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 ...

, multi-frequency signaling (MF) is a type of signaling that was introduced by the Bell System after

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...

. It uses a combination of audible tones for address (

telephone number A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...

) 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 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 Signalling System R2 is a signalling protocol for telecommunications that was in use from the 1960s mostly in Europe, and later also in Latin America, Asia, and Australia, to convey exchange information between two telephone switching systems for es ...

. 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 1975, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also perform ...

, 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 ...

with devices such as a

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. ...

. Multifrequency signaling is a technological precursor of dual-tone multi-frequency signaling (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. 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 cli ...

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 ...

with a

which generates the tones required to control remote telephone switches.

Multi-frequency signals

The Bell System published the following standards for MF tones: * KP (twice as long as digits) * Digits 0-9 : 55 ms * ST (same duration as digits) Space between the tones should be the size of the digits (0-9 ST) 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 (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 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 ...

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

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

. Benefits include higher connection establishment rate and better fraud security. Most 911

Public Safety Answering Point A public-safety answering point (PSAP), sometimes called public-safety access point, is a call center where emergency/non-emergency calls (like police, fire brigade, ambulance) initiated by any mobile or landline subscriber are terminated. It can ...

s (PSAPs) use the MF format to identify the calling party to the PSAP when processing calls from

Mobile Telephone Switching Office {{Refimprove, date=May 2015 The Mobile Telephone Switching Office (MTSO) is the mobile equivalent of a PSTN Central Office. The MTSO contains the switching equipment or Mobile Switching Center (MSC) for routing mobile phone calls. It also contains ...

s (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 Bell System 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 telecommunication service feature in North America by which a caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra d ...

. 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