127 lines
6.6 KiB
Plaintext
127 lines
6.6 KiB
Plaintext
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Section [6]: Introduction to Telecommunications
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Written February 1, 1992
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By: The Phone Knome
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Sysop of The Bill System
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This is the first in a series of articles about the Public Switched Telephone
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Network as it functions in the United States and Canada. The purpose of the
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series is to expand the horizons of any individual who desires to increase
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their awareness of how all these machines that are connected to each other
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function together as a network.
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These articles are not copyrighted. Let no one do so. They shall be free for
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distribution anywhere at anytime. They are dedicated to the millions whose
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need to communicate with each other have given me a career.
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********************** Reality Alert & Warning *******************************
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IT IS EXPRESSLY STATED THAT THE AUTHORS OF THIS ARTICLE & ANY THAT MAY
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FOLLOW IN THE SERIES, STRONGLY DISCOURAGE ANY AND ALL INDIVIDUALS FROM
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ENGAGING IN ACTS OF TOLL FRAUD OR ELECTRONIC PRIVACY INVASION. THESE
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ARTICLES ARE NOT BEING WRITTEN TO TEACH THE READERS HOW TO BEHAVE IN THE
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CRIMINAL ACTS THAT THEY MAY DISCUSS IN DEPTH.
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******************************************************************************
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On With the show...
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In the Science of telecommunications there are two major sub-sciences
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that work as a team to get the job done. They are ***Transmission***
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and ***Switching***. Transmission will be covered in the first few
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issues. Switching will be briefly defined but basicly dealt with later.
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Transmission is not a gear box as far as this series is concerned. It is
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the Transport of an electrical or accoustic energy (message) from one point
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to another. In its simplest applications it is copper wire. Due to the
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basic concept of Ohms Law & attenuation the range of wire alone is somewhat
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limited. The only thing I know of that has a wider bandwidth for direct
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connection is optical cable. The need to send a signal greater than it was
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possible useing wire alone caused the evolution of several generations of
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increasingly sophisticated transmission systems. The greed asociated with
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money added motivation to continue to develop systems that would increase
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the throughput of existing transmission components thereby cutting costs,
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and since our utility bills never go down if a Telco (Telphone Company)
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can cut operating cots their profit just goes up.
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By far one of the most important concepts that will serve as a foundation
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of enlightenment in this series is that of Digital Transmission with the
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MOST PREVAILING FORMAT being **** T1 ***** sometimes referred to as DS1.
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A T1 circuit is basicly a pipe that goes from one location to another to
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pass telephone calls, and or Private line Data Services Back and forth.
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In common applications it contains 24 telephone lines in a digital format.
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The format is serial synchronous in nature with a line rate of 1,544,000
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bits per second. Hauling ass compared to 14,400 huh?
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T1 ckts are used mostly to tie a Telephone Company Switch in one location
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to a switch in another location. Their use is also on the rise for Private
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Point to Point Communication in virtually endless possible applications.
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Most of the larger Telcos and LD Carriers now offer Line Services to Larger
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Commercial Customer in Metro areas in a T1 lie format.
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Many great enhancements with few drawbacks came with the introduction of T1
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technology in the early 70s. Line noise has all but disappeared in most
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connections useing the larger carriers. If we time warped 2 hst modems
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back to 1955 we would be lucky to get a 2400 baud connect. It alo brought
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about the ability to call coast to coast without spending a fortune. They
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dont require as much maintenance as analog networks so zillions of jobs
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went away through the years.
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One of the requested topics has been that of wire taps. T1 has made this
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a lot easier to implement and nearly impossible to detect by the tapped
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party. In nearly 90% of the USA a telephone signal is analog for a short
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distance on each end of the connection, From the serveing Telco to the
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telephone set or modem. A digital signal is the easiest to monitor with
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no affect on the analog lines. A digital test set with an impedance of
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1000 ohms on the input will not even cause a click when the test cord is
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inserted in the jack. most digital switches provide for Maintenance
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monitoring of analog lines by electronicly bridgeing a D to A convertor
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to the line just after it is converted to digital from analog in the
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transmit direction and just before it is converted from digital to analog
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in the receive direction. Bottom line is that anyone who can gain the
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cooperation of the telephone company with or without a court order can
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eavesdrop without haveing any affect at all on the circuit that goes out
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to the telephone set. This total circuit isolation makes it impossible
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to detect this kind of tap.
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There are many So Called Tap detectors on the market. They function on
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the premise of detecting abnormal voltage or current levels associated
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with the line inbetween the serving telephone company and the users phones.
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They are incapable of detecting any thing beyond the first line transformer.
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In effect even without telco help it is very easy to insert a device any
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where in the line betwwen the phone & the telco that can isolate the phone
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side of the circuit and alow monitoring equipment to be attatched with no
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side affects to your line. Such devices can even be field calibrated so
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that all voltage and attenuation characteristics that were present prior to
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its installation are repeated torwards the phone. They can be adjusted to
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within a millivolt for the DC characteristics an .05 db for the analog
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characteristics.
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At the risk of gossiping about the many law enforcement agencies that might
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try to tap a line I have an opinion or two.
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1. If the Feds want to listen they probably will, you cant tell, and if
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they are that interested you are probably toast any way, they are just
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trying to find out who your associates are by listening.
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2. Many States, Counties, Large Cities & even a few Fed agencies can contract
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with individuals who have the equipment needed to do this kind of dirty work.
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3. Most small town or county agencies would be likely to try it with an
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analog tap that a third grader could find with a multimeter. 20 bucks &
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a trip to Radio Shack would probably do the trick on building a gizmo that
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would make a light go on if someone tried a crude tap.
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