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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: net.dcom
Subject: Re: Phone line problem and modem
Message-ID: <1199@kitty.UUCP>
Date: Sat, 9-Aug-86 16:06:39 EDT
Article-I.D.: kitty.1199
Posted: Sat Aug  9 16:06:39 1986
Date-Received: Mon, 11-Aug-86 04:40:50 EDT
References: <416@aecom.UUCP>
Distribution: net
Organization: Recognition Research Corp., Clarence, NY
Lines: 59
Summary: Possible problem resulting from "pair gain" techniques

In article <416@aecom.UUCP>, naftoli@aecom.UUCP (Robert N. Berlinger) writes:
> I use a Multitech 224E modem to connect to our system from home at
> 2400 baud with group 3 MNP protocol.  This all worked fine until
> recently when it mysteriously stopped working.  The modem works fine
> on any other phone line, and also works fine at 1200 baud.  2400 is a
> no go.  I measured the voltage on the phone line and it was only ~13 volts.
> I thought that all phone lines are supposed to be 48 volts (all others
> that I tested were).  Could this be causing the problem?  It works ok
> (a bit noisy and low volume) for voice.

	I am assuming that you measured ~13 volts when the telephone line was
on hook (idle)...
	Sounds to me that you no longer have a "physical" pair from your home
to the central office.  In some congested or rapid-growth areas telephone
companies adopt techniques to effect "pair gain". 
	The most common technique is "added mainline carrier" (AML), which
allows one telephone line to serve more than one subscriber line.  The
simplest version allows one physical line to provide one additional carrier
line; the carrier line works by providing transmit and receive channels in
the frequency range of 25 to 100 kHz.  There is also a six channel version
of this technique which uses a somewhat larger bandwith.  In either of
these applications, a subscriber line carrier terminal is provided in the
subscriber premises.
	There is the older 1A Line Concentrator which uses an outdoor
pole-mounted "demultiplexer".  Transmission is not too bad on this type of
system since the multiplexing is "space division"; i.e., by means of a
crossbar switch, so there is no significant insertion loss or degredation
of transmission.
	There are newer digital multiplexing systems, which also use a
pole-mounted "demultiplexer".  I have no first hand experience with these,
but modem transmission impairment is a possibility anytime digital
sampling and analog reconstruction is used.
	In general, telephone companies use the above techniques for
low-traffic residential lines only (high-traffic business lines would
defeat the purpose of this "statistical" multiplexing) - but anything is
possible.
	AML is BAD NEWS for any type of data transmission; the reason is
that the carrier frequencies are determined solely by L-C networks, and
the circuitry can readily drift creating all sorts of transmission anomalies.
An AML line (like Superior-Continental equipment) would present between 6 and
14 volts to the local subscriber loop - so it sounds like this is what you
have.  I believe the 1A Line Concentrator presents 24 volts; I am not certain
what the digital "SLC" units present as a local loop voltage.

> How would I go about getting NY Tel. to fix this?  They usually are unwilling
> to listen to any problems dealing with data equipment.

	Sorry, but you gotta problem if all you are paying for is residential
service.  You could _try_ to talk the telephone company into giving you a
metallic pair, but they are not obligated to do so.  Your only guaranteed
solution is to request a business line with the specific representation that
it will be used for data transmission.  This will cost you more money, but
the charge won't really be absurd if you are making serious use of data
transmission.

==>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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