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Standard User VillageIT
(newbie) Wed 21-Oct-15 15:44:53
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How is FTTC deployed and how does it work in practice?


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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
Standard User Ribble
(fountain of knowledge) Wed 21-Oct-15 18:48:25
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Re: How is FTTC deployed and how does it work in practice?


[re: VillageIT] [link to this post]
 
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
Standard User Andrue
(eat-sleep-adslguide) Wed 21-Oct-15 20:14:15
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Re: How is FTTC deployed and how does it work in practice?


[re: VillageIT] [link to this post]
 
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 ).

---
Andrue Cope
Brackley, UK

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


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Standard User eckiedoo
(experienced) Wed 21-Oct-15 20:34:19
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Re: How is FTTC deployed and how does it work in practice?


[re: VillageIT] [link to this post]
 
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.
Standard User WWWombat
(knowledge is power) Wed 21-Oct-15 21:54:02
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Re: How is FTTC deployed and how does it work in practice?


[re: VillageIT] [link to this post]
 
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.
Standard User TheEulerID
(committed) Wed 21-Oct-15 23:02:48
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Re: How is FTTC deployed and how does it work in practice?


[re: Andrue] [link to this post]
 
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
Standard User RobertoS
(elder) Wed 21-Oct-15 23:28:07
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Re: How is FTTC deployed and how does it work in practice?


[re: TheEulerID] [link to this post]
 
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?

The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk. Domains, site and mail hosting - Tsohost.
Connection - AAISP Home::1 80/20. Sync 59999/14372kbps @ 600m. - BQM
Standard User TheEulerID
(committed) Thu 22-Oct-15 09:04:56
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Re: How is FTTC deployed and how does it work in practice?


[re: RobertoS] [link to this post]
 
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).
Standard User RobertoS
(elder) Thu 22-Oct-15 09:27:38
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Re: How is FTTC deployed and how does it work in practice?


[re: TheEulerID] [link to this post]
 
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 smile, founded on the basic definition.

The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk. Domains, site and mail hosting - Tsohost.
Connection - AAISP Home::1 80/20. Sync 59999/14372kbps @ 600m. - BQM

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

Standard User Andrue
(eat-sleep-adslguide) Thu 22-Oct-15 09:46:29
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Re: How is FTTC deployed and how does it work in practice?


[re: TheEulerID] [link to this post]
 
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.

---
Andrue Cope
Brackley, UK
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