[fa.teletext] LADT

teletext (06/20/82)

>From lauren@UCLA-Security Sat Jun 19 16:52:55 1982
I've been following LADT for quite some time.  It's actually nothing
more than the same technology used ("carrier") to provide a second
telephone line for a subscriber without adding another pair: multiplexing
another path "above" the "real" one.  It's main problems are that:

a) the equipment at the central office end is fairly expensive
b) the primary talk path is usually degraded since a sharp low pass
   filter is added on the line to separate the two paths.  

This latter problem can be solved through expensive digital carrier
systems, but I don't think Bell is considering that for LADT use.

The *really* big problem is that there is no evidence this service will
cost any less than just adding another "real" phone line.  In fact,
it will most likely cost FAR more since it will be rated as a data
service and requires more specialized equipment.  Current estimates of
its cost to the alarm industry have the alarm people screaming bloody
murder!  The established access charges for the new Bell packet network
are similarly very high and unsuitable for interconnecting "cheap"
videotext networks.

The main advantage of LADT will probably be that it saves telco trouble
by avoiding the need to install more outside plant just to serve
people who need second lines for videotext use.  However, there is no
reason to believe that the service will be priced in such a way
as to be of use to the "consumer" market -- in fact all available
info points in exactly the opposite direction.


P.S.  One other problem is that increasingly, people's BASIC telephone
service is provided by carrier (especially in dense metro areas) -- in
most cases they're not even aware of it (most people do not notice
the lack of frequency response on the filtered lines, they EXPECT
lousy service!)  If lines are already provided by carrier, then adding
LADT becomes a real problem.  An amazing percentage of new phones in
many areas are already "added" via carrier technology.


teletext (06/20/82)

>From SIRBU@MIT-MC Sun Jun 20 06:48:38 1982
I disagree with Lauren's evaluation of LADT technology.  In the first
place, if it were really the same as the techniques used by AT&T to
provide second phone lines, than clearly it is less expensive than a
real second line.  That establishes a bound on the cost; it's less than
that of a second phone.

However, LADT is really cheaper than FDM loop carrier.  For one thing,
when multiplexing two voice channels you need a reasonably linear mixer;
for LADT, all you have is an FSK device that's operating above the
voice band.  That's easier to build.  Indeed, the subscriber device is
no more complicated than a 300 baud modem.  Note also, that LADT offers
an RS-232 interface to the subscriber, thus saving the cost of the

At the central office end, I agree that you need a rather expensive
packet switch. The key question is, "Over how many subscribers is the
cost of that switch distributed?"  AT&T is expecting 60,000 for its
Coral Gables trial.  That will amortize a lot of capital. 

Finally, you note that there is increasing use of loop carrier for new
phones.  At the present time, roughly 70% of all subscriber loops are
unloaded copper.  The new subscriber loops use primarily digital carrier
(T1).  It's not to hard to imagine using LADT back to the T1 multiplexer
and then multiplexing several subscriber's data channels over one 56
kbps voice line back to the CO.  The use of subscriber loop carrier does
not inhibit this technology.

Marvin Sirbu

teletext (06/20/82)

>From lauren@UCLA-Security Sun Jun 20 16:03:19 1982
Date: 20 June 1982 1536-PDT (Sunday)
From: lauren (Lauren Weinstein)
Subject: LADT
In-reply-to: Your message of 20 June 1982 09:37-EDT

If you reread my last message, you'll see that I wasn't really
making a big deal about the "actual" costs of providing the service --
my concern is with the price of the service AS EVENTUALLY TARIFFED.
Such services will almost certainly not be considered to be included
within "basic" telephone service, and might well be catagorized as
fully enhanced services.  I have no reason to believe that such services
would be priced cheaply.  

Even with basic telephone service, the new moves toward making telco
charge what the service actually "costs" to provide are mainly resulting
in UPWARD pressures on residential phone bills.  If LADT is priced 
cheaply (fat chance!) it might not be so useful if associated with
$40/month local phone service and outrageous data packet charges.
The end result will almost certainly be a useful service for
businesses and some high-value applications (like large burglar
alarm systems -- the kind that use leased lines now), but I doubt
that the "average" consumer (and "simple" videotext systems) will
be helped much in a realistic sense.

This discussion has now moved beyond the realm of TELETEXT into
specifics of telecommunications technology.  I suggest we either
continue it privately or on TELECOM.


P.S.  In some metro areas, the percentage of new installs based on
conventional (non-digital) carrier is very high and rising rapidly.