How Telephones Work

8/7/2019 How Telephones Work

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HOW TELEPHONES WORK?

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In This Presentation

In the days of the manual switchboard A Simple Telephone

A Real Telephone

Telephone Wires And Cables

Digitizing and Delivering Creating Your Own Telephone Network Telephone Tones Bandwidth

The Main Animation - The Telephone network in action
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In the days of the manual switchboardv In the days of the manual switchboard, there was a pair of copper wires running

from every house to a central office in the middle of town.

v The switchboard operator sat in front of a board with one jack for every pair ofwires entering the office. Above each jack was a small light. A large batterysupplied current through a resistor to each wire pair .

v When someone picked up the handset on his or her telephone, the hook switchwould complete the circuit and let current flow through wires between the house

and the office. This would light the light bulb above that person's jack on theswitchboard.

v The operator would connect his/her headset into that jack and ask who theperson would like to talk to. The operator would then send a ring signal to thereceiving party and wait for the party to pick up the phone.

v Once the receiving party picked up, the operator would connect the two peopletogether in exactly the same way the simple intercom is connected.

Click here for figureClick here to go back
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A Simple Telephone

Click here for details
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A RealTelephone

v The only problem with the phone shown previously isthat when you talk, you will hear your own voicethrough the speaker.

v So any real phone contains a device called aduplex coil or something functionally equivalentto block the sound of your own voice fromreaching your ear.

v A modern telephone also includes a bell so it canring and a touch-tone keypad and frequencygenerator.

v In a modern phone there is an electronic

microphone, amplifier and circuit to replacethe carbon ranules and loadin coil. The

Click here for figureClick here to go back
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A Real

Telephone

Click here for details


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Telephone Wires And

Cablesv The telephone network starts in your house. A pair of

copper wires runs from a box at the road to a box(entrance bridge) at your house.

v From there, the pair of wires is connected to each phone jack

in your house (usually using red and green wires). If yourhouse has two phone lines, then two separate pairs ofcopper wires run from the road to your house. The secondpair is usually colored yellow and black inside your house.

v Along the road runs a thick cable packed with 100 or more

copper pairs. Depending on where you are located, this thickcable will run directly to the phone company's switch in yourarea or it will run to a box about the size of a refrigeratorthat acts as a digital concentrator.

v Now a days, copper cables are getting replaced with optical-

fibre cables. Click here for figureClick here to go back
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Telephone Wires And

Cables

Figure shows a typical phone company box that you see by the

side of the road. Click here to go back


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Digitizing and Deliveringv The concentrator digitizes your voice at a

sample rate of 8,000 samples per second and8-bit resolution.

v It then combines your voice with dozens ofothers and sends them all down a single wire(usually a coax cable or a fiber-optic cable) tothe phone company office.

v If you are calling someone connected to thesame office, then the switch simply creates aloop between your phone and the phone ofthe person you called.

v If it's a lon -distance call, then our voice isClick here to go back


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Creating Your Own Telephone Network

Click here to go back


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Telephone

Tonesv In a modern phone system, the operator has been

replaced by an electronic switch.

v When you pick up the phone, the switch sensesthe completion of your loop and it plays a dialtone sound so you know that the switch and your

phone are working. The dial tone sound is simplya combination of 350-hertz tone and a 440-hertztone.

v You then dial the number using a touch-tone

keypad. Different dialing sounds are made ofpairs of tones.

v If the number is busy, you hear a busy signal thatis made up of a 480-hertz and a 620-hertz tone,

with a cycle of one-half second on and one-half

Click here for table

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Telephone Tones

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Bandwidth

v In order to allow more long-distance calls tobe transmitted, the frequencies transmittedare limited to a bandwidth of about 3,000

hertz.

v All of the frequencies in your voice below 400hertz and above 3,400 hertz are eliminated.

That's why someone's voice on a phone hasa distinctive sound.

v This is because,

i. Audible range 20Hz to 20,000Hz

ii. Audio ran e 400Hz to 3000HzClick here to go back


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A Typical Network.Click Next tocontinue.

The signal leaves yourhouse to the telephone

box outside

The local exchange isconnected to all the

phones in a small

geographical area

Cell phone calls travel

to mobile towers andfrom there to a mobileexchange

From the local andmobile exchange callsare routed to the main

exchange.

The main exchange mayroute the call to another

main exchange orinternational exchange

The calls are sent to othercountries throughsatellites or under sea

cables,

By,

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Thank You

Press Esc To Exit


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Manual

Switchboards


How Telephones Work

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