computer networks - university of washington · computer networks 2 protocols and layers •...
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ComputerNetworksShyamGollakota
ComputerNetworks 2
ProtocolsandLayers• Protocolsandlayeringisthemainstructuringmethodusedtodivideupnetworkfunc>onality– Eachinstanceofaprotocoltalksvirtuallytoitspeerusingtheprotocol
– Eachinstanceofaprotocolusesonlytheservicesofthelowerlayer
ProtocolsandLayers(3)• Protocolsarehorizontal,layersarever>cal
ComputerNetworks 3
X
YY
XInstanceofprotocolX
Peerinstance
Node1 Node2
Lowerlayerinstance(ofprotocolY)
ProtocolX
ServiceprovidedbyProtocolY
ProtocolsandLayers(4)• Setofprotocolsinuseiscalledaprotocolstack
ComputerNetworks 4
ComputerNetworks 5
ProtocolsandLayers(6)• Protocolsyou’veprobablyheardof:
– TCP,IP,802.11,Ethernet,HTTP,SSL,DNS,…andmanymore
• Anexampleprotocolstack– UsedbyawebbrowseronahostthatiswirelesslyconnectedtotheInternet
HTTP
TCP
IP
802.11
Browser
ComputerNetworks 6
Encapsula>on• Encapsula>onisthemechanismusedtoeffectprotocollayering– Lowerlayerwrapshigherlayercontent,addingitsowninforma>ontomakeanewmessagefordelivery
– Likesendingale[erinanenvelope;postalservicedoesn’tlookinside
Encapsula>on(3)• Message“onthewire”beginstolooklikeanonion
– Lowerlayersareoutermost
ComputerNetworks 7
HTTP
TCP
IP
802.11
HTTP
TCP HTTP
TCP HTTPIP
TCP HTTPIP802.11
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
AdvantageofLayering• Informa>onhidingandreuse
ComputerNetworks 9
HTTP
Browser
HTTP
Server
HTTP
Browser
HTTP
Server
or
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
AdvantageofLayering(3)• Usinginforma>onhidingtoconnectdifferentsystems
ComputerNetworks 11
HTTP
TCP
IP
802.11
Browser
HTTP
TCP
IPEthernet
Server
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
ComputerNetworks 13
DisadvantageofLayering• ??
InternetReferenceModel• Afourlayermodelbasedonexperience;omitssomeOSIlayersandusesIPasthenetworklayer.
ComputerNetworks 14
4Applica>on–Programsthatusenetworkservice3Transport–Providesend-to-enddatadelivery2Internet –Sendpacketsovermul>plenetworks
1Link –Sendframesoveralink
InternetReferenceModel(3)• IPisthe“narrowwaist”oftheInternet
– Supportsmanydifferentlinksbelowandappsabove
ComputerNetworks 15
4Applica>on3Transport
2Internet
1Link Ethernet802.11
IP
TCP UDP
HTTPSMTP RTP DNS
3GDSLCable
ComputerNetworks 16
Layer-basedNames(2)• Fordevicesinthenetwork:
NetworkLink
NetworkLink
Link Link
Physical PhysicalRepeater(orhub)
Switch(orbridge)
Router
ComputerNetworks 17
Layer-basedNames(3)• Fordevicesinthenetwork:
Proxyormiddleboxorgateway
NetworkLink
NetworkLink
AppTransport
AppTransport
Buttheyalllooklikethis!
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ScopeofthePhysicalLayer• Concernshowsignalsareusedtotransfermessagebitsoveralink– Wiresetc.carryanalogsignals– Wewanttosenddigitalbits
…1011010110…
Signal
SimpleLinkModel• We’llendwithanabstrac>onofaphysicalchannel
– Rate(orbandwidth,capacity,speed)inbits/second– Delayinseconds,relatedtolength
• Otherimportantproper>es:– Whetherthechannelisbroadcast,anditserrorrate
CSE461UniversityofWashington 19
DelayD,RateR
Message
MessageLatency• Latencyisthedelaytosendamessageoveralink
– Transmissiondelay:>metoputM-bitmessage“onthewire”
– Propaga>ondelay:>meforbitstopropagateacrossthewire
– Combiningthetwotermswehave:
CSE461UniversityofWashington 20
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
CSE461UniversityofWashington 23
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
CSE461UniversityofWashington 24
Bandwidth-DelayProduct• Messagestakespaceonthewire!
• Theamountofdatainflightisthebandwidth-delay(BD)product
BD=RxD– Measureinbits,orinmessages– SmallforLANs,bigfor“longfat”pipes
CSE461UniversityofWashington 25
Bandwidth-DelayExample(2)• Fiberathome,cross-country
R=40Mbps,D=50msBD=40x106x50x10-3bits
=2000Kbit=250KB
• That’squitealotofdata“inthenetwork”!
110101000010111010101001011
weightsofharmonicfrequenciesSignalover>me
=
FrequencyRepresenta>on• Asignalover>mecanberepresentedbyitsfrequencycomponents(calledFourieranalysis)
26am
plitu
de
Lost!
EffectofLessBandwidth• Fewerfrequencies(=lessbandwidth)degradessignal
27
Lost!
27
Lost!Bandwidth
SignalsoveraWire(2)• Example:
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2:A[enua>on:
3:Bandwidth:
4:Noise:
Sentsignal
CSE461UniversityofWashington 29
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
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
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
35
Topic• We’vetalkedaboutsignalsrepresen>ngbits.How,exactly?– Thisisthetopicofmodula>on
…1011010110…
Signal
ASimpleModula>on• Letahighvoltage(+V)representa1,andlowvoltage(-V)representa0– ThisiscalledNRZ(Non-ReturntoZero)
36
Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
+V
-V
ASimpleModula>on(2)• Letahighvoltage(+V)representa1,andlowvoltage(-V)representa0– ThisiscalledNRZ(Non-ReturntoZero)
37
Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
+V
-V
Modula>on
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NRZsignalofbits
Amplitudeshipkeying
Frequencyshipkeying
Phaseshipkeying
39
Topic• Howrapidlycanwesendinforma>onoveralink?– Nyquistlimit(~1924)»– Shannoncapacity(1948)»
• Prac>calsystemsaredevisedtoapproachtheselimits
40
KeyChannelProper>es• Thebandwidth(B),signalstrength(S),andnoisestrength(N)– Blimitstherateoftransi>ons– SandNlimithowmanysignallevelswecandis>nguish
BandwidthB SignalS,NoiseN
41
NyquistLimit• Themaximumsymbolrateis2B
• ThusifthereareVsignallevels,ignoringnoise,themaximumbitrateis: R=2Blog2Vbits/sec
1010101010101010101
42
ClaudeShannon(1916-2001)• Fatherofinforma>ontheory
– “AMathema>calTheoryofCommunica>on”,1948
• Fundamentalcontribu>onstodigitalcomputers,security,andcommunica>ons
Credit:CourtesyMITMuseum
Electromechanicalmousethat“solves”mazes!
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
44
ShannonCapacity(2)• Shannonlimitisforcapacity(C),themaximuminforma>oncarryingrateofthechannel:
C=Blog2(1+S/(BN))bits/sec
Wired/WirelessPerspec>ve• Wires,andFiber
– EngineerlinktohaverequisiteSNRandB→ Canfixdatarate
• Wireless– GivenB,butSNRvariesgreatly,e.g.,upto60dB!→ Can’tdesignforworstcase,mustadaptdatarate
45
Wired/WirelessPerspec>ve(2)• Wires,andFiber
– EngineerlinktohaverequisiteSNRandB→ Canfixdatarate
• Wireless– GivenB,butSNRvariesgreatly,e.g.,upto60dB!→ Can’tdesignforworstcase,mustadaptdatarate
46
EngineerSNRfordatarate
AdaptdataratetoSNR
Puzngitalltogether–DSL• DSL(DigitalSubscriberLine)iswidelyusedforbroadband;manyvariantsoffer10sofMbps– Reusestwistedpairtelephonelinetothehome;ithasupto~2MHzofbandwidthbutusesonlythelowest~4kHz
47
DSL(2)• DSLusespassbandmodula>on(calledOFDM)
– Separatebandsforupstreamanddownstream(larger)– Modula>onvariesbothamplitudeandphase(calledQAM)– HighSNR,upto15bits/symbol,lowSNRonly1bit/symbol
48
Upstream Downstream
26–138kHz
0-4kHz 143kHzto1.1MHz
Telephone
Freq.
Voice Upto1Mbps Upto12Mbps
ADSL2:
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