How is FTTC deployed and how does it work in practice?

I have tried to read as many articles about this as I can including those on Think Broadband but cannot find the answers I need. In essence how does FTTC work e.g.
1) Is there a single dedicated fibre cable from the exchange to the cabinet (Star or radial topology in electrical terms)

2) I know that fibre can be joined (mechanically) and also via a repeater but can a fibre connection in one cabinet be extended to another cabinet at some distance? Or does Q1 hold true that each cabinet has a dedicated fibre from the exchange? I am trying to ascertain if it is possible for a fibre connection to one cabinet can be "extended" to another in the next village or if a new fibre has to be run from the exchange.

3) I read that each cabinet will have a mini DSLAM. Does this have DAC's (Digital to analogue convertors) that convert the digital light signal transmitted in the fibre to an analogue electrical signal sent along the copper pair? If so is there one DAC for each copper pair to a house or a single optical to digital circuit that splits the various "channels" transmitted simultaneously down the fibre and redirects them to the correct "Pair"?

4) How many connections can a single fibre support? I am referring each separate fibre strand in the bundle laid by the Teleco.

5) like copper do BT include an element of over provision to allow expansion later on or is the Fibre "sized" to the current population density?

6) Does anyone have a rough idea how much Openreach charge to lay a fibre from the exchange to an area approx 5.5km from the exchange? The last time we enquired for our village hall figures of £40,000 to£100,000 were banded about.

A lot of questions but I am trying to be prepared to accept or rebuff excuses from a forthcoming NotSpot seminar with our MP and representatives from BT, County Council and other alternative Broadband providers.
Many thanks
John

Each cabinet is connected directly to the head end by at least 1 dedicated fibre route, each fibre having 1Gb/s capacity , in tree and branch topology with the main spine cable branching off at aggression nodes and intermediate nodes . Cabinets are not interlinked or onward connected.
The Fibre does not connect directly via DA/AD converters .The Fibre terminates on the MSAN - which is very crudely a multiplexor - and the back haul (fibre capacity ) is shared amongst the copper connections on the attached line cards.
The number of connections depends on the cabinet type and size. Current cabinets can handle 96,128,256 or 288 connections.
There is a degree of over-provision of fibre.
What were you being priced for, FTTC, FTTP, Ethernet leased line? A lot would depend if that was 5.5km of new duct and cable , type of ground, any aerial route, road crossing / carriageway works. In short, highly variable

In reply to a post by VillageIT:

3) I read that each cabinet will have a mini DSLAM. Does this have DAC's (Digital to analogue convertors) that convert the digital light signal transmitted in the fibre to an analogue electrical signal sent along the copper pair?

No. Think of the cabinet as being a network switch, like you'd find on a LAN. The fibre is connected into one port. On the other ports are line cards that perform the conversion from analogue signal(*) to digital.

The signal from the fibre is converted into data packets by an adaptor which then presents it to the switch. The switch then sends these on to the apporpriate line card. The line card encodes that data into a form that the remote modem can understand (and vice versa for uploaded data).

(*)I'm not sure if there is an A/D conversion involved in the traditional sense. When sending data the hardware encodes the digital data into mutiple tones then sends those tones down the line. At the other end some clever maths are carried out to decompose the signal back again. It's a bit like sending red and green to someone. They see yellow but they can do the math to determine that you actually sent red and green.

It has something to do with fourier transforms ( https://en.wikipedia.org/wiki/Fourier_transform ).

Edited by Andrue (Wed 21-Oct-15 20:18:08)

There may be some degree of over-supply on the Fibre side; and historically at the PCP.

But taking the 288 FTTC mentioned, I suspect from what I have observed locally, the new, local copper link cables from FTTC to PCP are likely to be limited to 288 or possibly 300.

And these seem to be put in progressively, as demand builds, as "100 Pairs" according to the BT OR Technician, when the local FTTC was installed.

But with the limit of 288 VDSL circuits in that FTTC, any extra seem to be almost irrelevant.


Presently, that FTTC after about 18 months in use, can handle 96 VDSL circuits, needing furteher Filter/Links inside; and probably another "100 Pairs" installed, when the present 96 are fully allocated.

Upgrades to VDSL seem to average about 4 per month; and overall at that rate, it would appear that it will be about 2020 before the FTTC is fully utilised.

This ties in with the limited BT OR statistics I have seen.

You have some of these answers already, but i thought id clarify a little...

1) The cable from the exchange will have up to 288 fibres, that goes to form the basis of a tree & branch structure. Joints for branches happen at places called aggregation nodes, where some of the fibres (in groups of 12) will terminate, but most will pass through, running further along the spine to subsequent aggregation nodes.

4) For FTTC, a separate cable with just a few fibres (probably 4) will run from one of these underground aggregation nodes into the cabinet. One fibre will be plugged into the DSLAM, and spliced in the AN, offering 1 Gbps backhaul connectivity for all users ... if ever this gets fully utilised, a second fibre can add an extra 1Gbps. In the far distant future, an upgrade of the DSLAM could put 10Gbps down the same fibre.

The joints in the aggregation node are known as splices; the AN is just a big sealed plastic container that holds lots of splice trays for these joints.

5) The "spine" will have a lot of spare fibres, for future expansion. It represents the first steps in an eventual fttp distribution network. The current branch to the cabinet will have a little spare.

However, the DSLAM is not over-provisioned in the same way. BT don't expect every line to want fibre. The biggest FTTC cabinet supports 288 lines with 6 line cards. It will often be seen on PCP's (the existing green cab) with 500+ lines.

2) Fibre to another cabinet wouldn't run from the cabinet. It would run, instead, from an aggregation node further along the spine, or a separate branch off the spine.

6) I couldn't give you a number, but various communities have asked Openreach for installation of an FTTC cabinet, even when they're not included in the BDUK rollout. Reports suggest the funding could be anywhere from £10k to maybe £50k.

All DSL modems contain both DACs and ADCs to convert between digital and analogue and vice-versa (in fact all modern modems do). The really clever thing is that the the analysis and conversion of the waveform into data is all performed in the digital domain using something called DSP (digital signal processing). In earlier times all this was done in the analogue domain using complex tuned circuits full of capacitors, resistors and inductors. A 9,600 baud synchronous modem of the 1970s were often bolted into 19 inch racks and maybe 4U (7 inches high).


DSP is one of the great unsung technologies of the communications (and data processing world). It replaces very bulky analogue components with algorithms that are far more accurate, flexible and smaller. Very few so-called digital transmission systems are truly digital as it's not a very robust way of transmitting data over any distance (and also fairly inefficient). In a wider sense DSP is at the heart of any number of electronic systems, medical equipment, radar, sonar, video, audio and much else. (Fourier transforms play their part in all this).

https://en.wikipedia.org/wiki/Digital_signal_processing

In reply to a post by TheEulerID:

All DSL modems contain both DACs and ADCs to convert between digital and analogue and vice-versa (in fact all modern modems do).

[chuckle]That's wot a modem is, innit guv?

ADCs and DACs are just one part of what a modern modem. The ADC part just samples and produces a string of value representing the waveform. The demodulation process also involves decoding the symbols, correcting errors and so on. If all that a modem did was reproduce the waveform as digital values it would only be doing part of the job. The modulation/demodulation bit is a lot more than applying an DAC/ADC.

So no, a modem is more that just an ADC/DAC. (And in pre-DSP days this demodulation process didn't involve ADCs).

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.

A Model T Ford is a car, but a modern car does a lot more both in how it performs the basic function of a car and in the other useful facilities it provides whilst performing that function. Such as telling you where to turn right or left, and in the most modern not even requiring you to control it.

A modern bicycle such as Lance Armstrong's is a very sophisticated beastie, but it still basically does the same thing as Jeremy Corbyn's - transport a human being from point A to point B.

Edit: Don't forget my "innit" post was signalled as strongly as possible as a joke , founded on the basic definition.

Edited by RobertoS (Thu 22-Oct-15 09:49:38)

In reply to a post by TheEulerID:

All DSL modems contain both DACs and ADCs to convert between digital and analogue and vice-versa (in fact all modern modems do). The really clever thing is that the the analysis and conversion of the waveform into data is all performed in the digital domain using something called DSP.

Ah, yes, I remember them. We used them for a while back in my data recovery days to analyse hard disk platter encodings. That was an abortive attempt to create a generic read system for platters that we'd removed from the HDA.

What I was trying to allude to is that the copper signal is not a direct analogue equivalent of the digital signal in the sense that PCM is but I guess that when it comes down to it it's only a difference in complexity.

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)

Wireless doesn't need to be to an existing wireless connection. A lease line could be run to a building in the village and then the wireless built out from there.

You may also find that they will part fund satellite to make it more affordable - but it isn't an ideal solution even if it was cheaper due to the vast distances between you and the satellite.

If they went the mobile route then they could install additional masts in the area to boost coverage and speed.

Any of these are possible and only doing a full costing would show how it would compare to the FTTC/FTTP solutions.

Agreed but once you talk about installing a lease line then that dramatically increases the infrastructure costs for the wireless provider. This has to be recouped and requires a greater initial sign up to make it viable. I am not ignoring wireless as a solution and its clearly better than satellite, however If you are going to have to pay for lease line in order to get wireless to work then you may as well put the money into extending FTTC. People are fickle and despite moaning about poor speeds probably won't want the additional cost of installing a wireless antenna on the roof and drop to the router, when previously they just plugged something in to their phone line which already exists. After a while when their neighbour says they can now stream iPlayer or films from Netflix then they then start to consider upgrading.
The mobile route in my opinion is a no go as this would probably reverse a commercial decision on behalf of Mobile Phone operators. We had excellent reception from T-Mobile when they merged with Orange as there is a transmitter tower within the village. When EE took over and the bid for the 4g spectrum they rationalised (closed down) a lot of their masts to save money. The mast in the village was one of those shut down. There were numerous complaints to T-Mobile/Orange/EE but they deny it but have supplied nearly all customers with a boost box connected to their ADSL!! As with satellite broadband Mobile Broadband is very expensive in comparison with ADSL or FTTC and until the Mobile Phone operators come up with an unlimited tariff at a reasonable cost then 4g is only suitable for the occasional download and not for streaming.
John