ComputerNetworksShyamGollakota

ComputerNetworks 2

ProtocolsandLayers•  Protocolsandlayeringisthemainstructuringmethodusedtodivideupnetworkfunc>onality–  Eachinstanceofaprotocoltalksvirtuallytoitspeerusingtheprotocol

–  Eachinstanceofaprotocolusesonlytheservicesofthelowerlayer

ProtocolsandLayers(3)•  Protocolsarehorizontal,layersarever>cal

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X

YY

XInstanceofprotocolX

Peerinstance

Node1 Node2

Lowerlayerinstance(ofprotocolY)

ProtocolX

ServiceprovidedbyProtocolY

ProtocolsandLayers(4)•  Setofprotocolsinuseiscalledaprotocolstack

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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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Encapsula>on•  Encapsula>onisthemechanismusedtoeffectprotocollayering–  Lowerlayerwrapshigherlayercontent,addingitsowninforma>ontomakeanewmessagefordelivery

–  Likesendingale[erinanenvelope;postalservicedoesn’tlookinside

Encapsula>on(3)•  Message“onthewire”beginstolooklikeanonion

–  Lowerlayersareoutermost

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HTTP

TCP

IP

802.11

HTTP

TCP HTTP

TCP HTTPIP

TCP HTTPIP802.11

Encapsula>on(4)

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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

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HTTP

Browser

HTTP

Server

HTTP

Browser

HTTP

Server

or

AdvantageofLayering(2)•  Informa>onhidingandreuse

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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

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HTTP

TCP

IP

802.11

Browser

HTTP

TCP

IPEthernet

Server

AdvantageofLayering(4)•  Usinginforma>onhidingtoconnectdifferentsystems

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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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DisadvantageofLayering•  ??

InternetReferenceModel•  Afourlayermodelbasedonexperience;omitssomeOSIlayersandusesIPasthenetworklayer.

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4Applica>on–Programsthatusenetworkservice3Transport–Providesend-to-enddatadelivery2Internet –Sendpacketsovermul>plenetworks

1Link –Sendframesoveralink

InternetReferenceModel(3)•  IPisthe“narrowwaist”oftheInternet

–  Supportsmanydifferentlinksbelowandappsabove

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4Applica>on3Transport

2Internet

1Link Ethernet802.11

IP

TCP UDP

HTTPSMTP RTP DNS

3GDSLCable

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Layer-basedNames(2)•  Fordevicesinthenetwork:

NetworkLink

NetworkLink

Link Link

Physical PhysicalRepeater(orhub)

Switch(orbridge)

Router

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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

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DelayD,RateR

Message

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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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

weightsofharmonicfrequenciesSignalover>me

=

FrequencyRepresenta>on•  Asignalover>mecanberepresentedbyitsfrequencycomponents(calledFourieranalysis)

26am

plitu

de

Lost!

EffectofLessBandwidth•  Fewerfrequencies(=lessbandwidth)degradessignal

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Lost!

27

Lost!Bandwidth

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

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

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Topic•  We’vetalkedaboutsignalsrepresen>ngbits.How,exactly?–  Thisisthetopicofmodula>on

…1011010110…

Signal

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

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

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!

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

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

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: