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Standard User deiwise
(committed) Tue 09-Oct-12 16:57:51
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Understanding how BT phone exchanges actually work


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Hi,

I spend a lot of time trying to campaign for true broadband in my mother's village in Devon. Like many rural places, it is all but cut off from anything other than 512k DSL. Admittedly, it's in deepest darkest Devon, but I keep the faith in the hope her exchange (and others like it) will be upgraded. She relies on the internet to supplement her pension, as do many 65+ year olds.

What I really want to understand though is how exchanges actually work from a networking standpoint. Does each exchange, next to one another, connect to eachother - and if so, how? Is there any way of knowing what "path" the calls/data take from one exchange to another, to work out where the bottleneck potentially lies and/or which exchange might be able to be upgraded next, if pressure is put on BT (stop laughing)?

I see many exchanges improved/upgraded, some with fibre, in Devon - Dartmouth has recently been done which is near my mother's (Torcross). Does this mean, in theory, that her's will eventually be upgraded?

Here's a map which may explain what I mean. A is Kingsbridge, the nearest main town. Connections here are quite good. Dartmouth (C) is very good, with FTTC available. My mother's is some distance away. Do all the other exchanges need to be uploaded in a line, one by one, for her's to benefit? Or are rural BT phonelines way more complicated (with some lines going underground, some over, and most built by Victorian engineers!)?
Administrator MrSaffron
(staff) Tue 09-Oct-12 17:09:32
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Re: Understanding how BT phone exchanges actually work


[re: deiwise] [link to this post]
 
Kingsbridge exchange building has fibre going to the building already and has TalkTalk offering its ADSL2+ service, and BT offering their ADSL2+ service from (100+) providers.

Torcross will also have fibre, but is still only offering an up to 8 Mbps service, because it is a much smaller place, so less money to be made. Similar reason as to why you don't have a big B&Q, Cinema and large department stores.

Evidence of people in Chillington to the west of the exchange getting 6 Meg and better and 5 Meg towards Slapton Key.

So question is why is she only on a 512kbps service, and what is her distance from the exchange that appears to be just off the main cross roads with the A379 in Stokenham.

http://maps.thinkbroadband.com is how I can tell so much about the area.

The chances are that as part of the Devon broadband project there will be upgrades at some point in the next three years see http://www.thinkbroadband.com/news/5448-devon-and-so... for most recent update.

Though I would be less confident of getting anything more than 2 Mbps from this if they live in an area like Harleston which looks to be two or three properties around half a dozen fields.

Andrew Ferguson, andrew@thinkbroadband.com
www.thinkbroadband.com - formerly known as ADSLguide.org.uk
The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User WWWombat
(experienced) Tue 09-Oct-12 23:22:09
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Re: Understanding how BT phone exchanges actually work


[re: deiwise] [link to this post]
 
In reply to a post by deiwise:
What I really want to understand though is how exchanges actually work from a networking standpoint. Does each exchange, next to one another, connect to eachother - and if so, how?

Warning: technical overload approaching...

There are fundamental differences between 20CN voice, 20CN data and 21CN.

The 20CN digital telephone exchanges don't necessarily connect to their neighbours directly. Calls between exchanges go via a 2nd level of transit exchange, called tandems.

However, there are a couple of points that mean the "connections" do go from one "exchange building" along to the next "exchange building":

- There are some direct exchange-exchange connections, so some calls do route this way.
- Most of the smaller "exchange buildings" do not actually house a full exchange. They are actually remote concentrators, which have a direct connection back to a parent (larger, neighbouring) exchange proper.

Take a look at this description of 20CN. That describes calls, but not data - which follows a different path. This page includes a map that shows the topology of the BT Colossus network (the IP-over-ATM network connecting the 20CN ADSL DSLAMs back to ISPs).

21CN is a different kettle of fish, but it retains this "hierarchy" of transit connectivity.

The telephony side of 21CN has stalled, but the data side continues ahead using the same topology and ADSL2+.

Is there any way of knowing what "path" the calls/data take from one exchange to another, to work out where the bottleneck potentially lies and/or which exchange might be able to be upgraded next, if pressure is put on BT (stop laughing)?

Does this mean, in theory, that her's will eventually be upgraded?

Nope. While building the fibre "outwards" makes it obvious which order to upgrade exchanges in, the decision about *whether* to upgrade an exchange is all about the economic viability. And if BT can't afford to do it, the next question is whether BDUK subsidies will make it change to viable (assuming BT get the contract).


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Standard User deiwise
(committed) Tue 14-Jan-14 13:43:40
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Re: Understanding how BT phone exchanges actually work


[re: WWWombat] [link to this post]
 
Andrew and WWWombat,

I'm so embarrassed not to have replied to this. I'm absolutely convinced I did! I was really grateful for your input.

Things have progressed, much to my amazement. There's a planning application next to a BT cabinet in the village (Slapton) mentioning Harlequin, who confirmed to me they will be installing a fibre cabinet. Interestingly, they also said they "assumed" that the fibre run would already have been complete - running it from the exchange at Torcross. Do you think that would be the case? Seems odd to me.

Secondly, the Connecting Devon and Somerset map confirms Torcross is due for a Fibre upgrade, potentially allowing orders in September. All amazingly good news, and I can cancel the useless satellite connection very soon to boot!

Just to satisfy my geekery, would this mean that BT would have to run fibre all the way from a fibre-enabeld exchange to Torcross's? The nearest is Totnes, which is quite a trek. I am actually very sympathetic about BT and the government's rural broadband plans. The moment you step out of a city in this country, nothing is straightforward. You can dig under some roads, not others. You have access to some fields, but not the one you need. The whole process seems a nightmare, so I'd just be curious to know of the challenges they face laying all this cable.

And once it's in the exchange, they then have to lay it all the way to Slapton and the new fibre cabinet. How on earth will they do that?! There isn't a flat bit of land anywhere to be seen. It all seems impossible!

Anyway - thanks again for both of your replies all that time ago, was very helpful.

Will
Administrator MrSaffron
(staff) Tue 14-Jan-14 14:03:45
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Re: Understanding how BT phone exchanges actually work


[re: deiwise] [link to this post]
 
I have not looked for a telephone number confirm, but at a guess both Torcross and Slapton are served by the Torcross exchange which is actually located in Stokenham. Which I was to guess will probably be fed from Kingsbridge.

The fibre will go all the way to the cabinet, and go through existing ducting (think grey pipe similar size to a gutter downpipe), with smaller tubes inside this to carry and protect the fibre. Most areas have a good amount of this ducting in place, so its not that difficult to install.

In essence the fibre can actually bypass your exchange if they decide and then maybe in 20 years time once the voice is going over the fibre to, the exchange might actually close - a very long term goal.
Not being flat is not a problem, and even on the cost in Cornwall has some ducting that floods with sea water once the tide comes in. The range of the fibre is such that if they wanted to they could run the fibre all the way back to Dartmouth, but Kingsbridge is a very likely candidate as it has the WBC network and LLU operators present.

Andrew Ferguson, andrew@thinkbroadband.com
www.thinkbroadband.com - formerly known as ADSLguide.org.uk
The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User kitcat
(committed) Tue 14-Jan-14 14:42:21
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Re: Understanding how BT phone exchanges actually work


[re: MrSaffron] [link to this post]
 
Slapton may be served by Stoke Fleming due to the difficulties across Slapton Sands this would explain the low speed test result. ( or even East Allington)
Standard User deiwise
(committed) Tue 14-Jan-14 17:00:02
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Re: Understanding how BT phone exchanges actually work


[re: MrSaffron] [link to this post]
 
In reply to a post by MrSaffron:
I have not looked for a telephone number confirm, but at a guess both Torcross and Slapton are served by the Torcross exchange which is actually located in Stokenham. Which I was to guess will probably be fed from Kingsbridge.


Correct about Torcross, that serves Slapton. Good thought re Kingsbridge - didn't realise it was 20cn

In reply to a post by MrSaffron:
The fibre will go all the way to the cabinet, and go through existing ducting (think grey pipe similar size to a gutter downpipe), with smaller tubes inside this to carry and protect the fibre.


Direct from Torcross, right? So, fed from Kingsbridge to Torcross, then fed out to existing cabinets from Torcross.

In reply to a post by MrSaffron:
Most areas have a good amount of this ducting in place, so its not that difficult to install.


I'll bow to your better knowledge! I've looked on Google Streetmap for clues, with the overheads, but it's impossible to trace it. And the "exchange" at Torcross isn't a building as far as I can see. Or if it is, it's in someone's house.

In reply to a post by MrSaffron:
In essence the fibre can actually bypass your exchange if they decide and then maybe in 20 years time once the voice is going over the fibre to, the exchange might actually close - a very long term goal.


Bypass it how? By running it from Kingsbridge to the Slapton cabinet?

Thanks for the reply. Hugely informative.

Will
Standard User deiwise
(committed) Tue 14-Jan-14 17:00:36
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Re: Understanding how BT phone exchanges actually work


[re: kitcat] [link to this post]
 
Good thought, but it's definitely served by Torcross (WWTORX)
Administrator MrSaffron
(staff) Tue 14-Jan-14 17:45:54
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Re: Understanding how BT phone exchanges actually work


[re: deiwise] [link to this post]
 
By running it from Kingsbridge to the Slapton cabinet?

Yes potentially.

The fibre handover node that is the Falmouth Telephone exchange actually serves eight old traditional exchange areas.

Andrew Ferguson, andrew@thinkbroadband.com
www.thinkbroadband.com - formerly known as ADSLguide.org.uk
The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User deiwise
(committed) Tue 14-Jan-14 17:50:32
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Re: Understanding how BT phone exchanges actually work


[re: MrSaffron] [link to this post]
 
Interesting. It's a shame a list of BT's cabinets isn't available to see how many of them are in a particular area...
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