The line rate (512kb/s, 1mb/s etc) is the rate between your pc and the local head end. You don't share that. The contention ratio is the number sharing the link between the local head end and your isp's hub, and they don't tell you what speed that runs at.
The contention rate on my connection is 50:1, but I have never noticed any slowdown, so for the same money you would be better off getting 1mb at 50:1 rather than 512kb at 20:1.
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