{{short description|Audible telecommunication signal for control of telephone networks}}
'''2600 hertz''' ('''2600 Hz''') is a [[frequency]] in [[hertz]] (cycles per second) that was used in [[telecommunication signaling]] in mid-20th century [[Long-distance calling|long-distance]] telephone networks using [[carrier system]]s.<ref name="weaver">{{cite journal |title=In-Band Single-Frequency Signaling |last1=Weaver|first1=A. |last2=Newell|first2=N.A. |journal=Bell System Technical Journal |volume=33|issue=6 |pages=1309–1330 |date=1954-06-07 }}</ref>
{{Listen|filename=2600 Hz.ogg|title=2600 Hz|description=A 2600 Hz tone|format=[[Ogg]]}}
[[Tone signaling]] carrier systems operated in the standard telephony voice frequency range ({{gaps|300|Hz}} to {{gaps|3500|Hz}}). They replaced [[direct current]] (D.C.) signaling on toll [[trunk line]]s 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]] pulses, and on-hook/off-hook signaling. Common frequencies for this purpose were {{gaps|1600|Hz}}, {{gaps|2000|Hz}}, {{gaps|2400|Hz}}, {{gaps|2600|Hz}}, and {{gaps|3700|Hz}}, the latter being just outside the voice range.<ref name="weaver"/> 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]] methods, which do not require additional bandwidth for control of the network and benefit from a single amplification facility for speech and signaling.<ref name="Breen">{{cite journal |title=Signaling Systems for Control of Telephone Switching |last1=Breen|first1=C. |last2=Dahlbom|first2=C.A. |date=1960-05-17 |journal=Bell System Technical Journal |volume=39|issue=6 |pages=1381–1444 }}, p.1400</ref>
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 {{gaps|2600|Hz}} with a variety of lower frequencies in a compelled mode. Lower frequencies, such as {{gaps|1600|Hz}}, were used by the 1940s in composite signaling systems, however, the higher signal energy in speech at {{gaps|1600|Hz}} presented technical problems in protection against false operation.<ref>{{cite journal |title=Automatic Toll Switching Systems |last=Shipley|first=F.F. |date=1952-05-12 |journal=The Bell System Technical Journal |volume=31|issue=5 |pages=860–882 }}, p.869</ref> 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]]''.<ref>{{cite journal |title=Control Architecture |last1=Fluhr|first1=Z.C. |last2=Porter|first2=P.T. |date=1978-02-15 |journal=The Bell System Technical Journal |volume=56|issue=1|pages=43–69 }}, p.56</ref> In this instant, the call would be disconnected prematurely, and the trunk placed in idle condition. The [[Bell System]] 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.<ref name=Breen/> The condition was also often mitigated by narrow-band [[notch filter]]s during the seized line state.<ref name="zucker">{{cite journal |journal=Bell System Technical Journal |title=Time Domain Analysis and Synthesis of Notch Filters |date=1973-07-25 |last=Zucker|first=H. |volume=53|issue=2 |page=283 }}</ref>
The discovery of this phenomenon by technology-curious individuals in the 1960s, led to the abuse by [[phreaking]], a subculture that exploited the technology to explore national and international telephone networks and place cost-free long-distance telephone calls.<ref>{{cite book |title=Exploding the Phone: The Untold Story of the Teenagers and Outlaws who Hacked Ma Bell |last=Lapsley|first=Phil |date=2013-11-02 |publisher=Grove Atlantic |isbn=9780802120618 }}</ref>
Combating abuse, and improving communications, the telephone industry transitioned to [[out-of-band signaling]] systems, such as [[Signalling System 7]] (SS7), by the 1980s. This separated the voice and signaling [[Communication channel|channels]], making it impossible to generate control signals in the voice bearer channel. The development of the [[T-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.<ref>{{cite journal |title=History and Description of a New Signaling System |last1=Dahlbom|first1=C.A. |last2=Ryan|first2=J.S. |date=1977-05-07 |journal=The Bell System Technical Journal |volume=57|issue=2 |page=225 }}</ref>
In the 1970s, [[multi-frequency signaling]] systems came into use for [[IDDD|international direct distance dialing]] (IDDD) which used the frequency of {{gaps|2600|Hz}} in [[line signaling]] in two-frequency mode with {{gaps|2400|Hz}}, for line seizure during call setup, and for tear-down.
{{gaps|2600|Hz}} signaling was a standard for many international signaling systems,<ref>{{citation |title=Specifications of Signalling System R1 Line Signalling |work=ITU-T Recommendation Q.313 |publisher=International Telecommunication Union |year=1983 }}</ref> such as the ''Regional System R1''.<ref>{{cite book |last=Pearce|first=J. Gordon |title=Telecommunications Switching |publisher=Springer |date=2013 |page=243 }}</ref>
The prominent application of the {{gaps|2600|Hz}} frequency in telecommunications world-wide inspired the name of many [[hacker]] communities and publications, such as ''[[2600: The Hacker Quarterly]]'',<ref>{{cite book |last=Goldstein|first=Emmanuel |date=2008-07-28 |title=The Best of 2600: A Hacker Odyssey |publisher=Wiley & Sons |isbn=978-0470294192 }}</ref> and the late 20th-century [[counterculture]] ''2600''.
==See also== * [[Blue box]] * [[Signaling System No. 5]]
==References== {{Reflist}}
{{Telsigs}}
[[Category:Phreaking]] [[Category:Telecommunication protocols]] [[Category:Telephony signals]]