how do analogue telephone cables installed in the early 1900's still
support digital data transmission over long distances? How does this
effect the principle upon which a modern modem operates?

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Request for Question Clarification by mathtalk-ga on 25 Apr 2003 12:30 PDT

Hi, cccis-ga:

Are you asking how older telephone lines to a residence or business
can support DSL service without replacement?  Or you are perhaps
thinking of ISDN service?

Or, as your last question suggests, are you asking how analog modems
work on these lines (at speeds up to 56K)?

regards, mathtalk-ga

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Clarification of Question by cccis-ga on 25 Apr 2003 15:14 PDT

How does a modem allow digital transmission on these lines? Is it poss
to give little explanatoion about the principle on which a  odem
operates?
Ta cccis

Hi, cccis-ga:

The word "modem" is an abbreviation of "modulator/demodulator", an
indication of the way that these devices transmit "digital"
information over telephone lines, by amplitude modulation of audio
frequency signals.

[How to Build a Modulator/Demodulator]
http://www.spectrum-soft.com/news/Summer2000/modulator.shtm

The earliest commercially available modems appeared in 1962 and could
only transmit 300 "baud" or bits per second.  Each character required
7 data bits, 1 parity bit, and 2 stop bits (in a typical setup), so
this meant a mere 30 characters per second (probably slower than you
or I would read).

At first the Bell Telephone Co., a monopoly, wanted to charge
customers extra for the privilege of connecting modem devices to
"their" lines.  To get around this many early modems were designed to
use "acoustic coupling", meaning that a pair of rubber cups containing
a speaker and microphone were configured to hold an ordinary telephone
handset.  Users would dialup a remote connection manually and then
place the telephone handset in these cups, so that only sound waves
were entering or exiting the telephone company's equipment.

Eventually the federal courts found that such restrictive arrangements
were an abuse of monopoly powers, opening the way for the direct
connection of modems into telephone lines that we take for granted
today.

Nevertheless it clearly illustrates the underlying principle that the
digital information being transmitted is encoded into (and decoded
from) simple audio frequencies such as the phone lines have always
been designed to handle.

With the passage of time the encoding technology has gotten
increasingly sophisticated, starting with straightforward amplitude
modulation, and changing to incorporate adapative coupling,
compression, simple sideband signaling, and ultimately the multiple
carrier tone technology that allows the current 56K baud modems to
dominate dialup service.

The simple answer to your question is that modem technology only
requires phone lines to be able to transmit voice range signals
faithfully, in order to successfully present the encoded digital
information for terminal to host or host to host computer
communications.

For a more detailed discussion of how the modem protocols have
evolved, read here:

[Development of Modems]
http://www.cs.tcd.ie/courses/baict/bac/jf/projects/98/modems/develop.html

regards, mathtalk-ga


Search Strategy

Keywords: "history of modems"

Keywords: modulator demodulator

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Clarification of Answer by mathtalk-ga on 26 Apr 2003 07:02 PDT

Note that "amplitude modulation" is also the AM of AM radio, although
there the "carrier" frequency is a radio frequency rather than one in
the audio range (as with data and fax modems).

The higher the carrier frequency, the more information or "bandwidth"
can be encoded.  The use of "sideband" signals and combining multiple
carrier frequencies (which can be separated out by frequency tuning)
are important to getting the maximum bandwidth on a telephone line,
but these techniques cannot compete with the bandwidth available in a
cable modem connection (which uses radio frequency signals).

-- mathtalk-ga