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

, multi-frequency signaling (MF) is a type of

signaling A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. ...

that was introduced by the

Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the AT&T Corporation, American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America fo ...

after

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

. It uses a combination of audible tones for address (

telephone number A telephone number is the address of a Telecommunications, telecommunication endpoint, such as a telephone, in a telephone network, such as the public switched telephone network (PSTN). A telephone number typically consists of a Number, sequ ...

) transport and supervision signaling on

trunk line In telecommunications, trunking is a technology for providing network access to multiple clients simultaneously by sharing a set of circuits, carriers, channels, or frequencies, instead of providing individual circuits or channels for each clie ...

s between central offices. The signaling is sent ''

in-band 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 ...

'' over the same channel as the bearer channel used for voice traffic. Multi-frequency signaling defines electronic signals that consist of a combination of two audible frequencies, usually selected from a set of six frequencies. Over several decades, various types of MF signaling were developed, including national and international varieties. The

CCITT The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunicat ...

standardization process specified the American Bell System version as ''Regional Standard No. 1'', or Signalling System R1, and a corresponding European standard as Signalling System R2. Both were largely replaced by digital systems, such as

Signalling System 7 Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in the 1970s that is used to setup and teardown telephone calls on most parts of the global public switched telephone network (PSTN). The protocol also performs ...

, which operate out-of-band on a separate data network. Because of the in-band transmission characteristic of MF signaling, the systems proved vulnerable to misuse and fraud by

phone phreaking Phreaking is a slang term coined to describe the activity of a culture of people who study, experiment with, or explore telecommunication systems, such as equipment and systems connected to public telephone networks. The term ''phreak'' is a sen ...

with devices such as a

blue box A blue box is an Electronics, 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 voi ...

. Multifrequency signaling is a technological precursor of

dual-tone multi-frequency signaling Dual-tone multi-frequency (DTMF) signaling is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and Automatic telephone exchange, switching center ...

(DTMF, ''Touch-Tone''), which uses the same fundamental principle, but was used primarily for signaling address information and control signals from a user's telephone to the wire-center's Class-5 switch. DTMF uses a total of eight frequencies.

Operation

Digits are represented by two simultaneous tones selected from a set of five (MF 2/5), six (MF 2/6), or eight (MF 2/8) frequencies. The frequency combinations are played, one at a time for each digit, to the remote multi-frequency receiver in a distant

telephone exchange A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...

. MF is used for signaling in

trunking In telecommunications, trunking is a technology for providing network access to multiple clients simultaneously by sharing a set of circuits, carriers, channels, or frequencies, instead of providing individual circuits or channels for each clie ...

applications. Using MF signaling, the originating telephone switch sends a start signal to seize the line, taking the circuit off-hook. The terminating office acknowledges the seizure with a ready state by responding with a wink start signal, which is a momentary off-hook condition. The originating office then sends address information to the terminating switch. In R1 MF signaling this address information normally is a KP tone, the numeric digits of the destination number, and an ST tone to indicate the end of the address. Other information may also be added, such as the caller's number, using KP2 as a delimiter. MF is a type of

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 ...

. Depending on the type and configuration of switching equipment, it may or may not be audible to the telephone user, but the technology was vulnerable to abuse with a method called

phreaking Phreaking is a slang term coined to describe the activity of a culture of people who study, experiment with, or explore telecommunication systems, such as equipment and systems connected to public telephone networks. The term ''phreak'' is a se ...

with a

which generates the tones required to control remote telephone switches.

Multi-frequency signals

The

published the following standards for MF tone timing: * KP (110 ms) * Digits 0-9, ST: 55 ms The interval between digits is the same as the digit duration (55 ms) These standards are, for the most part, still in place where MF signaling is in use in legacy exchanges. MF signaling is still used in North America for inter-office signaling, although it is increasingly rare.

Demise

In-band signaling fell into disfavor in the

public switched telephone network The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists o ...

(PSTN) as electronic switching systems displaced electro-mechanical switching systems, but legacy offices may still exist in some countries that are still using some electromechanical and other legacy switching equipment. Out-of-band

Common Channel Signaling Common may refer to: As an Irish surname, it is anglicised from Irish Gaelic surname Ó Comáin. Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common, a central public park in Boston, Massachusetts * Cambridge Commo ...

(CCS) became nearly universal by the end of the 20th century in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. Benefits include higher connection establishment rate and better fraud security. Most 911 Public Safety Answering Points (PSAPs) use the MF format to identify the calling party to the PSAP when processing calls from Mobile Telephone Switching Offices (MTSOs) and landline telephone exchanges. This is based on an earlier system which used MF to identify the calling party to a

feature group A feature group, in North American telephone industry jargon, is most commonly used to designate various standard means of access by callers to competitive long-distance services. They defined switching arrangements from local exchange carrier ...

'D' (101xxxx) alternate long-distance provider.

References

External links

"Speeding Speech"
a 1950s

film, depicts a 2-1-1 long-distance operator manually entering a number on an MF keypad just prior to the introduction of

direct distance dialing Direct distance dialing (DDD) is a telecommunications service in North America by which a caller may call any other subscriber outside the local calling area without operator assistance, DDD was introduced in the United States in 1951, on a tri ...

. The keypad, visible at 0:01:41 and 0:05:20, has two columns of five digits plus KP (key pulse) and ST (start).

Details of MF signaling from Bell Laboratories Record. Pages 221-225 {{Telsigs Telephony signals pt:MF

Operation

Multi-frequency signals

Demise

See also

References

External links