>> I pay for 6Mbit/s but get only 3Mbit/s.
> 
> That's like my dad, he pays for 8Mbit/s (AFAIK all BT packages in the UK 
> offer 8Mbit/s) and only gets about 2.5 (according to the modem settings 
> page).

Presumably the link-layer speed depends on the line. (AFAIK, a large 
part of the modem handshake process consists of testing the various 
frequency bands to see which ones are clear enough to be useable. That's 
what dictates your transfer speed.)

The amount of IP data you can transfer depends not only on the 
link-layer speed, but also the speed of the server you're talking to, 
how much congestion there is on the entire route between you and the 
server, how responsive your own PC is going, etc etc etc.

(And, of course, the amount of "real" data is the amount of IP data 
minus all the protocol overhead - IP headers, TCP headers, handshakes, 
retransmits, timeouts, etc.)

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