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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 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. 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 with a blue box which generates the tones required to control remote telephone switches.

Multi-frequency signals

The Bell System 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 (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.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

• Signaling System No. 5
• Two-out-of-five code

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. The keypad, visible at 0:01:41 and 0:05:20, has two columns of five digits plus KP (key pulse) and ST (start).
• [1] Details of MF signaling from Bell Laboratories Record. Pages 221-225

References

1. Pearce, J. Gordon (2013). Telecommunications Switching. Springer. p. 243. https://books.google.com/books?id=fC4BCAAAQBAJ&pg=PA243 ↩

2. "In-Band Signaling in the former Soviet Union". Binary Revolution Forums. Archived from the original on 2021-05-16. Retrieved 2019-08-08.{{cite web}}: CS1 maint: bot: original URL status unknown (link) https://web.archive.org/web/20210516032817/https://www.binrev.com/forums/index.php?/topic/48787-in-band-signaling-in-the-former-soviet-union/ ↩

3. "NENA 03-002 v2: Enhanced MF Signaling, E9-1-1 Tandem to PSAP". http://www.nena.org/?page=EnhanceMF_Signaling ↩