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Re: modems

2002-06-11 13:44:22
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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