2600 hertz (2600 Hz) is a

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...

(cycles per second) that was used in telecommunication signaling in mid-20th century long-distance telephone networks using

carrier system A carrier system is a transmission system that transmission (telecommunications), transmits information, such as the voice signals of a telephone call and the video signals of television, by modulation of one or multiple carrier signals above the ...

s. Tone signaling carrier systems operated in the standard telephony voice frequency range ( to ). They replaced

direct current Direct current (DC) is one-directional electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...

(D.C.) signaling on toll trunk lines because they could be used with any type of toll facility over any length of transmission line that was suitable for voice transmissions. This included transmission through line repeaters and other facilities that would distort, block, or otherwise prohibit D.C. loop-disconnect signaling, such as

rotary dial A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone numb ...

pulses, and on-hook/off-hook signaling. Common frequencies for this purpose were , , , , and , the latter being just outside the voice range. These signaling systems were continuous tone methods, so that the idle condition of a trunk line could easily be detected by the presence of the appropriate signaling frequency, in contrast to conditions of a quiet line which could be due to pauses in speech, or arise from line faults. The tones were typically transmitted in the same physical and logical channel, which characterizes these systems as

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 o ...

methods, which do not require additional bandwidth for control of the network and benefit from a single amplification facility for speech and signaling., p.1400 The most common single-frequency signaling (SF) system in use in the United States from the 1950s to the 1970s used the frequency pairing of 2600 with 2400 hertz, while in other countries, some systems combined with a variety of lower frequencies in a compelled mode. Lower frequencies, such as , were used by the 1940s in composite signaling systems, however, the higher signal energy in speech at presented technical problems in protection against false operation. The in-band signaling method was vulnerable to ''talk-off'' conditions when the voice of a telephone user accidentally or intentionally generates the same tone or sufficiently strong spectral content at the frequency of the signaling system, a condition also known as ''

falsing In telecommunications, falsing is a signaling error condition when a signal decoder detects a valid input although the implied protocol function was not intended. This is also known as a false decode. Other forms are referred to as talk-off. Sig ...

''. In this instant, the call would be disconnected prematurely, and the trunk placed in idle condition. The

Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the AT&T Corporation, American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America fo ...

in the United States used special ''signal-to-guard'' arrangements in the signaling receiver to detect this condition by comparison with the energy in the frequency spectrum outside the signaling tone. The condition was also often mitigated by narrow-band

notch filter In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the inverse of a ''band-pass filter''. A notch filter is ...

s during the seized line state. The discovery of this phenomenon by technology-curious individuals in the 1960s, led to the abuse by

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 se ...

, a subculture that exploited the technology to explore national and international telephone networks and place cost-free long-distance telephone calls. Combating abuse, and improving communications, the telephone industry transitioned to

out-of-band signaling In telecommunications, 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. Classi ...

systems, such as Signalling System 7 (SS7), by the 1980s. This separated the voice and signaling channels, making it impossible to generate control signals in the voice bearer channel. The development of the

T-carrier The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell Syst ...

system in the 1960s helped obsoleting single-frequency (SF) signaling. SF signaling was replaced by advanced methods of common channel signaling, a technology in development since the 1920s. In the 1970s,

multi-frequency signaling 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 li ...

systems came into use for international direct distance dialing (IDDD) which used the frequency of in

line signaling Line signaling is a class of telecommunications signaling protocols. Line signaling is responsible for off-hook, ringing signal, answer, ground start, on-hook unidirectional supervision messaging in each direction from calling party to called ...

in two-frequency mode with , for line seizure during call setup, and for tear-down. signaling was a standard for many international signaling systems, such as the ''Regional System R1''. The prominent application of the frequency in telecommunications world-wide inspired the name of many

hacker A hacker is a person skilled in information technology who achieves goals and solves problems by non-standard means. The term has become associated in popular culture with a security hackersomeone with knowledge of bug (computing), bugs or exp ...

communities and publications, such as '' 2600: The Hacker Quarterly'', and the late 20th-century

counterculture A counterculture is a culture whose values and norms of behavior differ substantially from those of mainstream society, sometimes diametrically opposed to mainstream cultural mores.Eric Donald Hirsch. ''The Dictionary of Cultural Literacy''. Ho ...

''2600''.

References

{{Telsigs Phreaking Telecommunication protocols Telephony signals

See also

References