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1. POTS= Plain Old Telephone Service2. PSTN=Public Switched Telephone

Network3. Each pair of communications wires

consist of a tip and ring4. Tip is positive, Ring is negative5. The POTS (residential) color code is:

GREEN, RED, BLACK, YELLOW6. Talk voltage is 48VDC7. Ring voltage is 90VAC8. The bandwidth of an analog phone

signal is 3000Hz

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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

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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.

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• The basic design of the telephone has not changed much since its initial invention.

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50’s – 60’s – 70’s 80’s - 90’s – 2000’s

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Many calling features became available to POTS subscribers after computerization of telephone exchanges during the 1970s and 1980s.

Additional services include:1. Voicemail 2. Caller ID 3. Call waiting 4. Speed dialing 5. Conference call (three-way calling) 6. Enhanced 911 7. Centrex

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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).

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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.

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Ringer coils andringer bells

HandsetRotary dial

Hook switch

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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.

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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.

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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.

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How does a phone actually work?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

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The receiver is just a small speaker that converts the electrical pulses into acoustical energy.

FRONT OF SPEAKER BACK OF SPEAKER

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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

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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).

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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.

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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

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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.

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The number is dialed

The central office switching equipment translates the pulses

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DTMF was introduced by AT&T in 1964It 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.

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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

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

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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.

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Modern phones allow the user to select tone (DTMF)or pulse (Rotary), if you select pulse you will hear thepulse trains for each numberDialed.

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