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Duke CPS 004 - Computer Communications

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Computer CommunicationsSlide 2Basic CommunicationsConnection ModesEthernet (Bus Example)Internets – Networks of LANsTCP/IP: The Layers of the InternetTCP/IP: Nesting of PacketsTCP/IP: AddressingTCP/IP: Addressing (continued)ApplicationsSlide 12Client / Server ModelCompSci 437.1Computer CommunicationsComputer CommunicationsModes of CommunicationsComputer and humans communicateoEarly “time sharing”Computers talk to each otheroARPA Net (predecessor of Internet)Now, Humans use computer to talk to other humansoEmail, chat, ..oNews: started at Duke Computer ScienceoIRCoVoiceoVideoCompSci 437.2Computer CommunicationsComputer CommunicationsLike most computing, Layers Upon LayersHardware LayerSoftware Layers (several)Basic CommunicationsIn binaryRepresented by something over some medium by somethingStarted with terminals connect to computersCommunicated by voltages (currents) on wires (teletype)Then modems allowed communication over phone linesoTurn binary representing voltage into audio tonesoDial-up computer communicationsoTime sharing systemsoBulletin boardsoUSENET (first at Duke, UNC, then Bell Labs … the world!)CompSci 437.3Computer CommunicationsBasic CommunicationsMedium/HardwareFixed (hard wired)Telephone (dial up)Broadband Wired (phone: DSL, cable, LAN)Wireless (local, cell, satellite)TopographyStarPoint to PointRingBusNetworkCompSci 437.4Computer CommunicationsConnection ModesCircuit SwitchedPhysical connection madeE.g., telephone systemExclusive use of dedicated part of mediumInteractive use possibleMessage SwitchedStore and forward (email)Batch (non-interactive)Shared facilitiesPacket SwitchedShared mediumInteractive use possibleCompSci 437.5Computer CommunicationsEthernet (Bus Example)Also loosely called Local Area Network (LAN)Many nodes (hosts, computers, stations…) on same busEach node has an addressCalled machine or Mac addressGuaranteed to be unique!All messages “broadcast” in packets with destination (dst) and source (src) address(See diagrams on web)Everyone “listens” for their addressPotential security problem!Collisions possibleRandom back-off on collisionCompSci 437.6Computer CommunicationsInternets – Networks of LANsCan interconnect multiple LAN’s using a “LAN”Could be a WAN (wide area network)Interconnected LAN’s result in internetNote: not the same as the Internet(See diagrams on web)Packets: the currency of the InternetAt all levels, packets allow oSharing of bandwidthoDiscrete units of workoError checking and correctionEach packet includes destination and source addressCompSci 437.7Computer CommunicationsTCP/IP: The Layers of the InternetTCP/IP is the family of protocols used on the InternetMultiple Layers are Defined1. The Physical LayeroHardware (e.g., Ethernet - - - other used)oThe Ethernet packet2. The IP (Internet Protocol) LayeroThe IP PacketoProvides hardware independence3. The TCP (Transmission Control Protocol) LayeroThe TCP packetoValue added (connections, guaranteed results, …)4. The Application LayeroE.g., email, news, …CompSci 437.8Computer CommunicationsTCP/IP: Nesting of PacketsUse of the TCP/IP layers requires nesting of packetsEach packet on one layer carries the info of the next layer(s) as data(See the diagram on the web)ReliabilityChecksums to verify correctnessAcknowledgementsoHandshaking protocolsRe-sends as required when error detectedoErrors cut down effective bandwidthoErrors degrade (increase) response timeCompSci 437.9Computer CommunicationsTCP/IP: AddressingHardware Address (Ethernet Address)Unique 6 octet (8 bit byte) numberUsually 6 groups of two hex digits: e.g., b3 fe 13 21 a5 41o(Hexadecimal numbers …)Centrally administeredIP Address data4 octet number: e.g., 152.3.141.45Usually 4 decimal numbers separated by periods2 partsoNetwork partoHost partoVariable dividing line between network and host portionCentrally administeredCompSci 437.10Computer CommunicationsTCP/IP: Addressing (continued)Domain NameHierarchical systemRoot:oedu, com, gov, mil, net, org, or country codehost-name.sub-organization.organization. …E.g., dollar.cs.duke.eduPerson on host is, e.g., [email protected]Centrally administeredUse of these addressesEventually need hardware addressMost address automatically locatedHandling changes …CompSci 437.11Computer CommunicationsApplicationsStore and ForwardEmailNewsInteractive Utilitiesftp (file transport protocol) big before weboAnonymousoArchivesoFree softwareoWeather mapstelnet (secure form is ssh)oUse a remote computer interactivelyCompSci 437.12Computer CommunicationsApplicationsInteractive CommunicationsTalk (many variations: chat, AIM, …) IRCLD Telephone replacementsAmateur radio linksInformation servicesWWWOlder Predecessors: gopher, WAIS“Sharing” systemsLegality?CompSci 437.13Computer CommunicationsClient / Server ModelPrint ServerShare Expensive (was once) Printer oMachine with printer provides serveroOther machines with clients can use remote printerFile ServerUNIX: remote file systemsoNetwork File System (NFS): Computer Science uses thisoAndrews File System (AFS): OIT uses this Name ServerSupports domain addressingWeb ServerThe basis for all web accessClients like Netscape, Internet Explorer, new ones


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Duke CPS 004 - Computer Communications

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