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 CommunicationsModes of CommunicationsComputer 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 CommunicationsLike most computing, Layers Upon LayersHardware LayerSoftware Layers (several)Basic CommunicationsIn binaryRepresented by something over some medium by somethingStarted with terminals connect to computersCommunicated 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 CommunicationsMedium/HardwareFixed (hard wired)Telephone (dial up)Broadband Wired (phone: DSL, cable, LAN)Wireless (local, cell, satellite)TopographyStarPoint to PointRingBusNetworkCompSci 437.4Computer CommunicationsConnection ModesCircuit SwitchedPhysical connection madeE.g., telephone systemExclusive use of dedicated part of mediumInteractive use possibleMessage SwitchedStore and forward (email)Batch (non-interactive)Shared facilitiesPacket SwitchedShared mediumInteractive use possibleCompSci 437.5Computer CommunicationsEthernet (Bus Example)Also loosely called Local Area Network (LAN)Many nodes (hosts, computers, stations…) on same busEach node has an addressCalled machine or Mac addressGuaranteed to be unique!All messages “broadcast” in packets with destination (dst) and source (src) address(See diagrams on web)Everyone “listens” for their addressPotential security problem!Collisions possibleRandom back-off on collisionCompSci 437.6Computer CommunicationsInternets – Networks of LANsCan interconnect multiple LAN’s using a “LAN”Could be a WAN (wide area network)Interconnected LAN’s result in internetNote: not the same as the Internet(See diagrams on web)Packets: the currency of the InternetAt all levels, packets allow oSharing of bandwidthoDiscrete units of workoError checking and correctionEach packet includes destination and source addressCompSci 437.7Computer CommunicationsTCP/IP: The Layers of the InternetTCP/IP is the family of protocols used on the InternetMultiple 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 PacketsUse of the TCP/IP layers requires nesting of packetsEach packet on one layer carries the info of the next layer(s) as data(See the diagram on the web)ReliabilityChecksums to verify correctnessAcknowledgementsoHandshaking protocolsRe-sends as required when error detectedoErrors cut down effective bandwidthoErrors degrade (increase) response timeCompSci 437.9Computer CommunicationsTCP/IP: AddressingHardware Address (Ethernet Address)Unique 6 octet (8 bit byte) numberUsually 6 groups of two hex digits: e.g., b3 fe 13 21 a5 41o(Hexadecimal numbers …)Centrally administeredIP Address data4 octet number: e.g., 152.3.141.45Usually 4 decimal numbers separated by periods2 partsoNetwork partoHost partoVariable dividing line between network and host portionCentrally administeredCompSci 437.10Computer CommunicationsTCP/IP: Addressing (continued)Domain NameHierarchical systemRoot:oedu, com, gov, mil, net, org, or country codehost-name.sub-organization.organization. …E.g., dollar.cs.duke.eduPerson on host is, e.g., [email protected]Centrally administeredUse of these addressesEventually need hardware addressMost address automatically locatedHandling changes …CompSci 437.11Computer CommunicationsApplicationsStore and ForwardEmailNewsInteractive Utilitiesftp (file transport protocol) big before weboAnonymousoArchivesoFree softwareoWeather mapstelnet (secure form is ssh)oUse a remote computer interactivelyCompSci 437.12Computer CommunicationsApplicationsInteractive CommunicationsTalk (many variations: chat, AIM, …) IRCLD Telephone replacementsAmateur radio linksInformation servicesWWWOlder Predecessors: gopher, WAIS“Sharing” systemsLegality?CompSci 437.13Computer CommunicationsClient / Server ModelPrint ServerShare Expensive (was once) Printer oMachine with printer provides serveroOther machines with clients can use remote printerFile ServerUNIX: remote file systemsoNetwork File System (NFS): Computer Science uses thisoAndrews File System (AFS): OIT uses this Name ServerSupports domain addressingWeb ServerThe basis for all web accessClients like Netscape, Internet Explorer, new ones
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