telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

, single-frequency signaling (SF) is

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 pa ...

in which

dial pulse Pulse dialing is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name ...

s or supervisory signals are conveyed by a single

voice-frequency A voice frequency (VF) or voice band is the range of audio frequencies used for the transmission of speech. Frequency band In telephony, the usable voice frequency band ranges from approximately 300 to 3400 Hz. It is for this reason that ...

tone in each direction. SF and similar systems were used in 20th-century

carrier system A carrier system is a telecommunications system that 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 principal voice frequency or ...

s. An SF signaling unit converts DC signaling (usually, at least in

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-d ...

circuits, E&M signaling) to a format (characterized by the presence or absence of a single voice-frequency tone), which is suitable for transmission over an AC path, ''e.g.'', a carrier system. The SF tone is present in the

on-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...

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 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 number ...

or other DC dialing mechanism. The SF tone may occupy a small portion of the user

data channel A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for informat ...

spectrum, ''e.g.,'' 1600 Hz or 2600 Hz (SF "

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 ...

)". There may be a

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 opposite of a band-pass filter. A notch filter is a ...

at the precise SF

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 ...

, either filtering the circuit at all times or only when the circuit is

off-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...

, to prevent the user from inadvertently disconnecting a

call Call or Calls may refer to: Arts, entertainment, and media Games * Call, a type of betting in poker * Call, in the game of contract bridge, a bid, pass, double, or redouble in the bidding stage Music and dance * Call (band), from Lahore, Paki ...

if the users voice has a sufficiently strong spectral content at the SF frequency, a

falsing In telecommunications, falsing is when a decoder assumes that it is detecting a valid input even though one is not present. This is also known as a false decode. This article will discuss analog circuits used before digital signal processing. Exam ...

condition known as ''talk-off''. Notoriously, this property was exploited by

blue box A blue box is an electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voice circuits. ...

ers and other toll fraudsters. The SF tone may also be just outside the user voice band, ''e.g.,'' 3600 Hz. The

Defense Data Network The Defense Data Network (DDN) was a computer networking effort of the United States Department of Defense from 1983 through 1995. It was based on ARPANET technology. History As an experiment, from 1971 to 1977, the Worldwide Military Command an ...

(DDN) transmitted DC line signaling pulses or supervisory signals, or both, over carrier channels or

cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...

pairs on a

four-wire circuit In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path. The four-wire circuit gets its ...

basis using a 2600 Hz

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' ...

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. In-Band Single-Frequency Signaling
by A. Weaver and N. A. Newell, Bell System Technical Journal, June 7, 1954 It declined in the 1970s due to the adoption of

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 System ...

, and was largely abandoned late in the century in favor of

common-channel signaling In telecommunication, common-channel signaling (CCS), or common-channel interoffice signaling (CCIS), is the transmission of control information ''( signaling)'' via a separate channel than that used for the messages, The signaling channel usually ...

References

* {{telecommunications Telephony signals