FTTC/P handover exchanges

I've read that the exchange that the fibre is connected to isn't necessarily the same as for the copper phone line - does anyone know how widespread this is, or have a table showing which exchanges these are?

I was wondering whether this was just atypical for awkward exchanges or as part of a general plan by BT to simplify the network, which could have implications for LLU providers - only having to put equipment in a fraction of the exchanges could make things considerably cheaper.

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Richard J

some of the Openreach documentation shows two exchanges - the one whose area it is and the other which handles the fibre interconnect. The leaked "postcode PCP CSV" file includes this information IIRC.

In the long run fibre with 60km range is going to bypass many current exchanges. LLU will be history.

In reply to a post by Anonymous:

I've read that the exchange that the fibre is connected to isn't necessarily the same as for the copper phone line - does anyone know how widespread this is, or have a table showing which exchanges these are?

Mine would be one such example. My voice/ADSL exchange is Brackley (SMBY) but for FTTC the exchange is Banbury (SMBB) which is eight, going on nine miles away. They've just started going round marking the locations of the new cabinets. Mine is actually the first cabinet you come to when driving the route and interestingly there's now a dashed line leading away from the cab toward the main road.

It looks to me like BT are going to connect my cab to the distant exchange first and then work their way back to the exchange before branching out again.

I remember reading some papers from BT a few years back about a 100 km fibre "PON" (it had an amplifier), split between 1024 users - the idea was to basically eliminate all the exchanges and connect directly to the core network of about 100 nodes. GPON can't do that, but 20 km is still a lot longer reach than copper so I wouldn't be surprised if you'd need only a fifth or so of the exchanges.

The reason I was wondering was that it would seem to make economic sense to have fewer exchanges and a simpler network, but would rather mess up the current LLU model. Either for better (lower costs overall, more economic to do LLU with fewer, bigger exchanges) or worse (Openreach gets a larger fraction of the revenue).

Sadly, the exchange list BT hands out no longer seems to give the serving exchange information.

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Richard J

Its very typical and is technically desirable for BT. Reduces the handover points, which for BT means they can enable more areas without needing to also install 21CN at the same time in every exchange .
Also makes it feasible to close exchanges where there is only copper terminated and feed everything over FTTC/P, thats probably some time off (excluding the Fibre Only Exchange areas)

Less widespread in early phases, but now is quite common. Called a parent child releationship, it's done normally when the smaller exchange can't accommodate the head end, doesn't have the necessary power requirements or the exchange building is on BT's consolidation programe.

It's why you can sometimes have exchanges ready for service for FTTC, but not WBC. From a PR perspective, if you are in exchange x but the fibre head end is in exchange y, it will be exchange x listed in the press releases to reduce confusion.

The OR PPT slides (BT FTTC GEA Openhouse Slides Consolidatedv2.pdf, page 7) mention 845-1000 CP handover nodes which would suggest a 6:1 exchange consolidation (circa 6000 BT exchanges currently from memory).

One thing I'm not sure on is if the fibre is routed *directly* from the consolidated exchange or whether there is any aggregation via the 'child' exchange? It would be interesting to know given the potential long-term implications.

If I understand "One thing I'm not sure on is if the fibre is routed *directly* from the consolidated exchange or whether there is any aggregation via the 'child' exchange?" correctly, I believe the FTTC cabinet is connected directly to the head-end exchange.

The PSTN line continues to be via the child exchange, in the same way as when the FTTC cabinet is connected to there.

In reply to a post by RobertoS:

If I understand "One thing I'm not sure on is if the fibre is routed *directly* from the consolidated exchange or whether there is any aggregation via the 'child' exchange?" correctly, I believe the FTTC cabinet is connected directly to the head-end exchange.

The PSTN line continues to be via the child exchange, in the same way as when the FTTC cabinet is connected to there.

Well i would say the child exchange if enabled, as in our area only one of 4 exchanges has fiber, and only some homes within it's defined area have the choice of fttc , but looking at the latest info from bt the exchange that I'm connected is going to be enabled next year, and it will be the child of
the already enabled exchange,

There are handover exchanges not enabled for FTTC, maybe later.

In reply to a post by RobertoS:

If I understand "One thing I'm not sure on is if the fibre is routed *directly* from the consolidated exchange or whether there is any aggregation via the 'child' exchange?" correctly, I believe the FTTC cabinet is connected directly to the head-end exchange.

That's an interesting point. I can see that being the case for my cab because it's the first cab you'd come to when driving from the remote exchange toward my local exchange. However after that I'd have thought the fibre worked its way back to my local exchange before fanning out again. I assume that in most cases that's how the ducts are running.

Or are you just saying that there'd be no additional electronics in the local exchange?

Edited by Andrue (Thu 06-Oct-11 08:05:40)

If the plan is to close local exchanges then it does not make sense to route FTTC cables through them.

In reply to a post by Andrue:

Or are you just saying that there'd be no additional electronics in the local exchange?

A bit hazy as to where fibre routing and junctions may be sited, but there would certainly be no link into the national backhaul at the local one, therefore no need for any related electronics.

There are bound to be some posters around who know for a fact. This poster makes a good point.

Edited by RobertoS (Thu 06-Oct-11 09:24:34)

In reply to a post by Anonymous:

If the plan is to close local exchanges then it does not make sense to route FTTC cables through them.

Depends where the ducting is I suppose. Assuming that BT ducting is a star topography with the local exchange being the hub what other option is there? If you don't use the existing 'hub' then you've got a lot of digging to do. If the ducting is mostly below ground then it just sits there regardless of what's in the building above.

Granted it's not ideal to have cables underneath buildings but presumably all we're talking about is a relatively small box that splits/aggregates the signals. That could be sited in a cabinet just to one side. So perhaps what they would do is relocate the 'hub' so that it's on the edge of the exchange property then they close the exchange.

Edited by Andrue (Thu 06-Oct-11 10:12:24)

This diagram seems to suggest the local exchange is being bypassed (BTW anyone know what the "MDU" abbreviation stands for?), and there is an aggregation node outside the exchange?

Multiple Dwelling Unit - Flats

The exchange may or may not be bypassed. The eventual future will be less exchange buildings, replaced by smaller shed like buildings. Switching of line/services between providers handled electronically, rather than physically moving fibres around.

I only noticed recently that the deployment of the voice element of 21CN has stalled, and I did wonder if BT see FTTx as a way of dramatically reducing their physical plant & buildings once a credible voice over FTTx service is approved.

They might just as well sweat the System X/Y kit for a while longer because by the end of the decade I suspect things will look rather different.

Andre

Outside / alongside exchanges is a Cable chamber, on small sites this will just be a manhole. This is where the exterior Ducts will finish, cables can therefore bypass the building completely as not all cables will be required to enter every building.

All that will happen is the new fibres out to the cabinets will be spliced to the existing fibres back into the network if is there is capacity (or new cable if needed). As most exchanges already have fibre backhaul, (except extremely rural ones usually in Scottish highlands and Islands) many will not need additional backhaul fibres to the parent site.

If some cabinets are on existing fibre routes these can be spliced in at the nearest
Joint box big enough to fit a fibre joint in without going via the exchange cable chamber / manhole.

If/when you are able to empty the building you can sell / demolish without interfering with the cables.

In reply to a post by kitcat:

All that will happen is the new fibres out to the cabinets will be spliced to the existing fibres back into the network if is there is capacity (or new cable if needed). As most exchanges already have fibre backhaul, (except extremely rural ones usually in Scottish highlands and Islands) many will not need additional backhaul fibres to the parent site.

I understand that at a 'traditional' exchange where the exchange itself is being upgraded. What we're wondering about is what happens if the exchange itself is not being upgraded. My own exchange is one such example. Brackley is due to have FTTC in March 2012 but it's being provisioned from the Banbury exchange.

My question is what happens to the fibre when it arrives at Brackley. I can imagine it going straight to my cabinet because that's the sensible way to the town centre coming from Brackley. What I'm wondering about is what happens then? My assumption is that it will be blown the rest of the way to the cable chamber then split into multiple cables that are blown back to the remaining cabinets.

As most exchanges already have fibre backhaul

Of course but in this case my data will no longer be using Brackley's backhaul. I will be using Banbury's.

Edited by Andrue (Fri 07-Oct-11 15:35:45)

21CN voice, stalled I think due to reliability issues and time to roll-out at each exchange.

Andre

There will already be fibre cables from Banbury to Brackley, so that is the existing Backhaul for Brackley, If your cabinet is on the route between the two Openreach will splice a fibre from your Cabinet into the cable at the nearest Manhole. If your cabinet is 'beyond' the exchange manhole the fibres will be pulled / (blown if only a single cabinet) down (existing) duct to those cabinets. (Note blowing is only possible for small cables, 4 fibres, and short distances, so most of this will be pulled, FTTP could be blown to the house from the last splitter).

If there is an existing cable route out the other side of the exchange, say to Buckingham, Cabinets will be spliced into that cable to pick up fibres back through Brackley to Banbury

Imagine 3/4 of a spiders web hanging by a thread down the missing quadrant. Ypur cabinet is either connected to the hanging thread or the end of one one the radial threads.

They will only put a new cable in between Banbury and Brackley if all the existing fibres are used. I suspect that if this was the case Brackley would be later in the rollout!

The ability to use existing infrastructure is likely to have a big impact on where places sit in the rollout. So child sites are likely to have enough spare fibres existing to the parent to cater for all the cabs that are planned to be connected up. This would reduce the cost/manpower/capital needed to a cost in point.

Thanks Kitkat but that still doesn't quite answer my question

I would have thought that the ducting in a town was topologically a star (although some branches might be trees) and that therefore all the fibres from all the cabinets would eventually meet at a single point - the exchange. In your reply you state that cabinets on the other side of Brackley might be spliced into the Buckingham cable. That suggests something more akin to a bus or else that new ducting will be run from those cabinets to the main ducting for the backhaul.

http://en.wikipedia.org/wiki/Network_topology