Subject: modems

From: "Bill Cunningham" <[email-masked]>
Date: Tue, 11 Jun 2002 03:22:40 -0400

I know modems communicate on the physical layer by electrical pulses or
binaries sent on copper wires. Is that screeching you hear electrical
communication? Computers don't communicate by screeching...or do they?

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From: [email-masked]
Date: Tue, 11 Jun 2002 08:19:00 -0400

> communication? Computers don't communicate by screeching...or do they?

Any language that you don't understand sounds like screeching.

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From: Pete Resnick <[email-masked]>
Date: Tue, 11 Jun 2002 10:11:11 -0500

On 6/11/02 at 3:22 AM -0400, Bill Cunningham wrote:

>I know modems communicate on the physical layer by electrical pulses 
>or binaries sent on copper wires.

No, not at all. Modems communicate by sound. They MODulate the 
electrical pulses they get from the computer into sound, and the 
other end DEModulates the sounds back into electrical pulses to hand 
to the computer.

>Is that screeching you hear electrical communication?

The sound you hear is a sonic representation of the data. Changes in 
the frequency and amplitude of that sound represent the data. 
(Similarly, buttons on a touch-tone phone produce a sound that 
represent data.)

>Computers don't communicate by screeching...or do they?

They sure do. Back in the days of slower modems, it sounded more like 
a base-line "beep" sound with little "blip" sounds interspersed. 
Today it goes so fast that it just sounds like screeching (or 
hissing).

pr
-- 
Pete Resnick <mailto:[email-masked]>
QUALCOMM Incorporated - Direct phone: (858)651-4478, Fax: (858)651-1102

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From: Lloyd Wood <[email-masked]>
Date: Tue, 11 Jun 2002 21:04:38 +0100 (BST)

> >or binaries sent on copper wires.

You're confusing your physical and data-link layers. There
is no communication on the physical layer; only transmission...

> other end DEModulates the sounds back into electrical pulses to hand

You're confusing your modems and your acoustic couplers.

An electrical transmission in the ~3.5kHz bandpass range that equates
to the dominant frequencies used by the human voice, which the phone
system was engineered to convert and carry easily, is not a sound.
Modulating an electrical signal into said electrical transmission does
not involve sound.

> (Similarly, buttons on a touch-tone phone produce a sound that

buttons on a touch-tone phone connected to a traditional analogue
line cause electrical signals in that 3.5kHz bandpass range to be
produced.

> Today it goes so fast that it just sounds like screeching (or

I can't hear my gigabit ethernet.

L.

It's at times like this I think the IETF needs actual engineers.

> pr

<http://www.ee.surrey.ac.uk/Personal/L.Wood/><[email-masked]>

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From: Pete Resnick <[email-masked]>
Date: Tue, 11 Jun 2002 15:37:16 -0500

>Modulating an electrical signal into said electrical transmission does

OK, OK, of course that's exactly correct; almost all modems today 
completely bypass the issue of sound and transmit directly through 
the copper to the telephone switch. But let's get back to the 

>  >or binaries sent on copper wires.

The important feature of modems is that they send analog signals over 
those lines, not digital (which is what I took Bill to mean by 
"binaries"). And those analog signals correspond quite directly to 
things that create sound (if connected to a speaker of the right 
sort) and receive sound (if taken from a microphone of the right 
sort). It is the correspondence to the receiving and production of 
sound that makes modems interesting devices; that's why acoustic 
couplers worked on the old modems. Similarly touch tones are *tones* 
because they can pass through a system designed for transmitting 
analog electrical signals that can be turned into a sound. (Hence, 
you could go out and by those touch-tone producing boxes, program 
phone numbers into them, hold them up to your phone receiver, and get 
the number dialed.)

Yes, it is correct that most modems today deal with electrical 
transmission only and not sounds (except for their speakers). But it 
is the fact that those signals can easily become sounds that is key, 
at least to explain to Bill why his modem is screeching. (Although 
some of my friends in philosophy of science disagree, "explanation" 
is not a matter of reducing everything to physics.)

It's times like this I think the IETF needs more academics. :-)
-- 
Pete Resnick <mailto:[email-masked]>
QUALCOMM Incorporated - Direct phone: (858)651-4478, Fax: (858)651-1102