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1、Computer networks4(2.4):Transmission MediaOverviewzGuided wire zUnguided wireless zCharacteristics and quality determined by medium and signal zFor guided, the medium is more important zFor unguided, the data bandwidth produced by the antenna is more important zKey concerns are data rate and distanc
2、eDesign FactorszBandwidth yHigher bandwidth gives higher data rate zTransmission impairments yAttenuation zInterference zNumber of receivers yIn guided media yMore receivers (multi-point) introduce more attenuationElectromagnetic SpectrumGuided Transmission MediazTwisted Pair zCoaxial cable zOptical
3、 fiber导向介质光纤同轴电缆双绞线Twisted Pair：双绞线内导体芯线绝缘箔屏蔽铜屏蔽外套双绞线Twisted Pair - ApplicationszMost common medium zTelephone network yBetween house and local exchange (subscriber loop) zWithin buildings yTo private branch exchange (PBX) zFor local area networks (LAN) y10Mbps or 100Mbps100Hz 5MHzTwisted Pair - Pro
4、s and ConszCheap zEasy to work with zLow data rate zShort rangeEffect of Noise on Parallel LinesNoise on Twisted-Pair LinesTwisted Pair - Transmission CharacteristicszAnalog yAmplifiers every 5km to 6km zDigital yUse either analog or digital signals yrepeater every 2km or 3km zLimited distance zLimi
5、ted bandwidth (1MHz) zLimited data rate (100MHz) zSusceptible to interference and noiseUnshielded and Shielded TPzUnshielded Twisted Pair (UTP) yOrdinary telephone wire yCheapest yEasiest to install ySuffers from external EM interference zShielded Twisted Pair (STP) yMetal braid or sheathing that re
6、duces interference yMore expensive yHarder to handle (thick, heavy)UTPSTP UTP CategorieszCat 3 yup to 16MHz yVoice grade found in most offices yTwist length of 7.5 cm to 10 cm zCat 4 yup to 20 MHz zCat 5 yup to 100MHz yCommonly pre-installed in new office buildings yTwist length 0.6 cm to 0.85 cmNea
7、r End Crosstalk：近端串扰zCoupling of signal from one pair to another zCoupling takes place when transmit signal entering the link couples back to receiving pair zi.e. near transmitted signal is picked up by near receiving pairMaking and Testing 10base-TPatch Cable Coaxial Cable：同轴电缆同轴电缆100KHz 500MHzCo C
8、ableCoaxial Cable ApplicationszMost versatile medium：用途最广 zTelevision distribution：电视节目传送 yAriel to TV yCable TV zLong distance telephone transmission yCan carry 10,000 voice calls simultaneously yBeing replaced by fiber optic zShort distance computer systems links zLocal area networks：局域网Coaxial Ca
9、ble - Transmission CharacteristicszAnalog yAmplifiers every few km yCloser if higher frequency yUp to 500MHz zDigital yRepeater every 1km yCloser for higher data rates光传播特性-折射、临界角、反射Optical Fiber：光纤芯core防护层 jacket覆层 cladding-glass or plastic core -Laser or light emitting diode -Specially designed ja
10、cket -Small size and weightAngle of incidenceAngle of reflectionFiber ConstructionOptical Fiber Benefits优势zGreater capacity：大容量 yData rates of hundreds of Gbps zSmaller size & weight：体积更小，重量更轻 zLower attenuation：衰减更小 zElectromagnetic isolation：电磁隔离 zGreater repeater spacing：转发器间距更远Optical Fiber - Ap
11、plicationszLong-haul trunks：长途干线 zMetropolitan trunks：市区干线 zRural exchange trunks：农用交换干线 zSubscriber loops：用户环路 zLANs：局域网Optical Fiber - Transmission Characteristics：传输特性zAct as wave guide for 1014 to 1015 Hz yPortions of infrared and visible spectrum zLight Emitting Diode (LED)：发光二极管 yCheaper yWide
12、r operating temp range yLast longer zInjection Laser Diode (ILD)：注入型激光二极管 yMore efficient yGreater data rate zWavelength Division Multiplexing：波分复用Optical Fiber Transmission Modes：传输模式ModeSinglemodeStep-indexMultimodeGraded-index传输模式-多模突变Multimode step-index fiber传输模式-多模渐变Multimode graded-index fibe
13、r传输模式-单模Single-mode fiberWireless TransmissionzUnguided media：非导向介质 zTransmission and reception via antenna zDirectional：定向 yFocused beam：集成波束 yCareful alignment required：仔细校准 z Omnidirectional：全向 ySignal spreads in all directions yCan be received by many antennaeFrequencies：频率z2GHz to 40GHz yMicrow
14、ave yHighly directional yPoint to point ySatellitez30MHz to 1GHz yOmnidirectional yBroadcast radio z3 x 1011 to 2 x 1014 yInfrared yLocalVLFLF超低频、低频MFHF中频、高频VHFUHF甚高频、特高频SHF（Super high frequency)EHF（Extremely high frequency）超高频、极高频Terrestrial Microwave：地面微波zParabolic dish：抛物面“碟状”天线 zFocused beam：集成波
15、束 zLine of sight：视距 zLong haul telecommunications：长距离通信 zHigher frequencies give higher data rates地面微波接力两个地面站之间传送 距离：50 -100 km地球地球地面站之间的直视线路地面站之间的直视线路 微波传送塔微波传送塔Satellite Microwave：卫星微波zSatellite is relay station zSatellite receives on one frequency, amplifies or repeats signal and transmits on ano
16、ther frequency zRequires geo-stationary orbit yHeight of 35,784km zTelevision：电视传送 zLong distance telephone：长途电话传输 zPrivate business networks：专用商业网络Point-to-point link via satellite microwaveBroadcast link via satellite microwaveGeosynchronous OrbitCellular SystemBroadcast Radio：广播无线电zOmnidirectional zFM radio zUHF and VHF television zLine of sight zSuffers from multipath interference yReflectionsInfrared：红外zModulate nonc
