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TUCoPS :: Phreaking Technical System Info :: loops.txt

Loops Explained





          LOOPS EXPLAINED
          AUTHOR UNKNOWN
LOOPS OCCUR IN ALL AREA CODES AND CONSI
ST OF TWO PHONE NUMBERS.  THESE
NUMBERS ARE IN THE SAME EXCHANGE AND TH
E LAST FOUR DIGITS ARE USUALLY
SIMILAR.  A TYPICAL LOOP PAIR MIGHT LOO
K LIKE 212-555-9990 AND 555-9Y93.
THERE ARE USUALLY AT LEAST TWENTY LOOPS
 IN AN AREA CODE AND OFTEN ALL OF THE
LOOPS IN AN AREA CODE WILL HAVE IDENTIC
AL 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 FIR
ST 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 WILLJ=UHEAR IF YOU DIA
L 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 WWOW TWO
 PEOPLE TO TALK AND USUALLY SOUND 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 WH
AT TELCO USES LOOPS FOR.  THERE HAVE
BEEN A NUMBER OF THEORIES ADVANCED ON T
HIS 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 TRO
UBLE." (I HAVE ALWAYS ENJOYED THE
IMAGE  OF TWO LONELY PHONE LINES TIED T
OGETHER TO KEEP THEM COMPANY.)

LOOPS ARE USED TO SAVE TIME AND MANPOWE
R 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 CONN
ECT CENTRAL OFFICES AND RUN IN LENGTH
FROM A FEW THOUSAND FEET TO MANY MILES.
 HOME OR BUSINESS TELEPHONE IS A TTR
VOICE AND THE CALLER'S VOICE IN DIFFERE
NT DIRECTIONS.  ONCE THE LINE GETS TO
TELCO PREMISES THE SIGNAL IS DIVIDED UP
 INTO TWO CIRCUITS.  ONE CIRCUIT
CARRIES YOUR VOICE AND THE OTHER CARRIE
S 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 EQ
UALIZE FREQUENCIES. REPEATERS OCCUR
ABOUT EVERY TWO MILES ON AN "OLD STYLE"
 WIRE TRUNK LINE AND ABOUT EVERY 2000
FEET ON CARRIER TRUNKS. VERY SHORT TRUN
KS MAY NOT HAVE A REPEATER. REPEATERS
NEED TO BE TESTED AND ADJUSTED OCCASION
ALLY. IN THE OLD DAYS A TECH WOULD
TEST A TRUNK BY ARRANGING FOR SOMEONE T
O BE AT THE OTHER END.  HE WOULD THEN
SEND A 1000 HZ TEST TONE TO THE OTHER P
ERSON 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 SYST
EM GREW TELCO DECIDED TO CUT DOWN ON
MANPOWER BY TYING TWO LINES TOGETHER. T
HUS THE LOOP WAS BORN.  TRUNKS ARE
TIED TOGETHER VIA A THING CALLED A "ZER
O LOSS TERMINATOR WHICH CONNECTS LINES
SO THERE IS NO CHANGE IN VOLUME.  BY TH
E 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 H
ALF OF THE LOOP WITH A K NOWN GOOD
TRUNK. THEN HE REVERSES THE SIGNAL PATH
 TO COMPLETE THE TEST.

IT WASN'T LONG BEFORE SOME ORDINARY CIT
IZENS 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 M
ADE FREE CALLS TO FRIENDS BUT THERE
WAS SO LITTLE OF THIS THAT ITS EFFECT O
N THE PHONE COMPANY'S INCOME WAS WAY
"DOWN IN THE NOISE."  IT WASN'T UNTIL Y
EARS LATER IN THE EARLY SEVENTIES THAT
BELL WAS TO PUT BILLING CIRCUITRY ON LO
OP NUMBERS.
TO AVOID GIVING AWAY THEIR LOCATION MOS
T BOOKIES USED A CHEESEBOX A DEVICE
THAT CONNECTS TWO PHONE LINES TOGETHER.
  CHEESEBOXES WERE INSTALLED IN A
SMALL BUSINESS OFTEN A SMALL BUTCHER SH
OP 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 LI
NES TOGETHER WITH HIS CHEESEBOX AND
GAVE ONE OF THE NUMBERS TO HIS CLIENTEL
E. SOME BOOKIES EITHER COULDN'T AFFORD
A CHEESEBOX OR COUKDN'T LOCATE ONE AT A
NY PRICE SO THEY HIT UPON USING LOOPS.
 IT WAS GOOD WHILE IT L@STED. GRADUALLY
 HOWEVER MORE AND MORE SHADY
CHARACTERS STARTED USING LOOPS.  THE AU
THORITIES WEREN'T BLIND TO THIS AND
STARTED APPROACHING THE TELCO TO DO TA
CES ON THESE LOOPS. EVENTUALLY THE
PHONE COMPANY WAS SPENDING A LOT OF TIM
E ON CRIMINAL TRACES AND DECIDED TO DO
SOMETHING ABOUT THESE LOOPS.

AROU0B1
=STARTED INSERTING A BANDPASS FILTER TH
AT PASSED ONLY 1000
HZ IN THE TERMINATOR END OF ITS' LOOPS.
 WITH THIS CHANGE TTBLOCKED VOICES.
WE'RE GOING TO SEE THAT THE SOLUTION WA
S ONLY TEM@ORARY THOUGH.  THE OLD
STYLE FOUR WIRE TRUNKS COULD ONLY HANDL
E ONE CALL AT A TIME TAKING UP A LOT
OF WIRE AND SPACE. THERE HAD TO BE A WA
Y 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 SWITCH 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 FREQUENCIES 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.



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