer did see the other socket but didn't open it, he just said that's not the right one, presumably as it has a fancy faceplate on it.

My guess is this:
The outside socket is the main BT one and this links directly (pins 2, 3 and 5) to the other one on the internal wall. The internal wall one then links to the bedroom so using the new master socket only disconnects the bedroom; nothing is plugged into there anyway. However using the master socket does net me a 5Mbps gain, I can�t explain this. Plugging into the outer wall socket gives me the same sync speed as if I�d not used the master socket. Saying that, the new faceplate has the new bell wire inductor installed, could be that?

Would taking out the bell wire on either/both do anything, do you think?

Edited by deleted (Wed 24-Aug-16 21:40:18)

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Edited by deleted (Wed 24-Aug-16 22:41:56)

Can you take pictures of all sockets and upload them to an image sharing site?

Also if you can include the wiring that would be great, but only on the faceplates not the back wiring.

To confirm the outside socket is actually outside of the house right?

Is it possible to follow the wiring to see where it is going? It is possibly star wired which would result in poor speeds.

To confirm, all sockets work on infinity?