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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). DTMF 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 dtmf 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 dialThe 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: dial1.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 multifrequency dialIntroduced in 1964 by AT&T, Dual Tone Multifrequency 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 dtmf 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.
For example, if you press the number 1, both a 697 Hz and 1209 Hz tone will be sent. The dtmf 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. protocolsDTMF 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 usesDTMF is used in other applications as well. In voice mail systems, automated attendants and answering machines, DTMF 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. 
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