In telephony

Telephony

In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....

, multi-frequency signaling (MF) is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code

Two-out-of-five code

In telecommunication, a two-out-of-five code is an m of n code that provides exactly ten possible combinations, and thus is popular for representing decimal digits using five bits...

 for transmission from a multi-frequency sender, to be received by a multi-frequency receiver

Multi-Frequency Receiver

Multi-Frequency signalling, , is similar to the European version, CCITT Signaling System 5, . The original format was five tones used in pairs. This later evolved to six tones...

 in a distant telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

. MF was used for signaling in trunking

Trunking

In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...

 applications, and is a precursor of modern DTMF signaling (TouchTone) now used for subscriber signaling.

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

In-band signaling

In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...

 systems differ from Signaling System 7 (SS7) in that the routing digits are out-pulsed in MF format in the same voiceband

Voiceband

In electronics, voiceband means the typical human hearing frequency range that is from 20 Hz to 20 kHz. In telephony, it means the frequency range normally transmitted by a telephone line, generally about 200–3600 Hz. Frequency-division multiplexing in telephony normally uses...

 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

Common Channel Signaling

In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...

 is nearly universal today in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. Benefits include higher connection establishment rate, more fraud security (antiphreaking measures), and features such as Caller ID

Caller ID

Caller ID , also called calling line identification or calling number identification or Calling Line Identification Presentation , is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol applications, that transmits a caller's number to...

; however, some 911 Public Safety Answering Points (PSAPs) still use the MF format to process calls from Mobile Telephone Switching Office

Mobile Telephone Switching Office

The Mobile Telephone Switching Office is the mobile equivalent to a PSTN Central Office. The MTSO contains the switching equipment or Mobile Switching Center for routing mobile phone calls. It also contains the equipment for controlling the cell sites that are connected to the MSC.The systems...

s (MTSOs) and land telephone offices. Other countries may still use a version of in-band signaling.

MF signaling includes R2 signaling

R2 signalling

R2 is a 1950s- and 1970s-era channel-associated-signalling signalling protocol used outside of the former Bell System to convey information along a telephone trunk between two telephone switches in order to establish a single telephone call along that trunk....

, R1 (in North America), and Signaling System No. 5

Signaling System No. 5

CCITT5 was a multi-frequency telephone signalling system in use from the 1970s for International Direct Distance Dialing . It was sometimes nicknamed "Atlantic Code" because the first IDD connections between Europe and North America used it....

.