Multi-frequency signaling

In telephony, multi-frequency signaling (MF) is a type of signaling that was introduced by the Bell System after World War II. It uses a combination of audible tones for address (telephone number) transport and supervision signaling on trunk lines between central offices. The signaling is sent in-band over the same channel as the bearer channel used for voice traffic.

Multi-frequency signaling is a precursor of modern DTMF signaling (Touch-Tone), used for subscriber signaling. Both systems define signaling codes that consist of a combination of two frequencies. DTMF uses a total of eight frequencies, while the most common form of MF used a set of six frequencies. The individual MF and DTMF frequencies are different.

Over several decades, various types of MF signaling were developed. International versions followed, of which the CCITT standardization process specified the Regional Standard Number R1 (R1).[1] and a corresponding European standard as R2. Both were largely replaced by digital systems, such as Signalling System 7, which operate out-of-band on a separate data network.

Because of the inband transmission characteristic of MF signaling, the systems proved vulnerable to misuse and fraud by phone phreaking with devices such as a blue box.


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 applications.

Using MF signalling, the originating telephone switching office sends a starting signal to seize the line, taking the line off-hook. After the initial seizure, the terminating office acknowledges a ready state by responding with a wink (a momentary off-hook condition) and then goes back on-hook. This is called wink-start. The originating office then sends address information to the terminating switch. In R1 MF signalling this address information normally is a KP tone, the numeric digits of the destination number, and an ST tone to indicate the end of pulsing. Other information may also be added, such as the caller's number, using KP2 as a delimiter.

The R2 signalling suite, in use starting in the middle of the 20th century, included a compelled signalling version of multifrequency register signalling

MF is a kind of in-band signalling; depending on the switching equipment used it may or may not be audible to the telephone user. Tools such as a blue box allow telephone users to engage in phreaking; otherwise telephone users do not have a use for generating these tones.

Tone List

Tone List in Hz:

KP 1100 1700
KP2 1300 1700
Digit 1 700 900
Digit 2 700 1100
Digit 3 900 1100
Digit 4 700 1300
Digit 5 900 1300
Digit 6 1100 1300
Digit 7 700 1500
Digit 8 900 1500
Digit 9 1100 1500
Digit 0 1300 1500
ST 1500 1700

The Bell System published the following standards for MF tones:

KP (Twice as long as Digits) Digits 0-9 : 55ms ST (Same length as digits)

Space in 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.


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, such as in Russia and Italy, that are still using some electromechanical and other legacy switching equipment in the PSTN.[2]

Out-of-band Common Channel Signaling (CCS) became nearly universal by the end of the 20th century in the United States. 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.[3] This is based on an earlier system which used MF to identify the calling party to a feature group 'D' (101xxxx) alternate long distance provider.

See also


  1. Pearce, J. Gordon (2013). Telecommunications Switching. Springer. p. 243.
  2. "In-Band Signaling in the former Soviet Union". Binary Revolution Forums. Retrieved 2019-08-08.
  • "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. The keypad, visible at 0:01:41 and 0:05:20, has two columns of five digits plus KP (key pulse) and ST (start).
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