Why no ADSL line cards in DSLAM cabinets

This is probably a really silly question so I apologise in advance...

Is there any technical reason why offering an ADSL service from the cabinet (i.e. ADSL line card in a DSLAM rather than at the exchange) has never been offered as a solution to rural internet?

I know from my experience where we are about 6k from the exchange and 3k from the cabinet that ADSL all the way to the exchange (6k) gives about 4mbps (and frequent drop outs but that's another story). I eventually persuaded PlusNet to give us VDSL (to the cabinet so about 3k) which gives us about 3-4mbps (but no drop outs).

So ADSL over 6k gives a similar speed to VDSL over 3k. Why not offer ADSL from the cabinet? I understand many VSDL line cards are backwards compatible with ADSL so why not offer that as a service? Presumably that would give me 6-10mbps so a worthwhile boost without laying any new infrastructure.

So, what's the reason Openreach don't do this? There must be many rural cabinets like this and it seems they are doing all sorts of new infrastructure to deal with it including laying private fibre (Gigaclear in our case) to homes. Obviously I won't mind having a fibre link when it eventually comes in a couple of years but I am curious why they didn't go for a zero cost option instead or am I completely wrong about this option?

Thanks.

While Adsl variants are still being used from the exchange then it's a no due to the potentially destructive crosstalk that would be created form the cabinet based circuits. Though BT did investigate cabinet based Adsl for a while to increase range , I suspect they dropped it in favour of other solutions e.g. vectoring. as the limitations required to make it work were likely too restrictive

Currently, VDSL2 from the cabinet has to be modified so as to not interfere with ADSL2+ from the exchange.

The power has to be reduced where the spectrum overlaps, such that the cabinet can transmit at no higher power than the ADSL2+ signals are at when they pass the cabinet (so VDSL2 has to be at lower power for cabinets that are further from the exchange).

If BT wanted to transmit ADSL2+ from the cabinet, it would have to comply with the same rules (so as to not interfere with Sky or TT ADSL2+ from the exchange) ... and the rule would apply to the whole of the ADSL2+ spectrum.

Once you put that rule in place, the range of ADSL2+ from the cabinet becomes identical to the range of exchange-basedADSL2+ beyond that cabinet. So there's no point.

This rule is a technocratic one as part of requirements for competition from LLU providers, so can be changed with the right political will... if Sky and TT (and other LLU providers) can be persuaded to use BT cabinet-based hardware instead of their own exchange-based hardware.

However, as we've seen with the failure of LR-VDSL, Sky and TT refuse to accept this position and Ofcom refuse to impose it on them. BT gave up.

New Zealand is an example of a place which allows this to happen. The UK, unfortunately, is more interested in providing competition for the easiest 70% than in providing decent service to hardest 5%

Thanks guys, understood. At least I know why they don't do it now and the slow VDSL is about the same as I would get with cabinet based ADSL under those restrictions. All down to the pesky unsheilded copper then I suppose which I guess is where this crosstalk comes from (presumably not in the cabinet itself which could and should be shielded between circuits).

If ALL ADSL circuits where moved to cabinet based ADSL when the cabinet is upgraded this would not be a problem but that certainly wouldn't be a zero cost option! Not to mention the complications of the open market in final loop connections here.

In reply to a post by andyhurley:

Thanks guys, understood. At least I know why they don't do it now and the slow VDSL is about the same as I would get with cabinet based ADSL under those restrictions. All down to the pesky unsheilded copper then I suppose which I guess is where this crosstalk comes from (presumably not in the cabinet itself which could and should be shielded between circuits).

If ALL ADSL circuits where moved to cabinet based ADSL when the cabinet is upgraded this would not be a problem but that certainly wouldn't be a zero cost option! Not to mention the complications of the open market in final loop connections here.

What makes you think pairs are shielded in Openreach cabinets? Have you ever seen inside one?

.... and not to mention the size of the cabinet and ancillary equipment required.

I wasn't thinking of the pairs in the cards as such but the electronics in the cards. I used to work on network switching kit years ago (originally Nokia and then sold to Marconi) and each card in those had all the HF sections of the electronics shielded with a grounded metal cover (not unlike a tuner card in a STB/VCR). It was considered good practice and more or less essential to prevent all the high frequency stuff interfering from one line card to another. This was back in the late 90's so maybe things have got more casual since then but the physics hasn't changed as far as I am aware

If they could do away with ADSL from exchanges you wouldn't need a ADSL from the cabinet, just throttle the VDSL speed as required. VDSL could then use the lower frequencies as well which would improve speeds available for everyone, which is what long range VDSL was, but LLU and Ofcom killed that.

The other problem is capacity if everyone was supplied from a cabinet, we'd need bigger or more cabinets for a lot of locations.

Apologies for the double post, I've just asked a very similar question in a new thread.
My post has a bit more detail on my particular issue, so I'm hoping that it'll be allowed to stay.
In the mean time, does anyone know if there's been any movement on the situation re ADSL cards in All in One cabs?

you would also expect very little investment more in cabinets move will be more towatds FTTP and Fibre -- moving forward