Dual-Tone Multi-Frequency

Signalling is used to tell a local switching system that someone wants to place a call, give instructions to the public switched telephone network (PSTN) on how the call should be connected and tell the called party that a call is waiting.

There are 4 categories of signalling (Supervisory signalling, addressing signalling, alerting signalling and in progress signalling). Dual-tone multi-frequency signalling (and pulse dialing) is categorized as addressing signalling since it issues an address for the call using the dialed digits.

(By the way... A standard, single line phone that uses rotary dialing is called a 500 phone set. A standard, single line phone that uses Dual-tone multi-frequency dialing, is called a 2500 phone set. A single line telephone containing electronics to provde features such as last number redial and speed dialing lists is called a feature phone.)

The Dial

The dial was added to the telephone when automated switching systems were invented. First, the rotary dial which would signal the central office by interrupting the current flow in the line. It is heard as a series of clicks to the calling party. The dial register in the central office would count the interruptions to interpret the dialed digits.

Listen to a rotary phone: OMGADDTHISLINK.wav ([an error occurred while processing this directive])

Or pick up your own phone, set it to pulse (P) dial, and dial a number.

(One can also simulate pulse dialing by *rapidly* pressing the switch hook. Press the hook 1x for the number 1, 2x for the number 2, and so on up to 10x for the number 0.)

Dual-tone multi-frequency dial

Introduced in 1964 by AT&T, Dual-tone multi-frequency dial uses a keypad with 12 or 16 buttons to send tones to the dial register of a central office. The central office will respond to these tones unless the call sends an star as the first digit indicating that the following Dual-tone multi-frequency tones are to activate Custom Local Area Signalling Services (CLASS) (Such as Caller ID blocking, Call Forwarding, Automatic Callback, etc.) The keypad is arranged in a 3 rows/3 or 4 column matrix, depending on the amount of buttons. Each column and row is assigned it's own frequency. So, when a button is pressed, 2 frequencies are sent, depending on their row and column.

Hz	1209	1336	1477	1633
697	1	2	3	A
770	4	5	6	B
852	7	8	9	C
941	*	0	#	D

For example, if you press the number 1, both a 697 Hz and 1209 Hz tone will be sent. The Dual-tone multi-frequency reciever in the central exchange decodes the combonation of tones to determine the dialed digit.

Why Two Tones?

Why are two tones used instead of just one? A dual tone design requires fewer tone detectors in the dial register of the central office than a single tone design would.

dual tone: 12/16 buttons X 2 tones each = 7/ 8 tones
single tone: 12/16 buttons X 1 tone each = 12/16 tones

A dual tone design also reduces the sensitivity requirement of the reciever which allows it to recognize tone distortion caused by abnormal conditions such as line noise on a local loop.

Protocols

Dual-tone multi-frequency tones must be sounded at least 40 ms in order for the register at the central office to recognize the tones wth a 60 ms pause between digits. (33% faster than rotary dial, which takes an average of 1.5s per digit to dial.)

Other Uses

Dual-tone multi-frequency is used in other applications as well. In voice mail systems, automated attendants and answering machines, Dual-tone multi-frequency may be used for routing (calls or messages), addressing, and remote access. Like the central office, these systems decode the combonation of tones with a reciever chip to determine the digit dialed.

1209 hz 22K	1209 Hertz tone One of the two tones that compose Dual-tone multi-frequency 1, 4, 7, and *.
1336 hz 22K	1336 Hertz tone One of the two tones that compose Dual-tone multi-frequency 2, 5, 8 and 0.
1477 hz 22K	1477 Hertz tone One of the two tones that compose Dual-tone multi-frequency 3, 6, 9 and #.
1633 hz 22K	1633 Hertz tone One of the two tones that compose Dual-tone multi-frequency A, B, C and D.
0697 hz 22K	697 Hertz tone (697 Hz) One of the two tones that compose Dual-tone multi-frequency 1, 2, 3 and A.
0770 hz 22K	770 Hertz tone (770 Hz) One of the two tones that compose Dual-tone multi-frequency 4, 5, 6 and B.
0852 hz 22K	852 Hertz tone One of the two tones that compose Dual-tone multi-frequency 7, 8, 9 and C.
0941 hz 22K	941 Hertz tone One of the two tones that compose Dual-tone multi-frequency *, 0, # and D.
dual-tone multi-frequency 0 9.4K	Dual-tone multi-frequency 0
dual-tone multi-frequency 1 9.4K	Dual-tone multi-frequency 1
dual-tone multi-frequency 2 9.4K	Dual-tone multi-frequency 2
dual-tone multi-frequency 3 9.4K	Dual-tone multi-frequency 3
dual-tone multi-frequency 4 9.4K	Dual-tone multi-frequency 4
dual-tone multi-frequency 5 9.4K	Dual-tone multi-frequency 5
dual-tone multi-frequency 6 9.4K	Dual-tone multi-frequency 6
dual-tone multi-frequency 7 9.4K	Dual-tone multi-frequency 7
dual-tone multi-frequency 8 9.4K	Dual-tone multi-frequency 8
dual-tone multi-frequency 9 9.4K	Dual-tone multi-frequency 9
dual-tone multi-frequency pound 9.4K	Dual-tone multi-frequency Pound
dual-tone multi-frequency star 9.4K	Dual-tone multi-frequency Star
dual-tone multi-frequency a 9.4K	Dual-tone multi-frequency A
dual-tone multi-frequency b 9.4K	Dual-tone multi-frequency B
dual-tone multi-frequency c 9.4K	Dual-tone multi-frequency C
dual-tone multi-frequency d 9.4K	Dual-tone multi-frequency D
800-999-9965 292K Jenn	Female Voice: Hello. Cisco Unity Messaging System. From a touch tone telephone, you may dial an extension at any time. For a directory of exntensions, press 4. Otherwise, please hold to leave a general message. Please wait while I transfer your call.

This Is a Recording
thisisarecording.com
July 6, 2005

Touch Tone Tunes
touchtonetunes.com
July 18, 2006

The Web Page You Have Reached
twpyhr.com
October 26, 1997 - July 9, 2005

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