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 Section [6]: Introduction to Telecommunications
              Written February 1, 1992
              By: The Phone Knome
              Sysop of The Bill System

This is the first in a series of articles about the Public Switched Telephone
Network as it functions in the United States and Canada.  The purpose of the
series is to expand the horizons of any individual who desires to increase
their awareness of how all these machines that are connected to each other
function together as a network.

These articles are not copyrighted. Let no one do so.  They shall be free for
distribution anywhere at anytime.  They are dedicated to the millions whose
need to communicate with each other have given me a career.

********************** Reality Alert & Warning *******************************

IT IS EXPRESSLY STATED THAT THE AUTHORS OF THIS ARTICLE & ANY THAT MAY
FOLLOW IN THE SERIES, STRONGLY DISCOURAGE ANY AND ALL INDIVIDUALS FROM
ENGAGING IN ACTS OF TOLL FRAUD OR ELECTRONIC PRIVACY INVASION. THESE
ARTICLES ARE NOT BEING WRITTEN TO TEACH THE READERS HOW TO BEHAVE IN THE
CRIMINAL ACTS THAT THEY MAY DISCUSS IN DEPTH.

******************************************************************************

On With the show...

In the Science of telecommunications there are two major sub-sciences
that work as a team to get the job done.  They are ***Transmission***
and ***Switching***.  Transmission will be covered in the first few
issues.  Switching will be briefly defined but basicly dealt with later.

Transmission is not a gear box as far as this series is concerned.  It is
the Transport of an electrical or accoustic energy (message) from one point
to another.  In its simplest applications it is copper wire.  Due to the
basic concept of Ohms Law & attenuation the range of wire alone is somewhat
limited.  The only thing I know of that has a wider bandwidth for direct
connection is optical cable.  The need to send a signal greater than it was
possible useing wire alone caused the evolution of several generations of
increasingly sophisticated transmission systems.  The greed asociated with
money added motivation to continue to develop systems that would increase
the throughput of existing transmission components thereby cutting costs,
and since our utility bills never go down if a Telco (Telphone Company)
can cut operating cots their profit just goes up.

By far one of the most important concepts that will serve as a foundation
of enlightenment in this series is that of Digital Transmission with the
MOST PREVAILING FORMAT being **** T1 ***** sometimes referred to as DS1.

A T1 circuit is basicly a pipe that goes from one location to another to
pass telephone calls, and or Private line Data Services Back and forth.
In common applications it contains 24 telephone lines in a digital format.
The format is serial synchronous in nature with a line rate of 1,544,000
bits per second.  Hauling ass compared to 14,400 huh?

T1 ckts are used mostly to tie a Telephone Company Switch in one location
to a switch in another location.  Their use is also on the rise for Private
Point to Point Communication in virtually endless possible applications.
Most of the larger Telcos and LD Carriers now offer Line Services to Larger
Commercial Customer in Metro areas in a T1 lie format.

Many great enhancements with few drawbacks came with the introduction of T1
technology in the early 70s.  Line noise has all but disappeared in most
connections useing the larger carriers.  If we time warped 2 hst modems
back to 1955 we would be lucky to get a 2400 baud connect.  It alo brought
about the ability to call coast to coast without spending a fortune.  They
dont require as much maintenance as analog networks so zillions of jobs
went away through the years.

One of the requested topics has been that of wire taps.  T1 has made this
a lot easier to implement and nearly impossible to detect by the tapped
party.  In nearly 90% of the USA a telephone signal is analog for a short
distance on each end of the connection, From the serveing Telco to the
telephone set or modem.  A digital signal is the easiest to monitor with
no affect on the analog lines.  A digital test set with an impedance of
1000 ohms on the input will not even cause a click when the test cord is
inserted in the jack.  most digital switches provide for Maintenance
monitoring of analog lines by electronicly bridgeing a D to A convertor
to the line just after it is converted to digital from analog in the
transmit direction and just before it is converted from digital to analog
in the receive direction.  Bottom line is that anyone who can gain the
cooperation of the telephone company with or without a court order can
eavesdrop without haveing any affect at all on the circuit that goes out
to the telephone set.  This total circuit isolation makes it impossible
to detect this kind of tap.

There are many So Called Tap detectors on the market.  They function on
the premise of detecting abnormal voltage or current levels associated
with the line inbetween the serving telephone company and the users phones.
They are incapable of detecting any thing beyond the first line transformer.

In effect even without telco help it is very easy to insert a device any
where in the line betwwen the phone & the telco that can isolate the phone
side of the circuit and alow monitoring equipment to be attatched with no
side affects to your line.  Such devices can even be field calibrated so
that all voltage and attenuation characteristics that were present prior to
its installation are repeated torwards the phone.  They can be adjusted to
within a millivolt for the DC characteristics an .05 db for the analog
characteristics.

At the risk of gossiping about the many law enforcement agencies that might
try to tap a line I have an opinion or two.

1. If the Feds want to listen they probably will, you cant tell, and if
they are that interested you are probably toast any way, they are just
trying to find out who your associates are by listening.

2. Many States, Counties, Large Cities & even a few Fed agencies can contract
with individuals who have the equipment needed to do this kind of dirty work.

3. Most small town or county agencies would be likely to try it with an
analog tap that a third grader could find with a multimeter.  20 bucks &
a trip to Radio Shack would probably do the trick on building a gizmo that
would make a light go on if someone tried a crude tap.








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