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Re: [OM] In search of villains, was: web-pages: Flash or html ?

Subject: Re: [OM] In search of villains, was: web-pages: Flash or html ?
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Sun, 13 Jun 2010 17:52:27 -0500
Chuck wrote:
> 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.

The Wikipedia explanation is a little convoluted and poorly written.
It has nothing to do with higher voltage or crosstalk. At the
voice-switch, the signal is processed through an AD/DA converter. This
creates a hard bandpass filtering. The exact amount of bandwidth is
limited to a DS0 minus overhead. As this is a fully switched
environment your signal is 100% isolated through TDM (Time Division
Multiplexing) from any other signal.

The copper pairs themselves are not limited to 3khz in any way shape
or form. In fact, even the original ADSL format utilizes frequencies
as high a 1mHz. An HDSL (4-wire T1) will commonly interfere with or be
interferred with the original analog television channel 3 audio
broadcast. I think that's 1.44mHz.

> 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.

No changing out systems.  The DSL system is what we call an "overlay"
technology. Inotherwords, it leaves the existing system in place and
just adds this on top of what is already there. At the house, you have
a dsl filter which removes the dsl noise from the telephone handset,
and at the central office there is a splitter (same as the dsl filter)
that peels off the dsl and sends to to the dslam while putting the
voice-only signal on the line-card of the voice switch.

Now, however, we are dealing with VoIP, which is another critter
altogether. Trust me, VoIP isn't anywhere near as good as POTS. It's
failings are so numerous on so many levels. The only advantage to VoIP
over POTS is stat-muxing and the fact that packets are routable
without the SS7 control network.
AG
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