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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 Microwave Transmission Light Transmission Wireless vs. Wires/Fiber Electromagnetic Spectrum (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Different bands hav
