-- Note: This file arrived at ARGH garbled; I edited it to remove the garbage. If anyone has a better version, please send it to either 201@3464 WWIVNet or C579380@MIZZOU1.MISSOURI.EDU. (11/92) ------------------------------------------------------------------------------- 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.