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58 SUBSCRIBER SIGNALINGFigure 3.2-1. Components in a telephone, shown in the on-hook state: (a) dial telephone and (b)keypad telephone. C, Capacitor; KP/O, keypad and oscillator; CS, cradle switch; DS, dial switch;LC, line circuit; RCV, receiver; TR, transmitter; R, resistor; T, transformer; RR, ringer.When a subscriber starts to use the telephone, he lifts the handset out of its cradle.In this state, which is known as off-hook, switch (CS) connects transmitter (TR) andreceiver (RCV) to the subscriber line.When the telephone is off-hook, direct current can flow in the subscriber line.When the telephone is on-hook, capacitor (C) blocks direct current. At the exchange,line circuit (LC) determines the supervision state of the telephone from the presenceor absence of direct current in the line.3.2.3 Address SignalsAddress signaling takes place while the telephone is off-hook (CS closed). There aretwo types of address signals (digits). Today’s local exchanges handle both signaltypes.Dial-Pulse (DP) Address Signals. In early telephones, the address signals weregenerated by dials [1,2]. The dial switch (DS) in Fig. 3.2-1(a) is linked mechanicallyto the dial. When the dial is at rest, DS is closed, and the telephone presents a path fordirect current (dc) between points A and B. When the dial—after having been rotated bythe subscriber—spins back to its rest position, DS opens and closes a number oftimes, producing a string of breaks inthedcpath.Thenumberofbreaksinastringrepresentsthe value of the digit: one break for value 1, two breaks for value 2, ...,ten