6-1. communications, networks, & safeguards 6.1 from the analog to the digital age 6.2 networks...
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6-1
Communications, Communications, Networks, & Networks, & SafeguardsSafeguards
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
Chapte
r66
McGraw-Hill/Irwin © 2007 The McGraw-Hill Companies, Inc. All rights reserved.
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From the Analog to the Digital AgeFrom 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 wayLight works this wayHearing and phones work this
wayModems work this way
S1
-1
-0.5
0
0.5
1
Amplitude
Sine Wave
Frequency
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From the Analog to the Digital AgeFrom the Analog to the Digital Age
Digital signals send data in 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 signalsNewspaper photographs are
digital since they are made up of little dots
1 5 9
13 17 21 25 29 33 37 41
S10
1
Time
Data
Digital Signal
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From the Analog to the Digital AgeFrom 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 endThe process is called “modulation/demodulation”Modulation means to translate from digital to analogDemodulation means to translate from analog to digitalModems have to do all this just to use standard analog
voice phone lines
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From the Analog to the Digital AgeFrom 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 digitalThe analog-to-digital converter samples the sound and
converts the height of the wave to a numberSamples of the sound wave are taken at regular intervals
– about 44,100 times each secondBecause the digital samples are played back faster than
our ears can react, it sounds to us like a single continuous sound wave
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From the Analog to the Digital AgeFrom the Analog to the Digital Age
Digital sampling is similar to showing moviesMovies 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
motionDid 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!
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NetworksNetworks
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 computersPerson 1 saved their document to a floppy diskThen 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
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NetworksNetworks
Since the days of “sneakernet”, networks have become standard. They enable us to:Share peripheral devices such as laser printersShare programs and dataUse e-mail and other communication programsBackup critical information because it is stored centrallyAccess shared databases
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NetworksNetworks WAN – Wide Area Network
MAN – Metropolitan Area Network
LAN – Local Area Network
HAN – Home Area Network
PAN – Personal Area Network
Covers a wide geographic area, such as a country or the world
Covers a city or a suburb
Connects computers and devices 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, notebook PC, and printer
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NetworksNetworks
Client/ServerConsists of clients, which are computers that request
data, and servers, which are computers that supply dataFile servers act like a network-based shared disk driveDatabase servers store data but don’t store programsPrint servers connect one or more printers and schedule
and control print jobsMail servers manage email
Peer-to-PeerAll computers on the network communicate directly with
each other without relying on a serverFor fewer than 25 PCs
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NetworksNetworksPeer-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 officesLegal 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! Don’t download illegally!
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NetworksNetworks Intranets, Extranets, VPNsIntranets, Extranets, VPNs
IntranetsAn organization’s private network that uses the
infrastructure and standards of the internet and the webExtranets
Private internets that connect not only internal personnel but also selected suppliers and other strategic parties
Virtual Private NetworksPrivate networks that use a public network, usually the
internet, to connect remote sites
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NetworkNetwork ComponentsComponents
ConnectionsWired – twisted-pair, coaxial cable, or fiber-opticWireless – infrared, microwave (Bluetooth), broadcast
(Wi-Fi) or satelliteHosts & Nodes
Host: the central computer that controls the networkNode: a device that is attached to the network
PacketsThe format for sending electronic messagesA fixed-length block of data for transmission
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NetworkNetwork ComponentsComponents
ProtocolsThe 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 usingGovern the packet design and transmission standardsExamples 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
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NetworkNetwork PacketsPackets
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
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NetworkNetwork DevicesDevices
Daisy ChainHub
Switch
BridgeGatewayRouter
Backbone
Used in peer-to-peer networks – direct 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 typeConnects two networks of different typesConnects multiple LANs together. Routers are
the internet backboneThe main route that connects all the gateways,
routers, and other computers in an organization
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NetworkNetwork TopologiesTopologies
Bus – all nodes are connected to a single wire or cableRing – all nodes are connected in a continuous loopStar – all nodes are connected through a central host
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NetworkNetworkPacket Collision SchemesPacket Collision Schemes
Collisions happen when two data packets are going opposite directions on shared media
Ethernet – deals with collisionsAll devices send data at onceCollisions happen regularlyData is simply resent until it arrives
Token ring – avoids collisionsDevices take turns sending dataToken is sent around the ringWait to get the token, then send data
$$$ Pricier than Ethernet
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Wired Communications MediaWired Communications Media
Communications media carry signals over a communications pathTwisted-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
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Wired Communications MediaWired Communications Media
Communications media continuedFiber-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
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Wired Communications MediaWired Communications Mediafor the homefor 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
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Wireless Communications Wireless Communications MediaMedia
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
6-24
Wireless Communications MediaWireless Communications MediaBandwidthBandwidth
Narrowband (or Voiceband)Used for regular telephone communicationsTransmission rate < 100 kilobits per second
Medium BandUsed for long-distance data transmission or to
connect mainframe and midrange computersTransmission rate 100 kb to 1 megabit per second
BroadbandFor high-speed data and high-quality audio and videoTransmission rate 1 megabit per second to 100
megabits per secondUS households get 4 – 5 MB while Japanese get 100
MB per second
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Wireless Communications MediaWireless Communications Media
Infrared TransmissionSends signals using infrared lightFrequencies are too low to see (1-4 Mbits per second)
Broadcast RadioAM/FM, CB, ham, cellphones, police radioSends data over long distances using a transmitter and a
receiverMicrowave Radio
Superhigh frequency radio waves (1 gigahertz)Requires line-of-sight transmitters and receivers
Communications Satellites
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Wireless Communications MediaWireless Communications Media
Communications Satellites These are microwave relay stations in orbit around the earth -
Uplinking: transmitting a signal from ground station to a satellite Cover 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 22,300 miles up above earth Always above equator
MEO – medium-earth orbit 5,000 – 10,000 miles up
LEO – low-earth orbit 200 – 1,000 miles up Has less signal delay than GEO, MEO satellites
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Wireless Communications MediaWireless Communications MediaGPSGPS
Global Positioning System24 earth-orbiting satellites continuously transmitting timed
radio signalsEach satellite circles earth twice each day at 11,000 miles
upGPS receivers pick up transmissions from up to 4 satellites
and pinpoint the receiver’s locationAccurate within 3 – 50 feet, with a norm of 10 feet accuracyGPS receivers contain map files that are displayed based
on the GPS position to guide usersMany GPS receivers have speech chips, too
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Wireless Communications MediaWireless Communications MediaOne-wayOne-way PagersPagers
One-way pagers are radio receivers that receive data sent from a special radio transmitterRadio transmitter sends out signals over the special
frequencyPagers are tuned to that frequencyWhen a particular pager hears its own code, it receives
and displays the messageDiscussion 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
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Wireless Communications MediaWireless Communications MediaLong-Distance WirelessLong-Distance Wireless
Two-way pagers: Blackberry and Treo1G: First Generation Cellular
Analog cellphonesDesigned 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 off2G: 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
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Wireless Communications MediaWireless Communications Media2G Wireless2G Wireless
There are two competing, incompatible standardsCDMA – 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
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Wireless Communications MediaWireless Communications Media2.5G Wireless2.5G Wireless
Data speeds of 300–100 kilobits per secondGPRS – 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
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Wireless Communications MediaWireless Communications Media3G Wireless3G Wireless
Third generation wirelessHigh speed data: 144 kilobits per second up to 2
megabits per secondAccept e-mail with attachmentsDisplay color video and still picturesPlay music Two important upgrades:
EV-DO – Evolution Data Only Average speeds of 400 – 700 kilobits per second, peaks of 2
megabits per second UMTS – Universal Mobile Telecommunications System
Average speed of 220 – 320 kilobits per second
Discussion Question: If your cellphone can download and play music, do you still need an i-pod?
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Wireless Communications MediaWireless Communications MediaShort-range WirelessShort-range Wireless
Local Area NetworksRange 50 – 150 feetInclude Wi-Fi (802.11) type networks
Personal Area NetworksRange 30 – 32 feetUse Bluetooth, Ultra wideband, and wireless USB
Home Automation networksRange 100 – 250 feetUse Insteon, Zigbee, and Z-Wave standards
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Wireless Communications MediaWireless Communications MediaShort-range WirelessShort-range Wireless
Wi-Fi (802.11) networksWi-Fi b, a, and g correspond to 802.11b, 802.11a, and
802.11g802.11 is an IEEE wireless technical specification802.11b is older, transmits 11 megabits per second 802.11a is faster than b but with weaker security than g802.11g is 54 megabits per second and transmits 50 ftWi-Fi n with MIMO extends range of Wi-Fi using multiple
transmitting and receiving antennas – 200 megabits per second for up to 150 ft
Warning! Security is disabled by default on Wi-Fi
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Wireless Communications MediaWireless Communications MediaShort-range WirelessShort-range Wireless
Wi-Fi SecurityWhy is it disabled by default?
So non-technical users can get Wi-Fi working more easilyWhy 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/
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Wireless Communications MediaWireless Communications MediaPersonal Area WirelessPersonal Area Wireless
BluetoothShort-range wireless standard to link cellphones, PDAs,
computers, and peripherals at distances up to 30 ftNamed after King Harald Bluetooth, the Viking who
unified Denmark and NorwayTransmits 720 kilobits per secondWhen 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 networksTurn it off on your cellphone unless you need it at that
time
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Wireless Communications MediaWireless Communications MediaPersonal Area WirelessPersonal Area Wireless
Ultra Wideband (UWB)Developed for military radar systemsOperates in 480 megabit per second range up to 30 ftUses a low power source to send out millions of bursts of
radio waves each second100 times as fast as Bluetooth
Wireless USBUSB is the most used interface on PCsThe wireless version could be a hitRange of 32 ft and maximum data rate of > 480 megabits
per second
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Wireless Communications MediaWireless Communications MediaShort-Range Wireless for HomeShort-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!
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Cyberthreats, Hackers, & Cyberthreats, Hackers, & SafeguardsSafeguards
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.
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Cyberthreats, Hackers, & Cyberthreats, Hackers, & SafeguardsSafeguards
Denial of Service AttacksConsist 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 individualsWorms
A program that copies itself repeatedly into a computer’s memory or disk drive
May copy itself so much it crashes the infected computerFamous worms include: Code Red, SQL Slammer,
Nimda, MyDoom, SasserPrimarily target PCs running Microsoft Windows
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Cyberthreats, Hackers, & Cyberthreats, Hackers, & SafeguardsSafeguards
VirusesA 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” virusViruses are published at the rate of about one per dayTo see what the latest ones are, go to
http://www.symantec.com/avcenter/vinfodb.html
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Cyberthreats, Hackers, & Cyberthreats, Hackers, & SafeguardsSafeguards
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 spywareBackdoors and spyware allow others to access your
computer without your knowledge
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Cyberthreats, Hackers, & Cyberthreats, Hackers, & SafeguardsSafeguards
How they spreadVia e-mail attachmentsBy infected floppies or CDsBy clicking on infiltrated websitesBy downloading from infected files from websitesThrough infiltrated Wi-Fi hotspotsFrom 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
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Cyberthreats, Hackers, & Cyberthreats, Hackers, & SafeguardsSafeguards
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
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Cyberthreats, Hackers, & Cyberthreats, Hackers, & SafeguardsSafeguards
SafeguardsUse antivirus software, and keep it currentInstall a firewall to monitor network traffic and filter out
undesirable types of traffic and undesirable sitesUse robust passwords –
Minimum 8 characters with letters, numbers, characters 4cats is not a good password, but f0UrK@tTz is
Install antispyware softwareEncrypt financial and personal records so only you can
read themBack up your data, so if your PC is attacked and must be
reformatted, you can restore your dataNever download from a website you don’t trustConsider Biometric authentication
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Cyberthreats, Hackers, & Cyberthreats, Hackers, & SafeguardsSafeguards
EncryptionThe process of altering readable data into unreadable
form to prevent unauthorized access Uses powerful mathematical ciphers to create coded
messages that are difficult to breakTwo 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
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Future of CommunicationsFuture of Communications
This is a big area of developmentThere is a lot of money to be made from faster and
more secure broadband communicationsAreas of development include
Global high-speed low—orbital satellite networks for rural internet and voice connectivity
4G wireless technologyPhotonics to speed up fiber-optic linesSoftware-defined radioGrid computing
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