Black box (phreaking)
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The black box (as distinguished from blue boxes and red boxes), sometimes called an Agnew (see Spiro (device) for the origin of the nickname), was a device built by phone phreaks during the 1960s and 1970s in order to defeat long distance phone call toll charges, and specifically to block the supervision signal sent by the receiving telephone handset when the call was answered at the receiving end of the call.
The act of picking up the handset of a telephone causes a load to be put on the telephone line, so that the DC voltage on the line drops below the approximately 45 volts present when the phone is disconnected. The black box consisted of a large capacitor which was inserted in series with the telephone, thereby blocking DC current but allowing AC current (i.e., ringing signal and also audio signal) to pass. When the black box was switched into the telephone line, the handset could be picked up without the telephone system knowing and starting the billing process.
In other words, the box fooled the phone company into thinking no one had answered at the receiving end, and therefore billing was never started on the call.
In the earliest circuits, using a "black box" call meant talking between rings, since the phone company would continue to send rings (AC current) through the line which was heard as a loud recurring buzzing noise. But the circuit was later refined to solve this problem. The common electromechanical telephone switches (e.g. crossbar) used two relays to detect the line status and one of these was more sensitive than the other. Placing a resistor (the value of this resistor has been lost to history) in parallel with the capacitor allowed a small DC current to pass, enough to silence ringing without starting billing. However, this circuit did not work reliably on every phone line since the current could vary considerably. A refinement of this circuit replaced the resistor with a 36 volt Zener diode. This regulated the voltage drop to reliably prevent billing on most telephone switches.
Although digital switching systems, which were immune to being fooled by the black box, were developed in the 1970s, a great many electromechanical switches remained in service, allowing the black box to be used in many areas through the 1980s and beyond. Upgrades gradually eliminated all the electromechanical switches, rendering the black box a historical relic.
The incentive to circumvent long distance charges by fraud has also diminished with the advent of unlimited calling plans and VoIP.
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