Network technology November 12, 1998Course Theme“Harsh Realities” of Computer ScienceComputer systemSimple exampleQuestions about simple exampleGeneric networkProtocolsProtocol layeringEncapsulationProtocol stacksTransmission mediaShared vs switched mediaNetwork performance measuresExample performance measuresImportance of Overhead (+ Latency)Basic network typesAT&T Telephone HierarchyComputer-to-computer callsModulating digital signalsQuadrature amplitude modulation (QAM)Conventional ModemsT1 carrier (1.544 Mb/s)56KB “Modems”EthernetEthernet cablingRepeatersBridgesEthernet packet (frame) formatEthernet receiving algorithmEthernet sending algorithm (CSMA/CD)Binary exponential backoffWhy the 64 byte minimum packet size?Ethernet pros and consAsynchronous transfer mode (ATM)ATM overview (cont)ATM cell formatATM cell routingATM pros and consNetwork technologyNovember 12, 1998Topics•Overview•Telephone system•Ethernet•ATM15-213class24.pptCS 213 F’98– 2 –class24.pptCourse ThemeAbstraction is good, but don’t forget reality!Earlier courses to date emphasize abstraction•Abstract data types•Asymptotic analysisThese abstractions have limits•Especially in the presence of bugs•Need to understand underlying implementationsUseful outcomes•Become more effective programmers–Able to find and eliminate bugs efficiently–Able to tune program performance•Prepare for later “systems” classes–Compilers, Operating Systems, Networks, Computer ArchitectureCS 213 F’98– 3 –class24.ppt“Harsh Realities” of Computer Science•Int’s are not integers; float’s are not reals–Must understand characteristics of finite numeric representations•You’ve got to know assembly–Basis for understanding what really happens when execute program•Memory matters–Memory referencing bugs especially difficult»Violates programming language abstraction–Significant performance issues»E.g., cache effects•There’s more to performance than asymptotic complexity–Constant factors also matter•Computers do more than execute programs–Get data in and out–Communicate with each other via networksCS 213 F’98– 4 –class24.pptdiskcdromdiskDiskComputer systemLocal/IO BusLocal/IO BusMemoryMemoryNetworkadaptorNetworkadaptorIDE diskcontrollerIDE diskcontrollerVideoadaptorVideoadaptorDisplayDisplayNetworkNetworkProcessorand L1 cacheProcessorand L1 cacheInterruptcontrollerInterruptcontrollerSCSIcontrollerSCSIcontrollerSCSI busSCSI busSerial port controllerSerial port controllerParallel portcontrollerParallel portcontrollerKeyboardcontrollerKeyboardcontrollerKeyboardKeyboardMouseMousePrinterPrinterModemModemdiskDiskCS 213 F’98– 5 –class24.pptSimple exampleStarting Point: Want to send bits between 2 computers•FIFO queue on each end•Can send both ways (“full duplex”)•Name for standard group of bits sent: “packet”•Packet format and rules for communicating them (“protocol”)Simple request/response protocol and packet format:0/1 data/address0: please send the data word at “address”1: here is the data word you asked for.headerpayloadCS 213 F’98– 6 –class24.pptQuestions about simple exampleWhat if more than 2 computers want to communicate?•Need computer address field in packet?How do multiple machines share the interconnect?•multiple paths? arbitration? congestion control?What if a packet is garbled in transit?•Add error detection field in packet?What if a packet is lost?•More elaborate protocols to detect loss?What if multiple processes/machine?•one queue per process? separate field in packet to identify process?Warning: You are entering a buzzword-rich environment!!!CS 213 F’98– 7 –class24.pptGeneric networks/w interfaces/w interfaceh/w interfaceh/w interfaceInterconnectInterconnects/w interfaces/w interfaceh/w interfaceh/w interfaces/w interfaces/w interfaceh/w interfaceh/w interfacelink link linkhosthost hostprotocol stackadaptor/interface cardkernel codeCS 213 F’98– 8 –class24.pptProtocolsA protocol defines the format of packets and the rules for communicating them across the network.Different protocols provide different levels of service:•simple error correction (ethernet)•uniform name space, unreliable best-effort datagrams (host-host) (IP)•reliable byte streams (TCP)•unreliable best-effort datagrams (process-process) (UDP)•multimedia data retrieval (HTTP)CS 213 F’98– 9 –class24.pptProtocol layeringProtocols provide specialized services by building onservices provided by other protocols.Reliable byte streamdelivery(process-process)Unreliablebest effort datagramdelivery(host-host)Unreliablebest effortdatagramdelivery(process-process)Application (FTP, Telnet, WWW, email)Application (FTP, Telnet, WWW, email)User datagram protocol(UDP)User datagram protocol(UDP)Transmission control protocol(TCP)Transmission control protocol(TCP)Internet Protocol (IP)Internet Protocol (IP)Network interface (ethernet, ATM)Network interface (ethernet, ATM)hardwarehardwarePhysical connectionCS 213 F’98– 10 –class24.pptEncapsulationTCP segment headerTCP segment headerdatadatadatadataEthernet frameheaderEthernet frameheaderIP datagramheaderIP datagramheaderTCP segment headerTCP segment headerdatadataIP datagramheaderIP datagramheaderTCP segment headerTCP segment headerdatadataApplicationTCPIPNetwork interfaceCS 213 F’98– 11 –class24.pptProtocol stacksRepeaters/Bridges/RoutersHost A Host BXmit raw bitsFraming, Errorrecovery, mediaaccessRoutingFlow controlCongestionTelnet, FTP, HTTP, emailIPTCP/UDPapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalReliable, efficientend user servicenetworkdata linkphysicalnetworkdata linkphysicalCSMA/CD10Base-TCS 213 F’98– 12 –class24.pptTransmission media2 insulated copper wirestwisted pair:twisted pair:(1-2 Mb/s at 1 km)stiff copper wireinsulatorbraided outer conductorplastic covercoaxial cable:coaxial cable:(1-2 Gb/s at 1 km)silicalight sourcefiber:fiber:(100-200 Gb/s at 1 km)station wagon full of mag tapes hurtling down the highway:station wagon full of mag tapes hurtling down the highway:(15 Gb/s at 1 hour)7 GBytes/tape1000 tapes/station wagon (50x50x50cm)7,000 GBytes total7,000 GBytes/3600 minutes = 15 Gb/s$5/tape reused 10 times -> $500 tape cost$200 for shipping ->10 cents /GByteCS 213 F’98– 13 –class24.pptShared vs switched mediaShared media (e.g., Ethernet) Switched media (e.g.,
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