山东大学计算机网络英语课件02The Physical Layer

1、The Physical LayerChapter 2CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Theoretical Basis for Data CommunicationsGuided Transmission MediaWireless TransmissionCommunication SatellitesDigital Modulation and MultiplexingPublic Switched Telephone NetworkMobile Te

2、lephone SystemCable TelevisionRevised: August 2011The Physical LayerCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Foundation on which other layers buildProperties of wires, fiber, wireless limit what the network can doKey problem is to send (digital) bits using

3、 only (analog) signalsThis is called modulationPhysicalLinkNetworkTransportApplicationTheoretical Basis for Data CommunicationCommunication rates have fundamental limitsFourier analysis Bandwidth-limited signals Maximum data rate of a channel CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice

4、 Hall and D. Wetherall, 2011Fourier AnalysisA time-varying signal can be equivalently represented as a series of frequency components (harmonics):CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011a, b weights of harmonicsSignal over time=Bandwidth-Limited SignalsHa

5、ving less bandwidth (harmonics) degrades the signalCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 20118 harmonics4 harmonics2 harmonicsLost!BandwidthLost!Lost!Maximum Data Rate of a ChannelNyquists theorem relates the data rate to the bandwidth (B) and number of sig

6、nal levels (V):Shannons theorem relates the data rate to the bandwidth (B) and signal strength (S) relative to the noise (N):CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Max. data rate = 2B log2V bits/secMax. data rate = B log2(1 + S/N) bits/secHow fast signal

7、can changeHow many levelscan be seenGuided Transmission (Wires & Fiber)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Media have different properties, hence performanceReality checkStorage media Wires:Twisted pairs Coaxial cable Power lines Fiber cables Reality

8、Check: Storage mediaCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Send data on tape / disk / DVD for a high bandwidth linkMail one box with 1000 800GB tapes (6400 Tbit)Takes one day to send (86,400 secs)Data rate is 70 Gbps. Data rate is faster than long-distan

9、ce networks!But, the message delay is very poor.Wires Twisted PairCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Very common; used in LANs, telephone linesTwists reduce radiated signal (interference)Category 5 UTP cable with four twisted pairsLink TerminologyCN5

10、E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Full-duplex linkUsed for transmission in both directions at oncee.g., use different twisted pairs for each directionHalf-duplex linkBoth directions, but not at the same timee.g., senders take turns on a wireless channe

11、l Simplex linkOnly one fixed direction at all times; not commonWires Coaxial Cable (“Co-ax”)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Also common. Better shielding and more bandwidth for longer distances and higher rates than twisted pair.Wires Power LinesC

12、N5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Household electrical wiring is another example of wiresConvenient to use, but horrible for sending dataFiber Cables (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Common for h

13、igh rates and long distancesLong distance ISP links, Fiber-to-the-HomeLight carried in very long, thin strand of glassLight source(LED, laser)PhotodetectorLight trapped bytotal internal reflectionFiber Cables (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Fib

14、er has enormous bandwidth (THz) and tiny signal loss hence high rates over long distancesFiber Cables (3)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Single-modeCore so narrow (10um) light cant even bounce aroundUsed with lasers for long distances, e.g., 100km

15、Multi-modeOther main type of fiberLight can bounce (50um core)Used with LEDs for cheaper, shorter distance linksFibers in a cableComparison of the properties of wires and fiber:Fiber Cables (4)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011PropertyWiresFiberDist

16、anceShort (100s of m)Long (tens of km)BandwidthModerateVery HighCostInexpensive Less cheapConvenienceEasy to useLess easySecurityEasy to tapHard to tapWireless TransmissionCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Electromagnetic Spectrum Radio Transmission

17、 Microwave Transmission Light Transmission Wireless vs. Wires/Fiber Electromagnetic Spectrum (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Different bands have different uses:Radio: wide-area broadcast; Infrared/Light: line-of-sight Microwave: LANs and 3G/4G

18、; MicrowaveNetworking focusElectromagnetic Spectrum (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011To manage interference, spectrum is carefully divided, and its use regulated and licensed, e.g., sold at auction. Source: NTIA Office of Spectrum Management, 20

19、033 GHz30 GHz3 GHz300 MHzWiFi (ISM bands)Part of the US frequency allocationsElectromagnetic Spectrum (3)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Fortunately, there are also unlicensed (“ISM”) bands:Free for use at low power; devices manage interferenceWid

20、ely used for networking; WiFi, Bluetooth, Zigbee, etc.802.11b/g/n802.11a/g/nRadio TransmissionCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011In the HF band, radio waves bounce off the ionosphere.In the VLF, LF, and MF bands, radio waves follow the curvature of t

21、he earthRadio signals penetrate buildings well and propagate for long distances with path loss Microwave TransmissionMicrowaves have much bandwidth and are widely used indoors (WiFi) and outdoors (3G, satellites)Signal is attenuated/reflected by everyday objectsStrength varies with mobility due mult

22、ipath fading, etc.CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Light TransmissionCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Line-of-sight light (no fiber) can be used for linksLight is highly directional, has much band

23、widthUse of LEDs/cameras and lasers/photodetectorsWireless vs. Wires/FiberCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Wireless:Easy and inexpensive to deployNaturally supports mobilityNaturally supports broadcastTransmissions interfere and must be managedSign

24、al strengths hence data rates vary greatlyWires/Fiber:Easy to engineer a fixed data rate over point-to-point linksCan be expensive to deploy, esp. over distancesDoesnt readily support mobility or broadcastCommunication SatellitesCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. W

25、etherall, 2011Satellites are effective for broadcast distribution and anywhere/anytime communicationsKinds of Satellites Geostationary (GEO) Satellites Low-Earth Orbit (LEO) Satellites Satellites vs. Fiber Kinds of SatellitesCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wethe

26、rall, 2011Satellites and their properties vary by altitude:Geostationary (GEO), Medium-Earth Orbit (MEO), and Low-Earth Orbit (LEO)Sats needed for global coverageGeostationary SatellitesCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011GEO satellites orbit 35,000 k

27、m above a fixed locationVSAT (computers) can communicate with the help of a hubDifferent bands (L, S, C, Ku, Ka) in the GHz are in use but may be crowded or susceptible to rain.VSATGEO satelliteLow-Earth Orbit SatellitesCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall,

28、 2011Systems such as Iridium use many low-latency satellites for coverage and route communications via themThe Iridium satellites form six necklaces around the earth.Satellite vs. FiberCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Satellite:Can rapidly set up a

29、nywhere/anytime communications (after satellites have been launched)Can broadcast to large regionsLimited bandwidth and interference to manage Fiber:Enormous bandwidth over long distancesInstallation can be more expensive/difficultDigital Modulation and MultiplexingCN5E by Tanenbaum & Wetherall, Pea

30、rson Education-Prentice Hall and D. Wetherall, 2011Modulation schemes send bits as signals; multiplexing schemes share a channel among users.Baseband Transmission Passband Transmission Frequency Division Multiplexing Time Division Multiplexing Code Division Multiple Access Baseband TransmissionLine

31、codes send symbols that represent one or more bitsNRZ is the simplest, literal line code (+1V=“1”, -1V=“0”)Other codes tradeoff bandwidth and signal transitions CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Four different line codesClock RecoveryTo decode the s

32、ymbols, signals need sufficient transitionsOtherwise long runs of 0s (or 1s) are confusing, e.g.:Strategies:Manchester coding, mixes clock signal in every symbol4B/5B maps 4 data bits to 5 coded bits with 1s and 0s:Scrambler XORs tx/rx data with pseudorandom bitsCN5E by Tanenbaum & Wetherall, Pearso

33、n Education-Prentice Hall and D. Wetherall, 201110 0 0 0 0 0 0 0 0 0 um, 0? er, 0? DataCodeDataCodeDataCodeDataCode000011110010001010100010010110011010000101001010101011100110011110111011001010100011001110101010110111011100001110101011101111101110111111111101Passband Transmission (1)CN5E by Tanenbau

34、m & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Modulating the amplitude, frequency/phase of a carrier signal sends bits in a (non-zero) frequency rangeNRZ signal of bitsAmplitude shift keyingFrequency shift keyingPhase shift keyingBPSK2 symbols1 bit/symbolQPSK4 symbols2 bits/sy

35、mbolQAM-1616 symbols4 bits/symbolQAM-6464 symbols6 bits/symbolQAM varies amplitude and phaseBPSK/QPSK varies only phasePassband Transmission (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Constellation diagrams are a shorthand to capture the amplitude and pha

36、se modulations of symbols:Passband Transmission (3)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Gray-coding assigns bits to symbols so that small symbol errors cause few bit errors:ABCDEFrequency Division Multiplexing (1)CN5E by Tanenbaum & Wetherall, Pearson

37、Education-Prentice Hall and D. Wetherall, 2011FDM (Frequency Division Multiplexing) shares the channel by placing users on different frequencies:Overall FDM channelFrequency Division Multiplexing (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011OFDM (Orthogonal

38、 FDM) is an efficient FDM technique used for 802.11, 4G cellular and other communicationsSubcarriers are coordinated to be tightly packedTime Division Multiplexing (TDM)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Time division multiplexing shares a channel ov

39、er time:Users take turns on a fixed schedule; this is not packet switching or STDM (Statistical TDM)Widely used in telephone / cellular systemsCode Division Multiple Access (CDMA)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011CDMA shares the channel by giving us

40、ers a codeCodes are orthogonal; can be sent at the same timeWidely used as part of 3G networksA =+1-1+1-1B =+1+1-1-1+1+1-1-1C =-2+200S = +A -BS x A+2+2-2-2-2+200S x BS x CSum = 4A sent “1”Sum = -4B sent “0”Sum = 0C didnt sendSender CodesTransmittedSignalReceiver Decoding0000The Public Switched Telep

41、hone NetworkCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Structure of the telephone system Politics of telephones Local loop: modems, ADSL, and FTTH Trunks and multiplexing Switching Structure of the Telephone SystemCN5E by Tanenbaum & Wetherall, Pearson Educa

42、tion-Prentice Hall and D. Wetherall, 2011A hierarchical system for carrying voice calls made of:Local loops, mostly analog twisted pairs to housesTrunks, digital fiber optic links that carry callsSwitching offices, that move calls among trunksThe Politics of TelephonesIn the U.S., there is a distinc

43、tion for competition between serving a local area (LECs) and connecting to a local area (at a POP) to switch calls across areas (IXCs)Customers of a LEC can dial via any IXC they chooseCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Local loop (1): modemsCN5E by

44、Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Telephone modems send digital data over an 3.3 KHz analog voice channel interface to the POTSRates 56 kbps; early way to connect to the InternetLocal loop (2): Digital Subscriber LinesCN5E by Tanenbaum & Wetherall, Pearson

45、Education-Prentice Hall and D. Wetherall, 2011DSL broadband sends data over the local loop to the local office using frequencies that are not used for POTSTelephone/computers attach to the same old phone lineRates vary with lineADSL2 up to 12 MbpsOFDM is used up to 1.1 MHz for ADSL2Most bandwidth do

46、wnLocal loop (3): Fiber To The HomeCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011FTTH broadband relies on deployment of fiber optic cables to provide high data rates customersOne wavelength can be shared among many housesFiber is passive (no amplifiers, etc.)Tr

47、unks and Multiplexing (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Calls are carried digitally on PSTN trunks using TDMA call is an 8-bit PCM sample each 125 s (64 kbps)Traditional T1 carrier has 24 call channels each 125 s (1.544 Mbps) with symbols based o

48、n AMITrunks and Multiplexing (2)SONET (Synchronous Optical NETwork) is the worldwide standard for carrying digital signals on optical trunksKeeps 125 s frame; base frame is 810 bytes (52Mbps)Payload “floats” within framing for flexibilityCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall

49、 and D. Wetherall, 2011Trunks and Multiplexing (3)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Hierarchy at 3:1 per level is used for higher ratesEach level also adds a small amount of framingRates from 50 Mbps (STS-1) to 40 Gbps (STS-768)SONET/SDH rate hierar

50、chyTrunks and Multiplexing (4)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011WDM (Wavelength Division Multiplexing), another name for FDM, is used to carry many signals on one fiber:PSTN uses circuit switching; Internet uses packet switchingSwitching (1)CN5E by

51、Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011PSTN:Internet:Switching (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Circuit switching requires call setup (connection) before data flows smoothlyAlso teardown at end (not shown)Pa

52、cket switching treats messages independentlyNo setup, but variable queuing delay at routersCircuitsPacketsSwitching (3)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Comparison of circuit- and packet-switched networksMobile Telephone SystemCN5E by Tanenbaum & We

53、therall, Pearson Education-Prentice Hall and D. Wetherall, 2011Generations of mobile telephone systems Cellular mobile telephone systems GSM, a 2G system UMTS, a 3G system Generations of mobile telephone systemsCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 20111G,

54、analog voiceAMPS (Advanced Mobile Phone System) is example, deployed from 1980s. Modulation based on FM (as in radio).2G, analog voice and digital dataGSM (Global System for Mobile communications) is example, deployed from 1990s. Modulation based on QPSK.3G, digital voice and dataUMTS (Universal Mob

55、ile Telecommunications System) is example, deployed from 2000s. Modulation based on CDMA4G, digital data including voiceLTE (Long Term Evolution) is example, deployed from 2010s. Modulation based on OFDMCellular mobile phone systemsCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D

56、. Wetherall, 2011All based on notion of spatial regions called cellsEach mobile uses a frequency in a cell; moves cause handoffFrequencies are reused across non-adjacent cellsTo support more mobiles, smaller cells can be usedCellular reuse patternSmaller cells for dense mobilesGSM Global System for

57、Mobile Communications (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Mobile is divided into handset and SIM card (Subscriber Identity Module) with credentialsMobiles tell their HLR (Home Location Register) their current whereabouts for incoming callsCells kee

58、p track of visiting mobiles (in the Visitor LR)GSM Global System for Mobile Communications (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Air interface is based on FDM channels of 200 KHz divided in an eight-slot TDM frame every 4.615 msMobile is assigned up- and down-stream slots to useEach slot is 148 bits long, gives rate of 27.4 kbpsUMTS Universal Mobile T