In reply to a post by RobertoS:

The only place the two are in close-together wiring is between the PCP and the premises. Where the problem is far more manageable than inside an exchange.

This is where I am struggling to understand. All the evidence points to about 100 D-side pairs running parallel from the new FTTC cabinet, along the pavement for about 20m, then under the road in a single set of ducts, before diverging at the point where the old EO cabling will - other posters assure me - have been intercepted.

Why is it easier to manage VDSL-ADSL interference between 40m of parallel-running pairs than it would be if carefully routed inside an exchange - especially one, like most, built for Strowger so presumably with plenty of empty space these days?

In reply to a post by JohnGeddes:

Why is it easier to manage VDSL-ADSL interference between 40m of parallel-running pairs than it would be if carefully routed inside an exchange - especially one, like most, built for Strowger so presumably with plenty of empty space these days?

The rules say no VDSL in the exchanges. BT don't make the rules, but they have to follow them.

The rules don't necessarily make sense, just like the fact that the motorway speed limit is still 70 mph when cars can stop much quicker than they used to doesn't make sense.

You have a tiny cab of 50 people - you won't be high up on the list I'm afraid.

Network rearrangement is likely to be thousands.

In reply to a post by JohnGeddes:

Thanks for this - but I am still a bit confused. I understand that VDSL frequencies make interference with ADSL a real issue - but surely there are VDSL and ADSL pairs in the same cables that run along the street, and with only a single trench across the main road from exchange to FTTC cabinet, every ADSL pair on the cabinet is running parallel to every VDSL pair.

The crosstalk problems arise when frequencies overlap *and* the signal level (aka power) differs *and* the pairs lie in close proximity. If one line runs at a higher power level, crosstalk can swamp the signal on a nearby line with lower power. Difference in power is key.

In proper wiring, the power level transmitted by the cabinet is carefully adjusted (for all overlapping frequencies) such that it matches the power level of the ADSL signals as they pass the cabinets. Because both signals run in the same direction (away from the exchange, towards the home) the signal levels continue to match in the parallel pairs. Both attenuate at the same rate, and crosstalk is managed.

If we assume that the FTTC cabinet is correctly configured for area D, then the VDSL2 power levels will be reduced slightly (in the ADSL spectrum) so that they don't accidentally swamp the signals of exchange-based ADSL subscribers in area D.

What about area A?
If area A has their wiring entirely rerouted via the same cabinet, with new cables across the road, then back again, this remains true, because both the cabinet-based VDSL2 signals and the exchange-based ADSL signals follow the new route. However, it requires that all signals within one cable bundle go in just one direction. The trench across the road could have separate cables for each direction , as the pairs in each cable would have reasonable separation - separate ducting would help.

If, however, you send those VDSL2 signals (which are already reduced to match power with area D) back to the exchange frame, you cause a problem. The VDSL2 signals will have attenuated on their way back again, so will now be reduced in power even more. They will then share the frame with ADSL signals that have higher power (still at full, exchange-based, power). Because it is the ADSL signals that are higher power, they could swamp any reduced-power VDSL2 signals they run alongside.

If the crosstalk issues are manageable in such a topography, why is it unthinkable that VDSL should should be run via the MDF for those on non-FTTC cabinets?

Crossing the road, with careful cabling design, is one thing. Bringing different power signals to the MDF is a recipe for disaster. There is much less scope for careful design there, and much more risk of running pairs in a way that causes problems.

Plus the fact that signals above 2.2MHz aren't allowed there.

In reply to a post by WWWombat:

Bringing different power signals to the MDF is a recipe for disaster. There is much less scope for careful design there, and much more risk of running pairs in a way that causes problems.

There are some examples of MDF wiring here:
http://www.kitz.co.uk/adsl/telephone_exchange.htm
and here
http://www.telephonesuk.co.uk/exchange_inside.htm

All those loose green+yellow jumper wires show the problem - there is little way to manage those in a way that would guarantee proximity (or lack of) between services.

John

You are correct about the pairs running from the Old cable to the FTTC Cab, there will be two 100 pair cables one from EO cable to FTTC cab and one back to the cable from the cab to continue to the premises served by the Cab. There should be NO ADSL on these pairs as they have been provided to only take those customers having FTTC service.

The VDSL signals are only going away from the Cab/Exchange so can be configured to reduce conflict with the ADSL signals from the exchange once they travel on the cable out to customers from that point.

The rule were not designed for Equipment inside the exchange or right outside the exchange BUT only prohibit installation in the exchange.

BT therefore can place a cab outside the exchange and OFCOM have turned a 'blind eye' to this in order to help those customers on EO pairs.

BT takes the risk of the crosstalk affecting ADSL customers further away and has to overcome this with by reducing the power of the FTTC signal from cabs close to the exchange.

Consequentailly FTTC from an Cab outside the exchange ( serving EO lines) tends to be not as good on long lines from its location as a Cab a long way from the exchange would be on a similar lengh line from it's location.

This is why lines 1 mile from the Exchange sited cab may not get FTTC whilst lines 1 mile from a remote cab may get a resonable speed.

In reply to a post by kitcat:

John

You are correct about the pairs running from the Old cable to the FTTC Cab, there will be two 100 pair cables one from EO cable to FTTC cab and one back to the cable from the cab to continue to the premises served by the Cab. There should be NO ADSL on these pairs as they have been provided to only take those customers having FTTC service.

I may have completely misunderstood but why would there be no ADSL on those pairs? If those customers were ADSL subscribers then there would be. The lines aren't routed through to the new (copper) EO cab only when someone orders FTTC.

Icarus

You are correct in the short term, but the number islikely to drop towards Zero very fast so it is easier to say zero for simple understanding.

They do not route all pairs via the new cab. OR will pick the DPs that are most likely to benefit to move to the new Cab.

So discount all those on DPs a long way away that won't benefit as OR won't move those.

For a WBC or LLU exchange. Discount those close to the exchange that already have good speeds ( 12Mb+) as OR won't move these.

There will be some ADSl customers left but these are those that are likely to move to FTTC fairly quickly as they will benefit the most, ( medium distance 3-6Mb speeds in my opinion). So in the short term you are correct but in the longer term you can discount them also.

Those ADSL that are on the pairs are likely to see a degradation in their existing service ( first from the extra 100m of cable then from crosstalk with VDSL), there is some evidence of this on these forums, but not a lot, and will therefore move even faster to FTTC .

Those getting below 2Mb are likely to be too far away to benefit from FTTC, those above 12-15Mb are unlikely to move to a higher priced product.
( Most of of on here are speed junkies compared to the normal and we should not get our rush to speed confused with that of most customers who are price sensitive.)

There will always be exceptions to all the above cases of distance, speed and requirements BUT OR will only plan on the averages to maximise the cost / benefit equation even when subsidised by BDUK.

Frim what I've seen they do route all remaining EO lines through a cabinet eventually. I've not seen it where they leave some DPs still EO.

But I'll take your word for that. I work in an urban area so it could be different in rural exchanges, where like you say they'll only route lines through the new cab that are going to DPs that would benefit.