View source for Single-frequency signaling

{{Short description|Telephony signaling technique}}
{{Redirect|SF signal|text=For the science fiction blog and fanzine, see ''[[SF Signal]]''.}}

In [[telephony]], '''single-frequency signaling''' ('''SF''' or '''SF tone''') is [[line signaling]] in which [[dial pulse]]s or supervisory signals are conveyed by a [[single-frequency tone]] in each direction in the [[voice band|voice-band]].  SF and similar systems were used in 20th-century [[carrier system]]s.

An SF signaling unit converts [[direct current|DC]] signaling (usually, at least in [[long-distance calling|long-distance]] circuits, [[E and M signaling|E&M]] signaling) to a format (characterized by the presence or absence of a single voice-frequency tone), which is suitable for [[transmission (telecommunications)|transmission]] over an [[alternating current|AC]] path, ''e.g.'', a carrier system. The SF tone is present in the [[on-hook]] or idle state and absent during the seized state. In the seized state, dial pulses are conveyed by bursts of SF tone, corresponding to the interruptions in dc continuity created by a [[rotary dial]] or other DC [[Dialling (telephony)|dialing]] mechanism. 

The SF tone may occupy a small portion of the [[User (telecommunications)|user]] [[data channel]] spectrum, ''e.g.,'' 1600&nbsp;Hz or [[2600 Hz]] (SF "[[in-band signaling]])". There may be a [[notch filter]] at the precise SF [[frequency]], either filtering the circuit at all times or only when the circuit is [[off-hook]], to prevent the user from inadvertently disconnecting a [[telephone call|call]] if the users voice has a sufficiently strong spectral content at the SF frequency, a [[falsing]] condition known as ''talk-off''. Notoriously, this property was exploited by [[blue box]]ers and other toll fraudsters. The SF tone may also be just outside the user voice band, ''e.g.,'' 3600&nbsp;Hz.  

The [[Defense Data Network]] (DDN) transmitted DC line signaling pulses or supervisory signals, or both, over [[carrier signal|carrier]] channels or [[Electrical cable|cable]] pairs on a [[four-wire circuit]] basis using a 2600&nbsp;Hz [[Signaling (telecommunications)|signal]] tone. The conversion into tones, or vice versa, is done by SF signal units.

SF was developed in the early 20th century and standardized in middle century. <ref>[http://www.alcatel-lucent.com/bstj/vol33-1954/articles/bstj33-6-1309.pdf In-Band Single-Frequency Signaling] by A. Weaver and N. A. Newell, Bell System Technical Journal, June 7, 1954</ref> It declined in the 1970s due to the adoption of [[T-carrier]], and was largely abandoned late in the century in favor of [[common-channel signaling]].

SF tones can transmit data that only consists of everyday [[Telephone number|telephone numbers]], numbers which only have 0-9 in them. How many pulses are transmitted in one time is part of the number that is dialed. One pulse is 1, two pulses are 2, three pulses are 3 and so on, with 10 pulses being 0.

== References ==
{{Reflist}}
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*{{FS1037C MS188}}
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{{telecommunications}}

[[Category:Telephony signals]]