Transcript William Sawyer - McGraw Hill Higher Education

6-1
Chapter
6
Communications,
Networks, & Safeguards
6.1 From the Analog to the Digital Age
6.2 Networks
6.3 Wired Communications Media
6.4 Wireless Communications Media
6.5 Cyberthreats, Hackers, & Safeguards
6.6 The Future of Communications
McGraw-Hill/Irwin
© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
From the Analog to the Digital Age
 Analog signals use variation of
a wave form to send
information
 The wave forms look like the
sine wave shown at right
 In FM signals the Frequency
is varied (Modulated)
 In AM signals the Amplitude is
varied (Modulated)
 Radios send signals this way
 Light works this way
 Hearing and phones work this
way
 Modems work this way
Sine Wave
1
0.5
0
Amplitude
-0.5
-1
S1
Frequency
6-3
From the Analog to the Digital Age
 Digital signals send data in
Digital Signal
1
Time
6-4
41
37
33
29
25
21
17
13
9
0
5
Data
1
terms of 1s and 0s
 A digital signal may look like
this one at right
 Notice how the signal goes up
and down abruptly because
the only values that matter are
0 and 1 and the timing of
when they occur
 Computers use digital signals
 Newspaper photographs are
digital since they are made up
of little dots
S1
From the Analog to the Digital Age
Since computers use digital signals but phone lines
use analog, modems must translate from digital to
analog, send the signal along the phone line, then
translate back from analog to digital at the other end
 The
process is called “modulation/demodulation”
 Modulation means to translate from digital to analog
 Demodulation means to translate from analog to digital
 Modems have to do all this just to use standard analog
voice phone lines
6-5
From the Analog to the Digital Age
Tape recorders, voices, and musical instruments are
analog while CDs are digital
To burn a CD from a jam session, the digital recording
equipment must convert from analog to digital
 The
analog-to-digital converter samples the sound and
converts the height of the wave to a number
 Samples of the sound wave are taken at regular intervals
– about 44,100 times each second
 Because the digital samples are played back faster than
our ears can react, it sounds to us like a single
continuous sound wave 6-6
From the Analog to the Digital Age
Digital sampling is similar to showing movies
 Movies
show still pictures (frames)
 But they show them so fast that our eyes can’t react in
time
 So to us the series of still pictures look like continuous
motion
 Did you ever notice in movies when they show car tires in
motion they sometimes seem to move backwards?
 This
is because the tires are moving at a rate that is
incompatible with the frame rate of the movie, so our eyes
think the tires are really moving backwards when the car is
actually moving forward! 6-7
Networks
A system of interconnected computers, telephones, or
other communications devices that can communicate
with one another and share applications and data
Before we had computer networks, people used
“sneakernet” to share data between computers
 Person
1 saved their document to a floppy disk
 Then they walked over to person 2’s desk (wearing
sneakers, of course) and handed over the disk to
person 2
 Person 2 loaded the disk into their computer to read and
edit the document
6-8
Networks
Since the days of “sneakernet”, networks have
become standard. They enable us to:
 Share
peripheral devices such as laser printers
 Share programs and data
 Use e-mail and other communication programs
 Backup critical information because it is stored centrally
 Access shared databases
6-9
Networks
 WAN – Wide Area Network
 Covers a wide geographic area,
 MAN – Metropolitan Area
such as a country or the world
 Covers a city or a suburb
Network
 LAN – Local Area Network
 Connects computers and devices
 HAN – Home Area Network
 PAN – Personal Area Network
in a limited geographic area such
as an office, building, or group of
nearby buildings
 Uses wired, cable, or wireless
connections to link a household’s
digital devices
 Uses short-range wireless
technology to connect an
individual’s personal electronics
like cellphone, PDA, MP3 player,
6-10 notebook PC, and printer
Networks
Client/Server
 Consists
of clients, which are computers that request
data, and servers, which are computers that supply data
 File servers act like a network-based shared disk drive
 Database servers store data but don’t store programs
 Print servers connect one or more printers and schedule
and control print jobs
 Mail servers manage email
Peer-to-Peer
 All
computers on the network communicate directly with
each other without relying on a server
6-11
 For fewer than 25 PCs
Networks
 Peer-to-Peer (continued)
 Cheaper
than client/server since servers are typically more
expensive than PCs
 There are often problems with knowing who has the current
version of documents and files
 Too slow for use in larger offices
 Legal considerations
 Downloading copyrighted material without paying violates U.S.
copyright laws
 Server-based online file sharing sites such as Napster have
been shut down
 Peer-to-Peer file-sharing sites such as Kazaa, Grokster, and
Gnutella have been more difficult to control since there is no
central server to shut down
 So publishers are suing individual downloaders instead
Watch out!
6-12
Don’t download illegally!
Networks
Intranets, Extranets, VPNs
Intranets
 An
organization’s private network that uses the
infrastructure and standards of the internet and the web
Extranets
 Private
internets that connect not only internal personnel
but also selected suppliers and other strategic parties
Virtual Private Networks
 Private
networks that use a public network, usually the
internet, to connect remote sites
6-13
Network
Components
Connections
– twisted-pair, coaxial cable, or fiber-optic
 Wireless – infrared, microwave (Bluetooth), broadcast
(Wi-Fi) or satellite
 Wired
Hosts & Nodes
 Host:
the central computer that controls the network
 Node: a device that is attached to the network
Packets
 The
format for sending electronic messages
 A fixed-length block of data for transmission
6-14
Network
Components
Protocols
 The set of conventions governing the exchange of data
between hardware and/or software components in a
communications network
 Built into the hardware or software you are using
 Govern the packet design and transmission standards
 Examples are:
TCP/IP for LANs and internet
 AppleTalk for older Mac networks
 SIP for Voice over IP (VoIP)
 CDMA for cellphones
 IPX for older Novell networks

6-15
Network
Packets
 TCP/IP Packets carry four types of information
 Sender’s
address (source IP number)
 Address of intended recipient (destination IP number)
 Number of packets the original data was broken into
This happens because the amount of data the PC is sending can
be much larger than the space in a single packet
 So the data has to get broken up in one or more packets
 Then the packets have to be assigned a number like 1 of 6, 2 of 6,
3 of 6, 4 of 6, 5 of 6, and 6 of 6

 Packet
number and sequence info for each packet
Packets may arrive out of order (1, 6, 3, 2, 5, 4 for example)
 This information is used to resequence the packets and put them
back in the correct order (1, 2, 3, 4, 5, 6) so they can be read

6-16
Network
Devices
 Daisy
 Used in peer-to-peer networks – direct
Chain
 Hub
connections from one PC to the next
 Used in small LANs to connect PCs and LAN
segments to each other. Forwards to all ports
 Used in larger, busy LANs – faster than hubs
because it forwards only to correct destination
 Used to connect two networks of the same type
 Connects two networks of different types
 Connects multiple LANs together. Routers are
the internet backbone
 The main route that connects all the gateways,
routers, and other computers in an organization
 Switch
 Bridge
 Gateway
 Router
 Backbone
6-17
Network
Topologies
 Bus – all nodes are connected to a single wire or cable
 Ring – all nodes are connected in a continuous loop
 Star – all nodes are connected through a central host
6-18
Network
Packet Collision Schemes
Collisions happen when two data packets are going
opposite directions on shared media
Ethernet – deals with collisions
 All
devices send data at once
 Collisions happen regularly
 Data is simply resent until it arrives
Token ring – avoids collisions
$$$
 Devices
take turns sending data
 Token is sent around the ring
Pricier than Ethernet
 Wait to get the token, then send data
6-19
Wired Communications Media
Communications media carry signals over a
communications path
 Twisted-Pair
Wire
2
strands of insulated copper wire twisted around each other
 Twisting reduces interference (crosstalk) from electrical signals
 Data rates are 1 – 128 Megabits per second
 Coaxial
Cable
 Insulated
copper wire wrapped in a metal shield and then in an
external plastic cover
 Used for cable TV and cable internet electric signals
 Carries voice and data up to 200 megabits per second
6-20
Wired Communications Media
Communications media continued
 Fiber-optic
cable
 Dozens
or hundreds of thin strands of glass or plastic that
transmit beams of light, not electricity
 Can transmit up to 2 gigabits per second
 More expensive than twisted-pair or coax
 Lighter and more durable than twisted-pair or coax
 More difficult to tap into than twisted-pair or coax
6-21
Wired Communications Media
for the home
 Ethernet
 Pull
Cat5 cables through the house (yourself or contractor)
 Connect to PC’s Ethernet network interface card (nic)
 For several PCs, get a hub or switch to connect them all
 10 or 100 megabits per second
 HomePNA
 Uses existing telephone wiring and jacks
 Requires HomePNA nic in your PC
 Speeds of 10 – 240 megabits per second
 Homeplug
 Uses existing home electrical lines
 Speeds of 14 megabits per second
6-22
Wireless Communications Media
 Electromagnetic spectrum of radiation is the basis of all
telecommunications signals
 Includes the longest radio waves (9 kHz) and audio waves (sound), up
through gamma rays that come from nuclear decay (thousands of
gigahertz)
 Radio-frequency spectrum is the part of the electromagnetic spectrum
that we use for radio communication
6-23
Wireless Communications Media
Bandwidth
Narrowband (or Voiceband)
Used
for regular telephone communications
Transmission rate < 100 kilobits per second
Medium Band
Used for long-distance data transmission or to
connect mainframe and midrange computers
Transmission rate 100 kb to 1 megabit per second
Broadband
For high-speed data and high-quality audio and video
Transmission rate 1 megabit per second to 100
megabits per second
US households get 4 – 5 MB while Japanese get 100
6-24
MB per second
Wireless Communications Media
Infrared Transmission
 Sends
signals using infrared light
 Frequencies are too low to see (1-4 Mbits per second)
Broadcast Radio
 AM/FM,
CB, ham, cellphones, police radio
 Sends data over long distances using a transmitter and a
receiver
Microwave Radio
 Superhigh
frequency radio waves (1 gigahertz)
 Requires line-of-sight transmitters and receivers
Communications Satellites
6-25
Wireless Communications Media
 Communications Satellites
 These are microwave relay stations in orbit around
Uplinking: transmitting a signal from ground station to a satellite
 Cover
the earth -
broad service area
 Cost $300 million to $700 million each + launch costs
 Can be placed at different heights: GEO, MEO, LEO

GEO – geostationary earth orbit



MEO – medium-earth orbit


22,300 miles up above earth
Always above equator
5,000 – 10,000 miles up
LEO – low-earth orbit


200 – 1,000 miles up
Has less signal delay than GEO, MEO satellites
6-26
Wireless Communications Media
GPS
Global Positioning System
 24 earth-orbiting satellites continuously transmitting timed
radio signals
 Each satellite circles earth twice each day at 11,000 miles
up
 GPS receivers pick up transmissions from up to 4 satellites
and pinpoint the receiver’s location
 Accurate within 3 – 50 feet, with a norm of 10 feet accuracy
 GPS receivers contain map files that are displayed based
on the GPS position to guide users
 Many GPS receivers have speech chips, too
6-27
Wireless Communications Media
One-way Pagers
One-way pagers are radio receivers that receive data
sent from a special radio transmitter
 Radio
transmitter sends out signals over the special
frequency
 Pagers are tuned to that frequency
 When a particular pager hears its own code, it receives
and displays the message
Discussion Question: Why do airplane rules require you to turn off
pagers and cellphones during flight?
Answer: Pilots use radar and radio to determine their position and
communicate with ground control. Pager and cellphone signals use
radio, too, and competing signals can interfere with each other
6-28
Wireless Communications Media
Long-Distance Wireless
Two-way pagers: Blackberry and Treo
1G: First Generation Cellular
 Analog
cellphones
 Designed for voice communication using a system of
hexagonal ground-area cells around transmitter-receiver
cell towers
 Good for voice – less effective for data due to handing off
2G: Second Generation Cellular
 Use
same network of cell towers to send voice and data
in digital form over the airwaves
 Required digital receivers on original analog celltowers
6-29
Wireless Communications Media
2G Wireless
There are two competing, incompatible standards
 CDMA
– Code Division Multiple Access
 Transmission
rates 14.4 kilobits per second
 Used by Verizon and Sprint
 GSM
– Global System for Mobile Communications
 Transmission
rates of 9.6 kilobits per second
 Used by Cingular and T-Mobile, as well as Western Europe,
Middle East and Asia
 US GSM and European GSM use different frequencies
6-30
Wireless Communications Media
2.5G Wireless
Data speeds of 300–100 kilobits per second
 GPRS
– General Packet Radio Service
 An
upgrade to 2.5G
 Speeds of 30 – 50 kilobits per second
 EDGE
is Enhanced Data for Global Evolution
 A different
2.5G upgrade
 Speeds of up to 236 kilobits per second
6-31
Wireless Communications Media
3G Wireless
Third generation wireless
 High
speed data: 144 kilobits per second up to 2
megabits per second
Discussion
Question: If
 Accept e-mail with attachments
your cellphone
 Display color video and still pictures
can download
 Play music
and play music,
do you still
 Two important upgrades:
 EV-DO

need an i-pod?
Average speeds of 400 – 700 kilobits per second, peaks of 2
megabits per second
 UMTS

– Evolution Data Only
– Universal Mobile Telecommunications System
Average speed of 220 – 320 kilobits per second
6-32
Wireless Communications Media
Short-range Wireless
Local Area Networks
50 – 150 feet
 Include Wi-Fi (802.11) type networks
 Range
Personal Area Networks
30 – 32 feet
 Use Bluetooth, Ultra wideband, and wireless USB
 Range
Home Automation networks
100 – 250 feet
 Use Insteon, Zigbee, and Z-Wave standards
 Range
6-33
Wireless Communications Media
Short-range Wireless
Wi-Fi (802.11) networks
 Wi-Fi
b, a, and g correspond to 802.11b, 802.11a, and
802.11g
 802.11 is an IEEE wireless technical specification
 802.11b is older, transmits 11 megabits per second
 802.11a is faster than b but with weaker security than g
 802.11g is 54 megabits per second and transmits 50 ft
 Wi-Fi n with MIMO extends range of Wi-Fi using multiple
transmitting and receiving antennas – 200 megabits per
second for up to 150 ft
6-34 by default on Wi-Fi
Warning! Security is disabled
Wireless Communications Media
Short-range Wireless
Wi-Fi Security
 Why
is it disabled by default?
 So
 Why
non-technical users can get Wi-Fi working more easily
should this bother me?
 A person
with a $50 antenna can eavesdrop on everything
your computer sends over wireless from a block or two
away
 This is called “wardriving”
 To
read more about this problem, follow these links
 http://www.webopedia.com/TERM/W/wardriving.html
 http://www.wardriving.com/
6-35
Wireless Communications Media
Personal Area Wireless
Bluetooth
 Short-range
wireless standard to link cellphones, PDAs,
computers, and peripherals at distances up to 30 ft
 Named after King Harald Bluetooth, the Viking who
unified Denmark and Norway
 Transmits 720 kilobits per second
 When Bluetooth devices come into range of each other,
they negotiate. If they have information to exchange, they
form a temporary wireless network
 Bluetooth can also be used to eavesdrop on networks
 Turn it off on your cellphone unless you need it at that
6-36
time
Wireless Communications Media
Personal Area Wireless
Ultra Wideband (UWB)
 Developed
for military radar systems
 Operates in 480 megabit per second range up to 30 ft
 Uses a low power source to send out millions of bursts of
radio waves each second
 100 times as fast as Bluetooth
Wireless USB
 USB
is the most used interface on PCs
 The wireless version could be a hit
 Range of 32 ft and maximum data rate of > 480 megabits
6-37
per second
Wireless Communications Media
Short-Range Wireless for Home
 Insteon
 Combines
electronic powerline and wireless technology
 Can send data at 13.1 kilobits per second with 150 ft range
 Replaces X10
 ZigBee
 Entirely wireless very power-efficient technology
 Can send data at 128 kilobits per second with 250 ft range
 Z-Wave
 Entirely wireless power-efficient technology
 Can send data at 127 kilobits per second to range of 100 ft
 Allows you to remotely program your house!
6-38
Cyberthreats, Hackers, &
Safeguards
Problem: internet was begun to foster collaboration
among universities and scientists. They trusted each
other. No security was built into the internet.
Problem: the internet is open-access and is used by
some people who are not trustworthy, who take
advantage of the lack of built-in safeguards.
Problem: Most people connect to the internet and use
their computers in LANs. All it takes is one computer
on a LAN that has been compromised for all computers
on it to be vulnerable.
6-39
Cyberthreats, Hackers, &
Safeguards
Denial of Service Attacks
 Consist
of making repeated requests of a computer or
network device, thereby overloading it and causing
legitimate requests to be ignored
 Used to target particular companies or individuals
Worms
 A program
that copies itself repeatedly into a computer’s
memory or disk drive
 May copy itself so much it crashes the infected computer
 Famous worms include: Code Red, SQL Slammer,
Nimda, MyDoom, Sasser
 Primarily target PCs running Microsoft Windows
6-40
Cyberthreats, Hackers, &
Safeguards
Viruses
 A deviant
program that hides on a floppy, hard drive, CD,
or e-mail that causes unexpected side effects such as
destroying or corrupting data
 Viruses self-replicate and try to secretly distribute
themselves to other systems
 Famous viruses include the “I Love You” virus
 Viruses are published at the rate of about one per day
 To see what the latest ones are, go to
 http://www.symantec.com/avcenter/vinfodb.html
6-41
Cyberthreats, Hackers, &
Safeguards
Trojan Horses
 Programs
that pretend to be a useful program such as a
free game or screensaver.
 Carry viruses or malicious instructions that damage your
computer or install a backdoor or spyware
 Backdoors and spyware allow others to access your
computer without your knowledge
6-42
Cyberthreats, Hackers, &
Safeguards
How they spread
 Via
e-mail attachments
 By infected floppies or CDs
 By clicking on infiltrated websites
 By downloading from infected files from websites
 Through infiltrated Wi-Fi hotspots
 From one infected PC on a LAN to another
What can you do about it?
 Install
anti-virus software and subscribe to the automatic
anti-virus update service
6-43
Cyberthreats, Hackers, &
Safeguards
 Hackers are either
 Computer
enthusiasts, people who enjoy learning about
programming and computers (good)
 People who gain unauthorized access to computers or
networks, often for fun or to see if they can (not good)
 Crackers
 Malicious hackers who break into computers for malicious
purposes
Script kiddies are technically unsophisticated teenagers who use
downloadable software for perform break-ins
 Hacktivists are hacker activists who break into systems for a
political purpose
 Black-hat hackers are those who break into computers to steal or
destroy information or to use it for illegal profit
 Cyberterrorists attack computer systems so as to bring physical or
financial harm to groups, companies, or nations

6-44
Cyberthreats, Hackers, &
Safeguards
Safeguards
 Use antivirus software, and keep it current
 Install a firewall to monitor network traffic and
filter out
undesirable types of traffic and undesirable sites
 Use robust passwords –
 Minimum 8 characters with letters, numbers, characters
 4cats is not a good password, but f0UrK@tTz is
 Install antispyware software
 Encrypt financial and personal
records so only you can
read them
 Back up your data, so if your PC is attacked and must be
reformatted, you can restore your data
 Never download from a website you don’t trust
 Consider Biometric authentication
6-45
Cyberthreats, Hackers, &
Safeguards
Encryption
 The
process of altering readable data into unreadable
form to prevent unauthorized access
 Uses
powerful mathematical ciphers to create coded
messages that are difficult to break
 Two
forms:
 Private
Key encryption means the same secret key is used
by both the sender and receiver to encrypt and decrypt a
message
 Public Key encryption means that two keys are used
The public key of the recipient is published and is used by the
sender to encrypt the message
 The private key of the recipient is secret and is the only way to
decrypt the message
6-46

Future of Communications
This is a big area of development
There is a lot of money to be made from faster and
more secure broadband communications
Areas of development include
 Global
high-speed low—orbital satellite networks for rural
internet and voice connectivity
 4G wireless technology
 Photonics to speed up fiber-optic lines
 Software-defined radio
 Grid computing
6-47