Multi-frequency signaling

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 (TouchTone), 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 was used for signaling in trunking applications.

Using MF signaling, the originating telephone switching office sends a starting signal such as a seizure (off-hook) by toggling the AB bits. After the initial seizure, the terminating office acknowledges a ready state by responding with a wink (short duration seizure) and then goes back on-hook (wink start). The originating office sends the destination digits to the terminating switch.

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

MF signalling tones were vulnerable to being spoofed using blue boxes which generated a 2600 hertz tone to disconnect a toll call in progress and provided an operator-style MF keypad to dial another call using the same trunk.

MF and other in-band signaling systems differ from Signaling System 7 (SS7) in that the routing digits are out-pulsed in MF format in the same voiceband channel used for voice. The dialing user cannot detect these digits being out-pulsed because the audio connection is not established all the way to the user’s handset or device until after the connection is established with the terminating switch. Following a full connection, the same audio channel is connected to the user in order to communicate the voice, modem or fax data across that same 64-kbit channel previously used for the in-band MF signaling.

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