Dual-tone multi-frequency signaling (DTMF)

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.

When we consider dialing numbers in the telephone now days commonly uses DTMF. Using DTMF (dual tone multi frequency) when we press a key on our phone, it generates two tones of specific frequencies. Then the signal is encoded as a pair of sinusoidal (sine wave) tones from the table below which are mixed with each other. The mixed frequency is traveled to the exchange.

DTMF

Advantages of DTMF compare to Rotary Dialing 

The main advantage is easy to access the phone when we are taking a call. But using rotary dialing hard to make a call.

When we are taking a call, in rotary dialing we have to rotate the numbers. Rotary dial pulses cannot pass through a central exchange. It is time consuming. Normally it takes an average of about 1.5 seconds per digit, or 15 seconds for dialing a 10-digit number. But DTMF tones can pass through a central exchange. It is recognized by the register at the central exchange. A DTMF tone must be present for a minimum of 40 ms. Between two consecutive digits, a pause of 60 ms is required. Total time needed for transmission of one digit is about 100 ms. Compare to rotary dialing it takes very much low of time .It is more efficient than rotary dialing.

Telephone Switching Concept

Manual Switch – In the telephone exchange all subscribers are connected to a central switch and it is manually operated by an operator. If a person want to call another person first he or she should call to telephone exchange. There is a person who can connect the two subscribers together by plugging in the exchange.

Strowger Switch - Every subscriber line ended at the Strowger switch with a relay, referred to as line relay. This switch had stepping relays that worked according to the dial pulses. Upon completing one digit, the first stepping relay would activate another stepping relay. Complete connection was achieved by serial activation of several stepping relays. The line relays were connected to an electromechanical device called line finder.

Crossbar switch - Crossbar switch employed centralized common control equipment which encompassed a group of equipment engaged in establishing call set up. The equipment was released and was available for setting up other calls once setting up phase of a call was over. Crossbar switch replaced the line finder with line scanner, to check when a telephone line connected to a line link frame was off hook. 

SPC switch – after the evolution of electronics, especially the invention of the transistor, SPC switches came into use. The SPC switch entirely replaced the electromechanical common control equipment of the cross bar switch with electronic circuits, while retaining the same switching matrixes. These systems employ memories based on RAM (Random Access Memory) technology, tape and disk storage. SPC switching system was space division switching systems.