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.pdfEvery 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 architectureI'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.
NodesPage 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 OptionsPage 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 ManualsA 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.
SummaryNone 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.