And I thought I had done well. I distrusted the Wikipedia description
so did (I thought) my due diligence and cross-checked it. I found
someone who disputed the voltage as the cause of the limit but almost
immediately found another page which gave a rather involved technical
explanation of why voltage and cross-talk was the problem. So I
accepted what was there. Sounded at least as convincing as you. :-)
But I reject your comment about copper pairs. Not that it's wrong but
just that it's immaterial to the design of a modem designed for the
analog voice telephone system that is limited to 3 kHz... or maybe it's
5 kHz. Whatever, it's definitely slow, wire speed notwithstanding.
And maybe the existing system is left in place for DSL but that's
assuming it's not the same system that was installed in 1958. I
recently did convert to VoIP and am very happy. But when I wanted to
wire my VoIP modem into the house wiring what I discovered was not a
Bell System Network Interface Unit (already ancient technology) but a
good old Western Electric lighting protector and a falling off ground wire.
As to VoIP... it give me a good clean signal, costs only half as much as
POTS and gives me many more features to boot. I'm a happy camper.
Chuck Norcutt
Ken Norton wrote:
> 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
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|