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Standard User thomaswarne01
(regular) Sun 12-Feb-12 23:05:47
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How are Exchanges Linked?

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Hi there,
This must sound like a stupid question but, i was out walking today and noticed that bt have overhead cables aswell as manholes in the road and the road has been cemented back over where they obviously layed the ducts, but what is in the ducts?
fibre linking other exchanges?
in a rural area and i would imagin it links to another exchange but what are your thoughts?


Standard User chris6273
(member) Sun 12-Feb-12 23:54:40
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Re: How are Exchanges Linked?

[re: thomaswarne01] [link to this post]
It depends on how big the exchange is and where it is located. Larger exchanges require larger capacity backhauls (Multiple Fibres) and newer technology. I believe most large exchanges interlink (parent) with each other in one form or another, but I would have though that they have their own dedicated link back to at least a Metro PoP.

Small Rural Exchanges are another story though. Smaller exchanges in rural locations are in most cases piggy backed off each other to save having to lay fibre back to a larger one individually.

You will probably find the largest rural exchanges which are along main roads will be ones which handle multiple exchanges.

As far as I know, large exchanges which handle multiple separate smaller exchanges linking through them are upgraded to 21CN first because it allows the use of higher capacity multiplexing, which may be needed when other exchanges rely on it.

But yes in answer to your question. Picture this scenario:

A ----- B ----------------------- C

A: Very Small Exchange which might handle 0-1000 lines (20CN)
B: Medium Exchange which might handle 1000 - 10000 lines (Tier 2 & 3 MSAN PoPs)
C: Large Exchange which might handle 10000 - 50000 lines (Tier 1 MSAN PoP or Metro PoP)

(I hope I've named those right).

It would make sense linking A to B or Multiple A's to B because there are not very many lines in the A exchanges.
There would be little point in blowing a fibre from the A exchanges straight to C exchanges because it wouldn't be worth all the money and effort (The amount of backhaul which 1000 lines requires wouldn't be worth it).

What you might have seen is a collapsed duct which has been repaired. Ducts like you describe carry the E-Side cabling or D-Side Cabling (Probably Both). The E-Side are the big bulk cables which connect cabinets to exchanges and the D-Side are the cables which link properties to cabinets.

It may have been a duct which carries a backhaul fibre because if an exchange is enabled for ADSL or anything higher, it uses fibre to connect to other exchanges.

I hope this clears this up for you. Sorry if I've gone over the top a bit wink

My Broadband Speed Test

Downstream Upstream
Connection Speed 8128 kbps 448 kbps
Line Attenuation 13.0 db 10.0 db
Noise Margin 16.8 db 24.0 db
Max(Kbps): 11616 1056
Standard User RandomJointer
(eat-sleep-adslguide) Mon 13-Feb-12 20:51:41
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Re: How are Exchanges Linked?

[re: thomaswarne01] [link to this post]
I would imagine BT have overhead lines on local distribution, underground ducts and boxes on onward local distribution and deeper ducts and manholes on junctions between exchanges.

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