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RE: modems

2002-06-11 17:24:00
Are you sure the sound he is hearing is not the modem fan screeching?  :P

-----Original Message-----
From: Pete Resnick [mailto:presnick(_at_)QUALCOMM(_dot_)COM]
Sent: Tuesday, June 11, 2002 1:37 PM
To: Lloyd Wood
Cc: Bill Cunningham; ietf
Subject: Re: modems


On 6/11/02 at 9:04 PM +0100, Lloyd Wood wrote:

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.

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 
question Bill was asking and why he was asking it:

 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.

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:presnick(_at_)qualcomm(_dot_)com>
QUALCOMM Incorporated - Direct phone: (858)651-4478, Fax: 
(858)651-1102





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