There seems to be a massive difference betwen sync speed and line speed (profile) Currently I'm connect at 12617 (12 meg) and the line profile is 11000 (10 meg). I think I read that the maximum line speed is 21000 on a 24 meg connection. That seems crazy to me, why the big difference?
The ATM (Asynchronous Transfer Mode) overheads are nearly 10% and TCP/IP overheads around 3% making 13% overhead in total. 13% of 24000kbps is 3120kbps, and 24000-3120=20880kbps. That represents the available payload which BT round up to 21000kbps and call the IP Profile. The equivalent calculation for 12617kbps is 12617-1640=10977kbps and BT assign an 11000kbps IP Profile (or 11 meg colloquially). David