1. This diagram is based around the 155Mbps L2TP passthough product. The main difference between this and the 34Mbps is the way in which login/password data is conveyed between the BT and ISP RADIUS servers and is shown by the dotted line at the top of the diagram.
2. The sequence of the authentication process is indicated by the bracketed numbers.
3. Note that authentication will fail at stage (5) if the BT RADIUS server does not have the correct data stored for your username/ISP.
4. SSB and LSS are implemented at the BT RADIUS server.
5. Stage (15) is the point at which you have an IP address assigned to you.
6. Sequences (7) through (12) are the RAS and HG authenticating each other and are included for interest only. From a user perspective you can jump from (6) to (13). For hard-core techies you should note that there is actually another aggregation device between the RAS and the HG but for authentication purposes you can effectively treat this as transparent.
7. On the 34Mbps gateways the user authentication does not go through the L2TP tunnels - it is proxied across from the BT RADIUS to the ISP RADIUS, so sequences (13) through (15) follow the path shown by the dotted line at the top of the diagram.