Explain what is DTMF signalling.

Dual Tone Multi Frequency (DTMF) was initially introduced in 1963 with 10 buttons into Western Electric 1500 -type telephones. Dual Tone Multi Frequency was originally termed as Touch- Tone. It is a more efficient means than dial pulsing for transferring telephone numbers by a subscriber's location to central office switching the machine. It is an easy two-of-eight encoding scheme, here each digit is represented through the linear addition of two frequencies. This is strictly for signaling among a subscriber's location and message switching center or the nearest telephone office. Dual Tone Multi Frequency is sometimes confused along with other two-tone signaling system termed as multi frequency signaling (Mf) that is a two-of-six code designed to be utilized only to convey information among two electronic switching machines.

Figure demonstrates the four-row-by-four column keypad matrix used along with a DTMF keypad. As in the figure demonstrates, the keypad is comprised of 16 keys and eight frequencies. However, mainly household telephones are not equipped along with the special-purpose keys located into the fourth column (that is the A, B, C and D keys). Hence, most household telephones in fact use two-of-seven tone encoding scheme. The four vertical frequencies which are termed as the low group frequencies are 697 Hz, 852 Hz and 941 Hz and the 4-horizontal frequencies which are termed as the high group frequencies are 1209 Hz, 1336 Hz, 1477 Hz and 1633 Hz. The frequency tolerance of the oscillators is + .5 percent. As demonstrated in figure, the digits 2 by 9 can also be used to show 24 of the 26 letters (as Q and Z are omitted). These letters were originally used to specify one local telephone exchange from other.

Figure: DTMF Keypad Layout