Single-frequency signaling
Encyclopedia
Single-frequency signaling (SF) is line signaling
(in telephony
) in which dial pulses or supervisory signals
are conveyed by a single voice-frequency tone in each direction. SF and similar systems were used in 20th century carrier system
s.
An SF signaling unit converts DC
signaling (usually, at least in long distance 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
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 dialing mechanism.
The SF tone may occupy a small portion of the user
data
channel
spectrum, e.g., 1600 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 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 Hz.
The Defense Data Network
(DDN) transmitted DC line signaling
pulses or supervisory signals, or both, over carrier
channels or cable
pairs on a four wire circuit basis using a 2600 Hz 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. 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
.
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 party and vice versa...
(in telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
) in which dial pulses or supervisory signals
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 party and vice versa...
are conveyed by a single voice-frequency tone in each direction. SF and similar systems were used in 20th century carrier system
Carrier system
In telecommunication, a carrier system is a multichannel telecommunications system in which a number of individual channels are multiplexed for transmission...
s.
An SF signaling unit converts DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
signaling (usually, at least in long distance circuits, E&M signaling) to a format (characterized by the presence or absence of a single voice-frequency tone), which is suitable for transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
over an AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
path, e.g., a carrier system
Carrier system
In telecommunication, a carrier system is a multichannel telecommunications system in which a number of individual channels are multiplexed for transmission...
. The SF tone is present in the on-hook
On-hook
In telephony, the term on-hook has the following meanings:# The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook...
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
Rotary dial
The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes. Almon Brown Strowger filed the first patent...
or other DC dialing mechanism.
The SF tone may occupy a small portion of the user
User (telecommunications)
In telecommunications, a user is a person, organization, or other entity that employs the services provided by a telecommunication system, or by an information processing system, for transfer of information....
data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or 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...
spectrum, e.g., 1600 Hz or 2600 Hz (SF "in-band signaling
In-band signaling
In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...
)". There may be a notch filter at the precise SF frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, either filtering the circuit at all times or only when the circuit is off-hook
Off-hook
In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...
, to prevent the user from inadvertently disconnecting a call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
if the users voice has a sufficiently strong spectral content at the SF frequency, a falsing
Falsing
In telecommunications, falsing describes a decoder assuming that it is detecting a valid input when one is not present. This is also known as a false decode...
condition known as talk-off. Notoriously, this property was exploited by blue box
Blue box
An early phreaking tool, the blue box is an electronic device that simulates a telephone operator's dialing console. It functioned by replicating the tones used to switch long-distance calls and using them to route the user's own call, bypassing the normal switching mechanism...
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
Defense Data Network
The Defense Data Network was a computer networking effort of the United States Department of Defense from 1983 through 1995.-History:In 1975, the Defense Communication Agency took over operation of the ARPANET as it became an operational tool instead of a research project. In 1983, plans for a...
(DDN) transmitted DC line signaling
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 party and vice versa...
pulses or supervisory signals, or both, over carrier
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
channels or cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
pairs on a four wire circuit basis using a 2600 Hz 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. It declined in the 1970s due to the adoption of T-carrier
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
, and was largely abandoned late in the century in favor of common channel signaling
Common Channel Signaling
In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...
.