Funny enough a similar question was just asked in another thread, so I'll explain a wee bit more.
Ok.. you have all the links from all around the country. Each dslam/msan will obviously have a fibre link to the RAS.. so theres like thousands of these all over the place.
They all converge at the RAS. Although we think of the RAS as a location its actually a location which has huge amounts of routers etc, which will converge say the ATM links on to the Colossus core network.
I find it easier to explain on the old network, 21CN will be similar but it also has different tiers, so for simplicity (and time) I'll stick with the 20CN role model.
This is a very simplistic view, but it should give you an idea of what goes on.
Each msan has a fibre link to the RAS.
The fibre link coming in connects to an Edge Router, which converges a few hundred links MSAN into one.. Up to this point its ATM network.
This edge router then forwards these on to an Access Concentrator.
Each LAC will be taking in connections from several different Edge Routers - converging the traffic again.
The LAC is kind of the entry point to the BTw Colossus Network (IP) and puts your traffic on the main BT backbone, for forwarding on to your ISP.
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Both 20CN and 21CN require authentication on to the BTw network, authentication is carried out at the RAS when we first boot up our routers, regardless or not if the line is on 21CN and has a core node nearer your home location.