2600 hertz (2600 Hz) is afrequency inhertz (cycles per second) that was used intelecommunication signaling in mid-20th centurylong-distance telephone networks usingcarrier systems.[1]
Tone signaling carrier systems operated in the standard telephony voice frequency range (300Hz to3500Hz). They replaceddirect current (D.C.) signaling on tolltrunk 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 asrotary dial pulses, and on-hook/off-hook signaling. Common frequencies for this purpose were1600Hz,2000Hz,2400Hz,2600Hz, and3700Hz, the latter being just outside the voice range.[1] 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 asin-band signaling methods, which do not require additional bandwidth for control of the network and benefit from a single amplification facility for speech and signaling.[2]
The most commonsingle-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 combined2600Hz with a variety of lower frequencies in a compelled mode. Lower frequencies, such as1600Hz, were used by the 1940s in composite signaling systems, however, the higher signal energy in speech at1600Hz presented technical problems in protection against false operation.[3] The in-band signaling method was vulnerable totalk-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 asfalsing.[4] In this instant, the call would be disconnected prematurely, and the trunk placed in idle condition. TheBell System in the United States used specialsignal-to-guard arrangements in the signaling receiver to detect this condition by comparison with the energy in the frequency spectrum outside the signaling tone.[2] The condition was also often mitigated by narrow-bandnotch filters during the seized line state.[5]
The discovery of this phenomenon by technology-curious individuals in the 1960s, led to the abuse byphreaking, a subculture that exploited the technology to explore national and international telephone networks and place cost-free long-distance telephone calls.[6]
Combating abuse, and improving communications, the telephone industry transitioned toout-of-band signaling systems, such asSignalling System 7 (SS7), by the 1980s. This separated the voice and signalingchannels, making it impossible to generate control signals in the voice bearer channel. The development of theT-carrier 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.[7]
In the 1970s,multi-frequency signaling systems came into use forinternational direct distance dialing (IDDD) which used the frequency of2600Hz inline signaling in two-frequency mode with2400Hz, for line seizure during call setup, and for tear-down.
2600Hz signaling was a standard for many international signaling systems,[8] such as theRegional System R1.[9]
The prominent application of the2600Hz frequency in telecommunications world-wide inspired the name of manyhacker communities and publications, such as2600: The Hacker Quarterly,[10] and the late 20th-centurycounterculture2600.
