In reply to a post by RobertoS:

You mean a modern modem is a lot more sophisticated than older ones, and no-one can dispute that. The fact remains that a modem, by definition, converts signals from digital form to analogue and back again.

Well yes but the OP thought that it was just a matter of temporarily converting the digital signal to analogue while it passed over the copper wire. If you're going to correct a misunderstanding like that you have to go into more detail and the complexity can't really be glossed over.

If someone said 'Do I just walk to the hotel?' you'd be a bit remiss if you didn't explain about the car ride to the airport, the check-in process, the flight, the arrivals process, and the bus ride to the resort

I think if you go back to the final paragraph of the OP you will see this level of sophistication, whilst possibly of personal interest to him, isn't going to be relevant to his purpose.

The answer to part 1 of question 3 is "Yes". The second part of Q3 looks to be personal interest only.

Even "DSLAM" is likely to produce glazed-over eyes from most present at his discussions. The higher level descriptions in the thread seem to be what he needs.

As a technical discussion between yourself and TheEulerID, fine .

I'm leaving the discussion over the differences between a modem and a DAC well alone, but...

In reply to a post by RobertoS:

The answer to part 1 of question 3 is "Yes". The second part of Q3 looks to be personal interest only.

I thought I'd add a bit to this part, as the second part of Q3 doesn't look, to me, to be for personal interest only.

To Q3: Yes, there is a DAC for each copper line, but it should be seen merely as a component within a modem (just like the modem in the end-user's home). In the DSLAM, you might find 32, 48 or 64 such modems on one linecard, and up to 6 linecards. Of course, you probably don't find 64 separate chips on the linecard ... the "modems" will be running within one powerful DSP chipset instead.

There is likely to be a DAC that is a component in conversion of the optical signal to/from the digital backhaul signal too.

However, there is no channelisation of the optical backhaul signal: the signal from an individual copper line is not mapped 1:1 onto anything happening on the optical line, so there is no split of frequencies per channel, and no time multiplexing either. That kind of channelisation, in the telco world, died out with PDH and SDH hardware that is now being ripped out.

The DSLAM merely takes the packets off the incoming VDSL2 connections (at "up to" 80Mbps each), and sends the packets onward down the shared fibre connection (at 1Gbps); if there are too many packets to send, queuing happens (so delays happen to some packets). If there are still too many packets, some will be discarded, following some quality of service rules. Note, though, that we've been told that no cabinet in the country has reached congestion limits.

<sub>edit: The reason this aspect doesn't look like a question for personal benefit is because the presence of "channelisation" has capacity implications (like old-style 2Mbps lines could carry 30 voice channels, all of exactly 64Kbps). The number of "channels" that could be carried over the fibre would limit the number of properties that could be served.

Because DSLAMs work by forwarding packets, queueing them if necessary, they can make more beneficial use of a shared resource ... with the congestion limits only being approached in busy hour.

Hence, to me, the question was about understanding where the limitations appeared.</sub>

Edited by deleted (Thu 22-Oct-15 11:43:31)

Understood, and thanks .

No, I mean a DAC/ADC isn't a modem and a modem isn't a DAC/ADC. I'm not even sure they ever were. A DAC simply doesn't demodulate nor does an ADC modulate.

Firstly thank you all for such a comprehensive response to my questions. I have learnt a lot but obviously need to read a bit deeper into the technology employed. I think Andrue's likening the DSLAM to a network switch really made the light come on. If I had thought about it many network switches use fibre links between them as a backbone and the switch obviously does the routing. Also WWWombats description of an Aggregation Node being a "Fibre Splice Junction" presumably in an underground chamber is important. If I wanted to make a case for extending fibre to our existing cabinet it would not necessarily need to run from the exchange but from the nearest Aggregation Node which may or may not be near our adjacent village's cabinet.
I suppose Question 3 probably was for my own understanding of how FTTC was deployed and not necessarily important to make a case for extending to our village. However I am glad I asked as I am now much better informed.
I don't think Openreach or BT will be at this conference but the County Council officers who are working with BT on this Extended Rollout will be. Its these people I need to press to get BT to lay fibre to our village or negotiate a reasonable cost for us to raise privately.
Again many thanks to all who have answered my questions. Particularly when a Modem is not a Modem or digital to analogue converter
John

John, I hope you have better luck than I did.
I attended a meeting with local representatives, council officers working on our local BDUK project, head of council and regional managers for both BT and OpenReach.
It was hinted that our local problems would be solved with the second phase of BDUK and they admitted that the first phased map misrepresented the extent of 'superfast' and they would not be doing the same on the second phase map.
Turned out to be a load of bull.

What village is it?

FTTC is not always the best solution, depending on the geography of the area and what you are trying to achieve. The current Openreach alternative is to deploy FTTP which would have cost implications, but this is all down to things like what is the minimum speed you want everyone to be able to get, and do you want to potentially spend now, with maybe another upgrade needed in 8 to 10 years or got the money to go for full gold plated solution.

Hi Mr Saffron,
The village is Hunsdon in East Hertfordshire. The SEP Map here shows our two neighbouring villages Widford and Stanstead Abbotts both having fibre installed. In fact Openreach are laying fibre in Stanstead Abbotts now. Hunsdon however will not get fibre as part of the SEP and the conference is about the alternatives. These being Satellite and possibly Wireless. Satellite is very very expensive if you want reasonable monthly download limits e.g. 120 Gb (I work from home and regularly download 4 or five Windows 10 Upgrades per week for customers PCs and Laptops) so would soon bypass the regular 10 Gb allowance for £24.99 pm. If I want 100Gb allowance then I will have to fork out £99 per month!!.

Mobile Phone coverage is appalling too so 3g and 4g are almost non existent. I have to have a GSM booster connected to my 1Mbps ADSL in order to get a mobile signal at all.
I cannot see BT/Openreach installing FTTP despite approx 500 premises in the village. We are over 5Km from either exchange (Much Hadham and Stanstead Abbotts)
We looked at wireless ADSL for our Village Hall but there was no direct line of sight between the village and Harlow the nearest wireless node so even wireless broadband will be complicated unless there is a mast or tall building between the two points on which a repeater can be installed.
John

Wireless ADSL does not exist, ADSL by its very definition can only work across a metal wire, we presume you meant fixed wireless broadband.

Okay so FTTC looks feasible in terms of delivering reasonable speeds as area is relatively compact.

So if going the self funding route, just a case of getting enough people onboard to cover the funded element, also if there are really 500 premises the largest funded fibre cabinet is 288 lines, so if everyone wants you FTTC you need to ensure that Openreach will install the second cabinet when its needed without any further funding input from the community.

In the mean time people might find that switching from a BT Wholesale to a TalkTalk LLU ADSL2+ based while not giving better download connection speeds should boost upload speed some and removes the annoyance of the IP profile which is very slow to recover on an ADSL Max based broadband connection (i.e. one bad blip can mean slower speeds for 5 days)