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Signaling System No.7 Protocol Architecture And Sevices part 2

Subscriber Signaling Subscriber signaling takes place on the line between the subscribers and their local switch. Most subscribers are connected to their local switch by analog subscriber lines as opposed | Subscriber Signaling Subscriber signaling takes place on the line between the subscribers and their local switch. Most subscribers are connected to their local switch by analog subscriber lines as opposed to a digital connection provided by an Integrated Services Digital Network ISDN . As a result subscriber signaling has evolved less rapidly than network signaling. Subscriber signals can be broken down into the following four categories Address Signals Supervisory Signals Tones and Announcements Ringing Address Signals Address signals represent the called party number s dialed digits. Address signaling occurs when the telephone is off-hook. For analog lines address signaling is either conveyed by the dial pulse or Dual-Tone Multiple Frequency DTMF methods. Local switches can typically handle both types of address signaling but the vast majority of subscribers now use Dual-Tone Multi Frequency DTMF also known as touch-tone. The precursor to DTMF was dial pulse which is also known as rotary dialing. In rotary dialing the address signals are generated by a dial that interrupts the steady DC current at a sequence determined by the selected digit. The dial is rotated clockwise according to the digit selected by the user. A spring is wound as the dial is turned when the dial is subsequently released the spring causes the dial to rotate back to its original resting position. Inside the dial a governor device ensures a constant rate of return rotation and a shaft on the governor turns a cam that opens and closes switch contact. The current flowing into the telephone handset is stopped when the switch contact is open thereby creating a dial pulse. As the dial rotates it opens and closes an electrical circuit. The number of breaks in the string represents the digits one break for value 1 two breaks for value 2 and so on except for the value of 0 which is signaled using ten breaks . The nominal value for a break is 60 ms. The breaks are spaced with make intervals of .