2. The Telephone and Telephone Line Chapter 2

3. Overview of a Telephone System Telephone set’s major parts Transmitter Converting sound wave to electrical wave and sending it through a communication media Receiver Converting electrical to sound wave Dial pad Used to dial a called party by the calling party Switch hook Allowing to connect the phone to the central office Off-Hook: Establishing a closed circuit; we hear a dial tone (-48 to -52 V) On-Hook: Open circuit; Idle Ringer Notifying the called party – The called party will hear a Ring back tone

4. Dialing Mechanisms Rotary dial pads Invented in late 1800 by Strowger Uses pulses of specific lengths to indicate each number The switch counts the number of pulses generated Touchtone dial pads Introduced in 1972 Uses the dual-tone multi-frequency – DTMF – system Each number is represented by two specific frequencies Much faster than rotary pulses 1,209 Hz 1,336 Hz 1,477 Hz 697 Hz123 770 Hz456 852 Hz789 941 Hz*0#

5. Sound Wave Examples Each signal is represented by x(t) = sin (2  f.t) A dual tone signal with f1 and f2 is represented by x(t) = sin (2  f1.t) + sin (2  f2.t) f = 5Kz f = 1Kz

6. Ringing Ring tone Signal generated on the telephone of the called party 70V AC, 20 Hz ring voltage from the switch to the telephone Ring back Heard by the calling party Generated by the switch

7. Connecting Telephone

8. Telephone Signals Off-hook The voltage across Tip and Ring reads -50V DC and dial tone is present On-hook (idle) -48 to -52 V present on Ring and ground -5 V present on Tip and ground Upon Ringing 70V AC, 20 Hz ring voltage from the switch on the Ring wire to the telephone

9. TALKING Call Request Call Accepted Conversation Starts Call Termination Basic Call Processing Time

10. Global Telephone Network