计算机网络.docx
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计算机网络.docx
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计算机网络
•IntroductiontoComputerNetworks
•ThingsthatMakeComputerNetworksFascinatingandChallenging
•Mostpeopleuseit,butfewunderstandit(blackbox)
•Scale
–Billionsofusers,manyattackers
–Thousandsofapplications
–Hundredsofthousandsofrouters,millionsofswitches
–Hundredsofmillionsofendhosts(computer,phone,…)
•Complexity
–Networkhasmanyfunctions
–Networkismadeofmanytechnologies
–Networkhascomplexstructureanddynamics
•Distributed
–Networkisshared
–Networkhasnoonecentralcoordinationpoint
–Networkhasmanydynamicagents(applications,OS,humans)
–Routerscannotdotoomuchorwillbetooslow
•HistoryofComputerNetworks
•Communication
•Telecommunication
•Telecommunicationnetwork
•Computernetwork
•Convergencenetwork
•EarlyCommunicationoverLongDistance
•Betweenhumanbeings
•Letterandmessenger
–Informationcarriedbyphysicalobjects
–Speedlimitedbytransportationmeans:
horse,bird,train,car
–HarddisksoverFedEx?
–Bandwidth?
distance?
security?
•Fire
–Earlyopticalcommunication
–Speedoflight
–Bandwidth?
distance?
security?
•Telegraph:
CommunicationUsingElectrons
•Betweenhumanbeings
•Majormilestones:
–1827:
Ohm’sLaw
–1837:
“workable”telegraphinventedbySamuelMorse
–1838:
demonstrationover10milesat10w.p.m.
–1844:
CapitolHilltoBaltimore
–1851:
WesternUnionfounded
–1868:
transatlanticcablelaid
–1985:
lasttelegraphcircuitcloseddown
•Otherimportantdates
–1869:
transcontinentalrailway
–1876:
AlexanderBellinventedtelephone
•TelegraphEngineering
•Technicalissues
–Howtoencodeinformation?
–Howtofeed/inputinformationtothesystem?
–Howtooutputinformation?
–Howtoimprovethedistance?
–Howtoimprovethespeed?
•Commonissuesfacedbyalltelecommunicationsystems
•Telephony
•Interactivetelecommunicationbetweenpeople
•Analogvoice
–Transmitter/receivercontinuouslyincontactwithelectroniccircuit
–Electriccurrentvarieswithacousticpressure
•TelephonyMilestones
•1876:
AlexanderBellinventedtelephone
•1878:
PublicswitchesinstalledatNewHavenandSanFrancisco,publicswitchedtelephonenetworkisborn
–Peoplecantalkwithoutbeingonthesamewire!
•TelephonyMilestones
•1878:
Firsttelephonedirectory;WhiteHouseline
•1881:
Insulated,balancedtwistedpairaslocalloop
•1885:
AT&Tformed
•1892:
Firstautomaticcommercialtelephoneswitch
•1903:
3milliontelephonesinU.S.
•1915:
Firsttranscontinentaltelephoneline
•1927:
Firstcommercialtransatlanticcommercialservice
•TelephonyMilestones
•1937:
Multiplexingintroducedforinter-citycalls
–Onelinkcarriesmultipleconversations
•Summary
•Communicationcamelongbeforecomputer
•Evolutionsofmoderncommunicationandcomputerintertwined
•Importantconcepts
–Switching
–Multiplexing
–Analogvs.Digital
–Bandwidth
Latency
•DataorComputerNetworks
•Networksdesignedforcomputerstocomputersordevices
–vs.communicationbetweenhumanbeings
•Digitalinformation
vs.analogvoice
•WhatisaCommunicationNetwork?
(fromend-systempointofview)
•Networkoffersaservice:
moveinformation
–Bird,fire,messenger,truck,telegraph,telephone,Internet…
•Whatdistinguishdifferenttypesofnetworks?
–Theservicestheyprovide
•Whatdistinguishtheservices?
–Latency
–Bandwidth
–Lossrate
–Numberofendsystems
–Serviceinterface
–Otherdetails
•Reliability,unicastvs.multicast,real-time,messagevs.byte...WhatisaCommunicationNetwork?
(InfrastructureCentricView)
•Communicationmedium:
electron,photon
•Networkcomponents:
–Links–carrybitsfromoneplacetoanother(ormaybemultipleplaces):
fiber,copper,satellite,…
–Interfaces–attachdevicestolinks
–Switches/routers–interconnectlinks:
electronic/optic,crossbar/Banyan
–Hosts–communicationendpoints:
workstations,PDAs,cellphones,toasters
•Protocols–rulesgoverningcommunicationbetweennodes
–TCP/IP,ATM,MPLS,SONET,Ethernet,X.25
•Applications:
Webbrowser,XWindows,FTP,...
•NetworkComponents(Examples)
•JuniperRouters
•TypesofNetworks
•Geographicaldistance
–LocalAreaNetworks(LAN):
Ethernet,Tokenring,FDDI
–MetropolitanAreaNetworks(MAN):
DQDB,SMDS
–WideAreaNetworks(WAN):
X.25,ATM,framerelay
•Informationtype
–Datanetworksvs.telecommunicationnetworks
•Applicationtype
–Specialpurposenetworks:
airlinereservationnetwork,bankingnetwork,creditcardnetwork,telephony
–Generalpurposenetwork:
Internet,telecommunicationnetworks
•TypesofNetworks
•Righttouse
–Private:
enterprisenetworks
–Public:
telephonynetwork,Internet
•Ownershipofprotocols
–Proprietary:
SNA
–Open:
IP
•Technologies
–Terrestrialvs.satellite
–Wiredvs.wireless
•Protocols
–IP,ATM,X.25,AppleTalk,SNA
•TheInternet
•Globalscale,generalpurpose,heterogeneous-technologies,public,computernetwork
•InternetProtocol
–Openstandard:
InternetEngineeringTaskForce(IETF)asstandardbody(http:
//www.ietf.org)
•Developedbytheresearchcommunity
•HistoryoftheInternet
•70’s:
startedasaresearchproject,56kbps,<100computers
•80-83:
ARPANETandMILNETsplit
•85-86:
NSFbuildsNSFNETasbackbone,links6Supercomputercenters,1.5Mbps,10,000computers
•87-90:
linkregionalnetworks,NSI(NASA),ESNet(DOE),DARTnet,TWBNet(DARPA),100,000computers
•90-92:
NSFNETmovesto45Mbps,16mid-levelnetworks
•94:
NSFbackbonedismantled,multipleprivatebackbones
•Today:
backbonesrunat10Gbps,hundredsofmillionsofcomputers
•CommercialInternetafter1994
•GrowthoftheInternet
•NumberofHostsontheInternet:
Aug.1981213
Oct.19841,024
Dec.198728,174
Oct.1990313,000
Oct.19932,056,000
Apr.19955,706,000
Jan.199716,146,000
Jan.199956,218,000
Jan.2001109,374,000
•Jan2003171,638,297InternetApplications
•Sharedaccesstocomputingresources
–telnet(1970’s)
•Sharedaccesstodata/files
–FTP,NFS,AFS(1980’s)
•Communicationmediumoverwhichpeopleinteract
–email(1980’s),on-linechatrooms,instantmessaging(1990’s)
–audio,video(when?
)
•Amediumforinformationdissemination
–USENET(1980’s)
–WWW(1990’s)
–Fileswapping(2000)
•Today’sVision
•Everythingisdigital:
voice,video,music,pictures,liveevents
•Everythingison-line:
bankstatement,medicalrecord,books,airlineschedule,weather,highwaytraffic,toaster,refrigerator…
•Everyoneisconnected:
doctor,teacher,broker,mother,son,friends,enemies
•WhatisNext?
•Electroniccommerce
•Internetentertainment
•Worldasasmallvillage
–communityorganizedaccordingtointerests
–enhancedunderstandingamongdiversegroups
•Electronicdemocracy
–littlepeoplecanvoicetheiropinionstothewholeworld
–littlepeoplecancoordinatetheiractions
–bridgethegapbetweeninformationhave’sandhavenot’s
•Electronicterrorism
–hackercanbringthewholeworldtoitsknee
•IntroductiontoComputerNetworks
•AGenericSwitch
•CircuitSwitching:
Multiplexing/Demultiplexing
•Timedividedinframesandframesdividedinslots
•Relativeslotpositioninsideaframedetermineswhichconversationthedatabelongsto
–E.g.,slot0belongstoredconversation
•Needssynchronizationbetweensenderandreceiver
•Incaseofnon-permanentconversations
–Needstodynamicbindaslottoaconservation
–Howtodothis?
•Ifaconversationdoesnotuseitscircuitthecapacityislost!
CircuitSwitching
•Threephases
•circuitestablishment
•datatransfer
•circuittermination
•Ifcircuitnotavailable:
busy
•Examples
–Telephonenetworks
–ISDN(IntegratedServicesDigitalNetworks)
•TiminginCircuitSwitching
•PacketSwitching:
Multiplexing/DemultiplexingDatafromanyconversationcanbetransmittedatanygiventime
–Asingleconversationcanusetheentirelinkcapacityifitisalone
•Howtodemultiplex?
•Usemeta-data(header)todescribedataPacketSwitching
•Dataaresentasformattedbit-sequences,so-calledpackets.
•Packetshavethefollowingstructure:
HeaderandTrailercarrycontrolinformation(e.g.,destinationaddress,checksum)
•Ateachnodetheentirepacketisreceived,storedbriefly,andthenforwardedtothenextnodebasedontheheaderinformation(Store-and-ForwardNetworks)
•Allowsstatisticalmultiplexing
•PacketSwitch
•DatagramPacketSwitching
•Eachpacketisindependentlyswitched
–Eachpacketheadercontainsdestinationaddress
•TimingofDatagramPacketSwitching
•Virtual-CircuitPacketSwitching
•Hybridofcircuitswitchingandpacketswitching
•Dataistransmittedaspackets
•Allpacketsfromonepacketstreamaresentalongapre-establishedpath(=virtualcircuit)
•Packetheaderonlycontainslocalvirtualcircuitidentifier(VCI)
•DemultiplexingandswitchingbasedonVCI
•Guaranteesin-sequencedeliveryofpackets
•Example:
ATMnetworks
•Virtual-CircuitPacketSwitching
•Communicationwithvirtualcircuitstakesplaceinthreephases
•VCestablishment
•datatransfer
•VCdisconnect
•Note:
packetheadersdon’tneedtocontainthefulldestinationaddressofthepacketTimingofVirtual-CircuitPacketSwitching
•VirtualCircuitSwitching
•ATaxonomyofCommunicationNetworks
•Communicationnetworkscanbeclassifiedbasedonthewayinwhichthenodesexchangeinformation:
•Packet-Switchingvs.Circuit-Switching
•Mostimportantadvantageofpacket-switchingovercircuitswitching:
abilitytoexploitstatisticalmultiplexing
–Moreefficientbandwidthusage
•However,packet-switchingneedstobufferanddealwithcongestion
–Morecomplexswit
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