 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 3 ] Next Last |
A phreak or phreaker is a person who engages in the act of manipulating phones in this way. The tools of the phone phreak are electronic devices known as boxes, originally the blue box, but later the black box, red box, beige box and clear box and many others. However, phreaking does not necessarily mean that you are using a "color box" tool.
Most of the techniques formerly used in phreaking are no longer effective due to changes in the telephone system. Some of these changes were evolutionary, and some were designed specifically to disallow such access. Moreover, at least in the United States, the cost of telephone calls has diminished to the point where few would find it worthwhile to engage in toll fraud; and there are numerous competing providers of telephone service (except for most wired local service which remains controlled by regional Bell operating companies—remnants of the former AT&T telephony monopoly in the USA).
In the 1960s the US phone system used a mechanical device for call switching known as the crossbar. The crossbar system could control phone switching by watching the voltage on the lines connected to the user's phones. When the user picked up the handset, the voltage dropped from about 48 VThe volt is the SI derived unit for electric potential and voltage (derived from the ampere and watt). It is named in honor of Alessandro Volta, who, in 1800, invented the voltaic pile, the first chemical battery. The volt is defined as the potential diff to about 10 V, so the crossbar knew that person wanted to place a call. It would then play a dial tone and wait for the user to dial. It could also tell when the user had hung up when it saw the voltage increase back to about 48 V again. When a call was received, the crossbar would switch to an intermittent ring voltage of about 90 VAC at 20 HzHertz is also the name of a car rental company. See The Hertz Corporation''. The hertz (symbol Hz is the SI unit of frequency. It is named in honour of the German physicist Heinrich Rudolf Hertz who made some important contributions to science in the fiel to make the hammer repeatedly strike the bell inside the phone and cause the phone to ring.
Dialing worked in a similar fashion; the mechanical, spring-loaded rotary dial found on older telephones functioned by quickly connecting and disconnecting the line. At the phone company central office, the lines were connected to a series of mechanical disks ( stepping relay s) that rotated one position for every "click", so seven such clicks would turn the disk seven positions. After dialing several numbers in this way (typically seven in North America), the line would eventually be connected to another phone, which would start ringing. Anyone, with some practice, may to this day dial a telephone by repeatedly clicking the receiver, one click for a "1", two clicks in rapid succession for a "2", ten clicks in rapid succession for a "zero".
Switching through the use of electromechanical stepping relays only worked for "local" calls, telephones connected to the same central office shared a common crossbar. Long-distance calls, however, required a method of switching telephone calls that did not require a physical electrical connection.
Between central offices, long linesSee also long line (topology) and long-line fishing. In telephone systems nomenclature a long line is a transmission line in a long-distance communications network such as carrier systems, microwave radio links, geosynchronous satellite links, underground were employed which at first required the intervention of a human operator. In order to reduce or eliminate the need for operator assistance, AT&T began a system of "direct distance dialing" which relied on the use of area codeThe area code is a part of a telephone number normally occurring at the beginning of the number, that usually indicates a geographical area. It directs telephone calls to particular regions on a public switched telephone network (PSTN), where they are furs, special three-digit prefixes containing either "1" or "0" as the second digit.
No local telephone number could begin with any of the three-digit area codes, so they could be distinguished from long-distance calls. When detecting an area code, the line was switched to an outbound long line. Dialing a long distance became similar to dialing locally, with the exception that you are first switched to a remote central office who will handle the rest of the dialing. For instance if you dial 416-555-1212 the local central office switch will immediately forward your call to the 416 switch in Toronto over a long line, and from there the rest of the numbers will dial a Toronto call as if you were local.
Dialing pulses will not travel over long distances, which will filter them out due to capacitanceCapacitance is the ability of a capacitor to store potential difference or voltage for a given amount of stored charge. The SI unit of capacitance is the farad. where C is the capacitance, measured in farads Q is the charge, measured in coulombs V is the. During the 1960s, an increasing number of calls were being carried by microwave links and even satellite relays, in which case there was no electrical connection between the two end offices at all. In order to allow the dialing signals to travel between offices then, AT&T devised a device that translated the pulses into tones, which is, after all, what the phone system is built to handle. At the far end office another similar device translates the tones back into pulses, dialing the existing switch. These tones, known as multi-frequency, included not only numbers, but various commands for signaling things like hanging up the call.
In the 1970s, the area code system was augmented by requiring callers to dial "1" before the area code. This enabled all the former area codes to be used as local exchange prefixes, and theoretically enabled any three-digit combination to be used as an area code (although because of other limitations, area codes with a second digit other than "0" or "1" were not introduced until 1995). The prefix "011" was later implemented to permit overseas calls to be dialed without operator assistance in a similar fashion (though in some areas, for a time the code 011 would simply reach an overseas operator).
| Politics | Business | Finance |
 |
 |
 |
|