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Can someone educate me...??!!?? (FTTP technicalities)

pawhe955
Grafter
Posts: 111
Registered: ‎31-07-2007

Can someone educate me...??!!?? (FTTP technicalities)

In the twilight years of a long career in large corporate IP Networking, Network Security and associated infrastructures - but no exposure to, or real knowledge of, the practicalities/specifics of Broadband delivery infrastructure equipment, especially "last mile"....  (which uses specific technologies that are very different to Corporate Networking). So a techie, but not specifically in these technologies..... 

So - got into a discussion with someone about FTTP physical delivery, which I'd always presumed was limited to underground Fibre runs, with fibres going direct from house to exchange - so when they said that an OpenReach engineer had been implementing FTTP to a neighbour from the local telegraph pole, I wasn't so sure.  So did a bit of research, and am now aware that OpenReach do in fact deliver Fibre overhead, via local distribution poles. Also noted use of pole mounted Connectorised Block Terminals (CBTs) to facilitate that. 

(In Corp. Networking) have only ever implemented dedicated high-speed point-to-point Fibre (e.g. uplinks & trunks) that requires a powered termination unit at each end of the physical Fibre (e.g. transceivers). So whereas FTTC presumably has a powered termination unit in the local Cabinet(? - to terminate the Fibre link back to the exchange?) and the cabinet equipment also terminates the Copper-wire last mile runs to houses, I presumed that FTTP would mean a direct Fibre run between the House (with its mains powered termination unit) and the ISP's (powered) Fibre terminating device, presumably at the exchange (in essence, qualifying the difference between FTTC and FTTP).

So the bit that's missing for me is - I wasn't aware that Telegraph poles had any facility for power, so presume that a pole mounted CBT is unpowered(?).  But I saw that CBTs could be 8-port or 12-port, etc. - so what's going on there..???  Do CBT's perform formal multiplexing - consolidating multiple house fibres to a single Fibre back-haul to the local cabinet (or exchange?)..??  Or is it just a multiple ported Fibre termination/splicing enclosure? 

Are CBT's "active" telecomms devices?  e.g. are they powered?  Do they operate at layer 1/layer 2/layer 3..??  Comparing directly with the way that the ADSL modem in my house is (physically and electrically) Layer 1/Layer 2 connected to/terminated on a DSLAM modem port (in the local cabinet - or only at the Exchange?), what "active" device (and located where - pole, cabinet or exchange?) would the Fibre from my house actually terminate on?  Whereas the 2 copper wires into my house are "dedicated" to me/my house, and my broadband data only gets aggregated at the back-haul behind the DSLAM (pure ATM or IP, etc.), at what point (on/behind what device) does my (personal/dedicated) data originally sent over the Fibre that leaves my house, get aggregated back into the ISP backbone?  Or indeed becomes part of any infrastructure that is "shared" with anyone else...?Huh

Thanks in advance for any information (and apologies for any incorrect preconceptions, etc.)...

 

6 REPLIES 6
bmc
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Registered: ‎28-02-2017

Re: Can someone educate me...??!!?? (FTTP technicalities)

@pawhe955 

Can't begin to answer all your questions but I don't believe CBT's are powered. Also, the fibre run may go nowhere near your local Exchange - it'll route back to an Internet Head End via Aggretation Nodes.

 

CBT's differ in size depending on the number of properties served. The following is an 8 port underground CBT. When installing to the house an engineer will plug one end of the cable to a port.

 

DP.jpg

 

Brian

 

 

Champnet
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Re: Can someone educate me...??!!?? (FTTP technicalities)

@pawhe955  It's more similar to the old thick/thin ethernet topology................

 

 

 

 

 

 

 

 

 

 

 

RealAleMadrid
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Re: Can someone educate me...??!!?? (FTTP technicalities)

@pawhe955  Openreach FTTP is a passive optical network, there is no power requirement between the head end exchange and the terminating equipment in your premises known as an ONT.

The fibre runs to aggregation nodes and then to optical splitters which distribute the signals to end users, each single fibre network or (PON) can support up to 32 end users although usually only 30 are actually used.

All the users receive the data from every user on the PON but it is encrypted and the ONT only decodes the data for your own connection, the download rate is 2.4Gbps shared between about 30 users ( worst case). So if everyone attempts to use the full 900Mbps speed everything will slow down, in other words it is a shared contended service.

In normal circumstances it works perfectly well, at these speeds most downloads can be achieved in short bursts so the fact that you are sharing the connection is not normally a problem.

There are a lot more details involved but this is a brief introduction to FTTP.

pawhe955
Grafter
Posts: 111
Registered: ‎31-07-2007

Re: Can someone educate me...??!!?? (FTTP technicalities)

@RealAleMadrid  Many thanks for the excellent information.  I wasn't being totally lazy(!), and did continue trying to research myself, and eventually stumbled over the PON article on Wikipedia last night - which your post confirms is part of how OpenReach are deploying FTTP in the UK, so thanks for taking the time to respond. 

Even with passive splitting along the Fibre run, there must still be an electrical device at each end of the point-to-point Fibre run, to generate light to transmit, and convert the received light back to an electrical signal - so just for completeness (if I may)...

-- just to confirm where those powered end-points (i.e. transceivers, for want of a better word) - House (ONT) and (head-end?) exchange?

Trying to form an overall picture in my mind of the physical connectivity from exchange to house - i.e. from the PON article, I initially presumed that the CBT is where the optical splitting occurs (one fibre in from local cabinet, multiple Fibres out to houses? - but then why would pole based CBTs only be 8 or 12 port, if PON usually splits to 32?) - and I then re-read your answer and noted the reference to "aggregation node" - so can I ask where an "aggregation node" is located, and what it's actually aggregating?  Is it doing more passive Fibre splitting - or is the "aggregation node" an active component aggregating at a different layer (e.g. IP or ATM or some other layer/technology)....

-- so where are the PON "optical splitters" physically located (cabinets and not the CBT?) - and what does a CBT really do...??!!

Thanks for the information so far - much better educated than yesterday!  (which is not a bad thing.....)

RealAleMadrid
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Re: Can someone educate me...??!!?? (FTTP technicalities)

@pawhe955 Glad you found by brief description useful, however I feel far too many questions are coming my way so I am giving you a link to an Openreach Youtube video which will show the general arrangement of an FTTP service.

https://www.youtube.com/watch?v=InjZDBBgkps 

I will also give a few quick answers. 

The fibre is lit at the head end exchange by the OLT.(Optical Line Terminal) Which is the starting point for the passive optical network,  connected to a core switch through Ethernet cables. The ONT in your house performs the conversion back to an ethernet connection. There are no powered devices in between.

The aggregation node will have a number of fibres from the exchange, some go to FTTC cabinets others will be individual 32 user PON fibres. I don't believe there is any splitting, it is just a collecting point for fibres from the area to get back to the exchange. There are no active electronic devices in it.

CBTs are purely passive pluggable fibre optic connectors to take the final fibre run to houses, they are similar  in function to DPs ( distribution points) used for copper phone connections. They only serve a small number of houses within a couple of hundred metres of a pole or underground chamber. The PON fibre is first optically split into 4 then each one split again into 8 fibres. These splitters are mounted in green boxes on poles or underground. Openreach FTTP does not use any ground based cabinets and is not routed via FTTC cabinets.

There is a lot of stuff on the Internet, the Kitz website is very good for general and more technical information.

 

bmc
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Re: Can someone educate me...??!!?? (FTTP technicalities)

@pawhe955 

The CBT that I pictured serves 7 houses. They can be daisy chained in a circuit. My one is the last in a chain of 5 or 6 running most of the length of the street. The CBT deployed will depend entirely on the number of properties to be served.

 

The Aggregation nodes will be where ever they need to be (usually underground) on the way back to the Head End. I believe there are aount 400 Internet Head Ends serving Britain. Each then connects into the core backbone.

 

Note that the ONT serving properties is now much smaller and does no longer comes with a power backup unit. This might be the current model

https://www.ispreview.co.uk/index.php/2021/01/a-look-at-openreachs-future-4-port-ont-for-fttp-broadb...

 

In a related but not connected program OpenReach is withdrawing the PSTN network (but not the copper cables). Once done, I believe the idea is to drastically reduce the number of Telephone Exchanges. As FTTP generally does not run through these FTTP will not be a (real) problem for the closure program.

 

Brian