5.2.2 - Tone Dialling

Tone dialling, also called Dual Tone Multi-Frequency (DTMF) or ‘TouchTone’, is a relatively recent development. It is now accepted by virtually all exchanges and will eventually supersede pulse dialling altogether.

Each digit is represented by a combination of two tones from seven. Figure 3 shows a telephone key pad and the tones used.

	1	1	1	1
	2	3	4	6
	0	3	7	3
	9	6	7	3
	H	H	H	H
	z	z	z	z
697Hz
770Hz
852Hz
941Hz
Figure 3 Tone Dialling Matrix

Included in Figure 3 are the special keys A, B, C and D. These are used in some systems to perform special functions. They are included for completeness and will not be discussed here.

When a key is pressed the two tones which ‘cross’ at the key are transmitted to the exchange. E.g. dialling a 1 will send 697Hz and 1209Hz together, 2 will send 697Hz and 1336Hz and so on. Each digit is represented by a unique combination of tones. DTMF digits can be sent and detected more quickly than pulse digits. A detection time of only 50ms with an interdigit time of 50ms means that any digit can be dialled in 100ms. Compared to a pulse dialled 1 taking 800ms and pulse 0 taking 1.7sec.

DTMF dialling is therefore much faster than pulse dialling. The tones as they are transmitted give a distinctive chirping sound in the earpiece as they are sent.

5.2.3 - Ringing

To indicate the presence of an incoming call the exchange must make the telephones connected ring. This is done using a ring generator at the exchange that sends a 75Vac signal between 20Hz and 47Hz to the subscriber end. This triggers the ringer in the telephone.

Ringing is sent to the called party first, then the calling party. This allows the exchange ring generator to make most efficient use of the power required to ring the handsets. It also means the caller has ringing around 2 seconds before the calling party hears it. This can lead to the called party answering before the calling party even hears a ring tone if they are quick.

5.2.4 - Call Progress Tones

Another important function of the exchange is to inform the caller of the progress, success and failure of their call. this is performed using a series of tones. Each is different and indicates a specific condition. These are:

The precise nature of these tones varies between networks and systems. Modern equipment has replaced some with spoken messages giving a more friendly and understandable message.