textfiles/phreak/NUMBERS/telcoloo.one

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                               LOOPS EXPLAINED

Loops occur in all area codes and consist of two phone numbers.  These
numbers are in the same exchange and the last four digits are usually
similar.  A typical loop pair light look like 212-555-9990 and 555-9993.
There are usually at least twenty loops in an area code and often all of
the loops in an area code will have identical suffix pairs.  The basic
thing about any loop is that the two numbers are connected together.  If I
were to call one number and you were to call the other we'd beconnected.
It's all a bit eerie at first because most loops do not ring; if you dial a
loop and there is someone on the other end you will be instantly connected.
What will you hear if you dial a loop number and there's no one on the other
end?  That depends upon which of the numbers you dial.  If you dial the
higher number of the pair you will hear only silence; if you dial the lower
you will hear a 1000 HZ tone.  On most loops you can talk to one caller
after another on !iend wwow two people to talk and usually sound
like any other phone connection.  You may be asking so what? The answer to
yourquestion is that loops offer anonymity.  People use this anonymity for
many reasons.

    We are now to the point of wondering what telco uses loops for.  There
havebeen a number of theories advanced on this topic over the years but
fewpeople have bothered to ask telco.  One common theory has been akin to
the idea that the loops are somehow used to "tie up" unused phone lines at
thecentral office to "keep them out of trouble." (I have always enjoyed
theimage of two lonely phone lines tied together to keep them company.) 
Loops are used to save time and manpower in testing long distance trunks.
We're not talking about the phone line that connects your phone to the
central office but the trunks that connect central offices and run in
lengthfrom a few thousand feet to many miles.  Home or business telephone
lines are dwarfed by the much larger trunks.  The trunk carries yourvoice
and the caller's voice in different directions.  Once the line gets to telco
premises the signal is divided upinto two circuits. One circuit carries
your voice and the other carries your caller's voice. If the signalswere
kept on one circuit there would be problems with feedback and echoes.
Trunks may consist of two pairs of two wire circuits or may be radio
frequency carriers on a cable.  Trunks have repeaters along the way which
amplify the signal remove echoes and equalize frequencies. Repeaters occur
about every two miles on an "old style" wire trunk line and about every
2000feet on carrier trunks.  Very short trunks may not have a repeater.
Repeatersneed to be tested and adjusted occasionally. In the old days a
tech wouldtest a trunk by arranging for someone to be at the other end.
He would then send a 1000 hz test tone to the other person who would read
the volume on a meter.  To complete the test the othertech sends a signal
back on the other leg to the first tech as the phone system grew telco
decided to cut down on manpower by tying two lines together.  Thus the loop
was born.  Trunks are tied together via a thing called a "zero loss
terminator" which connects lines so there is no change in volume.  By the
mid fifties, the entire phone system had been equipped with loops, so a tech
at one end could test a trunk alone by dialing a loop.  He dials the other
half of the loop with a known goodtrunk.  Then he reverses the signal path 
to complete the test.  It wasn't long before some ordinary citizens
discovered that loops could also pass voices not just tones.  Since the
lines belonged to telco they weren't billed for the call.  So a few people
made free calls to friends but therewas so little of this that its effect
on the phone company's income was way "down in the noise."  It wasn't until
years later in the early seventies that Bell was to put billing circuitry on
loop numbers. To avoid giving away their location most bookies used a
cheesebox, a device that connects two phone lines together.  Cheeseboxes
were installed in a small business often a small butcher shop or a grocery.
The bookie arrangedwith the proprietor to have two phones installed in the
shop and would pay asmall monthly fee.  He then tied the lines together
with his cheesebox and gave one of the numbers to his clientele.  Some
bookies either couldn't afford a cheesebox or couldn't locate one at any
price so they hit upon using loops. It was good while it lasted.  Gradually 
however more and more shady characters started using loops.  The authorities
weren't blind to this and started approaching the telco to do traces on
these loops.  Eventually thephone company was spending a lot of time on
criminal traces and decided to do something about these loops.
Around ???? Bell started inserting a bandpass filter that passed only 1000 hz
in the terminator end of its loops. With this change they blocked voices.
We're going to see that the solution was only temporary though.  The old style
four wire trunks could only handle one call at a time taking up a lotof
wire and space.  There had to be a way to cram calls into a smaller space.
by the early 1960's bell had started sw itching to carrier trunks which put
many calls on a cable.  each signal mod ulated an am carrier on a different
frequency.  because carrier uses radio frequency transmitters and receo cram
ca lls into a smaller space. by the early 1960's bell had started sw
itching to carrier trunks which put many calls on a cable.  each signal mod
ulated an am carrier on a different frequency. because carrier uses radio
frequency transmitters and rece1000 hz f ilter so a switch was added to
switch it 'on and off.  normally the filter would be left  on.  when a tech
wished to test a trunk he would turn the swtch on bypas sing the filter.
when he was done he was expected to turn off the switch.  i f he forgets a
loop will continue to pass voice frequencie until ssoff.  let's look at
how loops are used nowada ys.  if a tech dials up the lower number he will
immediately ge a 1000 hz  tone coming back to him which is injected at a
specific volume known as "zero db" level. using his meter he can gauge if
there are any problems on the line and if he needs to do a complete test at
various frequencies he  then turns the filter bypass switch on.  most of
this work is done at night  when repair people are free from normal chores.
 in preparing this report i interviewed several security directors for new
york telephone company and spent some time o n loops talking to people who
use them.  it turns out there are people us ing loops for more things than i
had imagined. i have always wondered if spi es use loops but i haven't
encountered any yet.  when i started looking into l oops i was aware that
some radio pirates use loops.  especially in the n ew york city area you'll
often run into am and fm pirates on loops late at  night.  some local loop
numbers are pretty well  known and are passed around high schools and
colleges.  when students ge t bored at night or want to find a party they
call a loop and wait there t ill someone else calls. it may be someone they
know or a complete strange r. but it's someone to talk to. then there are
the loop habituates.  they re gulary meet with their circle of friends and
acquaintances on loops and tend to resent strangers on their loops. 
representatives are quick to point out  that loops belong to the phone
company.  anyone else using them is a t ransgressor.  since bell is the
aggrieved party it needn't have any qua lms about listening to loops nor
about tracing callers.  bell wishes to discou rage people from using them
and periodically programs its' billing comp uter to look for loop numbers.
any customer thus found is sent a card poin ting out that these numbers
belong to telco. with the exception of those stea ling services bell becomes
aware that some one is calling a loop using a fake d credit card number; or
sprint or mci will ask for help tracing someone illeg ally stealing their
services to call a loop.  then it's a matter of waiting fo r the person to
try again and tracing the call.  in these affairs the phone c ompany is very
aggressive and effective in tracking down offenders. bell has some very well
trained people who are most adept at keeping the offen der on the line until
a trace is complete.