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1、（数据与计算机通信）（数据与计算机通信）Dr. Y. XU， 2009DataandComputerCommunicationsI.OVERVIEW. 60.Introduction. 1.DataCommunicationsandNetworkingOverview. 2.ProtocolArchitecture. II.DATACOMMUNICATIONS. 12 3.DataTransmission. 4.GuidedandWirelessTransmission. 5.SignalEncodingTechniques. 6.DigitalDataCommunicationTechniq
2、ues. 7.DataLinkControl. 8.Multiplexing. 9.SpreadSpectrum. ContentsIII.WIDEAREANETWORKS. 1210.CircuitSwitchingandPacketSwitching. 11.AsynchronousTransferMode. 12.RoutinginSwitchedNetworks. 13.CongestionControlinSwitchedDataNetworks. 14.CellularWirelessNetworks. IV.LOCALAREANETWORKS. 1215.LocalAreaNet
3、workOverview. 16.High-SpeedLANs. 17.WirelessLANs. V.COMMUNICATIONSARCHITECTUREANDPROTOCOLS. 1218.InternetworkProtocols. 19.InternetworkOperation. 20.TransportProtocols. 21.NetworkSecurity. 22.DistributedApplications. Total54H3.1 Concepts and Terminology3.2 Analog and Digital Data Transmission3.3 Tra
4、nsmission Impairments3.4 Channel CapacityChapter3DataTransmission信息通过数据通信系统的传输过程信息通过数据通信系统的传输过程把携带把携带信息信息的的数据数据用物理用物理信号信号形式通过信道传送到目的地形式通过信道传送到目的地信息和数据（二进制位）不能直接在信道上传输信息和数据（二进制位）不能直接在信道上传输编码编码：数据：数据适合传输的数字信号适合传输的数字信号便于同步、识别、纠错便于同步、识别、纠错调制调制：数字信号：数字信号适合传输的形式适合传输的形式按频率、幅度、相位按频率、幅度、相位解调解调：接收波形：接收波形数字信号数
5、字信号解码解码：数字信号：数字信号原始数据原始数据信息信息数据数据信号信号在信道上传输在信道上传输信号信号数据数据信息信息数据编码数据编码调制调制解调解调数据解码数据解码01000001A01000001A信道信道3.1TerminologyTransmitter (发射器)/ ReceiverMediumGuided medium e.g. twisted pair, optical fiberUnguided medium e.g. air, water, vacuumDirect link (No intermediate devices)Point-to-pointDirect lin
6、k Only 2 devices share linkMulti-pointMore than two devices share the linkSimplexOne directione.g. TelevisionHalf duplexEither direction, but only one way at a timee.g. police radioFull duplexBoth directions at the same timee.g. telephoneFrequency,SpectrumandBandwidthTime domain (时域) conceptsAnalog
7、signal (Continuous signal,连续信号)Various in a smooth way over timeDigital signal(Discrete signal,离散信号)Maintains a constant level then changes to another constant levelPeriodic signal(周期信号)Pattern repeated over timeAperiodic signal (非周期信号)Pattern not repeated over time数据及计算机通信术语数据及计算机通信术语数据（数据（Data）：传递
8、（携带）信息的实体。：传递（携带）信息的实体。信息（信息（Information）：是数据的内容或解释。：是数据的内容或解释。信号（信号（Signal）：数据的物理量编码（通常为电编码），：数据的物理量编码（通常为电编码），数据以信号的形式在介质中传播。数据以信号的形式在介质中传播。模拟信号、数字信号模拟信号、数字信号基带（基带（Baseband）、宽带（）、宽带（Broadband）信道（信道（Channel）：传送信息的线路（或通路）。：传送信息的线路（或通路）。比特（比特（bit）：即一个二进制位。比特率为每秒传输的：即一个二进制位。比特率为每秒传输的比特数（即比特数（即数据数据传送速率
9、）。传送速率）。码元（码元（Codecell）：时间轴上的一个信号编码单元。：时间轴上的一个信号编码单元。波特（波特（Baud）：码元传输的速率单位。波特率：码元传输的速率单位。波特率为每秒传送的码元数（即信号传送速率）。为每秒传送的码元数（即信号传送速率）。比特率、波特率和信号编码级数的关系如下：比特率、波特率和信号编码级数的关系如下：Rbit=Rbaudlog2M上式中：上式中：M-信号的编码级数，信号的编码级数，Rbit-比特率，比特率，Rbaud-波特率波特率一个信号往往可以携带多个二进制位，所以在固定一个信号往往可以携带多个二进制位，所以在固定的信息传输速率下，比特率往往大于波特率。
10、换句的信息传输速率下，比特率往往大于波特率。换句话说，一个码元中可以传送多个比特。话说，一个码元中可以传送多个比特。例如：当波特率为例如：当波特率为9600时时若M=2，数据传输率为9600b/s若M=16，数据传输率为38.4kb/s误码率误码率：信道传输可靠性指标，是一个概率值。：信道传输可靠性指标，是一个概率值。信息编码信息编码：将信息用二进制数表示的方法。将信息用二进制数表示的方法。例如：例如：ASCII编码、编码、BCD编码等编码等数据编码数据编码：将数据用物理量表示的方法。：将数据用物理量表示的方法。例如：字符例如：字符A的的ASCII编码为编码为01000001，其数据，其数据编
11、码可能为编码可能为01000001t带宽（带宽（Bandwidth，BW）信道传输能力的度量。信道传输能力的度量。在传统的通信工程中：BW fmax fmin 单位：赫兹（Hz）在计算机网络中，一般用每秒允许传输的二进制位数作为带宽的计量单位。主要单位： b/s ，kb/s，Mb/s，Gb/s。例如：传统以太网理论上每秒可以传输1千万比特，它的带宽为10Mb/s。时延（时延（Delay）：信息从网络的一端传送到另一端所需信息从网络的一端传送到另一端所需的时间。的时间。时延时延=处理时延处理时延+排队时延排队时延+发送时延发送时延+传播时延传播时延处理时延处理时延=对数据进行处理和错误校验所需的
12、时间排队时延排队时延=数据在中间结点等待转发的延迟时间发送时延发送时延=数据位数/信道带宽传播时延传播时延=d/s d:距离，s:介质中信号传播速度(0.7c)bandwidth(带宽)本来是指信号具有的频带宽度，现在是数字信道所能传送的“最高数据率”的同义语，单位是“比特每秒”，或 b/s (bit/s)。(source bitrate: typing-40, reading 400, fast reading-4k, phone-64k, music-400k+, TV-90M+) 每秒 106 个比特时间1 0 1 0 1 11 sB1 Mb/s 时间每秒 4 106 个比特0.25 s
13、B4 Mb/s 往返时延（往返时延（Round-TripTime，RTT）：从发送端发送数据开始，到发送端收到接收端的确认所经历的时间RTT2传播时延传输可靠性传输可靠性两个含义：数据能正确送达数据能有序送达（当采用分组交换时）Signal&Information通信系统的任务是用信号（形式）传递信息（目的）。信息通信系统的任务是用信号（形式）传递信息（目的）。信息是指消息中所包含的有意义的内容，信号是信息的载体。是指消息中所包含的有意义的内容，信号是信息的载体。不同形式的信号，可以包含相同的信息，传输信息的多少使不同形式的信号，可以包含相同的信息，传输信息的多少使用信息量来衡量。用信息量来衡
14、量。信息及其度量信息及其度量离散信号可以看成是一种有限个状态的随机序列，它可以用离散信号可以看成是一种有限个状态的随机序列，它可以用离散型随机过程的统计特性来描述。离散型随机过程的统计特性来描述。如每一波形独立等概出现，则一个波形所传递的信息量为：如每一波形独立等概出现，则一个波形所传递的信息量为：Analogue&DigitalSignals(Continuous&DiscreteSignals)PeriodicSignalsSineWavePeak (峰值) Amplitude (幅值)( (A)maximum strength of signalvoltsFrequency (f)Rat
15、e of change of signalHertz (Hz) or cycles per secondPeriod = time for one repetition (T)T = 1/fPhase ()Relative position in timeVaryingSineWavess(t)=Asin(2 ft+ )Wavelength Distance occupied by one cycleDistance between two points of corresponding phase in two consecutive cyclesAssuming signal veloci
16、ty v = vTf = vc = 3*108 m/s (speed of light in free space) FrequencyDomainConceptsSignal usually made up of many frequenciesComponents are sine wavesCan be shown (Fourier analysis) that any signal is made up of component sine wavesCan plot frequency domain functionsAdditionofFrequencyComponents(T=1/
17、f)FrequencyDomainRepresentations ( early 19th century) the French mathematician Jean-Baptiste Fourier proved that any reasonably behaved periodic function, g(t) with period T can be constructed as the sum of a (possibly infinite) number of sines and cosines:g(t) = c + + f = 1/Tfundamental frequencya
18、n, bnthe sine and cosine amplitudes of the nth harmonics, and c is a constant. FourierAnalysisFrom g(t)， we can find c, an, bn:1) c = 2) an = 3)bn = 意义：周期为周期为T的信号可理解为若干周期为的信号可理解为若干周期为T/n的简谐信的简谐信号加权后的组合号加权后的组合。其中第n项称为n次谐波分量（0次谐波又称为直流分量，1次谐波又称为基波分量用三角函数（复指数函数）分解信号的优点：常用简谐函数，容易处理，可以表示时间和频率关系。频率和衰减：信号在通
19、道中的衰减是信号沿通道的损失量度。由于导线存在阻抗，阻碍信号的传输。当信号的频率增高，由于趋肤效应使电阻增大，又由于感抗增加、容抗减小，而使信号的高频分量衰减加大。衰减与传输信号的频率有关，也与导线的传输长度有关。随着长度的增加，信号衰减也随之增加。链路的衰减与频率的平方根成正比的，由电缆的结构、长度及传输信号的频率所决定的，在1M至100MHz频率范围内，不恰当的端接也会造成过量的衰减。Bandwidth-LimitedSignals(a) binary signal and its root-mean-square Fourier amplitudes.(b) (c) Successive
20、 approximations to the original signal.(d) (e) Successive approximations to the original signal.其傅立叶分析的系数为：an = cos(n/4) - cos(3n/4) + cos(6n/4) - cos(7n/4) bn = sin(3n/4) - sin(n/4) + sin(7n/4) - sin(6n/4) c = 3/8信号在信道上传输时的特性：对不同傅立叶分量的衰减不同，因此引起输出失真；信道有截止频率fc,0fc的振幅不衰减，fc以上的振幅衰减厉害，这主要由信道的物理特性决定，0fc称
21、为信道的有效带宽；实际使用时，可以接入滤波器，限制用户的带宽；通过信道的谐波次数越多，信号越逼真。Relation between data rate and harmonics (for a voice-grade line)Cut-off FrequencyCut-off Frequency（信道的截止频率）信道的截止频率）截截止止频频率率是是信号在信道传输过程中这个频率的分量的振幅衰减到原来的0.707时。这时输出信号的功率降低了一半。通信上认为这是这个频率对输出影响的截至点。此时：信号幅度An衰减的分贝数20lg(An/Amax）20 lg（0.707）=-3。该频率即为3分贝频率。而
22、这时信号功率Pn衰减值(Pn/Pmax）(An/Amax）2（0.707）20.5，正好衰减了一半。如果输入信号的带宽小于信道的带宽，则输入信号的全部频率分量都能通过信道，因而信道输出端得到的输出波形将是不失真的。但如果输入信号的带宽大于信道的带宽，则信号中某些频率分量就不能通过信道，这样输出得到的信号将与发送端发送的信号有些不同，即产生了失真。为了保证数据传输的正确性，必须限制信号的带宽。（一般要求是宽带宽，如音响系统，但在有些接收调谐电路和滤波电路里，要求通频带越窄越好。）分贝的知识的知识：分贝是一种比值，在表示电压比值是为log的20倍（在表示功率比值是为log的10倍）。如信号A强度是
23、信号B强度的10倍，则相差20dB。如信号相差100倍，即40dB。同理，如信号A功率是信号B功率的10倍，则相差10dB。如信号功率相差100倍，即20dB。这样的定义来自以前声学研究，由于人耳对声音强度的感觉与声音强度呈对数关系，而与声音强度的对数呈线性关系。为研究方便就对声强的比值取对数。但对数值又太小，就再乘20（如研究功率比就乘20）是为分贝（Decibeldb）db的标度大体上适合人耳对声音响度变化的感觉，因此音压的标度是与声能对数成正比。这意味著音压被以例为10，100，1000增加时，在主观的听感效果上 ，与它们的对数成正比。即分别为、。听力学上一般用分贝作为单位来表示声音的强
24、度级。声压级是最常用的声学参量，它所指的声压P与参考声压PO的比值取常用对数后以20倍乘的值，称为分贝。在隔音室中，听力正常的青年人所能听到的最小声音的强度叫听阈，约相当于0分贝听力级。通常一般耳语声约相当于听力级20分贝。普通对话约45分贝，低噪声街道约60分贝，繁忙交通要道约80分贝，地下铁道100分贝，飞机响声和雷鸣声约120分贝。信号幅度An衰减的分贝数20lg(An/Amax），信号功率Pn衰减的分贝数10lg(Pn/Pmax），这样，说一个放大器的电压增益为40分贝，即它可以放大电压100倍，但说一个通信系统信噪比为30分贝，即信号与噪声的功率比为1000。Spectrum&Ban
25、dwidthSpectrumrange of frequencies contained in signalAbsolute bandwidthwidth of spectrumEffective bandwidth (有效带宽)Often just bandwidthNarrow band of frequencies containing most of the energyDC ComponentComponent of zero frequencySignalwithDCComponentDataRateandBandwidthAny transmission system has a
26、 limited band of frequenciesThis limits the data rate that can be carried3.2AnalogandDigitalDataTransmissionData Entities that convey meaningSignalsElectric or electromagnetic representations of dataTransmissionCommunication of data by propagation(传播) and processing of signalsAnalogandDigitalDataAna
27、logContinuous values within some intervale.g. sound, videoDigitalDiscrete valuese.g. text, integersAcoustic(语音语音)Spectrum(Analog)AnalogandDigitalSignalsMeans by which data are propagatedAnalogContinuously variableVarious mediawire, fiber optic, spaceSpeech bandwidth 100Hz to 7kHzTelephone bandwidth
28、300Hz to 3400HzVideo bandwidth 4MHzDigitalUse two DC componentsAdvantages&DisadvantagesofDigitalCheaperLess susceptible to noiseGreater attenuationPulses become rounded and smallerLeads to loss of informationAttenuationofDigitalSignalsComponentsofSpeechFrequency range (of hearing) 20Hz-20kHzSpeech 1
29、00Hz-7kHzEasily converted into electromagnetic signal for transmissionSound frequencies with varying volume converted into electromagnetic frequencies with varying voltageLimit frequency range for voice channel300-3400HzConversionofVoiceInputintoAnalogSignalVideoComponentsUSA - 483 lines scanned per
30、 frame at 30 frames per second525 lines but 42 lost during vertical retraceSo 525 lines x 30 scans = 15750 lines per second63.5s per line11s for retrace, so 52.5 s per video lineMax frequency if line alternates black and whiteHorizontal resolution is about 450 lines giving 225 cycles of wave in 52.5
31、 sMax frequency of 4.2MHzBinaryDigitalDataFrom computer terminals etc.Two dc componentsBandwidth depends on data rateConversionofPCInputtoDigitalSignalDataandSignalsUsually use digital signals for digital data and analog signals for analog dataCan use analog signal to carry digital dataModemCan use
32、digital signal to carry analog data Compact Disc audioAnalogSignalsCarryingAnalogandDigitalDataDigitalSignalsCarryingAnalogandDigitalDataAnalogTransmissionAnalog signal transmitted without regard to contentMay be analog or digital dataAttenuated(衰减) over distance Use amplifiers to boost signalAlso a
33、mplifies noiseDigitalTransmissionConcerned with contentIntegrity (完整地) endangered (损伤) by noise, attenuation (衰减) etc.Repeaters (中继器) usedRepeater receives signalExtracts (解析) bit patternRetransmitsAttenuation (衰减) is overcomeNoise is not amplifiedAdvantagesofDigitalTransmissionDigital technology (数
34、据技术)Low cost LSI/VLSI technologyData integrity(完整性)Longer distances over lower quality linesCapacity utilization (容量利用率)High bandwidth links economicalHigh degree of multiplexing easier with digital techniquesSecurity & Privacy (安全和保密)EncryptionIntegration(综合性)Can treat analog and digital data simil
35、arly3.3TransmissionImpairments(损伤损伤)Signal received may differ from signal transmittedAnalog - degradation (损伤) of signal qualityDigital - bit errorsCaused byAttenuation (衰减) and attenuation distortion (衰减失真)Delay distortion (中继失真)Noise (噪声)Attenuation(衰减衰减)Signal strength (强度) falls off with distan
36、ceDepends on mediumReceived signal strength (强度) :must be enough to be detectedmust be sufficiently higher than noise to be received without errorAttenuation is an increasing function (函数变化) of frequencyDelayDistortionOnly in guided (导向) mediaPropagation velocity (速度) varies with frequencyNoiseAddit
37、ional signals inserted between transmitter and receiverThermal(热噪声): Due to thermal agitation (运动) of electrons, Uniformly (均匀地) distributed (分布); White noise (白噪声)Intermodulation(互调): Signals that are the sum and difference of original frequencies sharing a mediumCrosstalk (耦合) -A signal from one l
38、ine is picked up by anotherImpulse(冲击)Irregular (无规律) pulses or spikes (尖峰)e.g. External electromagnetic interference(干扰)Short duration (短的持续时间)High amplitude (高的幅度)3.4ChannelCapacityData rateIn bits per secondRate at which data can be communicatedBandwidthIn cycles per second of HertzConstrained by
39、 transmitter and mediumNoise通信通路上的平均噪声电平Error rate差错发生率NyquistBandwidthIf rate of signal transmission is 2B then signal with frequencies no greater than B is sufficient to carry signal rateGiven bandwidth B, highest signal rate is 2BGiven binary signal, data rate supported by B Hz is 2B bpsCan be in
40、creased by using M signal levelsC=2Blog2M ( 1924) Henry Nyquist derived an equation expressing the maximum data rate for a finite bandwidth noiseless channel. Nyquist proved that if an arbitrary signal has been run through a low-pass filter of bandwidth H, the filtered signal can be completely recon
41、structed by making only 2H (exact) samples per second. ShannonCapacityFormulaConsider data rate,noise and error rateFaster data rate shortens each bit so burst of noise affects more bitsAt given noise level, high data rate means higher error rateSignal to noise ration (in decibels): SNRdb=10 log10 (
42、signal/noise) CapacityC=Blog2(1+SNR)This is error free capacity(1948) Claude Shannon carried Nyquists work further and extended it to the case of a channel subject to random (that is, thermodynamic) noise.Shannon定理定理假设信道是有噪声的情况(SNR)db表示有用信号超出噪声值的量，以分贝位单位。SNR的值越高，表示信号的质量越好，所需中间转发器的数量越少。举例：见教材数字信号通过实际
43、的信道数字信号通过实际的信道失真不严重失真严重 实际的信道（带宽受限、有噪声、干扰和失真）输入信号波形输出信号波形(失真不严重)输入信号波形实际的信道（带宽受限、有噪声、干扰和失真）输出信号波形(失真严重)作业作业P88 3.43.9 (p61:Fig.3.4)3.133.153.23Chapter4TransmissionMedium4.1 Guided Transmission Media4.2 Wireless Transmission4.3 Wireless Propagation4.4 Line-of-sight(视距) TransmissionOverviewGuided (导向的
44、) - wireUnguided (非导向的) - wirelessCharacteristics and quality determined by medium and signalFor guided, the medium is more importantFor unguided, the bandwidth produced by the antenna(天线) is more importantKey concerns are data rate and distanceDesignFactorsBandwidthHigher bandwidth gives higher dat
45、a rateTransmission impairments(损伤)Attenuation (衰减)Interference (干扰)Number of receiversIn guided mediaMore receivers (multi-point) introduce more attenuationElectromagneticSpectrum(电磁波频电磁波频谱谱)4.1GuidedTransmissionMediaTwisted Pair (双绞线)Coaxial cable (同轴电缆)Optical fiber (光纤)TwistedPair(双绞线双绞线)CheapEas
46、y to work withLow data rateShort range双绞线双绞线（TwistPair，TP）-螺旋绞合的双导线螺旋绞合的双导线-每根每根4对、对、25对、对、1800对对-典型连接距离典型连接距离100m（LAN）-RJ45插座、插头插座、插头-优缺点：优缺点：成本低成本低组装密度高、节省空间组装密度高、节省空间安装容易（综合布线系统）安装容易（综合布线系统）平衡传输（高速率）平衡传输（高速率）抗干扰性一般抗干扰性一般连接距离短连接距离短应用领域：电话网络、计算机局域网应用领域：电话网络、计算机局域网内导体芯线内导体芯线绝缘绝缘内屏蔽内屏蔽外屏蔽外屏蔽外套外套Twist
47、edPair-ApplicationsMost common mediumTelephone networkBetween house and local exchange (subscriber loop,用户环路)Within buildingsTo private branch exchange (PBX ,专用分组交换机)For local area networks (LAN)10Mbps or 100MbpsTwistedPair-TransmissionCharacteristicsAnalog Amplifiers every 5km to 6kmDigitalUse eith
48、er analog or digital signalsrepeater every 2km or 3kmLimited distanceLimited bandwidth (1MHz)Limited data rate (100MHz)Susceptible to interference and noiseNearEndCrosstalk(串扰串扰)Coupling of signal from one pair to anotherCoupling takes place when transmit signal entering the link couples back to rec
49、eiving pairi.e. near transmitted signal is picked up by near receiving pairUnshieldedandShieldedTPUnshielded Twisted (无屏蔽) Pair (UTP)Ordinary telephone wireCheapestEasiest to installSuffers from external EM interferenceShielded Twisted (屏蔽) Pair (STP)Metal braid or sheathing that reduces interferenc
50、eMore expensiveHarder to handle (thick, heavy)UTPCategoriesCat 3up to 16MHzVoice grade found in most officesTwist length of 7.5 cm to 10 cmCat 4up to 20 MHzCat 5up to 100MHzCommonly pre-installed in new office buildingsTwist length 0.6 cm to 0.85 cmCat 5E (Enhanced) see tablesCat 6Cat 7ComparisonofS
51、hieldedandUnshieldedTwistedPairAttenuation (dB per 100 m)Near-end Crosstalk (dB)Frequency (MHz)Category 3 UTPCategory 5 UTP150-ohm STPCategory 3 UTPCategory 5 UTP150-ohm STP12.62.01.141625845.64.12.23253581613.18.24.4234450.42510.46.24147.510022.012.33238.530021.431.3TwistedPairCategoriesandClassesC
52、ategory 3 Class CCategory 5 Class DCategory 5ECategory 6 Class ECategory 7 Class FBandwidth16MHz100MHz100MHz200MHz600MHzCable TypeUTPUTP/FTPUTP/FTPUTP/FTPSSTPLink Cost (Cat 5 =1)0.711.21.52.2CoaxialCableA coaxial cable (1GHz-kms). 300450MHz，可传送100kmCoaxialCable(同轴电缆同轴电缆)CoaxialCableApplicationsMost
53、versatile mediumTelevision distributionAriel to TVCable TVLong distance telephone transmissionCan carry 10,000 voice calls simultaneouslyBeing replaced by fiber opticShort distance computer systems linksLocal area networksCoaxialCable-TransmissionCharacteristicsAnalogAmplifiers every few kmCloser if
54、 higher frequencyUp to 500MHzDigitalRepeater every 1kmCloser for higher data ratesOpticalFiber(光纤光纤)OpticalFiber-BenefitsGreater capacityData rates of hundreds of GbpsSmaller size & weightLower attenuationElectromagnetic isolation(隔离电磁场)Greater repeater spacing10s of km at leastOpticalFiber-Transmis
55、sionCharacteristicsAct as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrumLight Emitting Diode (LED ,发光二极管)CheaperWider operating temp rangeLast longerInjection Laser Diode (ILD ,入注型激光二极管)More efficientGreater data rateWavelength Division Multiplexing (波分多路复用)OpticalFiberTran
56、smissionModesFrequencyUtilizationforFiberApplicationsWavelength(invacuum)range(nm)Frequencyrange(THz)BandlabelFibertypeApplication820to900366to333MultimodeLAN1280to1350234to222SSinglemodeVarious1528to1561196to192CSinglemodeWDM1561to1620185to192LSinglemodeWDMAttenuationinGuidedMedia高密度多芯光缆高密度多芯光缆剖面结构
57、剖面结构芯芯封套封套外套外套加强芯加强芯光纤光纤外鞘外鞘加强芯加强芯光纤束光纤束4.2WirelessTransmissionFrequencies2GHz to 40GHzMicrowaveHighly directionalPoint to pointSatellite30MHz to 1GHzOmnidirectionalBroadcast radio3 x 1011 to 2 x 1014InfraredLocalTheElectromagneticSpectrum无线电微波红外线可见光紫外线X射线射线双绞线同轴电缆卫星地面微波 调幅无线电 调频无线电 海事无线电光纤电视(Hz)f(H
58、z)fLFMFHFVHF UHF SHFEHFTHF波段104105106107108109101010111012101310141015101610010210410610810101012101410161018102010221024 移动无线电 电信领域使用的电磁波的频谱电信领域使用的电磁波的频谱PoliticsoftheElectromagneticSpectrumThe ISM bands in the United States.【Story：美国无线频段拍卖：美国无线频段拍卖196亿美元亿美元新浪科技讯 北京时间3月19日消息：美国联邦通信委员会FCC拍卖700MHz频段。7
59、00MHz频段泛指698MHz-806MHz之间的频段，一共有108MHz。这些频段原来被美国电视台占用，由于美国计划2009年前完成电视台由模拟电视向数字电视迁移，FCC将这些频段收回，未来用于手机宽带互联网等业务。这一频段的信号有着很高的价值，既有合适的波长，可以较为便宜地覆盖广大农村地区；又有穿过厚墙的能力，可以较全面覆盖城市的各个角落，非常适合建立一个覆盖美国的手机宽带互联网络。108MHz中，24MHz用于公共安全，6MHz用于隔离公共安全频段与商用频段，其余78MHz商用。谷歌参与竞拍的是747MHz-752MHz和777MHz-782MHz之间的十个MHz的C频段。经过261轮竞
60、价之后，拍卖本周一正式结束，拍卖金额196亿美元。Verizon赢得了C频段的使用权，AT&T赢得了B频段的227张许可证，谷歌未能赢得任何许可证。E频段使用权则最归属美国卫星电视公司EchoStar的合作伙伴Frontier Wireless公司。至于涉及公共安全的D频段，则因无人出到底价而流拍。有分析师猜测，该频段遭受冷遇的原因是13亿美元的底价过高，以及该频段使用者需要与紧急应变部门合作的规定过于麻烦。】Antennas(天天线线)Electrical conductor (or system of.) used to radiate(辐射) electromagnetic energy
61、 or collect electromagnetic energyTransmissionRadio frequency energy from transmitterConverted to electromagnetic energyBy antennaRadiated into surrounding environmentReceptionElectromagnetic energy impinging on antennaConverted to radio frequency electrical energyFed to receiverSame antenna often u
62、sed for bothRadiation(发发光光)PatternPower radiated in all directionsNot same performance in all directionsIsotropic(同向性的) antenna is (theoretical) point in spaceRadiates in all directions equallyGives spherical(球形的) radiation patternParabolic(抛物面的抛物面的)ReflectiveAntennaUsed for terrestrial and satellit
63、e microwaveParabola is locus of point equidistant from a line and a point not on that lineFixed point is focusLine is directrix(准线)Revolve(旋转) parabola about axis to get paraboloid(抛物面)Cross section parallel to axis gives parabolaCross section perpendicular to axis gives circleSource placed at focus
64、 will produce waves reflected from parabola(抛物线) in parallel to axisCreates (theoretical) parallel beam of light/sound/radioOn reception, signal is concentrated at focus, where detector is placedParabolicReflectiveAntennaAntennaGainMeasure of directionality of antennaPower output in particular direc
65、tion compared with that produced by isotropic(同向性的) antennaMeasured in decibels (dB)Results in loss in power in another directionEffective area relates to size and shapeRelated to gainTerrestrialMicrowaveParabolic dishFocused beamLine of sightLong haul telecommunicationsHigher frequencies give highe
66、r data ratesSatelliteMicrowaveSatellite is relay (中继) stationSatellite receives on one frequency, amplifies or repeats signal and transmits on another frequencyRequires geo-stationary (地球同步) orbit (轨道)Height of 35,784kmTelevisionLong distance telephonePrivate business networksSatellitePointtoPointLi
67、nkSatelliteBroadcastLinkVSATs using a hub.BroadcastRadioOmnidirectional(单向的)FM radioUHF(UltraHigh Frequency,超高频) and VHF (Very High Frequency,特高频) televisionLine of sightSuffers from multipath interferenceReflectionsInfraredModulate noncoherent (不相干) infrared lightLine of sight (or reflection)Blocke
68、d (隔断) by wallse.g. TV remote control, IRD port (红外端口)4.3WirelessPropagationSignal travels along three routesGround waveFollows contour of earthUp to 2MHzAM radioSky waveAmateur(业余爱好者) radio, BBC world service, Voice of AmericaSignal reflected from ionosphere layer (电离层) of upper atmosphere(Actually
69、 refracted)Line of sightAbove 30MhzMay be further than optical(光学的) line of sight due to refraction(折射)More laterGroundWavePropagationSkyWavePropagationLineofSightPropagationRefraction(折射折射)Velocity of electromagnetic wave is a function of density of material3 x 108 m/s in vacuum, less in anything e
70、lseAs wave moves from one medium to another, its speed changesCauses bending(弯曲) of direction of wave at boundaryTowards more dense mediumIndex of refraction (refractive index) isSin(angle of incidence)/sin(angle of refraction)Varies with wavelengthMay cause sudden change of direction at transition
71、between mediaMay cause gradual bending if medium density is varyingDensity of atmosphere decreases with heightResults in bending towards earth of radio wavesOpticalandRadioHorizons(地平地平线线)4.4LineofSightTransmissionFree space lossSignal disperses with distanceGreater for lower frequencies (longer wav
72、elengths)Atmospheric Absorption(吸收)Water vapour and oxygen absorb radio signalsWater greatest at 22GHz, less below 15GHzOxygen greater at 60GHz, less below 30GHzRain and fog scatter radio wavesMultipathBetter to get line of sight if possibleSignal can be reflected causing multiple copies to be recei
73、vedMay be no direct signal at allMay reinforce or cancel direct signalRefractionMay result in partial or total loss of signal at receiverFreeSpaceLossMultipathInterference(干扰干扰)Chapter5SignalEncodingTechniques5.1 Digital data, digital signal5.2 Analog data, digital signal5.3 Digital data, analog sig
74、nal5.4 Analog data, analog signalDigitalData,DigitalSignalDigital signalDiscrete (离散), discontinuous voltage pulsesEach pulse is a signal elementBinary data encoded into signal elementsTermsUnipolar (单极性码): All signal elements have same signPolar (双极性码): One logic state represented by positive volta
75、ge the other by negative voltageData rate: Rate of data transmission in bits per secondDuration or length of a bit: Time taken for transmitter to emit the bitModulation rate (调制速率)Rate at which the signal level changesMeasured in baud = signal elements per secondMark传号) and Space(空号)Binary 1 and Bin
76、ary 0 respectivelyInterpretingSignalsNeed to knowTiming of bits - when they start and endSignal levelsFactors affecting successful interpreting of signalsSignal to noise ratio(信噪比)Data rateBandwidthComparisonofEncodingSchemesSignal Spectrum(信号频谱)Lack of high frequencies reduces required bandwidthLac
77、k of dc component allows ac coupling via transformer, providing isolationConcentrate power in the middle of the bandwidthClockingSynchronizing transmitter and receiverExternal clockSync mechanism based on signalError detectionCan be built in to signal encodingSignal interference and noise immunity (
78、抗噪声能力)Some codes are better than othersCost and complexityHigher signal rate (& thus data rate) lead to higher costsSome codes require signal rate greater than data rateEncodingSchemesNonreturn to Zero-Level (NRZ-L)Nonreturn to Zero Inverted (NRZI) (NRZI,不归零1制码)Bipolar -AMI (双极性AMI)Pseudoternary(伪三进
79、码)ManchesterDifferential ManchesterB8ZS(8零替换码)HDB3 (双极性3零码)NonreturntoZero-Level(NRZ-L)Two different voltages for 0 and 1 bitsVoltage constant during bit intervalno transition I.e. no return to zero voltagee.g. Absence of voltage for zero, constant positive voltage for oneMore often, negative voltag
80、e for one value and positive for the otherThis is NRZ-LNonreturntoZeroInverted(反转的反转的)Nonreturn to zero inverted on onesConstant voltage pulse for duration of bitData encoded as presence or absence of signal transition at beginning of bit timeTransition (low to high or high to low,跳变) denotes a bina
81、ry 1No transition denotes (表示) binary 0An example of differential (差分) encodingNRZDifferentialEncodingData represented by changes rather than levelsMore reliable detection of transition rather than levelIn complex transmission layouts it is easy to lose sense of polarityNRZprosandconsPros (优点)Easy t
82、o engineerMake good use of bandwidthCons (缺点)dc componentLack of synchronization capabilityUsed for magnetic recordingNot often used for signal transmissionMultilevelBinaryUse more than two levelsBipolar-AMI(交替传号反码)zero represented by no line signalone represented by positive or negative pulseone pu
83、lses alternate (交替) in polarityNo loss of sync (同步) if a long string of ones (zeros still a problem)No net dc component(净直流分量)Lower bandwidthEasy error detectionBipolar-AMIandPseudoternaryPseudoternary (伪三进码)One represented by absence of line signalZero represented by alternating positive and negati
84、veNo advantage or disadvantage over bipolar-AMITrade-OffforMultilevelBinaryNot as efficient as NRZEach signal element only represents one bitIn a 3 level system could represent log23 = 1.58 bitsReceiver must distinguish between three levels (+A, -A, 0)Requires approx. 3dB more signal power for same
85、probability of bit errorBiphase(双相位码双相位码)ManchesterTransition in middle of each bit periodTransition serves as clock and dataLow to high represents oneHigh to low represents zeroUsed by IEEE 802.3Differential ManchesterMidbit transition is clocking onlyTransition at start of a bit period represents
86、zeroNo transition at start of a bit period represents oneNote: this is a differential encoding schemeUsed by IEEE 802.5ConventionsforrepresentationofdataThere are two opposing conventions for the representations of data.The first of these was first published by G. E. Thomas in 1949 and is followed b
87、y numerous authors (e.g., Tanenbaum). It specifies that for a 0 bit the signal levels will be Low-High (assuming an amplitude physical encoding of the data) - with a low level in the first half of the bit period, and a high level in the second half. For a 1 bit the signal levels will be High-Low.The
88、 second convention is also followed by numerous authors (e.g., Stallings) as well as by IEEE 802.4 (token bus) and lower speed versions of IEEE 802.3 (Ethernet) standards. It states that a logic 0 is represented by a High-Low signal sequence and a logic 1 is represented by a Low-High signal sequence
89、.If a Manchester encoded signal is inverted in communication, it is transformed from one convention to the other. This ambiguity can be overcome by using differential Manchester encoding.-http:/ least one transition per bit time and possibly twoMaximum modulation rate is twice NRZRequires more bandw
90、idthProsSynchronization on mid bit transition (self clocking)No dc componentError detectionAbsence of expected transitionModulationRateScrambling(扰码技术扰码技术)Use scrambling to replace sequences that would produce constant voltageFilling (填充) sequence Must produce enough transitions to syncMust be recog
91、nized by receiver and replace with originalSame length as originalNo dc componentNo long sequences of zero level line signalNo reduction in data rateError detection capabilityB8ZS: Bipolar With 8 Zeros SubstitutionBased on bipolar-AMIIf octet of all zeros and last voltage pulse preceding was positiv
92、e encode as 000+-0-+If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+-Causes two violations of AMI codeUnlikely to occur as a result of noiseReceiver detects and interprets as octet of all zerosHDB3: High Density Bipolar 3 ZerosBased on bipolar-AMIString of four z
93、eros replaced with one or two pulsesB8ZSandHDB3DigitalData,AnalogSignalPublic telephone system300Hz to 3400HzUse modem (modulator-demodulator)Amplitude shift keying (ASK)Frequency shift keying (FSK)Phase shift keying (PK)ModulationTechniquesAmplitudeShiftKeyingValues represented by different amplitu
94、des of carrierUsually, one amplitude is zeroi.e. presence and absence of carrier is usedSusceptible to sudden gain changesInefficientUp to 1200bps on voice grade linesUsed over optical fiberBinaryFrequencyShiftKeyingMost common form is binary FSK (BFSK)Two binary values represented by two different
95、frequencies (near carrier)Less susceptible to error than ASKUp to 1200bps on voice grade linesHigh frequency radioEven higher frequency on LANs using co-axMultipleFSKMore than two frequencies usedMore bandwidth efficientMore prone to errorEach signalling element represents more than one bitFSKonVoic
96、eGradeLinePhaseShiftKeyingPhase of carrier signal is shifted to represent data.Binary PSKTwo phases represent two binary digitsDifferential PSKPhase shifted relative to previous transmission rather than some reference signalQuadraturePSKMore efficient use by each signal element representing more tha
97、n one bite.g. shifts of /2 (90o)Each element represents two bitsCan use 8 phase angles and have more than one amplitude9600bps modem use 12 angles , four of which have two amplitudesOffset QPSK (orthogonal QPSK)Delay in Q streamQPSKandOQPSKModulatorsExamplesofQPSFandOQPSKWaveformsPerformanceofDigita
98、ltoAnalogModulationSchemesBandwidthASK and PSK bandwidth directly related to bit rateFSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies(See Stallings for math)In the presence of noise, bit error rate of PSK and QPSK are abo
99、ut 3dB superior to ASK and FSKQuadrature(正交正交)AmplitudeModulationQAM used on asymmetric digital subscriber line (ADSL) and some wirelessCombination of ASK and PSKLogical extension of QPSKSend two different signals simultaneously on same carrier frequencyUse two copies of carrier, one shifted 90Each
100、carrier is ASK modulatedTwo independent signals over same mediumDemodulate and combine for original binary outputQAMModulatorQAMLevelsTwo level ASKEach of two streams in one of two statesFour state systemEssentially QPSKFour level ASKCombined stream in one of 16 states64 and 256 state systems have b
101、een implementedImproved data rate for given bandwidthIncreased potential error rateAnalogData,DigitalSignalDigitizationConversion of analog data into digital dataDigital data can then be transmitted using NRZ-LDigital data can then be transmitted using code other than NRZ-LDigital data can then be c
102、onverted to analog signalAnalog to digital conversion done using a codecPulse code modulationDelta modulationPulseCodeModulation(PCM)If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signalVoi
103、ce data limited to below 4000Hz，require 8000 sample per second4 bit system gives 16 levelsQuantizedQuantizing error or noiseApproximations mean it is impossible to recover original exactly8 bit sample gives 256 levelsQuality comparable with analog transmission8000 samples per second of 8 bits each g
104、ives 64kbpsPCMExampleNonlinearEncodingQuantization levels not evenly spaced(量化值间非等间隔)Reduces overall signal distortionCan also be done by companding (compress-exanding压扩)PCMBlockDiagramEffectofNon-LinearCodingTypicalCompandingFunctionsMagnitude: 量级Delta(增量增量)ModulationAnalog input is approximated by
105、 a staircase functionMove up or down one level () at each sample intervalBinary behaviorFunction moves up or down at each sample intervalDeltaModulation-exampleDeltaModulation-OperationDeltaModulation-PerformanceGood voice reproduction PCM - 128 levels (7 bit)Voice bandwidth 4khzShould be 8000 x 7 =
106、 56kbps for PCMData compression can improve on thise.g. Interframe coding techniques for videoAnalogData,AnalogSignalsWhy modulate analog signals?Higher frequency can give more efficient transmissionPermits frequency division multiplexing (chapter 8)Types of modulationAmplitudeFrequencyPhaseAnalogModulation作业作业P125：4.24.5
