Transcript The Telephone System

Unit 7 – Chapter 23
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Lesson 23-1
 The Telephone System
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Lesson 23-2
 Using Telephone Communications
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Lesson 23-3
 Exploring High-Speed Telecommunications
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Communicating by Telephone
 The first telephone message was sent in 1876
over a line connecting two rooms. Eventually
telephone cables were connected to a central
office. Operators in the office could connect calls
to anyone on the network. Today’s equipment can
connect telephones anywhere in the world.
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Telecommunications Formats
 Sending information over a telephone network is
called telecommunications. Telecommunications
has grown due to the demand for instant
communication. Today, people use many forms of
telecommunications to rapidly relay infromation:
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Telephones
Cell phones
Pagers
E-mail
Internet and fax machines.
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Making telephone calls.
 In the past, only a few companies provided
telephone services in the United States. Today,
people can choose from among many phone
companies that offer both local and long distance
service.
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Public Switched Telephone Network
 Many phone calls are made through the Public
Switched Telephone Network (PSTN). This
network is built from copper wires and other
cables and forms a circuit between the caller’s
telephone and another telephone.
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Local Calls
 For local calls, your phone company provides
directly wired services between the homes and
businesses that belong to the local network.
Within your neighborhood, telephones connect to
a common network for telephone service. This
common network, called the local loop, connects
to the phone company’s central office. Much of
the local loop is an analog system. An analog
system sends electrical signals that carry voice
and other sounds.
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Long-Distant Calls
 Outside the local loop, the long-distance
telephone system today is mostly digital. Digital
connections use computer code and can carry
voice, data, and video on a single line. When you
dial a long-distance number, computers figure out
how to complete your call. To connect analog and
digital networks, special equipment changes
analog signals into digital signals.

Contrasting Analog and Digital
Communications.
 People often confuse the terms, “analog” and “digital”
when they are talking about communications or
computers. The difference is important but easy to
understand. In analog communications, sounds (such
as a person’s voice or music) start as waves or
vibrations in the air. The vibrating air varies in
frequency or pitch (how high or deep the sound is)
and strength or loudness. A small microphone in the
telephone converts the sound waves into varying
patters of electrical signals or radio waves.
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Contrasting Analog and Digital
Communications.
 The patter of electrical signals or radio waves is
similar to the pattern of the sound waves. These
signals are converted back into sound waves by a
small loudspeaker in the receiver. In digital
communications, sounds are converted into binary
data (a series of 1’s and 0’s) at the caller’s end. The
stream of 1’s and 0’s is transmitted without any
variation in the pattern of electrical or radio waves.
The receiver converts the binary data back into sound
waves.
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The Wired – and the Wireless – World
 Wires connect the phone jack in your wall to an
interface box outside. Outside wires may be
above or below ground. Wires also connect your
local loop with distant places.
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Twisted Pair
 At first, the entire telephone system depended on
twisted pair technology. Twisted pair refers to a
pair of copper wires that are twisted together to
reduce interference, or outside noise. In the
United States today, most homes and business
buildings still have twisted pair wiring.
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Fiber-Optic Cables
 Fiber-optic cables are strands of fiberglass that
transmit digital data by pulses of light. These
cables can carry large quantities of information.
They work faster and more efficiently than copper
wires. As they get lower in price, fiber optic cables
will eventually replace copper.
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Wireless
 Wireless communication frees users from
traditional telephone lines. Messages are sent on
radio or infrared signals. Cell phones use radio
signals. Infrared signals are light waves that
cannot be seen by the human eye. Remote
control devices for TV’s, VCR’s, and DVD players
use infrared signals.
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Wireless
 Before fiber-optic cables, high-frequency radio
waves called microwave signals were used to relay
long-distance telecommunications. Microwaves
are broadcast from repeater tower to repeater
tower in a straight line.
 Satellites orbiting Earth also transfer voice and
data. Satellites provide an efficient means to
handle large amounts of phone calls and data.
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Using Modems
 Back in the early 1990’s when the Internet was still
“new,” most people used phone lines to connect
to it. Your computer is a digital device. The local
loop that connects you to the telephone system,
however, is analog. A device called a modem
makes it possible for your computer and
telephone lines to communicate, and for your
computer and TV cables to communicate.
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Using Modems
 The word modem actually names the work the device
does: modulation and demodulation. Through
modulation, the modem changes the digital signal of
the computer to the analog sounds used by
telephones. Then, the data-in the form of pictures,
audio, or video-can travel over the telephone wires.
When the data gets to it destination, the receiving
modem change the analog signals back to digital. This
process is called demodulation. The early dial-up
modems were usually internal, housed inside the
computer. Today’s modems for use with DSL and
cable (discussed later) are usually external.
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Drawbacks to Dial-up
 Today only 10% of U.S. households connect to the
computer using the local phone line loop, but dial-up
service is still a worthwhile backup. One problem with
using dial-up was that you could not be on your
computer and use your telephone simultaneously, so
the computer modem required a dedicated phone
line. Another larger drawback was slow speed.
 Modem speed is measured in bits per second, or bps,
which is the amount of data that can be sent in one
second. Dial-up modems can only transmit 56,600
bps, which has been surpassed by newer, speedier
technologies.
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Sending Faxes
 While dial-up is usually a thing of the past, there is
one machine that still used telephone lines to send
printed messages or visual images. A facsimile
machine, or fax machine, is a device that allows you
to send pages of information to a fax machine
anywhere in the world. Fax machines can send handwritten documents, printed text, pictures, blueprints,
or anything else on a page. Yet, even these machines
are largely being replaced by e-mail, by which you can
send attachments over the Internet.
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How Fax Machines Work
 As a document enters a fax machine, a sensor
scans it. The data becomes a digital signal. An
internal modem in the fax machine (or in an All-inOne printer with fax capability) changes the
digital signal to an analog signal. The receiving fax
machine accepts the analog signal, changes it
back to digital, and prints a copy of the original
document.
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Introducing Bandwidth
 People always want faster, better, and less
expensive telecommunication choices. Twisted
pair copper wiring in many homes and businesses
in the United States at first made it hard to
increase bandwidth. Then telephone, cable, and
satellite television companies began to compete
in offering higher speed communications. Now,
more than eight out of ten homes and businesses
are using high-speed connections for Internet
access.
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Understanding Bandwidth
 Bandwidth is the amount of data that can be sent
through a modem or network connection. The
more bandwidth, the faster the connection. It is
usually measured in bits per second (bps) or in
Megabits per second (Mbps). The more
bandwidth, the more information can be
transferred in a given amount of time.
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Understanding Bandwidth
 Imagine several people on different computers
connecting to the Internet to visit Web pages,
participate in video chats, or send e-mail. These
users need a lot of bandwidth. That’s why there is
always a race to find a technology that offers
services that transmit data faster. Currently the
fastest technology is broadband transmission.
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Working with Broadband Transmission
 Broadband is the general term for all high-speed
digital connections that transmit at least 1.5
megabits per second (Mbps), though current
broadband services transmit between 10 and 30
Mbps. Several broadband technologies are
available, and more are always on the drawing
board. This high transmission speed is required for
videoconferencing, video-on-demand, digital
television services, and high-speed Internet
connectivity in general.
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DSL
 Digital Subscriber Line, or DSL, uses the same copper
wires telephones use, but it transmits data in digital form
rather than analog. Voice calls and DSL can exist
simultaneously on copper lines, because each services has
its own frequency band. Unlike the old dial-up, DSL allows
for very fast connections to the Internet and features an
“always-on” connection. DSL service also requires a
modem, which translates the computer’s digital signals
into voltage sent across phone lines to a central hub. There
is one drawback to DSL: A user must be within a few miles
of a local telephone switching station for a connection to
be made.
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DSL
 Different companies offer DSL at different levels of
service and price. For instance, asymmetric DSL
(ADSL) allows download speeds of up to 1.5 Mbps and
upload speeds of 126 kilobits per second (Kbps). This
means you can receive data (download) faster than
you can send it (upload) with ADSL. Subscribers to a
Symmetric Digital Subscriber Line (SDSL) can send
data at the same speed at which they receive it. These
are used usually by businesses that need to send large
files, data, and programs.
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SONET
 Telephone companies that offer DSL and other
Internet connection methods rely on a digital
network called SONET. SONET stands for
Synchronous Optical Network. It uses fiver optics
to provide faster connections and greater
bandwidth—from 52 Mbps to up to a whopping 40
gigabits per second (Gbps).
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Cable and Satellite TV Connections
 Most cable and digital television companies offer
high-speed Internet connection through a cable
modem. Satellite television companies also offer
a similar service with signals sent through the
customer’s satellite dish. A cable modem
connects your computer to the local cable TV line
and supports data transfer rates of up to 30
Mbps—over 500 times faster than the old dial-up
modem.
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Cable and Satellite TV Connections
 However, this number can be misleading, because
most Internet Service Providers (ISPs) cap
subscribers’ transfer rates to less than 6 Mbps to
conserve bandwidth. However, this is more than
enough for the average home computer user.
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Voice over Internet Protocol
 Just as you can use your telephone to send and
receive messages over the Internet, you can use the
Internet to send and receive messages over the
telephone. Voice over Internet Protocol (VoIP)
technology allows you to have a telephone
connection over the Internet. VoIP uses data sent
digitally, with the Internet Protocol (IP), instead of
analog telephone lines. People use VoIP to talk to
each other from across the globe, sometimes without
having to pay a cent. With Web-cams callers can also
see each other during their calls.
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The Future of Bandwidth
 The demand for bandwidth is growing. People
want increased bandwidth for video-on-demand,
meetings via the Internet, and Web-based
learning. Thus, telephone and other high-tech
companies continue to look for new ways to
improve telecommunications services and data
transmission.