In telephony, multi-frequency signaling (MF) is an outdated, in-band signaling technique. It used a series of tones to represent the numbers in a phone number, allowing them to be sent over the same links as the phone call itself (thus "in-band"). Numbers were represented two simultaneous tones selected from a set of five frequencies in a two-out-of-five code. The tones were played, one at a time for each number, to the remote multi-frequency receiver in a distant telephone exchange. MF was used for signaling in trunking applications, and is a precursor of modern DTMF signaling (TouchTone) now used for subscriber signalling. TouchTone uses eight frequencies.
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.
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 user dialing 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.
Out-of-band Common Channel Signaling is nearly universal today in the United States. Benefits include higher connection establishment rate, more fraud security (antiphreaking measures), and features such as Caller ID; however, some 911 Public Safety Answering Points (PSAPs) still use the MF format to process calls from Mobile Telephone Switching Offices (MTSOs) and land telephone offices. Other countries may still use a version of in-band signaling.
MF signaling includes R2 signaling, R1 (in North America), and Signaling System No. 5.