计算机网络英文版课件：ch2-physical

1、Chapter 2The Physical LayerPhysical LayerFundamental Concepts Theoretical Basis for Data Communication3 kinds transmission media Guided Transmission MediaTwisted pairCoaxfiber opticswireless (terrestrial radio)satelliteExamples of communication systemstelephone systemThe Local Loop, Modems, ADSL Tru

2、nks and Multiplexing Switchingcable television systemPhysical InterfaceFundamental Concepts (1)信道(Channel)：传送信息的媒体带宽(Bandwidth)：信号占用的频率范围 (Hz) 数据率(Bit Rate)：数据传输速率 (bps)波特率(Baud)：码元传输速率 1 Baud = (log2V) bps，其中V是信号的电平级数信道容量(Channel Capacity)：信道的最大数据率传播速度(Propagation Speed)：信号单位时间内传送的距离（米/秒）媒体不同速度不同同种

3、媒体，频率不同的电磁波速度也有差异误码率BER (Bit Error Rate)：信道传输可靠性指标 BER=传送错的位数 / 传送总位数Fundamental Concepts (2)时延(Delay)：从向网络中发送数据块的第一比特开始，到最后一位数据被接收所经历的时间时延的组成：传播时延、发送时延、处理时延、排队时延传播时延：信号通过传输介质的时间。发送时延：设备发送一个数据块所需要的时间（数据块长度/信道带宽)处理时延：交换机/路由器检查数据、选路的时间排队时延：在交换机/路由器中排队等待的时间Simplex, Half-duplex, Full-duplexSimplexTransm

4、ission occurs in only one directioneg. FM Radio broadcasting Half-duplexTransmission can be made in both direction, but at one time only one direction is availableeg. talkie and walkie （对讲机）Full-duplexTransmission can be made in both directions simultaneouslyeg. Phone conversation Parallel vs. Seria

5、l transmissionAsync. & Synchronous Transmission同步串行传输和异步串行传输同步串行传输以时钟信号线对传输的数据线上的信号进行比特同步以数据块（帧或分组）为单位传输异步串行传输独立时钟，无须同步以字符为单位进行传输发送两个字符之间的间隔是任意的接收方依靠字符中的起始位和停止位来同步2.1 The Theoretical Basis for Data CommunicationFourier AnalysisBandwidthNo transmission facility can transmit signal without losing some

6、 powerAll transmission facilities diminish different Fourier components by different amountsRange of frequencies transmitted without being strongly (0.5) attenuated is called bandwidthTelephone wires bandwidth: 1 MHz UTP 5: 100MHzbandwidth is a physical property of the transmission medium, depends o

7、nconstructionthicknesslengthbandwidth (Hz, bps)Bandwidth-Limited Signals (1)(a) A binary signal (b) and its Fourier amplitudes.(b)-(e) Successive approximations to the original signal.Bandwidth-Limited Signals (2)Bandwidth-Limited Signals (3)T=8/bps, First harmonic f=bps/8 for voice-grade line, band

8、width 3000 Hz Number of harmonics sent=3000/f=24000/bAt data rates 38.4 kbps, there is no hope at all for binary signals, even if the transmission facility is completely noiselessMaximum Data Rate of a ChannelNyquists theorem (1924) If a signal has been run through a low-pass filter of bandwidth H,

9、the signal can be completely reconstructed by making 2H samples per second C=2Hlog2VShannons Theorem (1948)Example: Voice phone line 300Hz-3300Hz, S/N dB=30dB作业2、3、4、5、9 2.2 Guided Transmission Media Magnetic MediaBandwidth (bps)A tape can hold 200 GB,A box can hold 1000 tapes: 200 TB,24小时快递,bandwid

10、th=200T x 8bits/86400s sec=19 GbpsCosttape cost $4000 per box per usage$1000 for shippingshipping 1 GB for under 3 centsTwisted Pair (1)Category 3 UTP Category 5 UTP Since 1865: Single wire -Twisted pairTwisted Pair: Most common transmission mediaFor analog/digital signals transmitting差分传输减弱环境噪音对自身信

11、号的影响When the wires are twisted, the waves from 2 wires cancel out, so the wire radiates less effectively(串扰：自身信号对环境影响) A B CTwisted Pair (2)UTP & STP (Unshielded / shielded twist pair)BandwidthCategory 3 UTP: 16MHzCategory 5 UTP: 100MHzCategory 6 UTP: 250MHzCategory 7 UTP: 600MHzCoaxial Cable It can

12、 span longer distances at higher speeds Two kinds of coaxial cable are widely used 50-ohm cable, is commonly used when it is intended for digital transmission 75-ohm cable, is commonly used for analog transmission and cable television High bandwidth (750MHz, 1GHz) Excellent noise immunityFiber Cable

13、s From 1981 to 2000CPU (4.77M to 2G Hz)A gain of a factor of 20 per decadePhysical limits: speed of light, heat dissipationData communication (56k to 2G bps)A gain of a factor of 125 per decadeError rate: 10-5 to almost 0Electrical and optical signals by photodiode: 40GbpsMultimode & single-mode fib

14、er (50/8um) (a) Side view of a single fiber(b) End view of a sheath with three fibersTransmission of Light through FiberAttenuation of light through fiber depends on the wavelength of the light3 bands are used for optical communicationBandwidth: All 3 bands are 2530THz wide2.3 Wireless TransmissionT

15、he Electromagnetic SpectrumElectromagnetic spectrum and its uses for communication In general, as we go from long-wave radio toward visible light, the waves behave more and more like light and less like radioTransmission Frequency BandUsing narrow frequency band (radio, TV, .)Most transmission use n

16、arrow frequency band (f/f1) to get the best receptionSpread Spectrum (bluetooth, wlan, cdma, .)FHSS: Frequency hopping spread spectrumDSSS: Direct sequence spread spectrumHedy Lamarr (1913-2000) A B C D Radio Transmission (VLF,LF,MF,HF)(a) In the VLF, LF, and MF bands, radio waves follow the curvatu

17、re of the earth (1000km， BeijingWuhan)(b) In the HF band, they bounce off the ionosphere at a height of 100500 kmMicrowave Transmission(100M10GHz)Before fiber optic, microwaves formed the heart of the long-distance telephone transmission systemwaves travel in nearly straight lines, the transmitting

18、& receiving antennas must be accurately alignedDistance between repeaters: 80kmMultipath fading is often a serious problem (weather and frequency dependent) (At about 2.4GHz, absorbed by rain and tree leaves)Politics of Electromagnetic SpectrumNational governments allocate spectrumWorldwide:ITU-R; U

19、SA: FCCbeauty contest; holding a lottery; competitive biddingISM (Industrial, Scientific, Medical) bandssome frequency bands for unlicensed usageFCC mandates that all devices in ISM bands use spread spectrum techniques to minimize interference between uncoordinated devicesPower is under 1 watt（限制频率范

20、围，调制技术，信号强度） ISM bands in USALightwave Transmission(Unguided optical signaling)Aiming a 1-mm-wide laser beam at a small target 500m away, .Laser beams cannot penetrate rain or thick fog Convection currents interfere with laser communication systems2.4 Communication Satellites Communication Satellite

21、s Act as a big microwave repeater in the skyContains multiple antennas and multiple transpondersTransponderlisten to some portion of the spectrumamplifies the incoming signalsrebroadcasts it at another frequency (downward beams)Downward beamsBroad: a substantial fraction of the earths surfaceNarrow:

22、 spot beams cover an area hundreds of kms in diameter (bent pipe)Station keepingLifetime: about 10 yearsCommunication Satellites and Some of Their Properties Altitude above the earth, round-trip delay time and number of satellites needed for global coverageThe Principal Satellite BandsA modern satel

23、lite has around 40 transponders, each with an 80-MHz bandwidthGEO: VSATs Using a HubProblemMicro-station do not have enough power to communicate directly with one another via the satelliteLEO: IridiumLEOThe ground stations do not need much powerRound-trip delay is only a few msIridium Altitude: 750k

24、m6 satellite necklaces, 11x6=66 satellitesEach satellite has a max of 48 cells (spot beams)1628 moving cells cover the earthCommunication between distant customers takes place in space实施情况90年策划，97年发射，98年运营，99年破产受到移动电话网的冲击Iridium Cells(a) Iridium satellites from six necklaces around the earth(b) 1628

25、 moving cells cover the earthIridium & Globalstar(a) Relaying in space. (b) Relaying on the ground.LEO: Globalstar48 LEOs satellitesRelaying on the groundputs much of the complexity on the ground, ease to managelower-powered telephones can be used (a few milliwatts)2.5 Public Switched Telephone Netw

26、ork (PSTN)(a) Fully-interconnected network.(b) Centralized switch.(c) Two-level hierarchy.Structure of the Telephone System(1)A typical circuit route for a medium-distance call5-level hierarchyEnd officeToll officePrimary officeSection officeRegional officeStructure of the Telephone System(2)Compone

27、nts of Telephone SystemLocal loopsAnalog twisted pairs going to houses and businesses,UTP-3 (总长度光速43分钟)TrunksDigital fiber optics connecting the switching officesSwitching officesWhere calls are moved from one trunk to anotherLocal Loops MODEM, ADSLProblems Transmission lines suffer from 3 major pro

28、blemsAttenuation(衰减, 衰减程度与频率相关)delay distortion(延迟畸变，与频率相关的传播速度造成)NoiseThermal noiseCrosstalkImpulse noiseModemsThe use of both analog and digital transmissions for a computer to computer callConversion is done by the modems and codecs.CODEC:8000 samples per second, all time intervals within the tel

29、ephone system are multiple of 125us1 sample = 7 or 8 bits (非线性编码)Modulations (a) A binary signal (b) Amplitude modulation(c) Frequency modulation(d) Phase modulationCombination of Modulation Techniques Modems use a combination of modulation techniques to transmit multiple bits per baud（multiple ampl

30、itudes and multiple phase shifts）QPSK: Quadrature Phase Shift Keying-相移键控QAM : Quadrature Amplitude Modulation-正交幅度调制Modems sample 2400 times/sec and focus on getting more bits per sample V.32: 2400 baud * (5-1) bits = 9.6KbpsV.32bis: 2400 baud * (7-1) bits = 14.4KbpsV.34: 2400 baud * 12 bits = 28.8

31、KbpsV.34bis: 2400 baud * 14 bits = 33.6Kbps (Shannon limit for voice level channel)V.90(56kbps)QPSK & QAM(a) QPSK. (b) QAM-16. (c) QAM-64.V.32 & V.32bis(a) V.32 for 9600 bps (b) V32 bis for 14,400 bps.(a)(b)DSL: Digital Subscriber LinesBandwidth versus distanced over category 3 UTP for DSLDMT: Discr

32、ete Multitone Modulation Divide 1.1 MHz into 256 channels of 4312.5 HzChannel 0 for POTS15 not used, to keep voice signal and data signals from interfering with each other250 channelsUpstream control (1) Downstream control (1)User data (248) ADSL Modulation Scheme Within each channel, a modulation s

33、cheme similar to V.34 is usedSampling rate is 4000 baud (other than 2400)Channel quality is monitored and data rate adjusted continuouslyThe actual data are sent with QAM modulation, with up to 15 bits/baudANSI T1.413 and ITU G.992.18Mbps, 1MbpsStandard service: 512 kbps, 64 kbpsPremium service: 1 M

34、bps, 256 kbpsTypical ADSL Equipment Configuration 作业22、23、25、26 Trunks and Multiplexing Frequency Division Multiplexing(a) The original bandwidths.(b) The bandwidths raised in frequency.(b) The multiplexed channel.Wavelength Division Multiplexing(1)ProblemnGHz-wide fiber: it is currently impossible

35、to convert between electrical and optical media any fasterBandwidth of a single fiber band is about 25,000 GHzWDM (Wavelength Division Multiplexing) DWDM (Dense WDM)Wavelength Division Multiplexing(2)Time Division Multiplexing: T1T1 carrier (1.544Mbps), 24 voice channelsBit 1 (frame synchronization)

36、, pattern 01010.7-bit voice samplesignaling: 24 bits x 8000 frames/sec = 192 bpsWhen a T1 system is being used entirely for data, only 23 of the channels are used for data, the 24th one is used for synchronizationVoice EncodingPCM (Pulse Code Modulation):7 bits or 8 bitsDPCM (Differential pulse code

37、 modulation): 5 bitsDelta modulation: 1 bitPredictive encodingDelta modulationTime Division Multiplexing (T2T4)Multiplexing T1 streams into higher carriers4 T1 streams are joined to form a T2 stream7 T2 streams are joined to form a T3 stream6 T3 streams are joined to form a T4 streamAt each step a s

38、mall amount of overhead is added for framing and recovery if synchronization between sender and receiver is lost 三大数字体系2.0488.44834.368139.264564.9921.5446.31244.736274.1761.5446.31232.06497.728397.200J1E1T1J2E2T2J3E3T3J4E4T4J5E5x32x24x24x 4x 4x 4x 4x 7x 5x 4x 6x 3x 4x 4USA日本56k56k64k作业28、31、34 Swit

39、ching Circuit Switching(a) Circuit switching (b) Packet switchingPacket switchingA node in a packet switching networkincoming linksoutgoing linksNodeMemoryMessage Switching(a) Circuit switching (b) Message switching (c) Packet switchingCircuit Switched and Packet-switched NetworksCDMA Code Division

40、Multiple AccessA form of spread spectrum communicationBit timeEach bit time is subdivided into m short intervals called chips(m=64,128)Chip sequenceEach station is assigned a unique m-bit code called a chip sequenceOrthogonality比较站A和站B的片序列分量：一半相同，一半相反Send dataTransmit bit 1: sends its chip sequenceT

41、ransmit bit 0: sends the complement of its chip sequenceNo other patterns are permittedCDMA Code Division Multiple Access(a) Binary chip sequences for four stations(b) Bipolar chip sequences (c) Six examples of transmissions(d) Recovery of station Cs signalOrthogonalityA*A=1A*(A)=-1A*B=0A*(B)=0作业42、43、532.7 Cable TelevisionCommunity Antenna TelevisionAn early cable television systemA big Antenna: pluck TV signal out of the