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= Loops Explained =
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anonymous
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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 might 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 be
connected. 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 the other end, very much like any other phone connection. You
may be asking so what? The answer to your question 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
have been a number of theories advanced on this topic over the years but few
people 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 the central
office to "keep them out of trouble." (I have always enjoyed the image 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 length
from a few thousand feet to many miles. When you talk on the phone, your voice
and the caller's voice go in different directions. Once the line gets to
telco premises the signal is divided up into two circuits. One circuit
carries your voice and the other carries your caller's voice. If the signals
were 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 2000
feet on carrier trunks. Very short trunks may not have a repeater. Repeaters
need to be tested and adjusted occasionally. In the old days a tech would
test 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 other tech 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 good
trunk. 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 be longed to telco they
weren't billed for the call. So a few people made free calls to friends but
there was so little of this that its effect on the phone company's income was
negligible. 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 arranged with the proprietor to have two phones installed
in the shop and would pay a small monthly fee. He then tied the lines together
with his cheesebox and gave one of the numbers to his clientel. 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 the
phone company was spending a lot of time and money on criminal traces and
decided to do something about these loops.
In the late 50's, the phone company started inserting a bandpass filter
that passed only 1000 Hz in the terminator end of its' loops. With this change
it successfully 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 lot
of wire and space. There had to be a way to cram calls into a smaller space.
By the early 1960's bell had started switching to carrier trunks which put
many calls on a cable. Each signal modulated an AM carrier on a different
frequency. Because AM carriers use radio frequency transmitters and recievers,
they could no longer pass a 1000 Hz tone through the bandpass filter. 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 switch on, bypassing the
filter. When he was done he was expected to turn off the switch. If he
forgets a loop will continue to pass voice frequencies until it is switched
off.
Let's look at how loops are used nowadays. 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. 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.
It turns out there are people using loops for more things than I had
imagined. I have always wondered if spies use loops but i haven't encountered
any yet. When i started looking into loops I was aware that some radio
pirates use loops. Especially in the New 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 get bored at night or want to find a
party they call a loop and wait there till someone else calls. It may be
someone they know or a complete stranger, but it's someone to talk to. Then
there are the loop habituates. They regulary meet with their circle of friends
and aquaintances 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 transgressor. Since Bell is the
aggrieved party it needn't have any qualms about listening to loops nor about
tracing callers. Bell wishes to discourage people from using them and
periodically programs its billing computer to look for loop numbers. Any
customer thus found is sent a card pointing out that these numbers belong to
the telco. With the exception of those stealing services, Bell becomes aware
that some one is calling a loop using a faked credit card number; or Sprint or
MCI will ask for help tracing someone illegally stealing their services to
call a loop. Then it's a matter of waiting for the person to try again and
tracing the call. In these affairs the phone company is very aggressive and
effective in tracking down offenders. Bell has some very well trained people
who are most adept at keeping the offender on the line until a trace is
complete.
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