Digital Subscriber Line how is DSL "speed" defined? the data rate?

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Subject Author Date
how is DSL "speed" defined? the data rate? efffemm 06-04-07
Posted by on June 4, 2007, 9:16 pm
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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.


Posted by Rick Wintjen on June 4, 2007, 11:28 pm
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efffemm@f-m.fm wrote:
> 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.
>
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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