2600 hertz (2600 Hz) is a

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...

(cycles per second) that was used by

AT&T AT&T Inc. is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company by revenue and the third largest provider of mobile tel ...

as a steady

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

to mark currently unused

long-distance Long distance or Long-distance may refer to: *Long-distance calling *Long-distance operator *Long-distance relationship * Long-distance train *Long-distance anchor pylon, see dead-end tower Footpaths *Long-distance trail *European long-dista ...

telephone line A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...

s. During the 1960s,

in-band signaling In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even ov ...

was used, so the same line for both voice conversations and telephone connection management signals. Since a pause in a voice conversation would produce silence, another method was required for

switches In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...

to determine whether a line was in use. The solution AT&T created was to produce a 2600 Hz tone on idle trunks.

In phreaking

phreaking Phreaking is a slang term coined to describe the activity of a culture of people who study, experiment with, or explore telecommunication systems, such as equipment and systems connected to public telephone networks. The term ''phreak'' is a ...

, a device known as a blue box was used to generate the 2600 Hz signal on a line being used. This indicated to switch that the line was idle. After the tone, the switch believed another call was starting and used the subsequent dialed digits to connect the call. This technique only affected interoffice

multi-frequency In telephony, multi-frequency signaling (MF) is a type of signaling that was introduced by the Bell System after World War II. It uses a combination of audible tones for address (telephone number) transport and supervision signaling on trunk lines b ...

(MF) trunks; local calls that originate and terminate on the same switch do not use a 2600 Hz signal. By placing a call to a non-local

toll-free A toll-free telephone number or freephone number is a telephone number that is billed for all arriving calls. For the calling party, a call to a toll-free number from a landline is free of charge. A toll-free number is identified by a dialing prefi ...

number, interoffice trunks were used for free. Using a blue box would then disconnect the toll-free call and let any other number be dialed. Since the phone was never physically hung up, the connection was still toll-free. At one point in the mid-1960s, packets of the

Cap'n Crunch Cap'n Crunch is a maize, corn and oat breakfast cereal manufactured by Quaker Oats Company, a subsidiary of PepsiCo since 2001. After introducing the original cereal in 1963, marketed simply as ''Cap'n Crunch'', Quaker Oats has since introduced n ...

breakfast cereal Cereal, formally termed breakfast cereal (and further categorized as cold cereal or warm cereal), is a traditional breakfast food made from processed cereal grains. It is traditionally eaten as part of breakfast, or a snack food, primarily in ...

included a free gift: a small whistle that (by coincidence) generated a 2600 Hz tone when one of the whistle's two holes was covered. The phreaker Captain Crunch adopted his

nickname A nickname is a substitute for the proper name of a familiar person, place or thing. Commonly used to express affection, a form of endearment, and sometimes amusement, it can also be used to express defamation of character. As a concept, it is ...

from this whistle. In the 1970s and 80s some trunks were modified to filter out

SF tone In telephony, single-frequency signaling (SF) is line signaling in which dial pulses or supervisory signals are conveyed by a single voice-frequency tone in each direction. SF and similar systems were used in 20th-century carrier systems. An SF ...

arriving from a caller. Later in the 20th century, long-distance companies adopted the

out-of-band signaling In telecommunication, signaling is the use of signals for controlling communications. This may constitute an information exchange concerning the establishment and control of a telecommunication circuit and the management of the network. Classif ...

system

Signaling System 7 Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in 1975, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also perform ...

. This system separated the voice and signaling channels, making it impossible to generate these signals from an ordinary phone line.

References

{{Telsigs Phreaking

In phreaking

See also

References