I suspect the router's config page picks up the information from the 'line' and displays it
You're correct Dennis. The rate displayed will be the data rate at which the modem synchronizes to the DSLAM at the central office, in other words the "raw" bit rate.
the actual throughput and performance of the line during normal use will probably be less - unless you live next door to the telephone exhange.
The throughput in terms of your own data will
always be less than the line sync rate even if you live
inside the telephone exchange!
Any communications protocol has a certain overhead in terms of data bits which are needed simply to run the link and keep data going where it should. Even a very simple system will have an effective bit rate of
user data which is less than the actual line bit rate (e.g. some of the old point-to-point teletype systems ran at 110bps, but only 8 out of every 11 bits were actually user data, giving an effective rate of 10 characters per second, not the 13.75 you might infer from the basic bit rate and 8 bits per byte).
When you start getting to the much more complex protocols we're talking about here, then the overhead is that much greater and the effective rate -- as far as user's data is concerned -- is that much less. With packet-driven systems you have not only the overhead of the bits which need to be added to let the network know the origin and destination of each packet, among other things, but you then also have to allow for packet propagation time with transfer protocols which involve multiple packets and the receipt of acknowledgments.
Basically then, if your line rate is 512kbps, that doesn't mean that you will achieve file transfers at a rate of 64 kilobytes per second as you might infer from dividing the line rate by 8. Your
effective rate in terms of how long it takes to transfer a file of X kilobytes will always be less than the basic line rate.
