Operang Systems and Networks Networks Part 2: Physical Layer · 2 Overview • Important concepts...

Opera&ngSystemsandNetworksNetworksPart2:PhysicalLayer

AdrianPerrigNetworkSecurityGroupETHZürich

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Overview•  Importantconceptsfromlastlecture

–  Sta&s&calmul&plexing,sta&s&calmul&plexinggain–  OSI7layermodel,interfaces,protocols–  Encapsula&on,demul&plexing

•  Thislecture–  Socketprogrammingoverview–  Physicallayer

•  Onlinelecturevideos:hJp://computernetworks5e.org

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Network-Applica&onInterface•  Defineshowappsusethenetwork

–  Letsappstalktoeachotherviahosts;hidesthedetailsofthenetwork

host

appapp

host

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Mo&va&ngApplica&on•  Simpleclient-serverconnec&onsetup

request

reply

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Mo&va&ngApplica&on(2)•  Simpleclient-serverconnec&onsetup

–  Clientappsendsarequesttoserverapp–  Serverappreturnsa(longer)reply

•  Thisisthebasisformanyapps!–  Filetransfer:sendname,getfile(§6.1.4)–  Webbrowsing:sendURL,getpage–  Echo:sendmessage,getitback

•  Let’sseehowtowritethisapp…

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SocketAPI•  Simpleabstrac&ontousethenetwork

–  ThenetworkserviceAPIusedtowriteallInternetapplica&ons–  PartofallmajorOSesandlanguages;originallyBerkeley(Unix)~1983

•  Supportstwokindsofnetworkservices–  Streams:reliablysendastreamofbytes–  Datagrams:unreliablysendseparatemessages.(Ignorefornow.)

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SocketAPI(2)•  Socketsletapplica&onsaJachtothelocalnetworkat

differentports

Socket,Port#1

Socket,Port#2

SocketAPI(3)

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Primi%ve MeaningSOCKET Createanewcommunica&onendpointBIND AssociatealocaladdresswithasocketLISTEN Announcewillingnesstoacceptconnec&ons;givequeuesizeACCEPT Passivelywaitforanincomingconnec&onCONNECT Ac&velyaJempttoestablishaconnec&onSEND Sendsomedataovertheconnec&onRECEIVE Receivesomedatafromtheconnec&onCLOSE Releasetheconnec&on

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UsingSocketsClient(host1) Server(host2)Time

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UsingSockets(2)Client(host1) Server(host2)Time

request

reply

disconnect

1 1

2

3

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connect

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UsingSockets(3)Client(host1) Server(host2)Time

5:connect*

1:socket 2:bind1:socket

3:listen

9:send

6:recv*

4:accept*

7:send8:recv*

10:close 10:close

request

reply

disconnect

connect

*=callblocks

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ClientProgram(outline)socket() //makesocketgetaddrinfo() //serverandportname

//www.example.com:80connect() //connecttoserver[block]…send() //sendrequestrecv() //awaitreply[block]… //dosomethingwithdata!close() //done,disconnect

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ServerProgram(outline)socket() //makesocketgetaddrinfo() //forportonthishostbind() //associateportwithsocketlisten() //preparetoacceptconnec&onsaccept() //waitforaconnec&on[block] …recv() //waitforrequest…send() //sendthereplyclose() //eventuallydisconnect

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WhereweareintheCourse•  Beginningtoworkourwayupstar&ngwiththePhysicallayer

PhysicalLink

NetworkTransportApplica&on

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ScopeofthePhysicalLayer•  Concernshowsignalsareusedtotransfermessagebitsoveralink– Wiresetc.carryanalogsignals– Wewanttosenddigitalbits

…1011010110…

Signal

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Topics1.  Proper&esofmedia

– Wires,fiberop&cs,wireless2.  Simplesignalpropaga&on

–  Bandwidth,aJenua&on,noise3.  Modula&onschemes

–  Represen&ngbits,noise4.  Fundamentallimits

– Nyquist,Shannon

SimpleLinkModel•  We’llendwithanabstrac&onofaphysicalchannel

–  Rate(orbandwidth,capacity,speed)inbits/second–  DelayorLatencyinseconds,relatedtolength

•  Otherimportantproper&es:– Whetherthechannelisbroadcast,anditserrorrate

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DelayD,RateR

Message

MessageLatency•  LatencyL:delaytosendamessageoveralink

–  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,2forstorageordatasize–  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•  “Dialup”withatelephonemodem:

–  D=5ms,R=56kbps,M=1250bytes

•  Broadbandcross-countrylink:–  D=50ms,R=10Mbps,M=1250bytes

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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–  Owen,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•  Fiberathome,cross-country

R=40Mbps,D=50msBD=40x106x50x10-3bits

=2000Kbit=250KB

•  That’squitealotofdata“inthenetwork”!

110101000010111010101001011

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How“Long”isaBit?•  Interes&ngtrivia:how“long”istherepresenta&onofabitonawire?

•  Consideringafiberop&ccable–  Signalpropaga&onspeed:200’000’000m/s–  Sendingrate:1Gbpsàdura&onofsendingonebit:1ns–  Bit“length”:1ns*200’000’000m/s=0.2m–  “Length”ofa1Kbpacket:0.2m*8*210=1.6km

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TypesofMedia(§2.2,2.3)•  Mediapropagatesignalsthatcarrybitsofinforma&on•  We’lllookatsomecommontypes:

– Wires–  Fiber(fiberop&ccables)– Wireless

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Wires–TwistedPair•  Verycommon;usedinLANsandtelephonelines

–  Twistscanreduceradiatedsignalorreduceeffectofexternalinterferencesignal

Category5UTPcablewithfourtwistedpairs

Wires–CoaxialCable•  Alsocommon.BeJershieldingforbeJerperformance

•  Otherkindsofwirestoo:e.g.,electricalpower(§2.2.4)

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Fiber•  Long,thin,purestrandsofglass

–  Enormousbandwidth(highspeed)overlongdistances

Lightsource(LED,laser)

Photo-detector

Lighttrappedbytotalinternalreflec&on

Op&calfiber

Fiber(2)•  Twovarie&es:mul&-mode(shorterlinks,cheaper)andsingle-mode(upto~100km)

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FiberbundleinacableOnefiber

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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:IndustryScienceMedicine)frequencies,e.g.,WiFi,arewidelyusedforcomputernetworking

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802.11b/g/n

802.11a/g/n

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Signals(§2.2,2.3)•  Analogsignalsencodedigitalbits.Wewanttoknowwhathappensassignalspropagateovermedia

…1011010110…

Signal

weightsofharmonicfrequenciesSignalover&me

=

FrequencyRepresenta&on•  Asignalover&mecanberepresentedbyitsfrequencycomponents(calledFourieranalysis)

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amplitu

de

Lost!

EffectofLessBandwidth•  Fewerfrequencies(=lessbandwidth)degradessignal

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Lost!

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Lost!Bandwidth

SignalsoveraWire•  Whathappenstoasignalasitpassesoverawire?

1.  Thesignalisdelayed(propagatesat⅔c)2.  ThesignalisaJenuated3.  FrequenciesaboveacutoffarehighlyaJenuated4.  Noiseisaddedtothesignal(later,causeserrors)

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EE:Bandwidth=widthoffrequencyband,measuredinHzCS:Bandwidth=informa&oncarryingcapacity,inbits/sec

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SignalsoverFiber•  Lightpropagateswithverylowlossinthreeverywidefrequencybands–  Useacarriertosendinforma&on

Wavelength(μm)

AJenua&on(dB/km

1,5um=0,2dB/km

BySVG:SassospiccoRaster:Alexwind,CC-BY-SA-3.0,viaWikimediaCommons

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SignalsoverWireless•  Travelatspeedoflight,spreadoutandaJenuatefasterthan1/dist2

Signalstrength

DistanceA B

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SignalsoverWireless(2)•  Mul&plesignalsonthesamefrequencyinterfereatareceiver

Signalstrength

DistanceA BC

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SignalsoverWireless(3)•  Interferenceleadstono&onofspa&alreuse(ofsamefreq.)

Signalstrength

DistanceA BC

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SignalsoverWireless(4)•  Variousothereffectstoo!

– Wirelesspropaga&oniscomplex,dependsonenvironment

•  Somekeyeffectsarehighlyfrequencydependent–  E.g.,mul&pathatmicrowavefrequencies

WirelessMul&path•  Signalsbounceoffobjectsandtakemul&plepaths

–  SomefrequenciesaJenuatedatreceiver,varieswithloca&on– Messesupsignal;handledwithsophis&catedmethods(§2.5.3)

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Modula&on(§2.5)•  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

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ManyOtherSchemes•  Canusemoresignallevels,e.g.,4levelsis2bitspersymbol

•  Prac&calschemesaredrivenbyengineeringconsidera&ons–  E.g.,clockrecovery

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ClockRecovery•  Um,howmanyzeroswasthat?

–  Receiverneedsfrequentsignaltransi&onstodecodebits

•  Severalpossibledesigns–  E.g.,Manchestercodingandscrambling(§2.5.1)

1000000000…0

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ClockRecovery–4B/5B•  Mapevery4databitsinto5codebitswithoutlongrunsofzeros–  0000à11110,0001à01001,1110à11100,…1111à11101

–  Hasatmost3zerosinarow–  Alsoinvertsignallevelona1tobreakuplongrunsof1s(calledNRZI,§2.5.1)

ClockRecovery–4B/5B(2)•  4B/5Bcodeforreference:

–  0000à11110,0001à01001,1110à11100,…1111à11101

•  Messagebits:111100000001

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CodedBits:

Signal:

111011111001001

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PassbandModula&on•  Whatwehaveseensofarisbasebandmodula&onforwires–  Signalissentdirectlyonawire

•  Thesesignalsdonotpropagatewellonfiber/wireless–  Needtosendathigherfrequencies

•  Passbandmodula&oncarriesasignalbymodula&ngacarrier

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PassbandModula&on(2)•  Carrierissimplyasignaloscilla&ngatadesiredfrequency:

•  Wecanmodulateitbychanging:–  Amplitude,frequency,orphase

PassbandModula&on(3)

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NRZsignalofbits

Amplitudeshiwkeying

Frequencyshiwkeying

Phaseshiwkeying

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FundamentalLimits(§2.1)•  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/N)bits/sec

Wired/WirelessPerspec&ve•  WiresandFiber

– EngineerlinktohaverequisiteSNRandB→ Canfixdatarate

•  Wireless– GivenB,butSNRvariesgreatly,e.g.,upto60dB!→ Can’tdesignforworstcase,mustadaptdatarate

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EngineerSNRfordatarate

AdaptdataratetoSNR

Pu~ngitalltogether–DSL•  DSL(DigitalSubscriberLine,see§2.6.3)iswidelyusedforbroadband;manyvariantsoffer10sofMbps–  Reusestwistedpairtelephonelinetothehome;ithasupto~2MHzofbandwidthbutusesonlythelowest~4kHz

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DSL(2)•  DSLusespassbandmodula&on(calledOFDM§2.5.1)

–  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: