Computer Networks - University of Washington · Computer Networks 2 Protocols and Layers •...
Transcript of Computer Networks - University of Washington · Computer Networks 2 Protocols and Layers •...
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ComputerNetworksShyamGollakota
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ComputerNetworks 2
ProtocolsandLayers• Protocolsandlayeringisthemainstructuringmethodusedtodivideupnetworkfunc>onality– Eachinstanceofaprotocoltalksvirtuallytoitspeerusingtheprotocol
– Eachinstanceofaprotocolusesonlytheservicesofthelowerlayer
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ProtocolsandLayers(3)• Protocolsarehorizontal,layersarever>cal
ComputerNetworks 3
X
YY
XInstanceofprotocolX
Peerinstance
Node1 Node2
Lowerlayerinstance(ofprotocolY)
ProtocolX
ServiceprovidedbyProtocolY
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ProtocolsandLayers(4)• Setofprotocolsinuseiscalledaprotocolstack
ComputerNetworks 4
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ComputerNetworks 5
ProtocolsandLayers(6)• Protocolsyou’veprobablyheardof:
– TCP,IP,802.11,Ethernet,HTTP,SSL,DNS,…andmanymore
• Anexampleprotocolstack– UsedbyawebbrowseronahostthatiswirelesslyconnectedtotheInternet
HTTP
TCP
IP
802.11
Browser
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ComputerNetworks 6
Encapsula>on• Encapsula>onisthemechanismusedtoeffectprotocollayering– Lowerlayerwrapshigherlayercontent,addingitsowninforma>ontomakeanewmessagefordelivery
– Likesendingale[erinanenvelope;postalservicedoesn’tlookinside
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Encapsula>on(3)• Message“onthewire”beginstolooklikeanonion
– Lowerlayersareoutermost
ComputerNetworks 7
HTTP
TCP
IP
802.11
HTTP
TCP HTTP
TCP HTTPIP
TCP HTTPIP802.11
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Encapsula>on(4)
ComputerNetworks 8
HTTP
TCP
IP
802.11
HTTP
TCP HTTP
TCP HTTPIP
TCP HTTPIP802.11
HTTP
TCP
IP
802.11(wire)
HTTP
TCP HTTP
TCP HTTPIP
TCP HTTPIP802.11
TCP HTTPIP802.11
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AdvantageofLayering• Informa>onhidingandreuse
ComputerNetworks 9
HTTP
Browser
HTTP
Server
HTTP
Browser
HTTP
Server
or
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AdvantageofLayering(2)• Informa>onhidingandreuse
ComputerNetworks 10
HTTP
TCP
IP
802.11
Browser
HTTP
TCP
IP
802.11
Server
HTTP
TCP
IP
Ethernet
Browser
HTTP
TCP
IP
Ethernet
Server
or
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AdvantageofLayering(3)• Usinginforma>onhidingtoconnectdifferentsystems
ComputerNetworks 11
HTTP
TCP
IP
802.11
Browser
HTTP
TCP
IPEthernet
Server
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AdvantageofLayering(4)• Usinginforma>onhidingtoconnectdifferentsystems
ComputerNetworks 12
HTTP
TCP
IP
802.11
Browser
IP
802.11
IP
Ethernet
HTTP
TCP
IPEthernet
Server
IP TCP HTTP
802.11 IP TCP HTTP Ethernet IP TCP HTTP
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ComputerNetworks 13
DisadvantageofLayering• ??
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InternetReferenceModel• Afourlayermodelbasedonexperience;omitssomeOSIlayersandusesIPasthenetworklayer.
ComputerNetworks 14
4Applica>on–Programsthatusenetworkservice3Transport–Providesend-to-enddatadelivery2Internet –Sendpacketsovermul>plenetworks
1Link –Sendframesoveralink
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InternetReferenceModel(3)• IPisthe“narrowwaist”oftheInternet
– Supportsmanydifferentlinksbelowandappsabove
ComputerNetworks 15
4Applica>on3Transport
2Internet
1Link Ethernet802.11
IP
TCP UDP
HTTPSMTP RTP DNS
3GDSLCable
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ComputerNetworks 16
Layer-basedNames(2)• Fordevicesinthenetwork:
NetworkLink
NetworkLink
Link Link
Physical PhysicalRepeater(orhub)
Switch(orbridge)
Router
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ComputerNetworks 17
Layer-basedNames(3)• Fordevicesinthenetwork:
Proxyormiddleboxorgateway
NetworkLink
NetworkLink
AppTransport
AppTransport
Buttheyalllooklikethis!
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ScopeofthePhysicalLayer• Concernshowsignalsareusedtotransfermessagebitsoveralink– Wiresetc.carryanalogsignals– Wewanttosenddigitalbits
…1011010110…
Signal
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SimpleLinkModel• We’llendwithanabstrac>onofaphysicalchannel
– Rate(orbandwidth,capacity,speed)inbits/second– Delayinseconds,relatedtolength
• Otherimportantproper>es:– Whetherthechannelisbroadcast,anditserrorrate
CSE461UniversityofWashington 19
DelayD,RateR
Message
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MessageLatency• Latencyisthedelaytosendamessageoveralink
– Transmissiondelay:>metoputM-bitmessage“onthewire”
– Propaga>ondelay:>meforbitstopropagateacrossthewire
– Combiningthetwotermswehave:
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MessageLatency(2)• Latencyisthedelaytosendamessageoveralink
– Transmissiondelay:>metoputM-bitmessage“onthewire”
T-delay=M(bits)/Rate(bits/sec)=M/Rseconds
– Propaga>ondelay:>meforbitstopropagateacrossthewire
P-delay=Length/speedofsignals=Length/⅔c=Dseconds
– Combiningthetwotermswehave:L=M/R+D
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MetricUnits• Themainprefixesweuse:
• Usepowersof10forrates,2forstorage– 1Mbps=1,000,000bps,1KB=210bytes
• “B”isforbytes,“b”isforbits
Prefix Exp. prefix exp. K(ilo) 103 m(illi) 10-3
M(ega) 106 µ(micro) 10-6
G(iga) 109 n(ano) 10-9
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LatencyExamples(2)• “Dialup”withatelephonemodem:
D=5ms,R=56kbps,M=1250bytes
L=5ms+(1250x8)/(56x103)sec=184ms!
• Broadbandcross-countrylink:D=50ms,R=10Mbps,M=1250bytes
L=50ms+(1250x8)/(10x106)sec=51ms
• Alonglinkoraslowratemeanshighlatency– Open,onedelaycomponentdominates
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CSE461UniversityofWashington 24
Bandwidth-DelayProduct• Messagestakespaceonthewire!
• Theamountofdatainflightisthebandwidth-delay(BD)product
BD=RxD– Measureinbits,orinmessages– SmallforLANs,bigfor“longfat”pipes
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Bandwidth-DelayExample(2)• Fiberathome,cross-country
R=40Mbps,D=50msBD=40x106x50x10-3bits
=2000Kbit=250KB
• That’squitealotofdata“inthenetwork”!
110101000010111010101001011
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weightsofharmonicfrequenciesSignalover>me
=
FrequencyRepresenta>on• Asignalover>mecanberepresentedbyitsfrequencycomponents(calledFourieranalysis)
26am
plitu
de
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Lost!
EffectofLessBandwidth• Fewerfrequencies(=lessbandwidth)degradessignal
27
Lost!
27
Lost!Bandwidth
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SignalsoveraWire(2)• Example:
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2:A[enua>on:
3:Bandwidth:
4:Noise:
Sentsignal
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SignalsoverWireless• Signalstransmi[edonacarrierfrequency,likefiber
• Travelatspeedoflight,spreadoutanda[enuatefasterthan1/dist2
• Mul>plesignalsonthesamefrequencyinterfereatareceiver
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SignalsoverWireless(5)• Variousothereffectstoo!
– Wirelesspropaga>oniscomplex,dependsonenvironment
• Somekeyeffectsarehighlyfrequencydependent,– E.g.,mul>pathatmicrowavefrequencies
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WirelessMul>path• Signalsbounceoffobjectsandtakemul>plepaths
– Somefrequenciesa[enuatedatreceiver,varieswithloca>on– Messesupsignal;handledwithsophis>catedmethods(§2.5.3)
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Wireless• Senderradiatessignaloveraregion
– Inmanydirec>ons,unlikeawire,topoten>allymanyreceivers
– Nearbysignals(samefreq.)interfereatareceiver;needtocoordinateuse
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WiFi
WiFi
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Wireless(2)• Microwave,e.g.,3G,andunlicensed(ISM)frequencies,e.g.,WiFi,arewidelyusedforcomputernetworking
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802.11b/g/n
802.11a/g/n
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Topic• We’vetalkedaboutsignalsrepresen>ngbits.How,exactly?– Thisisthetopicofmodula>on
…1011010110…
Signal
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ASimpleModula>on• Letahighvoltage(+V)representa1,andlowvoltage(-V)representa0– ThisiscalledNRZ(Non-ReturntoZero)
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Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
+V
-V
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ASimpleModula>on(2)• Letahighvoltage(+V)representa1,andlowvoltage(-V)representa0– ThisiscalledNRZ(Non-ReturntoZero)
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Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
+V
-V
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Modula>on
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NRZsignalofbits
Amplitudeshipkeying
Frequencyshipkeying
Phaseshipkeying
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Topic• Howrapidlycanwesendinforma>onoveralink?– Nyquistlimit(~1924)»– Shannoncapacity(1948)»
• Prac>calsystemsaredevisedtoapproachtheselimits
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KeyChannelProper>es• Thebandwidth(B),signalstrength(S),andnoisestrength(N)– Blimitstherateoftransi>ons– SandNlimithowmanysignallevelswecandis>nguish
BandwidthB SignalS,NoiseN
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NyquistLimit• Themaximumsymbolrateis2B
• ThusifthereareVsignallevels,ignoringnoise,themaximumbitrateis: R=2Blog2Vbits/sec
1010101010101010101
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ClaudeShannon(1916-2001)• Fatherofinforma>ontheory
– “AMathema>calTheoryofCommunica>on”,1948
• Fundamentalcontribu>onstodigitalcomputers,security,andcommunica>ons
Credit:CourtesyMITMuseum
Electromechanicalmousethat“solves”mazes!
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ShannonCapacity• Howmanylevelswecandis>nguishdependsonS/N
– OrSNR,theSignal-to-NoiseRa>o– Notenoiseisrandom,hencesomeerrors
• SNRgivenonalog-scaleindeciBels:– SNRdB=10log10(S/N)
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0
1
2
3
N
S+N
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ShannonCapacity(2)• Shannonlimitisforcapacity(C),themaximuminforma>oncarryingrateofthechannel:
C=Blog2(1+S/(BN))bits/sec
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Wired/WirelessPerspec>ve• Wires,andFiber
– EngineerlinktohaverequisiteSNRandB→ Canfixdatarate
• Wireless– GivenB,butSNRvariesgreatly,e.g.,upto60dB!→ Can’tdesignforworstcase,mustadaptdatarate
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Wired/WirelessPerspec>ve(2)• Wires,andFiber
– EngineerlinktohaverequisiteSNRandB→ Canfixdatarate
• Wireless– GivenB,butSNRvariesgreatly,e.g.,upto60dB!→ Can’tdesignforworstcase,mustadaptdatarate
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EngineerSNRfordatarate
AdaptdataratetoSNR
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Puzngitalltogether–DSL• DSL(DigitalSubscriberLine)iswidelyusedforbroadband;manyvariantsoffer10sofMbps– Reusestwistedpairtelephonelinetothehome;ithasupto~2MHzofbandwidthbutusesonlythelowest~4kHz
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DSL(2)• DSLusespassbandmodula>on(calledOFDM)
– Separatebandsforupstreamanddownstream(larger)– Modula>onvariesbothamplitudeandphase(calledQAM)– HighSNR,upto15bits/symbol,lowSNRonly1bit/symbol
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Upstream Downstream
26–138kHz
0-4kHz 143kHzto1.1MHz
Telephone
Freq.
Voice Upto1Mbps Upto12Mbps
ADSL2: