BASIC TELECOMMUNICATIONS

BASIC TELECOMMUNICATIONS The components of a phone and DTMF vs. PULSE

BASIC TELECOMMUNICATIONS • While POTS provides limited features, low bandwidth and no mobile capabilities, it provides greater reliability than other telephony systems (mobile phone, VoIP, etc.). • Many telephone service providers attempt to achieve "dial-tone availability" more than 99.999% of the time This is equivalent to having no dial-tone available for less than five minutes each year

BASIC TELECOMMUNICATIONS • Today the function of the POTS local loop presented to the customer for connection to telephone equipment is practically unchanged and remains compatible with old Pulse dialing telephones, even ones dating back to the early 20th century. • Often considered second in impact only to the printing press for revolutionizing human communication, the telephone made it possible to connect people to others without dependency on the postal service.

BASIC TELECOMMUNICATIONS • The basic design of the telephone has not changed much since its initial invention. 50’s – 60’s – 70’s 80’s - 90’s – 2000’s

BASIC TELECOMMUNICATIONS Many calling features became available to POTS subscribers after computerization of telephone exchanges during the 1970s and 1980s. Additional services include: • Voicemail • Caller ID • Call waiting • Speed dialing • Conference call (three-way calling) • Enhanced 911 • Centrex

BASIC TELECOMMUNICATIONS The subscriber loop typically carries a "load" of about 300 Ohms, and does not pose a threat of electrocution to human beings (although shorting the loop can be felt as an unpleasant sensation).

BASIC TELECOMMUNICATIONS How might you as a telephone technician be exposed to a an electric shock on a voice line?

BASIC TELECOMMUNICATIONS Here is a disassembled rotary phone showing the handset, hook switch, dial assembly, and the base, the majority of which is comprised of the bell (ringer) assembly. Handset Rotary dial Hook switch Ringer coils and ringer bells

BASIC TELECOMMUNICATIONS • The mechanical ringer in a rotary phone receives the 90VAC, 20Hz from the CO which is passed through a 470nf capacitor to filter the DC talk voltage. • Then it energizes two coils that causes a rocker arm to swing back and forth against two bell assemblies.

BASIC TELECOMMUNICATIONS • The ringer circuits in the modern telephones have the same basic idea, but the coil controlled bell is replaced by modern electronic ringing chip and small speaker. • The capacitor is still used in series with a ring IC input to allow only AC pass to the ring chip.

BASIC TELECOMMUNICATIONS • Speak into the mouthpiece • The acoustic vibrations push the metallic coating slightly closer to the electrode, resulting in variations in voltage and therefore a speedy conversion from acoustic to electric energy. • The electric pulses are conveyed through a wire to the speaker on the other end, where electric pulses are converted into acoustic energy again.

BASIC TELECOMMUNICATIONS The telephone operates on simple principles. A telephone mouthpiece contains a thin metallic coating separated from an electrode by a thin barrier (today we use plastic) which connects to a wire carrying an electric current. FRONT OF THE MOUTH PIECE BACK OF THE MOUTH PIECE

BASIC TELECOMMUNICATIONS The receiver is just a small speaker that converts the electrical pulses into acoustical energy. FRONT OF SPEAKER BACK OF SPEAKER

BASIC TELECOMMUNICATIONS • A dial tone is the signal you hear on a telephone before you dial a phone number. In most instances, the signal disappears after you dial the first number • The dial tone was widely established as telephone systems became automated, starting around 1947 • Prior to that, a person had to talk with a switchboard operator in order to connect with another party

BASIC TELECOMMUNICATIONS The dial tone was created so the caller knew the phone was indeed connected and working before making the call. • The early dial tone resembled a buzzing sound and was created by a motor driven machine. Today the sound of the dial tone in North America is that of two interfering frequencies of 350 and 440 Hz (Hertz).

BASIC TELECOMMUNICATIONS • Other common telephone tones include the disconnect tone that the telephone makes when the other party hangs up, the busy signal, and the loud tone that the phone makes when the receiver is off the hook. • There are also several special information tones. These are the three-beep tones used by the phone company prior to the recorded messages that explain why the call did not go through.

BASIC TELECOMMUNICATIONS • Pulse dialing is associated with rotary phones, though it is still available on all phone sets connected to land lines. • The standard pulse rate in the U.S. is 10 PPS (Pulses Per Second) with a make/break ratio of 60/40 • If the number 0 is dialed a pulse train of 10 pulses is generated by the opening (breaking) and closing (making) of the switch contacts

BASIC TELECOMMUNICATIONS • Each pulse is 100ms long with 60ms break and 40ms make • The breaking and making (60/40) of the switch contacts opens and closes the local loop to the CO switch, which in turn translates each pulse train back to the digit being dialed. • After the last pulse is transmitted the telephone remains in the off-hook condition until another digit is dialed or the phone is hung up and returned to the on-hook condition.

BASIC TELECOMMUNICATIONS The number is dialed The central office switching equipment translates the pulses

BASIC TELECOMMUNICATIONS • DTMF was introduced by AT&T in 1964 • It was developed to allow rapid transmission across microwave links and use by computer controlled exchanges. • DTMF is commonly referred to as “Touch Tone Dialing” which utilizes a keypad to generate a single tone for a digit.

BASIC TELECOMMUNICATIONS • The DTMF touch tone keypad is an analog in-band signaling technique • In-band signaling uses signals that are audible to the caller • Out-of –band signaling uses signals that are not audible to the caller

BASIC TELECOMMUNICATIONS The tones are created by combining two different frequencies for each digit of the keypad

BASIC TELECOMMUNICATIONS Each column and row of the keys are assigned a frequency and each single digit tone is 100ms long with a break/make ratio of 60ms/40ms just like a rotary dial.

BASIC TELECOMMUNICATIONS Modern phones allow the user to select tone (DTMF) or pulse (Rotary), if you select pulse you will hear the pulse trains for each number Dialed.

BASIC TELECOMMUNICATIONS