Ken said:
It was called an acoustic coupler.
Almost nothing that is today called a modem is actually a modem. The
term "modem" is short for "modulator-demodulator." Technically, it's a
device that converts the analog signal on the telephone line to the
digital signal needed by a computer, and vice-versa. Technically, any
device that doesn't do that analog to digital conversion is not a
modem.
A device that connects to a high-speed internet connection is properly
called a "gateway," not a modem, because that high-speed internet
connection is digital to begin with. So there's no analog to digital
conversion, no modulating or demodulating is required, and the term
"modem" is technically inappropriate.
However, the difference between a modem and a gateway is not widely
known, and the term "modem" is widely used for both types of devices.
Some people strenuously object to this usage, because it's not
technically correct. My personal feeling is that, leaving aside the
analog to digital conversion issue, both devices do essentially the
same thing--they connect a computer (or network) to the internet.
Since there is no term that is really correct for any device that
connects a computer to the internet, and since the term "modem" is so
widely used for this, I think insisting that a gateway not be called a
modem is just rigid and inflexible.
Despite the original meaning of the term, for all practical purposes,
calling that DSL or cable device on your desk a "modem" is far and
away the best thing to do. Like so many English words, the word
"modem" has changed its meaning over time.
I'm not sure I'd agree with this.
ADSL and dialup have a lot of similarities.
On dialup, 33K (or less) modems could be connected straight though
the analog telephone network. Even if the telephone company
equipment happened to be digital, all it needed to emulate
was 4KHz of analog bandwidth (8KHz Nyquist sampling).
At 56K, backhauling is used to get around data rate limitations
in one direction. By not sending the signal (in one of the
two directions) through the analog network, a higher rate
was possible. I think in that case, there might have been
a T1 or higher rate digital cabling to the ISP, to make it possible.
So a portion of the analog telephone network was being bypassed.
And this was a precursor to the ADSL generation (the notion
of total bypass, at each line card termination point).
In analog days, the wiring on the telephone network, only
had to pass the 4KHz signal.
When we fast forward to ADSL, now, the telephone network
analog portion, is bypassed in both directions at the
line card on the DSLAM access mux. The line bandwidth is
anywhere from 1.5MHz to 12MHz, rather than the puny 4KHz
limitations of analog phone service. The loading coils
have been removed. We still have frequency bins (like on
dialup modems), but there are a hell of a lot more bins,
and a lot more data rate possible (currently, up to
52Mbit/sec download rate, with no change to telco
wiring from the corner to your house).
http://home.earthlink.net/~mark.wagnon/dmt.jpe
Both ADSL and dialup, use similar protocols. Both
with PPP in the name. On dialup, it was simple Point
to Point Protocol. Back in dialup days, you could even
view the log, as your session was logged into the modem
pool. ADSL uses PPPOE or PPPOA, which is a stream to
packet protocol as well, complete with login sequence.
Only, we don't get to watch the log like we used to.
What comes *after* a modem, is a distinguishing feature.
The dialup, had nothing but computer software after
the modem. In the case of the Winmodem, even the
DSP conversion function was done by the computer portion.
On a Winmodem, all the modem was contributing, was
a place to capture analog samples, for analysis by
the DSP code in the Winmodem driver.
X ---- dialup ---- RS232 -------------------- computer
Ethernet
X ---- ADSL --- router ---------------------- computer
It's the router that makes you think things are different.
There were some products made, where a dialup modem
was "shared" with two computers. That would be the
logical equivalent of the ADSL modem/router style
solution. Only those sharing boxes were never popular,
and were discontinued before they could sell any.
(And, who would want to share 5KB/sec anyway!)
Same thing as happened to ISDN - obsolete out of the
gate. As a result, people will only remember the
fairly "raw" experience provided by the dialup
modem. No "networking features" to speak of.
All networking features, such as ICS, where done
on the computer end.
X ----- dialup --- RS232 --- Computer #1 ---------- Computer #2
PPP & ICS Ethernet
That's an example of sharing a modem, using networking
protocols in the OS (ICS or Internet Connection Sharing).
The dialup modem is "dumb as dirt" in that picture.
Provides a stream of bytes. To be reconstructed via PPP.
http://en.wikipedia.org/wiki/Point-to-point_protocol
HTH,
Paul
