How are exchanges connected?

Very random question today as i've been walking in the Peaks. Lot's of mini exchanges in villages that have now been upgraded to Fibre, but where do they go?

Do these smaller, rural exchanges connect to bigger ones close by? What kinda fibre cables run under the roads? Is it 10GB etc?

Any Superfast broadband in the area (FTTP/FTTC) likely bypasses the small exchanges completely and go to larger headend exchanges.
ADSL services may use microwave or fibre using SDH fibre systems, and PSTN could be microwave, SDH/PDH or even some old copper DLS for really small sites.
If they really are small exchanges there wont be anything like 10Gbs, maybe as little as 34Mbits for ADSL services, unless there is an LLU operator that's chucked in 100mbit or 1gb for unbundling.

How far can FTTP go in terms of bypassing the smaller exchanges 10 miles, 50 miles etc ?

In reply to a post by mwarby:

How far can FTTP go in terms of bypassing the smaller exchanges 10 miles, 50 miles etc ?

Around 20km using the standard deployment method.

They can increase this distance by using Subtended Head Ends, making it more like 20km from where they install the S.H.E (ideally next to an existing Huawei FTTC cabinet).

mwarby

Technically 50-60 miles, but usually much less than this.

Exchanges use different transmission tech for different services. So PSTN ( Voice) will still be on SDH or even PDH using multiple 2Mb, sometimes over copper in small exchanges.

As Witchunt says exchanged based ADSL could be 34Mb (SDH) or Ge ( Multiples) Bigger exchanges may have 10Ge for LLU backhaul. VDSL could be Ge (Multiples) from each Cab bypassing the local exchange to a Head end. FTTP is 2.5Ge bck to the Head end.

From Head end to core is now likely to be 10Ge (Multiples) or 40Ge in larger sites.

Leased lines most lcan be anything from 2Mb to 155Mb for old TDM circuits or 100Mb to 10Gb for ethernet

Looks like in some areas, everything will be FTTP, which may mean those exchanges are a thing of the past.

Edited by mwarby (Thu 15-Oct-20 21:43:04)

In reply to a post by mwarby:

Looks like in some areas, everything will be FTTP, which may mean those exchanges are a thing of the past.

Future sale of 5000+ copper exchange sites, might dent that little pension pot issue

In reply to a post by Pheasant:

In reply to a post by mwarby:

Looks like in some areas, everything will be FTTP, which may mean those exchanges are a thing of the past.

Future sale of 5000+ copper exchange sites, might dent that little pension pot issue

Most (maybe all) exchanges are rented anyway.

https://www.egi.co.uk/news/bt-to-end-3700-telephone-...

So it will improve their profitability won't directly reduce their pension deficit.

Edited by Andrue (Fri 16-Oct-20 08:16:14)

In reply to a post by Pheasant:

might dent that little pension pot issue

In fairness to BT the pension shortfall has been reducing over recent years because of lump sum payments from BT

Pension Shortfall
June 2017 - £11.2bn
June 2018 - £8.3bn
June 2019 - £7.7bn

Between now and 2030 they will make additional payments of no less than £700m each year

Edit: Changed 'Shortfall Reductions' to 'Pension Shortfall' as it could be misleading.

Edited by deleted (Fri 16-Oct-20 13:47:34)

Cheers for the detail there. must admit it was a bit of a throw away remark on my part. All good.

Pheasant

Many of the exchanges that will be bypassed are very small and the buildings are basically sheds. They are practically all leased and the rent is insignificant on these 1500..

With 5500 buildings in total and keeping 1000-1200 as head ends. Take off the 1500 'sheds' and you get 2800-3000 that actually reduce your costs. The sale and leaseback agreement was stacked with the highest rents on those hardest to get out of , ( except for a few that were high value like Chelsea, were there were lease breaks put with escalating rents to encourage BT to leave). There were also limits on how many BT could 'handback' in any period to protect the rental yield. There should be a end point to those but no idea when that would be, vaguely remember it was reported as a 30 or 40 year lease when signed..

So not a pot of gold to solve the pension issue .

I thought that these days, the PSTN was converted into VOIP at the earliest opportunity and all the switching was IP. I'd imagined that every exchange was now just racks of analogue terminal adaptors - are there still non-IP links between exchanges?

That was the plan with 20CN but it didn't happen. Not entirely sure why, heard rumours they just could get it working back then . Then NGA became the focus and it all got shelved until the move from PSTN to VOIP over broadband.

In reply to a post by witchunt:

That was the plan with 20CN but it didn't happen. Not entirely sure why, heard rumours they just could get it working back then . Then NGA became the focus and it all got shelved until the move from PSTN to VOIP over broadband.

Given conditions this year, are there still a significant number of exchanges left to convert under the 21CN programme? A December 2019 article on ispreview mentioned somewhere around 51 exchanges left on 20CN by this year.

I am struggling to find any information that can help me understand how fibre is going to be deployed in the UK if the objective of moving away from copper altogether is achieved. Whilst I am familiar with the exchange network that supports copper I have no idea what the future fibre network might look like. Are physical exchanges still required or will there be some form of computer controlled master distribution system? Presumably each fibre has a finite capacity and there must be aggregation points of some sort. I am in a rural area that is 100% EO. As we eventually get FTTP which is the current plan, does the existing exchange have a role in the future. Will the term connected to XYZ exchange have any meaning?
Where can I find a guide that might help me understand what the country's future communication network might physically look like even if I just concentrate on what Openreach is building.

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

In reply to a post by chriswillsher:

Presumably each fibre has a finite capacity and there must be aggregation points of some sort.

The fibres are lit from larger, "head end" exchanges. That's where you'll find the Optical Line Termination (OLT) equipment that drives the fibres. Each OLT port drives one fibre, and each fibre can serve up to 32 properties.

Fibre cables carrying many fibres run from there to Fibre Aggregation nodes. Smaller cables run from those to Splitters, where the light from a single fibre is split up to 32 ways. Smaller cables again run from those run to Connectorised Block Terminals (CBTs), which is where the individual cables that connect to each house are connected.

In short, the FTTP network spans out as a tree from the head-end exchanges. It bypasses smaller exchanges.

In reply to a post by chriswillsher:

I am struggling to find any information that can help me understand how fibre is going to be deployed in the UK if the objective of moving away from copper altogether is achieved. Whilst I am familiar with the exchange network that supports copper I have no idea what the future fibre network might look like.

Are physical exchanges still required or will there be some form of computer controlled master distribution system?

Yes physical exchanges are still required, but there are fewer of these "head end" exchanges that serve larger geographic areas, rather than many exchanges serving smaller geographic areas. Smaller local exchanges play no role with FTTP.

In reply to a post by chriswillsher:

Presumably each fibre has a finite capacity and there must be aggregation points of some sort.

Optical fibre has tremendous information carrying capability, in the many terabits/sec for a single strand of glass.

However the architecture deployed for FTTP at least by Openreach, uses a passive optical network (PON) design that passively splits the optical fibre path in the journey from the exchange to the subscriber. The physical connection of a PON is thus "shared" amongst the users connected to it. It is not a dedicated 1-to-1 fibre connection from exchange to user.

The fibre network serving premises is indeed aggregated at various points on the fibres routes from the exchange, called unsurpringsly Aggregation Nodes. Up to 32 premises are served by each individual strand (or core) of fibre that terminates back on an equipment port in the head end exchange.

In reply to a post by chriswillsher:

I am in a rural area that is 100% EO. As we eventually get FTTP which is the current plan, does the existing exchange have a role in the future. Will the term connected to XYZ exchange have any meaning?

Your local rural exchange, unless it serves much larger area, will typically not be a head end exchange and will serve no role for FTTP.
You will still be connected to XYZ exchange, but that exchange may be 15 or 20 miles away, rather than 3 or 4 miles away.

In reply to a post by chriswillsher:

Where can I find a guide that might help me understand what the country's future communication network might physically look like even if I just concentrate on what Openreach is building.

Google is actually a good ally. Theres plenty of good information out there.

Thanks for that helpful information. Will it be possible to know where the head-end exchanges are located in the same way that you can now map the copper exchange network?
I would like to try and get an idea of where the fibre cable to our village comes from and if possible, the route it will take. Currently we are being told that there is a section of duct that needs replacing but I have failed to find out where this might be.

We were in scope to be connected under the DSSB programme and already have the local infrastructure including an aggregation node in the manhole and CBTs on the poles but when they hit the blockage everything was cancelled. I am in the process of setting up a CFP and the initial cost indication is silly money for 40 properties. I understand that the initial figure is based on a robotic operation so am now awaiting a final cost which I believe involves some form of human sanity check.

In reply to a post by chriswillsher:

Thanks for that helpful information. Will it be possible to know where the head-end exchanges are located in the same way that you can now map the copper exchange network?

One trick is if there is any FTTP only near you (eg new build), to put one of those addresses in the BT Wholesale checker. It will say 'Connected to XXXXX exchange' - that is the fibre headend.

In reply to a post by Nick_W789:

In reply to a post by chriswillsher:

Thanks for that helpful information. Will it be possible to know where the head-end exchanges are located in the same way that you can now map the copper exchange network?

One trick is if there is any FTTP only near you (eg new build), to put one of those addresses in the BT Wholesale checker. It will say 'Connected to XXXXX exchange' - that is the fibre headend.

Actually no it is not the head end exchange (necessarily).

In reply to a post by Pheasant:

Actually no it is not the head end exchange (necessarily).

Your response took me by surprise.

I was also lead to believe that for a fttp only property (without any copper) the DSL checker shows the head end exchange instead of the local exchange, if the exchange shown on the DSL checker is not the head end exchange nor the local exchange what purpose does it serve and why is it in the results on the DSL checker.

In reply to a post by dect:

In reply to a post by Pheasant:

Actually no it is not the head end exchange (necessarily).

Your response took me by surprise.

I was also lead to believe that for a fttp only property (without any copper) the DSL checker shows the head end exchange instead of the local exchange, if the exchange shown on the DSL checker is not the head end exchange nor the local exchange what purpose does it serve and why is it in the results on the DSL checker.

Apologies I missed the ‘“only” in the statement, otherwise if FTTP is enabled (and copper is present however not necessarily active) then just the local exchange is shown on the checker. A better system imho would be to indicate both where the property is served by fibre and copper, but no doubt too complex

The headend is in Perth.

Thanks I suspected as much. Our local exchange is really only a small hut and is obviously not part of the fibre network at all. We are about 12 miles from the centre of Perth. These head end exchanges must cover quite an area. One can only speculate the cost savings that will eventually be realised by removing the copper network throughout the country. Hardly a day goes by without seeing an Openreach engineer mending a faulty copper connection.

I have found an excellent description of the physical construction aspect of laying fibre networks throughout the country. This is a few years old and some of the connection technology might have changed but the basics are still very relevant.

Building fibre networks

witchunt

COST

It worked in S Wales ( think it still does) but it cost too much and as usual with certain suppliers the promises exceeded the delivery.

The actual cost of transferring from one technology ( RCU/RSS) to another (MSAN) by rejumpering was too much to ever cost in with VOIP on the horizon. (and OFCOM wouldn't let the cost be recovered).

So decision was to sweat the assets until they wore out sometime around 2025 and move to VOIP. With call revenue trending towards zero the was no profit in voice to pay for anything else.

Pheasant

I think you are getting confused there were around 51 exchanges still on (20CN) ADSL last year. The rest were all on 21CN broadband ( ADSL2+ / VDSL / FTTP)

These were all very small ultra rural places.

PSTN Voice apart from a small subset (in S Wales) are all still on 20CN. Salisbury is likely to be the first Voice area to be migrated completely to VOIP ( via FTTP) and Mildenhall ( Via FTTC) then the is a published programme to close new provision on 118 further exchanges announced in May. This will then be a continual programme. OFCOM are just starting a consultation of the ability of OR to start forcing migration 2 years after the closure of new provision.

In reply to a post by j0hn83:

In reply to a post by mwarby:

How far can FTTP go in terms of bypassing the smaller exchanges 10 miles, 50 miles etc ?

Around 20km using the standard deployment method.

They can increase this distance by using Subtended Head Ends, making it more like 20km from where they install the S.H.E (ideally next to an existing Huawei FTTC cabinet).

Wonder if Openreach ever did or do specify any distance limits with their FTTP infrastructure?

I ask because I stumbled on an old (ancient really, 2013!) Openreach PIA reference offer document online that had a GEA FTTP schematic diagram, where a 28dB optical loss budget was noted, from OLT to the ONT.

This max figure would have included various necessary PON splitter losses, presumably up to 32 way splitting per core (but 128 way is noted, I guess in line with the GPON standards) as well as splices and connectors along in the full link.

Not sure what splitter losses they allow for, but a quick search says something in the order of 17 dB for combo of two series PON splits, so some fag pack maths and you're down to around 11 dB budget for the fibre attenuation @ around 0.36 dB per km, which is around 30 km or so, with no margins....

All back of fag packet of course!

In reply to a post by kitcat:

Pheasant

I think you are getting confused there were around 51 exchanges still on (20CN) ADSL last year. The rest were all on 21CN broadband ( ADSL2+ / VDSL / FTTP)

Yep that's what I thought I had said, around 51 exchanges left to convert to 21CN...no?

Interesting about the PSTN wind down.