Slide Title,Body Text,Second Level,Third Level,Fourth Level,Fifth Level,Sep10 CN,Weihong,Chen,Hunan City University,Advanced Computer Networks,计算机网络,The Physical Layer,Chapter 2,C,B,Contents,The Basis for Data Communication,1,Transmission Media,2,Public Switched Telephone Network,3,1 The Basis for Data Communication,（1）Nyquist Theorem:,for a finite bandwidth noiseless channel.,C,=2,H,log,2,V,C,Data rate,，,bps,H,Bandwidth,，,Hz,V,Signal discrete levels,V,Max.data rate,2 6200 bps,4 12400 bps,8 18600 bps,16 24800 bps,32 31000 bps,Nyquist,Theorem described an equation expressing,the maximum data rate,.,e.g,.A noiseless 3.1kHz channel cannot transmit binary(I.e.,two level)signals at a rate exceeding 6200 bps.,（2）Shannon Theorem:,f,or the channel subject to random noise.,The maximum data rate of a channel,C,=,H,log,2,（1+,S,/,N,）,C,Data rate,，,bps,H,Bandwidth,，,Hz,S/NSignal-to-noise ratio,S/R is given as,10log,10,S/N,and its unit is dB.,e.g.,A channel of 3000 Hz bandwidth with a signal to thermal noise radio of 30 dB can never transmit much more than 30,000 bps.,10log,10,S/N=30,S/N=1000,C,=3000 log,2,（,1+,1000,）,30000bps,2.,Guided Transmission Media,Magnetic Media,Twisted Pair,Coaxial Cable,Fiber Optics,2.1 Magnetic Media,Features:,The bandwidth characteristics are excellent,The delay characteristics are poor.Transmission time is measured in minutes or hours.,An Example:,200GB,1000,24Hours,19Gbps,1Hour,400Gbps,2.2 Twisted Pair,Can run several kilometers without amplification,Transmit either analog or digital signals,The bandwidth depends on the thickness of the wire and the distance traveled,Fig.1(a)Category 3 UTP (b)Category 5 UTP,双绞线由两根绝缘铜导线按螺旋状缠绕在一起,一对双绞线形成一条通信链路。,双绞线上可以传输模拟信号和数字信号；,带宽：与导线的粗细、长度及螺旋的密度有关；,非屏蔽双绞线（,UTP,）和屏蔽双绞线（,STP,）；,应用,连接电话机和电话端局，传输模拟信号，可以传输几公里；,用于住宅因特网接入，如使用普通模拟电话线和拨号调制解调器的住宅接入（,56Kbps,），使用数字用户线的宽带住宅接入（几个,Mbps,）。,用于组建计算机局域网,3,类双绞线：支持,10Mbps,；,5,类双绞线：支持,100Mbps,，是目前高速局域网连网的主要方式；,超,5,类双绞线：支持,1000Mbps,传输距离,100,米,；,.,双绞线制作方法,:,线序标准：,568A,白绿、绿、白橙、兰、白兰、橙、白棕、棕,568B,白橙、橙、白绿、兰、白兰、绿、白棕、棕,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,交叉线,EIA-568B,直连线,EIA-568A,交叉线,：交换机,交换机、,PC-PC,、,HUB-HUB(,标准端口,),直连线,：,PC/,路由器,交换机,/HUB,、,HUB-HUB(,级连端口,),双绞线的连接,2.3 Coaxial Cable,Fig.2 A coaxial cable,50-ohm,commonly used for digital transmission,75-ohm,commonly used for analog transmission,The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity.The bandwidth possible depends on the cable quality,length,and signal-to-noise ratio of the data signal.Modern cables have a bandwidth of close to 1 GHz.,同轴电缆由一对导体按照“同轴”的形式构成。,两类同轴电缆：,基带同轴电缆：阻抗为,50,，一般传输数字信号。传输距离较短，数据速率较低。曾被广泛用作电话系统的长途干线和用于传统局域网的组网，但现在基本上已被光纤和双绞线所替代。,宽带同轴电缆：阻抗为,75,，一般传输模拟信号。传输距离长，速率高，最常用做有线电视电缆。采用有线电视网络提供因特网接入，是宽带住宅接入的另一种可选方式。,2.4 Fiber Optics,(1)Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.,(2)Light trapped by total internal reflection.,The amount of refraction depends on the properties of the two media(in particular,their indices of refraction),For angels of incidence above a certain critical value,Fig.3,光线在光纤中的折射,折射角,入射角,包层,（低折射率的媒体）,包层,（低折射率的媒体）,纤芯,（高折射率的媒体）,包层,纤芯,光纤的工作原理,高折射率,(,纤芯,),低折射率,(,包层,),光线在纤芯中传输的方式是不断地全反射,输入脉冲,输出脉冲,单模光纤,多模光纤与单模光纤,输入脉冲,输出脉冲,多模光纤,Multimode fiber,Fibers core diameter is typically 50 microns,Any light ray incident on the boundary above the critical angle will be reflected internally.,Many different rays will be bouncing around at different angles.,Each ray is said to have a different mode,the fiber is called a multimode fiber,The distance is 2km without the relay,Single-mode fiber,Fibers core diameter is typically 8 to 10 microns,The fiber acts like a wave guide,and the light can propagate only in a straight line without bouncing,More expensive,used for longer distances,The distance is 100km without the relay,Fiber Cables,Fig.4(a)Side view of a single fiber.,(b)End view of a sheath with three fibers.,光纤传输系统由,3,个部分组成：光源，光纤和光检测器。,光源：发光二极管，激光。,光检测器：光电二极管，将光信号转换成电信号。,光纤：由两种折射系数不同的石英玻璃纤维组成，光线在光纤中全反射地向前传播。,单模光纤和多模光纤：纤芯直径分别为,8-10,m,和,50,m,。,光纤的最大优点是大容量、长距离、抗干扰，缺点是光设备成本高。,光纤的,3,种连接方式：光接头，机械方法，融合。,光纤网络较常使用环形拓扑和星形拓扑。,环形光纤网络,A fiber optic ring with active repeaters.,星形光纤网络,A passive star connection in a fiber optics network.,Example,Gigabit Ethernet cabling.,无线传输的频率范围大致从无线电到可见光（,10,4,10,15,Hz,）,频率越高，每赫兹编码的比特数越多，相同波带下所获得的带宽也越高（,f=c,/,2,），,所以高速数据传输通常发生在高频区域。,与计算机网络关系较密切的无线通信介质包括：,陆地无线电信道：,覆盖局部区域，通常跨越数十到几百米，应用在无线局域网中，如,802.11,；,运行在广域网，跨越几万米，应用在广域无线接入网中，如,3G,等；,卫星无线电信道,短距离内可使用红外线、蓝牙无线电等，距离在,10,米以内，数据为,1Mbps,。,2.3 Wireless Transmission(,无线传输,),卫星通信方式,每颗卫星连接两个或多个位于地球的微波发射,/,接收站；,卫星上携带转发器，监听频段中的一部分，放大输入的信号，并且以另一频率重新广播出去。,根据卫星所处的高度不同，有三类卫星：,地球同步卫星：目前远距离通信基本上依靠这一类卫星（,图,），缺点是传输延迟长；,中轨道卫星：目前这一类卫星没有用于电信通信，,GPS,卫星属于这一类,低轨道卫星：铱系统，,Globalstar,，,Teledesic,等，希望全面替代地面通信系统，提供世界范围内的电信服务；传输延迟短。,2.4 Communication,Satelites,(,通信卫星,),Geostationary Satellites,与地面站相对固定位置,使用三颗卫星即可覆盖全球,传输延迟时间长（,270ms,）,广播式传输,应用领域：,电视传输,长途电话,专