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[elpact]

So firstly have to say this site is an immense resource about telecoms infrastructure, I've learnt more from this site than any other.  Thanks so much to Kitz.

I'm trying to get a clearer picture in my head about the backend and what exactly goes on there (for both adsl and fttc).  This lovely picture gives a nice overview:



I'm pretty clear what happens up to the MSAN (for adsl) and up to the OLT (for fibre), but beyond there, is where it gets murky for me.

For example, the cloud where it says "Broadband backhaul", what does this refer to exactly?  And as for the B-RAS, is this also where the Radius sits (for authentication)?  I know the Radius protocol (using AAA) has both a main Radius Server and a Radius Client (also known as NAS), but not sure where that would fit into the diagram above?

I understand some SPs may have slightly different setups, so perhaps we can take TTB as an example please. Also, on some sites I've read that the B-RAS (which they also refer to BNG) is in actual fact the Network Access Server (NAS / Radius Client).  Would this be accurate?

Just trying to get some more clarity here please, from the best site around.

Thanks!

[j0hn]

For example, the cloud where it says "Broadband backhaul", what does this refer to exactly

In the case of FTTC/FTTP it's the network between the OpenReach L2S and your ISP end point, which is almost always in London.

Many ISP's don't have their own backhaul and will use a Wholesale provider. The main 2 Wholesale backhaul providers are BT Wholesale and Talktalk Business.

BT Wholesale have their own fibre that criss crosses the UK. They provide the backhaul for hundreds of ISP's.

Talktalk don't actually own the majority (or maybe any) of the fibre they use for backhaul. It's made of mainly of dark fibre that they rent/lease from multiple networks.

Some providers use multiple backhaul providers. Some let you pick which 1 your line will use, others don't. Some will even let you change this mid-contract.

For example I could buy 2 lines from AAISP and have 1 line use BT Wholesale backhaul and the other use Talktalk Business backhaul.
Both lines connect to the exchange on the same OLT and L2S but they leave the exchange on different backhaul networks, taking different routes across the country.
Both would end up routing my traffic to AAISP.

Other large providers with their own backhaul network are Sky and Vodafone.
Sky don't Wholesale theirs like BT/Talktalk. I believe Vodafone do.

BRAS is done differently by different backhaul providers.

[Reformed]

BT Wholesale lease at least some circuits, John. Don't believe they own their own fibre. They've certainly been buying OSA FC and DFX from Openreach via BT Networks.

Think actually BT Networks run the core based on Openreach products and their own kit with Wholesale running the access layer.

Not 100% sure but reasonably sure the core is run by another unit in BT.

[kitz]

>> For example, the cloud where it says "Broadband backhaul", what does this refer to exactly?

The various paths/routes between locations.  Basically lots of routers and fibre optic cable as the paths between A and B.


There's more information about BTw's 21CN network on this page BT 21CN - Network Topology & Technology which shows some of the different types of routing depending upon size and location of the exchange.

There's also various methods of handover to the ISP which are covered here: BT 21CN & the ISP Network - WBC/WBMC

There's also other bits and pieces about BTw's backhauls and locations in the same BT 21CN section, but you may be interested in the Converged Network page which shows a diagram how all the various paths join together and the type of equipment used at each node.

Finally all ISP's are different but there is an old page that shows Plusnets old network here prior to 21CN.  It is outdated because Plusnet now purchased their own MSILs at various locations around the UK so use dedicated WBMC.
But if you replace BT's Centrals with WBMC Shared Host links, it may help give an idea how BTW based ISPs such as AAISP and IDnet etc may set up their network. 

[kitz]

Think actually BT Networks run the core based on Openreach products and their own kit with Wholesale running the access layer.

Not 100% sure but reasonably sure the core is run by another unit in BT.

The water gets very muddy with the split of Openreach/BTw as to who owns what. I do believe this is the area where splitting off Openreach wasn't quite so easy.

 The backhaul and Core has always been reliant on BT for technology..... and Openreach gained from many BT depts such as R&D.  Openreach uses their version of the BTw DLM.   In days of old it all came under BTw, but now Im not certain exactly who owns what when it comes to 21CN and since the split.    When I did one of the visits, the CEO arranged for a BTw rep to explain/answer any questions about the Backhauls and Core and I was under the impression that at that time it all came under BTw (eg the MSE BRASes belong to BTw), but things may well have changed since then.   I've been saying for years I'd love to do an updated visit now that 21CN has matured...  unfortunately due to my health its now unlikely to ever happen :/  Not even sure if they would be so accommodating these days.  I was very fortunate back then as the CEO at the time was very open about their new network and made sure I had access to people from both Openreach and BTw over the course of a few days.   You hear and know far more than me now.

[Weaver]

I should know this, but as you know, my memory is shot: Is the BRAS the node that is the endpoint of the L2TP link to the ISP’s LNS?

[Reformed]

Yes indeed.

[andrew-AAISP]

This diagram may help fill in some of the blanks at the ISP side:

[Weaver]

Thanks Andrew. Extremely helpful. Is that picture on the www somewhere?

[andrew-AAISP]

Hello! It's from here: https://www.firebrick.co.uk/support/knowledge-base/lns/