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[c6em]

That's just the duct that is orange
The jacketed fibre cable is blown through

[Black Sheep]

Hmmm ?? I've only ever come across actual orange fibre cables, blown down a black (dark-coloured) duct. Or, 'sub-duct' to use its correct term.

This is not my area of expertise though.

[waltergmw]

I will reply more fully later but the large black BT tube is only 25 mm sub duct through which any blowable fibre can be inserted.

B4RN do use single mode fibre to every individual house and hence they all have 1 Gbps available but few can use much of that unless all the children are on their devices at the same time. B4RN have two access points on the Manchester fibre cable and have already lit multiple 10 Gbps wavelengths from each. From the two main nodes dual diverse routes feed all the intervening village cabinets diversely using some bundles for ongoing trunk routes and other fed and back-fed to individual properties. There are no intervening electronics anywhere other than in the village cabinets. B4RN are able to offer a 10 Gbps service to any commercial entity wishing that. N.B. the network is being very actively expanded (now with well over 800 properties lit) all the time now into Yorkshire and the edge of Cumbria too. A cabinet is powered up just as soon as one 10 Gbps fibre is available. The second diverse fibre will arrive by happenstance often as other properties are lit en route.

The mouse - attacked 16 mm orange tube contains (I think) a 144 single mode fibre on its way to Arkholme. It has obviously been replaced now !

I'm off with the wheelbarrow now.

Kind regards,
Walter

[waltergmw]

Gentlefolk,

I'll just add a bit on fibre cables and ducts in general but I can provide more information if anybody has specific questions they would like answered - provided that is not used in a propaganda war some other web sites seem to enjoy !

Runs to each house use 7 mm orange tube underground, black UV resistant 7 mm on house walls etc. and white LSOH 5 mm tube inside routed to the CPE equipment. Every domestic feed includes a gas block to seal the fibre just in case B4RN end up in Scottish or Devon granite with radio active radon gas !  I suspect methane from peat bogs etc. is far more likely and is just as undesirable. The CPE is similar to routers we have all met with 4 * 1 Gbps ethernet ports and a WiFi unit. The latter is often useless in old stone farmhouses. The actual fibres are either two fibre or four fibre bundles which is the maximum practical limit even though 12 fibre bundles might in theory be used.

There are very many different combinations of larger tubes usually with different colours on multi-tube sheathed bundles. Popular formats are 12 * 7 mm tubes and 24 7 mm tubes with a central 14 mm tube for the main supply 96 fibre bundle. Inter-cabinet routes, again sometimes with multiple sheathed bundles, consisting of 16 mm rifled tubes to assist air flow. (BT's sub duct has a smooth bore but is often used with a blower using toothed "tractor wheels".

Fibre cable sheaths can be black, orange, red or yellow and all have their description and distance figures printed every metre along the length. Most fibre cables consist of individual 12 fibre bundles wound in multiples so capacities are usually 48, 96, 144, or 192. The latter sometimes consist of 24 fibre bundles but with the standard colours repeated in stripes for the second set. BT often use a 12 * 20 format providing 240 fibres in total and I think is the largest that is used with the 25 mm subduct.

There are many different types of blowing machines ranging from a small highly portable device for the domestic blows - but with a heavier air compressor through to larger ones. If tubes have been mole ploughed on a straight run several km can be blown from an access chamber but B4RN usually plan for chambers at about 500 m intervals to allow for bendy routes. A route is started with all the tubes connected with hose joiners in the chambers. If the fibre is observed slowing too much the next chamber joiner is opened thus increasing the air flow and allowing the cable to be pulled out and fleeted in a figure of 8. Once the first chamber has had sufficient length blown through, the blower is moved to the next fleet to repeat the process. If by any chance the tube has been kinked sufficiently enough to stop the fibre then the length is estimated and an additional chamber dug in with the tube cut at that point.

Blowing is actually a misnomer as the air pressure mainly just keeps the fibre cable freely floating in the centre of the tube. The cable propulsion is usually done with friction feed belts or tractor wheels.

I have plenty of pictures if anybody would like to PM me with a return e-mail address. Commercial illustrations can be found as fibreflow at www.emtelle.com

Kind regards,
Walter

[WWWombat]

Thanks Walter. Interesting to see what B4RN are up to.

Here's a long diatribe on what I think I know about the equivalent for BT's access network, and a couple of other comments to linked subject-areas. I hope it's readable.

considering the loop length to my cabinet is about 4km, something doesnt add up

It certainly wont be one single 4km run of OM4 if thats what you are thinking.  There will be various nodes along the way consisting of splitters/repeaters/manifolds.

I have no idea about the use of single-mode, multi-mode, OM3 or OM4 within the BT network. However...

I thought the whole point of BT's FTTP access network (ie specifically the bit between the exchange/head-end and the subscriber's property) was that it was a PON - a passive optical network - where the "P", passive, is the important bit. No electronics, no power, so no repeaters or amplifiers, but there can be passive splitters. The current use of the PON architecture is to carry GPON signals (or are they termed lights?), but they are equally capable of carrying 10Gbps XGPON too.

Because the first part of the architecture (head-end along the fibre spine to the aggregation node) is identical between the FTTP, FTTC and FTTP-on-demand architectures, this implies the fibres within the spine cable could be used for any of these jobs.
 
Elsewhere, I have seen reference to the fact that the maximum reach of the PON is 20km. That would fit with the idea that BT are deploying fewer head-ends than exchanges (whose original distribution around the country was defined by the reach of voice signal over copper).

Ah - I found a presentation that I've obviously seen before. It has reference to the 20km reach and more: http://resource.npl.co.uk/docs/networks/foton/080717/campbell.pdf

Every area is different, and its entirely up to BT what they use.   Without doubt someone will have had to have sat down to plan and cost the various options available depending upon the number of cabs and their distance from the exchange and locality.

They may use single mode fibre to a node for some really long runs...  or they may use say OM4 to cab 1 and then use a splitters and/or fibre optic repeater to reach cab2 etc.   There will be nodes for both fttc & fttp.. and no 2 area's will be the same.

I'm cheating, because I know what I'm going to write further down. However, there is probably a great value to BT by make all areas identical - with no need for any special cases within each exchange area. That way leads to problems far in the future - when you need to keep engineers with all the appropriate skills. So much easier if there is one homogonous network.

Today, Openreach have problems with maintenance of some areas, because very few engineers have the skills for sealing joints with lead - as was done on old, paper-core E-side cables. We are seeing timescale problems in rolling out all the EO-replacement PCPs in BDUK because there aren't enough engineers with the skills to do the copper re-arrangement.

Having staff trained in the right fibre skills will be an issue in 20-30 years time, if they go ahead now and install odd solutions.

Anyway, on to what I have gleaned about ...

The BT Fibre Access architecture

I've seen quite a few pictures of BT's access network architecture, with some differences. However, this presentation puts a lot together in one PDF.
It seems to have been the subject of a BT presentation at IWCS (International Wire & Cable Symposium) in 2010:
http://iwcs.omnibooksonline.com/data/papers/2010/1_8P.pdf

(BTW: They made a series of presentations from 2005 through to 2010 that give a great insight into the changes in thinking that went on.)

Pages 9 and 10 of that presentation show the 2010 picture. TBB's Mr Saffron has an image in amongst his pictures of actual FTTP equipment.

Some important points don't seem to change between the various images:

• The fibre spine cable, from head-end to aggregation node, and onwards to further aggregation nodes in the spine, is labelled COF-200
• The fibre cable used from aggregation node to splitter node, and onwards to further splitters in a chain, and then (after splitting) onwards to Fibre DPs, is labelled Mini-Cable, which elsewhere seems to get a label of COF-205. Alternatively, this could be distributed overhead with an "aerial cable"
• Blown-Fibre-Tubing, BFT, for carrying blown-fibre-units (BFU, of 4 fibres, it seems) exists from the fibre DP to the manifolds, in groups of 7 or 12 tubes, which may be in chambers or at the top of poles.
• Blown-Fibre-Drop, BFD, is the term for the tubing used for the final drop from the manifold to each house.
• The BFD (either underground or aerial) then terminates at a splice point on the customer wall
• It looks like the COF-200 cable, and the COF-201 mini-cable can be blown down sub-ducting, out as far as fibre DPs.
• It looks like empty BFT tubing is blown (?) down sub-ducting from the fibre DPs to the foot of the poles or to underground chambers, for termination at a manifold.
• Once an order is placed, empty BFD is installed from the manifold to the house, and joined to the tubing in the manifold.
• It looks like the BFU (the 4-fibre unit) is the part blown down tubing from the fibre DP, through a joint in the tubing in the manifold, into the splice point on the subscriber's wall.

So "blowing fibre" could be a term that applies to bigger, multi-cored cables (with fibres in multiples of 12, up to 288 in total) being blown down sub-ducting. Or it could apply to the final push of blowing a single BFU through BFT/BFD tubing from the DP into the home.

It looks the "black cable with yellow stripes" that people often see could actually be fibre cables or mini-cables, sub-ducting for these cables, or empty BFT or BFD.

Page 8 in the presentation refers to FTTC, and suggests that a 4-fibre BFU is blown in BFT to the cabinet, from the aggregation node.

Nodes
Page 11 shows how many end-users are supported by the various nodes.

Things don't seem to be too much different today.

None of the pictures gives a clue about how a number of aggregation nodes are wired back to the exchange - is it a star network, a tree+branch look, or a linear spine? Likewise for the splitter nodes that are wired out of each aggregation node, and likewise for the DPs wired out of each splitter.

Cable Options
Page 12 shows a few cable options. From the documents I link below, it appears that the largest cable carries 288 individual fibres, rather than the one suggested on page 12 to be "360?". I think Walter is right that the largest sizes are aimed at 25mm subducting, while 144-fibre cable is aimed at 16mm subduct, and anything below 96-fibre is aimed at 14mm subduct.

Page 24 shows more data about the final BFD and BFU tubing parts.

Note: I've tried to be careful in this post to refer to tubing, when only the 4-fibre BFU's are blown to a property, and sub-ducting, when the larger multi-core cables are blown between nodes.

BT/Tyco Instruction Manuals

A while back, I found a variety of instruction manuals to the various nodes, showing how fibre cables should be treated, complete with things like the exact lengths needed for each different size of footway chamber, the sealing of cable in the port, and looping-through of fibre elements (ie a group of 12 fibres) to nodes further down the chain.

Reading these instruction manuals, with reference to the architecture pictures above, reveals a lot of detail of the way things are meant ti be put together. It suggests that fibre spines are indeed linear spines, with fibre looped-through (unspliced) early aggregation nodes on the way to later ones. The same applies to chains of splitter nodes, and even to chains of fibre DPs.

I originally posted about these documents on TBB. I thought I'd done the same, but a search does not find them, so I'll copy my TBBB text:

Anyone interested in some of the nuts & bolts of the fibre distribution network that BT are building?

I found instruction guides (with lots of photos) for some of the nodes that BT are using in their fibre access network; these give us some clues as to the architecture BT intend for fibre.

Aggregation Node
http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=tc-924-sip-uk&DocType=SS&DocLang=English&s_cid=1046

This shows there are separate areas for "NGA Spine" fibre and "P2P BAU" fibre.

Generic Joint
http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=tc-699-sip-bt-uk&DocType=SS&DocLang=English

This shows some of the fibre options being chosen by BT for the access network. For example, the larger cables seem to consist of 240, 276 or 288 fibres.

Splitter Node
http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=tc-923-sip-uk&DocType=SS&DocLang=English&s_cid=1046

It seems that a splitter node is supplied with one 2x32 splitter - 32 end-users, with (I assume) a way to have 2 separate parent fibres back to the AGN for redundancy. Perhaps this allows for dual-parenting, or separate head-ends.

Three further splitter devices can be added, so one node can supply up to 128 properties. The box is designed for fibre from the AGN to loop-through, on its way to further SPNs down the chain.

Splitter Node for MDUs
http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=tc-1042-sip-uk&DocType=SS&DocLang=English

Similar function, but housed in a way suitable for MDU basements.

Fibre DP
http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=tc-921-sip-uk&DocType=SS&DocLang=English&s_cid=1046

The bit I haven't figured out is the distinction (in the aggregation node) between "P2P BAU" fibre and "NGA Spine" fibre.

Summary

None of the architecture diagrams, nor the installation manuals, show any reference to any kind of powered hardware. I suspect they really have gone for a homogeneous PON network - which in turn means that the 20km capability must come from single-mode fibre, and it looks like it gets used everywhere.

[kitz]

Thank you very much www for all the above info.  Its too late for me to digest it properly and pay it the proper attention that it needs, but I just wanted to say I much appreciate the time it must have taken you to type all that out.

One thing that does strike is that in some of my earlier posts (ie in particular the one to chrys about OM4 - I was at that point thinking FTTC and the aqua cables that we do sometimes see.    Thinking about it (even in my very tired state) you are highly likely to be correct about PON for FTTP.   Ive seen an image - (in fact I think its on the main site) implying that for fttp they use 7BFT single mode locally and 1BFT single mode to the premise.

When I get chance, I will read your post in much more detail, and look at the links that you kindly provided.

[WWWombat]

No problem. There's no point in acquiring all this knowledge if you can't share it with someone. But it has taken a while to acquire and assimilate!

One question... can multi-mode fibre be spliced onto single-mode fibre? And still work, obviously...

[kitz]

Not directly spliced, but there are plenty of converters which can do this, some specifically designed for FTTx networks.

You can also use WDM on single mode fibre.. although that is expensive because it doesnt just split, it also forms the light ray into different colour frequencies so you need a MUX/DEMUX but its major benefit is no dispersion like with multi-mode. Its more akin to DSL/DMT in how it works as multimode simply bounces the light rays at slightly different angles.  BT use WDM extensively for long runs.. or more specifically all Tier 1 MSANs use WDM for their backhauls.    I believe there are cheaper forums of WDM, but whether or not they'd use it in the local area I dunno.. perhaps in future for FTTP in an area where there's lots of users?
 
I doubt it (WDM) would be used with FTTC..  the architecture is different.  I'm also pretty sure that I can recall reading that BT already use a mix of single/multi modes when it comes to some of their provisioning for business lines... although I'd have to go digging through SINET to recall which of the services. 

Edit
I just did a really quick search on Openreach, FTTP and WDM..  and came up with this.{page 21}  I've not read it all, but it would appear BT are looking at using WDM for single mode fibre on FTTP in the future.

[c6em]

This is some seriously good quality information you lot have dug up - especially @wwwombat.
XGPON or 10GPON uses different transmission standards so can be used alongside GPON on the same fibre.
This will allow in the future individual users to be upgraded to XGPON without affecting existing GPON users.

Of course the 'problem' with a GPON architecture is in the upload in that it can never be symmetrical due to the broadcast nature of the GPON system.
The users ONT has to "request" allocation of upstream bandwidth otherwise its transmissions may collide with others doing the same.
(and the need in the future for much more powerful lasers at the user's ONT end)
I'm sure though that this issue in so much as it is a problem will be solved - do you really need umpteen 100's of Mbsp upload?

[WWWombat]

Yes - WDM would be an obvious way to improve the existing PON. We can expect that BT will have planned for that (as shown in the fisher document), so it seems to be another tick in the box for single-mode.

If multi-mode needs a media converter before it can be connected to single-mode, then I'd say that was an indicator that the FTTC cabs *don't* make use of multimode. I'm still pretty sure their fibre comes from the same main spine cable in the AN as the PON fibre will.

Upstream: do we really need umpteen 100's of Mbps? Some would say yes ... but I suspect that our needs for that would be far more bursty than the downstream. Which means we may well survive on sharing a variable permission-based setup.

[sorc]

They have been doing exactly that for the last couple of years It all depended on whether your own Exchange was ear-marked for the full 'FTTP' (Fibre To The Premises), or 'FTTC' (Fibre To The Cab) product ?? I couldn't give a percentage as to how many Exchanges are, or going to be, FTTP in comparison to FTTC ....... but I will bet it's 5% or under ??.
I feel for the people of Rochdale for example, as they were ear-marked for FTTP, but when it was found their Exchanges 3-phase electric supply was already running at 90+%, which put the estimated installation costs a lot higher as they would have had to upgrade the supply. Instead, they went for FTTC ..... that's business for you.

I am in Cornwall. There's a lot of mixed FTTP/FTTC, with the FTTP being seemingly totally random. Areas like mine where the cabinet serves probably a km of road would seem like great FTTP candidates since the speeds attained at the other end of the road are not likely to be great. But no, it's FTTC - and it doesn't look like they bothered with FTTP when they had to put in new copper for a new housing estate. Elsewhere in the village, Openreach has decided that precisely one road gets FTTP (and there are no exchange-only lines or underground ducting/wiring to the home)

I've also seen rural farmhouses where Openreach has spared no expense in running FTTP, yet the single potential customer hasn't taken up the service (yet other very rural people have been directed to satellite internet) Or fibre to a pole that has no customers on it at all. Or some streets where one half is one one PCP, the other half is on another. One got FTTP, the other got FTTC. It seems totally random (and it makes me want to disbelieve the "FTTP would cost too much" brigade because BT is certainly capable of it when they want to do it)

Once the FTTC (and it's sister, BDUK) programmes have completely rolled out, then I would suspect 'FTTP (oD) - On Demand will be the next logical step. I've no idea at all on pricing for FTTP (oD) at the minute, but would hope it gets cheaper as time goes by

I can already get FTTP on demand. Aren't the install/rental costs meant to be stratospheric? How does Openreach pick which areas can get it, because IIRC it isn't across 100% of the FTTC network yet. (it's been cynically suggested that the pricing is designed such to prevent people considering it as an alternative to a much more lucrative leased line)