In telephony, multi-frequency signaling (MF) is a signaling system that was introduced by the Bell System after World War II. It uses a combination of tones for address (phone number) and supervision signaling. 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), now used for subscriber signalling. DTMF uses eight frequencies.
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.
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 in Hz:
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.
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. 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.