Grab your Phillips, it's...
The Latest from The Screwdriver List!
MODEM=modulator/demodulator
That's the easy part. The harder part is explaining what a modern, 56kbps
dial-up modem actually does.
The earliest modems converted bits, one-by-one, into corresponding tones in
the middle of the range of human hearing, and thus amenable to transmission
over a voice phone line. As the speed of the data to be sent increased this
became less than optimal, because as the bits became shorter there wasn't
enough time to make the tones sufficiently clearly defined. (Those of you
who know Fourier's Theorem will understand this right away. For the others,
simply let me say that a very brief tone sounds more like a click, and
distinguishing the different clicks corresponding to different very brief
tones is virtually impossible.)
Later modems have used a variety of techniques, mostly involving two ideas.
First is to group the bits together and use each group to pick not just one
of two tones, but one of eight or sixteen different "tones." That means
each of those tones gets to persist for eight or sixteen times as long, and
as a result, become more clearly defined and more easily discernable by the
receiver. Second is to use phase encoding as well as amplitude
encoding--which is more-or-less like switching to FM from AM in radio
stations. The combination of these two techniques, along with some pretty
complex mathematical manipulations, has allowed a general strategy for
encoding digital data for transmission over voice phone lines from its
initial limitation of less than 100 bits per second to the present limit of
around 50,000 bits per second.
The only way to go faster is to use more overall bandwidth--and that is
what DSL is all about. The phone line copper wire pairs can actually carry
more than a million bits per second, but not through normal telco voice
equipment.
John
At 10:29 PM 1/29/01 -0600, you wrote:
>Grab your Phillips, it's...
>The Latest from The Screwdriver List!
>
>
>It's time once again for The Screwdriver List Trivia Question of the Week!
>As usual, there's no million dollars if you answer it right, but I think
>that each of these questions will help at least one member on The List.
>
>Here's this week's puzzler:
>
>What does the word "modem" actually stand for, and what conversion does a
>standard 56KBps dial-up modem perform?
>
>Know the answer? Then speak up! The first member that posts the right
>answers is The List Trivia Guru for the week.
>
>Good luck!
>
>The Chief
>
>Mark L. Chambers
>Author, Technical Editor and Consultant
>Home Page: http://mlcbooks.somewhere.net
>
>Mark's Thought for the Millenium: What good is a Web search engine that
>returns 324,909,188 "matches"? That's like saying, "Good news, I've located
>the information you want. It's on Earth."
>++++++++++++++++++++++++++++++++++++++++++++
>That's all for now from The Screwdriver List
>"Red Stripe to Pin 1"
>++++++++++++++++++++++++++++++++++++++++++++
>
>
>
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John M. Goodman, Ph.D., author of "Peter Norton's Inside the PC," Seventh
Edition (Sams 1997, ISBN 0-672-31041-4), and Eighth Edition (Sams 1999,
ISBN 0-672-31532-7).
=======
++++++++++++++++++++++++++++++++++++++++++++
That's all for now from The Screwdriver List
"Red Stripe to Pin 1"
++++++++++++++++++++++++++++++++++++++++++++
