Kĩ thuật lập trình - Communications, networks, & safeguards

1Communications, Networks, & Safeguards6.1 From the Analog to the Digital Age6.2 Networks6.3 Wired Communications Media6.4 Wireless Communications Media6.5 Cyberthreats, Hackers, & Safeguards6.6 The Future of CommunicationsChapter6McGraw-Hill/Irwin© 2007 The McGraw-Hill Companies, Inc. All rights reserved.6-2From the Analog to the Digital AgeAnalog signals use variation of a wave form to send informationThe wave forms look like the sine wave shown at rightIn 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 wayFrequency3From the Analog to the Digital AgeDigital signals send data in terms of 1s and 0sA digital signal may look like this one at rightNotice how the signal goes up and down abruptly because the only values that matter are 0 and 1 and the timing of when they occurComputers use digital signalsNewspaper photographs are digital since they are made up of little dots4From the Analog to the Digital AgeSince 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 lines5From the Analog to the Digital AgeTape recorders, voices, and musical instruments are analog while CDs are digitalTo 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 wave6From the Analog to the Digital AgeDigital 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!7NetworksA system of interconnected computers, telephones, or other communications devices that can communicate with one another and share applications and dataBefore 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 2Person 2 loaded the disk into their computer to read and edit the document8NetworksSince 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 9NetworksWAN – Wide Area NetworkMAN – Metropolitan Area NetworkLAN – Local Area NetworkHAN – Home Area NetworkPAN – Personal Area NetworkCovers a wide geographic area, such as a country or the worldCovers a city or a suburbConnects computers and devices in a limited geographic area such as an office, building, or group of nearby buildingsUses wired, cable, or wireless connections to link a household’s digital devicesUses short-range wireless technology to connect an individual’s personal electronics like cellphone, PDA, MP3 player, notebook PC, and printer10NetworksClient/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 emailPeer-to-PeerAll computers on the network communicate directly with each other without relying on a serverFor fewer than 25 PCs11NetworksPeer-to-Peer (continued)Cheaper than client/server since servers are typically more expensive than PCsThere are often problems with knowing who has the current version of documents and filesToo slow for use in larger officesLegal considerationsDownloading copyrighted material without paying violates U.S. copyright lawsServer-based online file sharing sites such as Napster have been shut downPeer-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 downSo publishers are suing individual downloaders insteadWatch out!Don’t download illegally!12Networks Intranets, Extranets, VPNsIntranetsAn organization’s private network that uses the infrastructure and standards of the internet and the webExtranetsPrivate internets that connect not only internal personnel but also selected suppliers and other strategic partiesVirtual Private NetworksPrivate networks that use a public network, usually the internet, to connect remote sites 13Network ComponentsConnectionsWired – twisted-pair, coaxial cable, or fiber-opticWireless – infrared, microwave (Bluetooth), broadcast (Wi-Fi) or satelliteHosts & NodesHost: the central computer that controls the networkNode: a device that is attached to the networkPacketsThe format for sending electronic messagesA fixed-length block of data for transmission14Network ComponentsProtocolsThe set of conventions governing the exchange of data between hardware and/or software components in a communications networkBuilt into the hardware or software you are usingGovern the packet design and transmission standardsExamples are:TCP/IP for LANs and internetAppleTalk for older Mac networksSIP for Voice over IP (VoIP)CDMA for cellphonesIPX for older Novell networks15Network PacketsTCP/IP Packets carry four types of informationSender’s address (source IP number)Address of intended recipient (destination IP number)Number of packets the original data was broken intoThis happens because the amount of data the PC is sending can be much larger than the space in a single packetSo the data has to get broken up in one or more packetsThen 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 6Packet number and sequence info for each packetPackets 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 read16Network DevicesDaisy ChainHubSwitchBridgeGatewayRouterBackboneUsed in peer-to-peer networks – direct connections from one PC to the nextUsed in small LANs to connect PCs and LAN segments to each other. Forwards to all portsUsed in larger, busy LANs – faster than hubs because it forwards only to correct destinationUsed 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 organization17Network TopologiesBus – 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 host18NetworkPacket Collision SchemesCollisions happen when two data packets are going opposite directions on shared mediaEthernet – deals with collisionsAll devices send data at onceCollisions happen regularlyData is simply resent until it arrivesToken ring – avoids collisionsDevices take turns sending dataToken is sent around the ringWait to get the token, then send data$$$ Pricier than Ethernet19Wired Communications MediaCommunications media carry signals over a communications pathTwisted-Pair Wire2 strands of insulated copper wire twisted around each otherTwisting reduces interference (crosstalk) from electrical signalsData rates are 1 – 128 Megabits per secondCoaxial CableInsulated copper wire wrapped in a metal shield and then in an external plastic coverUsed for cable TV and cable internet electric signalsCarries voice and data up to 200 megabits per second20Wired Communications MediaCommunications media continuedFiber-optic cableDozens or hundreds of thin strands of glass or plastic that transmit beams of light, not electricityCan transmit up to 2 gigabits per secondMore expensive than twisted-pair or coaxLighter and more durable than twisted-pair or coaxMore difficult to tap into than twisted-pair or coax21Wired Communications Mediafor the homeEthernetPull 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 all10 or 100 megabits per secondHomePNAUses existing telephone wiring and jacksRequires HomePNA nic in your PCSpeeds of 10 – 240 megabits per secondHomeplugUses existing home electrical linesSpeeds of 14 megabits per second22Wireless Communications MediaElectromagnetic spectrum of radiation is the basis of all telecommunications signalsIncludes 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 communication23Wireless Communications MediaBandwidthNarrowband (or Voiceband)Used for regular telephone communicationsTransmission rate 480 megabits per second37Wireless Communications MediaShort-Range Wireless for HomeInsteonCombines electronic powerline and wireless technologyCan send data at 13.1 kilobits per second with 150 ft rangeReplaces X10ZigBeeEntirely wireless very power-efficient technologyCan send data at 128 kilobits per second with 250 ft rangeZ-WaveEntirely wireless power-efficient technologyCan send data at 127 kilobits per second to range of 100 ftAllows you to remotely program your house!38Cyberthreats, Hackers, & SafeguardsProblem: 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.39Cyberthreats, Hackers, & SafeguardsDenial of Service AttacksConsist of making repeated requests of a computer or network device, thereby overloading it and causing legitimate requests to be ignoredUsed to target particular companies or individualsWormsA program that copies itself repeatedly into a computer’s memory or disk driveMay copy itself so much it crashes the infected computerFamous worms include: Code Red, SQL Slammer, Nimda, MyDoom, SasserPrimarily target PCs running Microsoft Windows40Cyberthreats, Hackers, & SafeguardsVirusesA deviant program that hides on a floppy, hard drive, CD, or e-mail that causes unexpected side effects such as destroying or corrupting dataViruses self-replicate and try to secretly distribute themselves to other systemsFamous 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 41Cyberthreats, Hackers, & SafeguardsTrojan 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 knowledge42Cyberthreats, Hackers, & SafeguardsHow 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 anotherWhat can you do about it?Install anti-virus software and subscribe to the automatic anti-virus update service43Cyberthreats, Hackers, & SafeguardsHackers are eitherComputer 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)CrackersMalicious hackers who break into computers for malicious purposesScript kiddies are technically unsophisticated teenagers who use downloadable software for perform break-insHacktivists are hacker activists who break into systems for a political purposeBlack-hat hackers are those who break into computers to steal or destroy information or to use it for illegal profitCyberterrorists attack computer systems so as to bring physical or financial harm to groups, companies, or nations44Cyberthreats, Hackers, & SafeguardsSafeguardsUse 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, characters4cats is not a good password, but f0UrK@tTz isInstall 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 authentication45Cyberthreats, Hackers, & SafeguardsEncryptionThe process of altering readable data into unreadable form to prevent unauthorized accessUses 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 messageThe private key of the recipient is secret and is the only way to decrypt the message 46Future of CommunicationsThis is a big area of developmentThere is a lot of money to be made from faster and more secure broadband communicationsAreas of development includeGlobal high-speed low—orbital satellite networks for rural internet and voice connectivity4G wireless technologyPhotonics to speed up fiber-optic linesSoftware-defined radioGrid computing47