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From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: sci.electronics
Subject: Two phones on 4-wire D station wire
Message-ID: <728@neoucom.UUCP>
Date: Sun, 11-Oct-87 01:15:17 EDT
Article-I.D.: neoucom.728
Posted: Sun Oct 11 01:15:17 1987
Date-Received: Mon, 12-Oct-87 22:03:16 EDT
Organization: Northeastern Ohio Universities College of Medicine
Lines: 50
Keywords: duplexer coils

Hello,

My house is wired with that skinny gray 4 wire stuff (I think Ma
Bell calls it D Station Wire or some such thing).  I also have two
phone lines.  One line was installed in 1930 (you should see the
arncane thing it has for a protector block!), the other lead was
installed in 1977.  All the wiring in the house carries both lines.
I often have modems going on both lines, and I don't see any
interaction.  About the only problem is when somebody picks up an
extension on the same line as an active modem.

In many places where there aren't many free pairs, the telco uses
duplexer coils.  What they do is use a balanced transmission  line
to place two conversations on one pair and use the ground as the
return path.  All is well as long as the mess stays in balance.  In
my case, since one of my lines was ancient, the telco opted to make
the second line completely new.  If you bug the telco enough, you
might be able to get them to give you dedicated lines, but you'll
probably have to talk to quite a few telco personell who insist it
isn't their problem first.  The telco, in general, isn't terribly
sympathetic to residential lines used for modems.  They only
guarantee least common denominator service of being able to hear
and talk to a human on the other end.

In some places, i think, the term "phantom circuit" is used to
refer to duplexed lines.  (Anybody know if the terms are
equivaltent?)  Also, sometimes loading coils are used for line
equalization and can produce abberant modem behavior if things get
out of adjustment.

I just got through running some tests with a trellis modulated high
speed modem, a Telebit Trailblazer in fact.  I was surprised at how
much better it did on crummy lines than regular Hayes modems.  Its
performance was superior even when talking to a standard 212 1200
baud modem on the other end.  There is a line at the office that I
simply can not use from home via a Hayes because the continuous
stream of ~r{_i stuff prevents even logging in.  Even in 1200 bps
mode without nmp, I used that line with the Telebit and never got
any noise.  Perhaps the more sophisticated Viterbi algorithm used
in the Telebit's detector was able to sort out the apparent phase
jitter on that line.  Naturally, you pays for what ya get.  The
suggested price for the Telebit is >$1K.  (There is currently a
price incentive for registered UUCP sites on the Telebit, but I am
unsure of the details.  If you're interested, you should talk to
Telebit-- I don't have any info.)

Hope this is useful,
Bill

(wtm@neoucom.UUCP)