Transcript Chapter6 - Department of Electrical Engineering & Computer

Chapter 6
Telecommunications and
Networks
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Communications
 Communications
 The message (data and information) is
communicated via the signal
 The transmission medium “carries” the signal
Transmission
medium
Sender
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Signal
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Receiver
Communications ‘Discussion’
The transmission of data from one computer to another, or from one
device to another. A communications device, therefore, is any
machine that assists data transmission. For example, modems,
cables, and ports are all communications devices. Communications
software refers to programs that make it possible to transmit data.
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Telecommunications
 Telecommunications
 The electronic transmission of signals for
communications, including such means as:
• Telephone
• Radio
• Television
 Telecommunication medium
• Anything that carries an electronic signal and interfaces
between a sending device and a receiving device
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Communications and
Telecommunications
• In human speech, the sender transmits a signal through
•
the transmission medium of the air
In telecommunications, the sender transmits a signal
through the transmission medium of a cable
Schematic
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Data Communications
 Data communications
 A specialized subset of telecommunications
that refers to the electronic collection,
processing, and distribution of data -- typically
between computer system hardware devices
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Elements of a
Telecommunications System
 Telecommunication devices
 Relay signals between computer systems and
transmission media
Schematic
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Computer Network
 Computer network…
 The communications media, devices, and
software needed to connect two or more
computer systems and/or devices
 Used to share hardware, programs, and
databases across the organization
 Fosters teamwork, innovative ideas, and new
business strategies
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Types of Telecommunications
Media (1)
 Twisted pair wire cable
• Insulated pairs of wires historically used in
telephone service and to connect computer
devices
 Coaxial cable
• Consists of an inner conductor wire
surrounded by insulation, called the dielectric
• The dielectric is surrounded by a conductive
shield, which is surrounded by a nonconductive jacket. Coaxial cable has better
data transmission rate than twisted pair
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Twisted-pair ‘Discussion’
A type of cable that consists of two independently insulated wires
twisted around one another. One wire carries the signal while the
other wire is grounded and absorbs signal interference. Twisted-pair
cable is used by older telephone networks and is the least expensive
type of local-area network (LAN) cable. Other types of cables used
for LANs include coaxial cables and fiber optic cables.
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Coaxial Cable ‘Discussion’
A type of wire that consists of a centre wire surrounded by
insulation and then a grounded shield of braided wire. The shield
minimizes electrical and radio frequency interference.
Coaxial cabling is the primary type of cabling used by the cable
television industry and is also widely used for computer networks.
Although more expensive than standard telephone wire, it is much
less susceptible to interference and can carry much more data.
Because the cable television industry has already connected millions
of homes with coaxial cable, many analysts believe that they are
the best positioned to capitalize on the much-heralded information
highway.
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Types of Telecommunications
Media (2)
 Fiber-optic Cable
• Many extremely thin
strands of glass or plastic
bound together in a
sheathing which
transmits signals with
light beams
• Can be used for voice,
data, and video
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Fiber Optic ‘Discussion’ (1)
A technology that uses glass (or plastic) threads (fibers) to transmit
data. A fiber optic cable consists of a bundle of glass threads, each
of which is capable of transmitting messages modulated onto light
waves.
Fiber optics has several advantages over traditional metal
communications lines:
 Fiber optic cables have a much greater bandwidth than metal cables. This
means that they can carry more data
 Fiber optic cables are less susceptible than metal cables to interference
 Fiber optic cables are much thinner and lighter than metal wires
 Data can be transmitted digitally (the natural form for computer data) rather
than analogically.
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Fiber Optic ‘Discussion’ (2)
The main disadvantage of fiber optics is that the cables are
expensive to install. In addition, they are more fragile than wire and
are difficult to split.
Fiber optics is a particularly popular technology for local-area
networks. In addition, telephone companies are steadily replacing
traditional telephone lines with fiber optic cables. In the future,
almost all communications will employ fiber optics.
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Types of Telecommunications
Media (3)
 Microwave Communications
• Line-of-sight devices which must be placed in
•
relatively high locations
Microwave usage
– Information is converted to a microwave signal, sent
through the air to a receiver, and recovered
Pretty picture
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Types of Telecommunications
Media (4)
 Satellite transmission
• Communications satellites are relay stations that
•
receive signals from one earth station and
rebroadcast them to another
They use microwave signals
Pretty picture
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Types of Telecommunications
Media (5)
 Cellular transmission
• Signals from cells are transmitted to a receiver and
integrated into the regular network
Pretty picture
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Cellular ‘Discussion’
Refers to communications systems, especially the Advance Mobile
Phone Service (AMPS), that divide a geographic region into sections,
called cells. The purpose of this division is to make the most use out
of a limited number of transmission frequencies. Each connection, or
conversation, requires its own dedicated frequency, and the total
number of available frequencies is about 1,000. To support more
than 1,000 simultaneous conversations, cellular systems allocate a
set number of frequencies for each cell. Two cells can use the same
frequency for different conversations so long as the cells are not
adjacent to each other.
For digital communications, several competing cellular systems exist,
including GSM and CDMA.
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Types of Telecommunications
Media (6)
 Infrared transmission
• Involves sending signals through the air via light
•
•
waves
Requires line-of-sight and short distances (a few
hundred yards)
Used to connect various computing devices such
as handheld computers
Sorry, no pretty picture!
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Terminology
 Analog Signal
 A continuous, curving signal
 Digital Signal
 A signal represented by bits
 Modems
 Devices that translate data from digital to
analog and analog to digital
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Analog ‘Discussion’ (1)
Almost everything in the world can be described or represented in
one of two forms: analog or digital. The principal feature of analog
representations is that they are continuous. In contrast, digital
representations consist of values measured at discrete intervals.
Digital watches are called digital because they go from one value to
the next without displaying all intermediate values. Consequently,
they can display only a finite number of times of the day. In
contrast, watches with hands are analog, because the hands move
continuously around the clock face. As the minute hand goes
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Analog ‘Discussion’ (1)
around, it not only touches the numbers 1 through 12, but also the
infinite number of points in between.
Early attempts at building computers used analog techniques, but
accuracy and reliability were not good enough. Today, almost all
computers are digital.
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Digital ‘Discussion’ (1)
Describes any system based on discontinuous data or events.
Computers are digital machines because at their most basic level
they can distinguish between just two values, 0 and 1, or off and
on. There is no simple way to represent all the values in between,
such as 0.25. All data that a computer processes must be encoded
digitally, as a series of zeroes and ones.
The opposite of digital is analog. A typical analog device is a clock in
which the hands move continuously around the face. Such a clock is
capable of indicating every possible time of day. In contrast, a
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Digital ‘Discussion’ (2)
digital clock is capable of representing only a finite number of times
(every tenth of a second, for example).
In general, humans experience the world analogically. Vision, for
example, is an analog experience because we perceive infinitely
smooth gradations of shapes and colors. Most analog events,
however, can be simulated digitally. Photographs in newspapers, for
instance, consist of an array of dots that are either black or white.
From afar, the viewer does not see the dots (the digital form), but
only lines and shading, which appear to be continuous. Although
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Digital ‘Discussion’ (3)
digital representations are approximations of analog events, they are
useful because they are relatively easy to store and manipulate
electronically. The trick is in converting from analog to digital, and
back again.
This is the principle behind compact discs (CDs). The music itself
exists in an analog form, as waves in the air, but these sounds are
then translated into a digital form that is encoded onto the disk.
When you play a compact disc, the CD player reads the digital data,
translates it back into its original analog form, and sends it to the
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Digital ‘Discussion’ (4)
amplifier and eventually the speakers.
Internally, computers are digital because they consist of discrete
units called bits that are either on or off. But by combining many
bits in complex ways, computers simulate analog events. In one
sense, this is what computer science is all about.
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How a Modem Works
 Modem
 Modulates a digital signal into an analog signal
for transmission via analog medium, then
demodulates the signal into digital for
receiving
Pretty picture
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Modem ‘Discussion’ (1)
Acronym for modulator-demodulator. A modem is a device or
program that enables a computer to transmit data over telephone
lines. Computer information is stored digitally, whereas information
transmitted over telephone lines is transmitted in the form of analog
waves. A modem converts between these two forms.
Fortunately, there is one standard interface for connecting external
modems to computers called RS-232. Consequently, any external
modem can be attached to any computer that has an RS-232 port,
which almost all personal computers have. There are also modems
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Modem ‘Discussion’ (2)
that come as an expansion board that you can insert into a vacant
expansion slot. These are sometimes called onboard or internal
modems.
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Multiplexer
 Multiplexer
 Allows several telecommunications signals to
be transmitted over a single communications
medium at the same time
Pretty picture
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Communications
link
Multiplexor
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Front-end
processor
Host
computer
Multiplexor ‘Discussion’
A communications device that multiplexes (combines) several signals
for transmission over a single medium. A demultiplexor completes
the process by separating multiplexed signals from a transmission
line. Frequently a multiplexor and demultiplexor are combined into a
single device capable of processing both outgoing and incoming
signals.
A multiplexor is sometimes called a mux.
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Front-End Processor
 Front-end processor…
 Special purpose computers that manage
communication to and from a computer system
Pretty picture
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Incoming and
outgoing jobs
Job 1
Job 2
Job 3
Main
system
Front-end
processor
Job 4
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Carriers and Services (1)
 Carriers
• Organizations that take the responsibility of ensuring telecommunications can
effectively take place between enterprises
 Common carriers
• Long-distance telephone companies
 Value-added carriers
• Companies that have developed private telecommunications systems and offer
their services for a fee
 Switched lines
• Lines that use switching equipment to allow one transmission device to be
connected to other transmission devices (e.g., standard telephone line)
 Dedicated line
• A line that provides constant connection between two points. No switching or
dialing is needed
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Carriers and Services (2)
 Private branch exchange (PBX)
 Communication system that can manage both voice and data transfer
within a location (e.g. a building) and to outside lines
 Wide area telecommunication service (WATS)
 Billing method for heavy users of voice services
 Phone and dialing services
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Includes automatic number identification (a.k.a. caller ID)
Integration of telephones and personal computers
Access code screening
Call priorities
One number portability (use anywhere)
Intelligent dialing (auto re-dial for a busy number)
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Carriers and Services (3)
 ISDN
• ISDN = Integrated Services Digital Network
• Technology that uses existing common-carrier lines
to simultaneously transmit voice, video, and image
data in digital form
Pretty picture
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Carriers and Services (4)
 T1 carriers
• An expensive service developed by AT&T to
increase the number of voice calls that could be
handled through existing cables
 Digital subscriber lines (DSL)
• Uses existing phone wires going into today’s homes
and businesses to provide transmission speeds
exceeding 500 Kbps at a cost of $100 - $300 per
month
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Networks and Distributed
Processing
 Centralized processing
 Data processing that occurs in a single location or
facility
 Decentralized processing
 Data processing that occurs when devices are placed at
various remote locations
 Distributed processing
 Data processing that occurs when computers are
placed at remote locations but are connected to each
other via telecommunications devices
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Network Concepts and
Considerations
 Network Topology
 A logical model that describes how networks
are structured or configured
 Topologies…
• Ring (see chapter 1)
• Bus (see chapter 1)
• Star (see chapter 1)
• Hierarchical
• Hybrid
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Hierarchical
• Uses treelike structures
with messages passed
along the branches of the
hierarchy
Hybrid
• Network made up of
various types of
topologies
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Network Types
 Local area network (LAN)
 Connects computer systems and devices in the same
geographic area (can be Ring, Bus, Hierarchical, Star,
Hybrid)
 Wide area network (WAN)
 Ties together large geographic regions using
microwave and satellite transmission or telephone
lines
 International network
 Links systems between countries
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Terminal-to-Host Connection
 Applications and databases reside on the same
host computer
 User interacts with the application using a
“dumb terminal”
Target PC
“dumb terminal”
Host
computer
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File Server Connection
 Applications and databases reside on the same
host computer
 File server transfers data and programs to PCs
on the network, where these target PCs
perform most of the processing
Target PC
File downloaded
to user
File server
Host
computer
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Client/Server Connection
 Applications and databases reside on
specialized host computers
 Servers do most or all of the processing and
transmit the results to the client
Client
Server
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Server
Client
Server
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Advantages & Disadvantages of
Client/Server
Advantages
Disadvantages
 Reduced cost potential
 Improved Performance
 Increased Security
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 Increased cost potential
 Loss of control
 Complex multi-vendor
environment
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Communications Software and
Protocols (1)
 Communications software
• Provides error checking, message formatting, communications
logs, data security and privacy, and translation capabilities for
networks
 Network operating system (NOS)
• Systems software that controls the computer systems and
devices on a network and allows them to communicate with
each other
 Network management software
• Enables a manager on a networked desktop to monitor the use
of individual computers and shared hardware, scan for viruses,
and ensure compliance with software licenses
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Communications Software and
Protocols (2)
 Protocol
• Rules that ensure communications among
computers of different types and from different
manufacturers.
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Communications Software and
Protocols (3)
 Open Systems Interconnection (OSI) Model
 Serves as a standard model for network architectures
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and is endorsed by the International Standards
Committee
Communication functions are represented in seven
layers to promote the development of modular
networks. Designed to permit communication among
different computers from different operating systems
Seven layers (see figure 6.20)
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Communications Software and
Protocols (4)
 TCP/IP
• TCP/IP = Transmission Control Protocol/Internet Protocol
Standard originally developed by the U.S. government to link
defense research agencies; it is the primary communication
protocol of the Internet
 Systems Network Architecture (SNA)
• IBM communication protocol
 Ethernet
• Protocol standard developed forLANs using a bus topology
 X.400 and X.500
• An international standard for message handling and network
directories
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Bridges, Routers,
Gateways and Switches
 Bridge
• Connects two or more networks, with the same protocol, at the
media control portions of the data link layer
 Router
• Operates at the network level of the OSI model and features
more sophisticated addressing software than bridges. Can
determine preferred paths
 Gateway
• Operates at or above the OSI transport layer and links LANs
or networks that employ different architectures and use
dissimilar protocols
 Switch
Schematic
• Routes or switches data to its destination
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Machine
#1
Gateway
box
Machine
#2
Machine
#3
Application 7
7
7
7
7
7
Presentation 6
6
6
6
6
6
Session 5
5
5
5
5
5
Transport 4
4
4
4
4
4
Bridge
box
Network 3
Router
box
Machine
#4
3
3
3
3
3
3
3
Data link 2
2
2
2
2
2
2
2
2
2
Physical 1
1
1
1
1
1
1
1
1
1
Media
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Telecommunications
Applications (1)
 Linking personal computers to mainframe
computers
 Download and upload information
 Voice mail
 Enables users to leave, receive, and store verbal
messages for and from other users
 Electronic mail (e-mail)
 Enables a sender to connect a computer to a network,
type messages, and send it to another person on the
network
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Telecommunications
Applications (2)
 Electronic software distribution
• Involves installing software on a file server for users to share
by signing onto the network and requesting that the software
be downloaded onto their computers over a network
 Electronic document distribution
• Transporting documents -- such as sales reports, policy
manuals, and advertising brochures -- over communications
lines and networks
 Telecommuting
• Enables employees to work away from the office using
personal computers and networks to communicate via
electronic mail with other workers and to pick up and deliver
results
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Telecommunications
Applications (3)
 Videoconferencing
 Allows participants to conduct long-distance
meetings “face to face” while eliminating
Pretty picture
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Telecommunications
Applications (4)
 Electronic Data Interchange (EDI)
• Uses network systems and follows standards and
procedures that allow output from one system to be
processed directly as input to other systems, without
human intervention
Pretty picture
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EDI link
Vendor
Customer
(a)
EDI link
EDI link
Third-party
clearing house
(b)
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Vendor
Customer
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Telecommunications
Applications (5)
 Public network services
 Services that give personal computer users access to
vast databases and other services, usually for an initial
fee plus usage fees
 Specialized and regional information services
 Specialized electronic bulletin boards and e-mail
services targeting particular interests.
 Distance learning
 Use of telecommunications to extend the classroom
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End of Chapter 6
Chapter 7
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