Chris is apparently looking for villains where there are none. Yes,
there is a law in the US restricting analog telephone modems to, not 56
kbps, but 53.3 kbps. The reason has to do with preventing cross-talk at
the higher voltage required to actually get to 56 kbps.
To understand even the 56 kbps speed limit one must understand how the
telephone system works. From your house to the phone company's "central
office" (a switching station technicallly limited to being not more
than, IIRC, about 3-5 miles away) your analog telephone signal is
carried on what's called the local loop. This analog signal is limited
to 3 kHz (perfectly adequate for carrying human speech). At the central
office switching station the analog signal is converted to an equivalent
signal carrying capacity 64 kbps digital signal. Since the telephone
system is a point-to-point switching system you "own" this 64 kpbs
digital channel as long as you are using your phone or modem. The modem
becomes limited to 56 kbps because the 64 kbps digital channel uses part
of the bandwidth for signals outside of the voice signal. The further
restriction to 53.3 kbps (9.5% loss) has to do with preventing cross-talk.
But is 53.3 kbps a real world restriction. Not really. For those who
have used 56 kbps modems does anyone ever remember actually connecting
at a speed greater than about 33 kbps? It's possible but the gods have
to be willing and the wind blowing in the right direction. The real
problem is 50 year old telephone wires and that 3 kHz analog signal.
That modem is not a digital device. It's an analog device and 3 kHz is
all it has to work with. If you read this history of modem development
on Wikipedia you'll gain an understanding of how amazing it is that we
can even get what we do out of the analog phone system.
<http://en.wikipedia.org/wiki/Modem>
Now about DSL. DSL stands for Digital Subscriber Line. Which simply
means that the phone company has converted your personal DSL line from a
3 kHz analog line to a digital line with something much more than 64
kbps capacity on your end. That means they had to change the switching
system for your line and also means that your connection takes up some
multiple of those 64 kbps digital lines. At least logically. In the
real world those lines get multiplexed together onto even faster
physical connections. So, yes, they had to actually change the system
to accommodate your use of DSL and you're also using greater capacity on
their system once you connect and your data starts rolling in. But what
they're justified in charging you for that is a different matter. Their
business practices and obsolete depreciation practices are slowly
consigning them to the dustbin of history.
Chuck Norcutt
Chris Crawford wrote:
> Copper can carry either Dial-Up, which is limited in the USA to 56kb/s
> downloads (its a law, not a technical limit), or it can carry DSL, which can
> be very fast...as fast as Cable TV internet service, though not as fast as
> fiber. There is no technical reason for DSL to cost more or for dial up to
> be slower...its just the way the phone companies set it up, and it really is
> illegal here to offer dial up service faster than 56kb/s! Why? I think the
> phone companies lobbied congress for that so they can be justified in
> charging 3 times as much for DSL.
>
>
--
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