Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching center. The version of DTMF that is used in push-button telephones for tone dialing is known as Touch-Tone. It was developed by Western Electric and first used by the Bell System in commerce, using that name as a registered trademark. DTMF is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4. Other multi-frequency systems are used for internal signaling within the telephone network. Introduced by AT&T in 1963,[1] the Touch-Tone system using the telephone keypad gradually replaced the use of rotary dial and has become the industry standard for landline service.

Contents [hide] 1 Multifrequency signaling 2 #, *, A, B, C, and D 3 Binary data transfer 4 Keypad 5 Special tone frequencies 6 See also 7 References 8 Further reading 9 External links

[edit] Multifrequency signaling Prior to the development of DTMF, numbers were dialed on automated telephone systems by means of pulse dialing (Dial Pulse or DP in the U.S.) or loop disconnect (LD) signaling, which functions by rapidly disconnecting and re-connecting the calling party's telephone line, similar to flicking a light switch on and off. The repeated interruptions of the line, as the dial spins, sounds like a series of clicks. The exchange equipment interprets these dial pulses to determine the dialed number. Loop disconnect range was restricted by telegraphic distortion and other technical problems[which?], and placing calls over longer distances required either operator assistance (operators used an earlier kind of multi-frequency dial) or the provision of subscriber trunk dialing equipment. Multi-frequency signaling (see also