how is DSL "speed" defined? the data rate?

When your broadband connection is supposed to be say

512 kbps, is this the data rate, or does it include packet headers, error-correction codes and so on? I am curious because I get a maximum throughput only 80% of 512 kbps. Before with a dialup modem, I got connections at 52 kbps initial and final speeds (I am close to telephone exchange). So I don't believe the telephone line is crud.
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There are framing bit periods with dialup, plus a parity bit if set, so actual byte transfer rate is not the connect speed, in bps, divided by a factor of 8; it's closer to a factor of 10 or 11. All of the packet info is added by the provider, so it doesn't normally show up in the transfer rate. ADsl and cable transfers include all of the packet headers, so the bytes per second is about 80% of the bps divided by 8, assuming the data transferred is large enough to be split into full packets, on average.

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