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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:
