In telephony Multi-Frequency (MF) is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a Multi-Frequency Sender, to be received by a Multi-frequency receiver in a distant telephone exchange. MF was used for trunk signaling, and is a precursor of modern DTMF "touch" tones now used for subscriber signaling.

Using MF signaling, the originating telephone switching office would send 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 would send the destination digits to the terminating switch. What makes MF and in-band signaling different than SS7 is that the routing digits are out-pulsed in MF format in the same 64 kilobit channel used by the voice audio connection. The user dialing can not 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 over to the user in order to communicate the voice, modem or fax data across that same 64kb 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 (phreaking) and features such as caller I.D.; however, some 911 PSAPs (Public Safety Answering Point) still use the MF format to process calls from MTSOs (Mobile Telephone Switching Office) and land telephone offices. Other countries may still use a version of in-band signaling.

MF signaling includes R2 signalling, R1 [ ITU-T Recommendation Q.310-Q.332] - Specification of Signalling System R1] (in North America), and Signaling System No. 5.

For details on R1, see blue box.


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