The Phreaker's Guide 
             to Loop Lines     
            by  Lefty Carlson  
                  &            
              Landau Ltd       
_______________________________________
Revised: April 5, 1985

    A loop is a wonderous device which
the telephone company created as test
numbers for telephone repairmen when
testing equipment.  By matching the
tone of the equipment with the tone of
the loop, repairmen can adjust and test
the settings of their telephone
equipment.

    A loop, basically, consists of two
different telephone numbers.  Let's
use A and B as an example.  Normally
if you call A, you will hear a loud
tone (this is a 1004 hz tone), and if
you call B, the line will connect, and
will be followed by silence.

   This is the format of a loop line.
Now, if somebody calls A and someone
else calls B--Viola!--A and B loop
together, and one connection is made.
Ma Bell did this so repairmen can
communicate with each other without
having to call their own repair ofnice.
They can also use them to exchange programs, like for ANA or Ringback.
Also, many CO's have a "Loop Assign}ent Center".  If anyone has any information on these centers please tell me.


    Anyway, that is how a loop is
constructed.  From this information,
anyone can find an actual loop line.
Going back to the A and B example,
Note: the tone side and the silent side
can be either A or B.  Don't be fooled
if the phone company decides to
scramble them around to be cute.

   As you now know, loops come in pairs
of numbers.  Usually, right after each
other.

 For example: 817-972-1890
                 and
              817-972-1891

   Or, to save space, one loop line can
be written as 817-972-1890/1 (according to the Landau Shorthand For Numbers.


   This is not always true.  Sometimes,
the pattern is in the tens or hundreds,
and, occaisionally, the numbers are
random.


   In cities, usually the phone company
has set aside a phone number suffix
that loops will be used for.  Many
different prefixes will correspond
with that one suffix.

   In Arlington, Texas, a popular
suffix for loops is 1893 and 1894, and
a lot of prefixes match with them to
make the number.

 For Example:  817-460-1893/4
               817-461-1893/4
               817-465-1893/4
               817-467-1893/4
               817-469-1893/4

        ...are all loops...

   or a shorter way to write this is:

              817-xxx-1893/4

      xxx= 460, 461, 465, 467, 469


   Note: You can mix-and-match a
popular suffix with other prefixs in a
city, and almost always find other
loops or test numbers.

   Note: For Houston, the loop suffixes are 1499 and 1799.  And for Detroit it's 9996 and 9997.

   When there are a large number of
loops with the same prefix format,
chances are that many loops will be
inter-locked.  Using the above example
of Arlington loops again, (I will write
the prefixes to save space) 460, 461,
and 469 are interlocked loops.  This
means that only one side can be used at
a given time.  This is because they are
all on the same circuit. 

   To clarify, if 817-461-1893 is
called, 817-460 and 469-1893 cannot be
called because that circuit is being
used.  Essentialy, interlocked loops
are all the same line, but there are a
variety of telephone numbers to access
the line.


   Also, if the operator is asked to
break in on a busy loop line he/she
will say that the circuit is
overloaded, or something along those
lines.  This is because Ma Bell has taken the checking equipment off the line.  However, there are still many rarely used loops which can be verfied and can  have emergency calls taken on them.


   As you have found out, loops come in
many types.  Another type of loop is a
filtered loop.  These are loop lines
that the tel co has put a filter on, so
that normal human voices cannot be
heard on either line.  However, other
frequencies may be heard.  It all
depends on what the tel co wants the
loop to be used for.  If a loop has
gotten to be very popular with the
local population or used frequently for
conferences, etc. the tel co may filter
the loop to stop the unwanted
"traffic".  Usually, the filter will be
removed after a few months, though.
The wise and knowledgeable phreaker
can use loops in many ways.  If there
is a person he/she wants to talk to,
but does not want to give them their
home telephone number, then they can
meet on a loop.  Conferences can be set
up on loops.  Collect calls can be
accepted on loops.  If a loop is
unmonitered, then it can be called free
from a pay phone (this has many good
uses if you think about it). 
Therefore, loops can be a powerful tool
if used correctly, and not abused.
They can make a good phreak's life a
lot easier.


   I hope that this tutorial is of use
to everyone who reads it, and was
enjoyable.  We, here at the Landau Ltd
Mansion, enjoyed writing it.  Look for
more articles, files, and tutorials
from myself, Lefty Carlson, and Landau
Ltd in the future.


  PS: Look out for a part 2 of this series.  Of which, I plan it to include as large a list of working loops as possible.  I'm not exactly sure how to do it, but if anyone has any ideas please leave me mail.  Remember: Comments, critisisms, critiques, or er criteria are always welcome!







           ---> The End <---
_______________________________________

R. 1985/LL