1. This might be a good point to highlight the importance of the BT RADIUS server. This is actually where SSB and LSS are implemented. A more detailed description of the authentication process is shown here. You can also bypass your ISP for testing purposes by using the bt_test@startup_domain login. To see what happens when you do this then click here.
2. It is important to note that there are several RAS's. I believe they are organised at a regional level and serve many exchanges. Up to the RAS you share your PVC with users from other ISPs, but after the RAS the traffic is segregated into tunnels to individual ISPs.
3. If all of these abbreviations are confusing you then look at the glossary.
4. This is a very simplified diagram. The ATM network has loads of switches and paths which are well outside the scope of this diagram.
5. The labels (1), (2), & (3) on the diagram indicate the sequence of events.
6. In the diagram the RAS and Home Gateway are so named because those are the BT assigned terms. In "real" networking terms the RAS is actually a LAC and the Home Gateway is a LNS.