30-01-2013, 12:54 PM
Telephone
[attachment=48882]
INTRODUCTION
The telephone, or phone, is a telecommunications
device that transmits and receives sounds, usually the
human voice. Telephones are a point-to-point
communication system whose most basic function is to
allow two people separated by large distances to talk to
each other. Developed in the mid-1870s by Alexander
Graham Bell and others, the telephone has long been
considered indispensable to businesses, households and
governments, is now one of the most common
appliances in the developed world. The word
"telephone" has been adapted to many languages and is
now recognized around the world.
All modern telephones have a microphone to speak into,
an earphone (or 'speaker') which reproduces the voice
of the other person, a ringer which makes a sound to
alert the owner when a call is coming in, and a keypad
(or on older phones a telephone dial) to enter the telephone number
of the telephone to be called. The microphone and earphone are
usually built into a handset which is held up to the face to talk. The
keypad may be part of the handset or of a base unit to which the
handset would be connected. A landline telephone is connected by a
pair of wires to the telephone network, while a mobile phone (also
called a cell phone) is portable and communicates with the telephone
network by radio. A cordless telephone has a portable handset which
communicates by radio transmission with the handset owners base
station which is connected by wire to the telephone network, and can
only be used within about 50 feet from the base station.
The microphone converts the sound waves to electrical signals and
then these are sent through the telephone network to the other phone and there converted by an earphone, or
speaker, back into sound waves. Telephones are a duplex communications medium, meaning they allow the
people on both ends to talk simultaneously. The telephone network, consisting of a worldwide net of telephone
lines, fiberoptic cables, microwave transmission, cellular networks, communications satellites, and undersea
telephone cables connected by switching centers, allows any telephone in the world to communicate with any
other. Each telephone line has an identifying number called its telephone number. To initiate a telephone call the
user enters the other telephone's number into a numeric keypad on the phone.
History
Credit for the invention of the electric telephone is frequently disputed,
and new controversies over the issue have arisen from time to time.
As with other influential inventions such as radio, television, the light
bulb, and the computer, there were several inventors who did
pioneering experimental work on voice transmission over a wire
and improved on each other's ideas. Innocenzo Manzetti, Antonio
Meucci, Johann Philipp Reis, Elisha Gray, Alexander Graham Bell,
and Thomas Edison, among others, have all been credited with
pioneering work on the telephone. An undisputed fact is that
Alexander Graham Bell was the first to be awarded a patent for the
electric telephone by the United States Patent and Trademark Office
(USPTO) in March 1876.[1] That first patent by Bell was the master
patent of the telephone, from which other patents for electric
telephone devices and features flowed.
Basic principles
A traditional landline telephone system,
also known as "plain old telephone
service" (POTS), commonly carries
both control and audio signals on the
same twisted pair © of insulated
wires: the telephone line. The signaling
equipment, or ringer, (see figure 1)
consists of a bell, beeper, light or other
device (A7) to alert the user to
incoming calls, and number buttons or a
rotary dial (A4) to enter a telephone
number for outgoing calls. Most of the
expense of wire-line telephone service
is the wires, so telephones transmit both
the incoming and outgoing voice
channels on a single pair of wires. A twisted pair line rejects electromagnetic interference (EMI) and crosstalk
better than a single wire or an untwisted pair. The strong outgoing voice signal from the microphone does not
overpower the weaker incoming speaker signal with a sidetone because a hybrid coil (A3) subtracts the
microphone's signal from the signal sent to the local speaker. The junction box (B) arrests lightning (B2) and
adjusts the line's resistance (B1) to maximize the signal power for the line's length. Telephones have similar
adjustments for inside line lengths (A8). The wire's voltages are negative compared to earth, to reduce galvanic
corrosion. Negative voltage attracts positive metal ions toward the wires.
[attachment=48882]
INTRODUCTION
The telephone, or phone, is a telecommunications
device that transmits and receives sounds, usually the
human voice. Telephones are a point-to-point
communication system whose most basic function is to
allow two people separated by large distances to talk to
each other. Developed in the mid-1870s by Alexander
Graham Bell and others, the telephone has long been
considered indispensable to businesses, households and
governments, is now one of the most common
appliances in the developed world. The word
"telephone" has been adapted to many languages and is
now recognized around the world.
All modern telephones have a microphone to speak into,
an earphone (or 'speaker') which reproduces the voice
of the other person, a ringer which makes a sound to
alert the owner when a call is coming in, and a keypad
(or on older phones a telephone dial) to enter the telephone number
of the telephone to be called. The microphone and earphone are
usually built into a handset which is held up to the face to talk. The
keypad may be part of the handset or of a base unit to which the
handset would be connected. A landline telephone is connected by a
pair of wires to the telephone network, while a mobile phone (also
called a cell phone) is portable and communicates with the telephone
network by radio. A cordless telephone has a portable handset which
communicates by radio transmission with the handset owners base
station which is connected by wire to the telephone network, and can
only be used within about 50 feet from the base station.
The microphone converts the sound waves to electrical signals and
then these are sent through the telephone network to the other phone and there converted by an earphone, or
speaker, back into sound waves. Telephones are a duplex communications medium, meaning they allow the
people on both ends to talk simultaneously. The telephone network, consisting of a worldwide net of telephone
lines, fiberoptic cables, microwave transmission, cellular networks, communications satellites, and undersea
telephone cables connected by switching centers, allows any telephone in the world to communicate with any
other. Each telephone line has an identifying number called its telephone number. To initiate a telephone call the
user enters the other telephone's number into a numeric keypad on the phone.
History
Credit for the invention of the electric telephone is frequently disputed,
and new controversies over the issue have arisen from time to time.
As with other influential inventions such as radio, television, the light
bulb, and the computer, there were several inventors who did
pioneering experimental work on voice transmission over a wire
and improved on each other's ideas. Innocenzo Manzetti, Antonio
Meucci, Johann Philipp Reis, Elisha Gray, Alexander Graham Bell,
and Thomas Edison, among others, have all been credited with
pioneering work on the telephone. An undisputed fact is that
Alexander Graham Bell was the first to be awarded a patent for the
electric telephone by the United States Patent and Trademark Office
(USPTO) in March 1876.[1] That first patent by Bell was the master
patent of the telephone, from which other patents for electric
telephone devices and features flowed.
Basic principles
A traditional landline telephone system,
also known as "plain old telephone
service" (POTS), commonly carries
both control and audio signals on the
same twisted pair © of insulated
wires: the telephone line. The signaling
equipment, or ringer, (see figure 1)
consists of a bell, beeper, light or other
device (A7) to alert the user to
incoming calls, and number buttons or a
rotary dial (A4) to enter a telephone
number for outgoing calls. Most of the
expense of wire-line telephone service
is the wires, so telephones transmit both
the incoming and outgoing voice
channels on a single pair of wires. A twisted pair line rejects electromagnetic interference (EMI) and crosstalk
better than a single wire or an untwisted pair. The strong outgoing voice signal from the microphone does not
overpower the weaker incoming speaker signal with a sidetone because a hybrid coil (A3) subtracts the
microphone's signal from the signal sent to the local speaker. The junction box (B) arrests lightning (B2) and
adjusts the line's resistance (B1) to maximize the signal power for the line's length. Telephones have similar
adjustments for inside line lengths (A8). The wire's voltages are negative compared to earth, to reduce galvanic
corrosion. Negative voltage attracts positive metal ions toward the wires.