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From: larry@kitty.UUCP
Newsgroups: mod.telecom
Subject: Re: what is an AML and how does it work?
Message-ID: <8511092153.AA01450@ucbvax.berkeley.edu>
Date: Sat, 9-Nov-85 16:53:31 EST
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Posted: Sat Nov  9 16:53:31 1985
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> I moved into a new apartment last month and had quite a bad experience
> getting phone service.  ...
> After 2 weeks of
> calling (from work) and badgering them almost every day, NET decided to
> provide service to me by way of an AML.  An AML is apparently some sort of
> multiplexor which is able to provide service for 2 (or more) phone numbers
> over a single pair of wires.  The AML takes one number as input and gives
> another number as output.  (There is apparently another AML or similar
> device at the central office end of the circuit).  ...
> One more interesting thing to report about this whole affair is its effect
> on my telephone answering machine.  For some reason unknown to both me and
> the phone company, my answering machine will not answer calls when it is
> hooked up to the line which is output from the AML, but works fine on the
> primary incoming line.  Anyone have any ideas?

	Oh yucch, AML.  AML (Added Main Line carrier) is indeed what you
mentioned - a method of multiplexing a second telephone circuit over an
existing physical cable pair.  Most AML apparatus is manufactured by
Superior-Continental; I don't believe WECO ever made any AML per se.  The
system works by frequency division multiplex, with the CO generally sending
at 76 kHz, and the station sending at 28 kHz.  Modulation is typically AM
double sideband.
	The yucchy part about AML is what your station apparatus sees.  Unlike
a conventional CO line which presents 48 volts DC in an on-hook condition, AML
provides typically 6 to 10 volts DC.  Since there is essentially negligible loop
resistance between the AML and the station, the off-hook telephone excitation
CURRENT is sufficient for operating most devices.  HOWEVER, certain electronic
telephone sets and anciliary devices which look at T/R loop voltage for line
status purposes will NOT work because the low open circuit voltage makes the
device think the circuit is permanently off-hook.
	Another bad part is ringing.  Many AML devices - as a result of making
the subscriber unit as simple as possible - do NOT provide superimposed
ringing; i.e., instead of 20 Hz ringing being over T+R, the ringing runs between
the R lead and an auxiliary Y lead.  There is no way around this situation if
that is the way your AML is designed.  Newer telephone answering machines and
modems no longer provide a separate ringing detector lead which could be
connected to the Y lead; as a result, many newer telephone answering machines
and modems will NOT detect ringing when connected to AML.  Period.
	Yet another bad part is that while the PHYSICAL telephone line
associated with the AML is OFF-HOOK, your AML is being powered solely by an
internal rechargeable battery.  If the physical line - which need not be your
own line - is in use for excessive periods of time, your battery will not be
sufficiently charged, thereby causing assorted troubles for the AML circuit.
There are auxiliary AC-powered chargers which the telephone company can
install to solve this latter problem, but usually it takes a lot of complaints
to the telephone company in order for them to do this.
	I don't know if there is anything you can do to force the telephone
company to provide you with a physical pair service instead of AML; it all
depends upon what their tariffs say, or what class of service you are paying
for.
	Fortunately, the use AML devices like the above is on the decline.
While there are still subscriber line multiplex (i.e., `pair gain') devices
being used, the trend is to locate them in outside enclosures for groups of
subscriber lines; these newer devices are also much better designed, use
digital techniques, and do not have the pitfalls of the AML devices above.

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