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[burakkucat]

* ADSL2, or ADSL2+, or both?

* Which do I choose? (Ultra-long line 4.6 mi from the exchange.)

ADSL2 and not ADSL2+.

At a length of 4.6 miles, you will not be able to use all the tones of ADSL2+ so ADSL2 will be the correct choice. Although ADSL2 uses the same number of tones as G.Dmt (a.k.a. ADSL1), they are used more efficiently.

For the circuit from EABSE to The Cattery, Beattie Bellman states "Recorded Line Length: 2481m, Estimated Attenuation: 34dB". The Huawei MSAN at the exchange end is configured for ADSL2+ mode by default but will drop back to ADSL2 or G.Dmt if the CPE requests it. Due to the length and quality of the circuit none of the higher tones (of ADSL2+) are used. Hence I have configured the modem to use ADSL2 mode.

[Weaver]

@Burakkucat -many thanks for confirming my suspicion, as I seem to remember this from long ago.

I will presumably have to somehow get into all the DLink modems and lock them to ADSL2-only mode, not "auto". God knows how. I'll maybe have to get AA on the case.

Just so I understand, why is ADSL2+ actually a _bad thing_ rather than just being of no advantage?

What would happen if I simply left things on auto?

[Weaver]

I remember ADSL2 allows a single bit in a bin, not requiring two bits min as G.Dmt does. So a bit of extra performance there.

Any chance of SRA ever?

[Weaver]

My other questions was, do I definitely go 21CN?

[burakkucat]

Just so I understand, why is ADSL2+ actually a _bad thing_ rather than just being of no advantage?

What would happen if I simply left things on auto?

In my own case, I noticed better performance with ADSL2 rather than with either G.Dmt and ADSL2+.

The number of bits per tone (bin) comes into the consideration . . . and I know that Kitz has a write up, in the main site, which explains all.

As for the "badness" of ADSL2+ in your case?   Consider that you have a wide band receiver (modem(s)) listening to what the wide band transmitter (DSLAM) is sending. Now due to the length of the circuit, the top 50% of the frequency range cannot be used due to excessive attenuation. Your receiver, being configured to use those higher frequencies, turns up its gain in an attempt to "hear" something. But fails. It persists. Now Hamish, in his hire car, is taking old Mother McDonald to see the doctor. Hamish's car, due to its age, tends to act as a wide band spark transmitter similar to that of Marconi's best from the beginning of the 20th century. You wide receiver happily picks up the signal transmitted by Hamish's car and, as a result, you have a burst of CRCs. As no useful signal will ever be received in that top of the frequency range, it makes sense to turn off the receiver . . .  i.e. configure the modem not to use them. 

SRA. I always configure that as on, on the off-chance that one day it may be implemented! 

20CN & 21CN. That question can be sub-divided into Broadband access and telephony.

The latter first. My understanding is that Beattie Bellman had big plans to move to VoIP for 21CN telephony. After a number of reviews, technical, financial, etc, the idea was dropped for the foreseeable future. So all classical telephony is very much 20CN.

Broadband access. I believe (but will be happy to be corrected, if wrong) that the equipment for ADSL2+ and upwards (i.e. VDSL2) is all termed 21CN. 

[Weaver]

@burakkucat - re ADSL2+ got it. The wider the bandwidth of the rx system the more crap flies in. I didn't think about hardware being reconfigured much, I just imagined that and ADSL2+ system had wider-band capable hardware and the differences are down to software only. Obviously your design is far superior, with variable bandwidth hardware.

[Weaver]

@burakkucat -just for me being super dim;

Q1: if you are going to offer FTTC, then is it true that your exchange, at least the xDSL side of it, ignoring PSTN, is fit to be called 21CN? This is the crux of things. I get 50% off daytime peak dats from AA if the exchange "is 21CN".

Q2: if you are offering FTTC, do non-FTTC users on the exchange always get offered an option of ADSL2 ?  (or only g.dmt)

[c6em]

Not necessarily.
Smaller exchanges may take the fibre incoming supply instead from another nearby 'head end' larger exchange.
So the FTTC cabs will be supplied from a different exchange to the one the user's phone and ADSL comes from.
In this case the local exchange will remain on ADSLmax, and the users will have the options only of ADSLmax or FTTC.
So no, in this case the exchange remains on 20CN.

Indeed very long term I see this as the modus operandi for BT once the entire UK has FTTC.
Large regional exchanges supplying maybe 1000+ cabinets in the entire surrounding area.
Eventually these FTTC units will be able to supply the voice service as well leading to all these smaller local exchanges being closed.
All users will be by then have been swapped to FTTC compulsorily so the local exchange building becomes redundant and can be sold off for housing plus the E sides cabling in ducts recovered and sold.

[burakkucat]

b*cat bows, in gratitude, to c6em for answering Weaver's most recent queries. 

[NewtronStar]

Mother McDonald and Hamish

It would have been better if you had used Father Ted and Mrs Brown in this example can already here Mrs Brown say to father will you feckin speak up can't here you with all that p00 


https://www.youtube.com/watch?v=Jreqe8rTy3Y

[sorc]

Not necessarily.
Smaller exchanges may take the fibre incoming supply instead from another nearby 'head end' larger exchange.
So the FTTC cabs will be supplied from a different exchange to the one the user's phone and ADSL comes from.
In this case the local exchange will remain on ADSLmax, and the users will have the options only of ADSLmax or FTTC.
So no, in this case the exchange remains on 20CN.

Indeed very long term I see this as the modus operandi for BT once the entire UK has FTTC.
Large regional exchanges supplying maybe 1000+ cabinets in the entire surrounding area.
Eventually these FTTC units will be able to supply the voice service as well leading to all these smaller local exchanges being closed.
All users will be by then have been swapped to FTTC compulsorily so the local exchange building becomes redundant and can be sold off for housing plus the E sides cabling in ducts recovered and sold.

Some places did get 21CN ADSL2+ after most of the area served by the exchange has either got FTTC or FTTP, like mine - and since IPstream has been retired here it's ADSL2+ or FTTC/FTTP only. I have no idea what drives BT's decision making on that, just as it's unclear why they cherry picked certain streets here for the full FTTP treatment but decided others (with people on much longer lines) can make do with FTTC. I am reasonably sure that the FTT* comes from a more distant exchange, so that would not be why.

TalkTalk also came in afterward and LLU'ed my exchange too so they're on the same wavelength (or doing it to avoid paying to use BT's voice network).

[WWWombat]

On 21CN:
At a previous address, the exchange (Brookwood) was slow (for a large exchange) to get 21CN. FTTC went live around 6 months before 21CN.

Ordinarily, I'd think the cabinets could have been wired to a different head-end (Woking), but that exchange was even later to get FTTC.

On optical isolation:
Would it be enough to just use some media converters for the ethernet connection (on the PPPoE leg)? Something that did 100base-tx to 100base-fx, leaving a short fibre leg?
http://www.cclonline.com/product/85222/INSIXTMC100SC/Wired-Accessories/Dynamode-Insixt-INSIXTMC100SC-100TX-100FX-SC-Multimode-Media-Converter/NET0664/

On the future prospects:
If the superfast coverage stays 4 miles away, it is at least only 4 miles away. Any prospect of setting yourself up with a wireless backhaul to a friendly bod near the cab?

On 21CN:
I think b*cat got the history (with voice and data) correct. For me, the distinguishing feature *now* is that 20CN was based on an ATM core, while 21CN is based on an IP core.

As BT have said they expect most subscribers to be using IP-based voice by 2025, the question is ... how? Do they intend to re-invoke the original 21CN picture, with voice converted at the MSAN (or, presumably, now including the DSLAM MSAN in the FTTC cabinet)? Or do they introduce voice gateways in the house, and convert it to IP before it ever leaves the home!

[Weaver]

@WWombat  on the friendly bod idea, I have one in mind, but it would be a really cheeky big request I feel, putting her in a very awkward place when she wants to say no. The other thing is that the 3 mile link to her is very difficult, there's no line of sight, so there would have to be a two pairs of dish antennae high on the moor or something, and god only knows how I would get power to the units, plus the fact that the 130mph winds would rip them to pieces.

We are due for >85 mph winds this afternoon, my brother-in-law has been going round the place looking for potential missions and anything loose. I'm going to get someone to do a dress rehearsal with the generator. Am also hoping the new fat UPS will prove its worth. Have two UPSs now, one for a PC and its display, and another huge one for all the networking equipment.

Need to refuel the generator and make sure it starts ok. Really need a more expensive push-button-start one as this pull-cord thing is too difficult for my wife to start.

[benji09]

  Weaver, Slightly off topic, but reference your lightning strike problem, do you leave a long wave radio on during the day - possibily tuned to Radio 4 ? 

[Weaver]

@benj09  - should I Benj?