数据与计算机通信.ppt

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1、1,数据通信与网络,Data and Computer Communications,传输媒体 4. Transmission Media,导向媒体（双绞线、同轴电缆、光纤） 无线传输（地面微波、卫星、广播无线电、红外） 无线电传播方式 视距传输,3,Overview,Guided - wire Unguided - wireless Characteristics and quality determined by medium and signal For guided, the medium is more important For unguided, the bandwidth pr

2、oduced by the antenna is more important Key concerns are data rate and distance,4,传输 媒体种类,双绞线 Twisted Pair 同轴电缆 Coaxial Cable 光纤 Optical Fiber,无线电 Radio 微波 Microwave 卫星 Satellite Microwave 红外 Infrared,有线媒体 Guided,信道通常是抽象的描述， 传输媒体即是物理信道。,无线媒体 Unguided,信道与传输媒体,5,Design Factors,Bandwidth 带宽 Higher band

3、width gives higher data rate Transmission impairments 传输损伤 Attenuation, etc. Interference 干扰 Intermodulation noise Crosstalk Number of receivers 接收器数目 In guided media More receivers (multi-point) introduce more attenuation,6,频谱及其可用于通信的频段,7,Electromagnetic Spectrum,特低频 音频 甚低频 低频 中频 高频 甚高频 超高频 极高频 特高频

4、 红外线 可见光,8,Overview,Guided Transmission Media导向媒体 （双绞线、同轴电缆、光纤） Wireless Transmission无线传输 （地面微波、卫星、广播无线电、红外） Wireless Propagation无线电传播方式 Line-of-Sight Transmission视距传输,9,导向传输媒体 Guided Transmission Media,10,Transmission Characteristics of Guided Media,11,典型的美国线规(AWG)特性,12,Twisted Pair,Physical Struct

5、ure Applications Pros and cons UTP and STP UTP Categories Transmission Characteristics Media Specifications UTP Practice,13,由两根互相绝缘的铜导线，用规则的方法扭绞起来构成。,双绞线(Twisted-pair),14,Twisted Pair - Twist Length,15,Twisted Pair - Applications,Most common medium Telephone network Between house and local exchange

6、(subscriber loop) Within buildings To private branch exchange (PBX) For local area networks (LAN) 10Mbps or 100Mbps,16,Twisted Pair - Pros and Cons,Cheap Easy to work with Low data rate Short range,17,Twisted Pair - Transmission Characteristics,Analog Amplifiers every 5km to 6km Digital Use either a

7、nalog or digital signals repeater every 2km or 3km Limited distance Limited bandwidth (1MHz) Limited data rate (100MHz) Susceptible to interference and noise,18,Near End Crosstalk,Coupling of signal from one pair to another Coupling takes place when transmit signal entering the link couples back to

8、receiving pair i.e. near transmitted signal is picked up by near receiving pair,19,Unshielded and Shielded TP,Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference Shielded Twisted Pair (STP) Metal braid or sheathing that reduces inte

9、rference More expensive Harder to handle (thick, heavy),20,UTP(Unshielded Twisted-Pair )电缆,21,Physical UTP,22,STP(Shielded Twisted-Pair)电缆,23,STP Physical Structure,衬铂,24,Physical STP,25,UTP Categories,Cat 3 up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cm Cat 4 up to 20

10、 MHz Cat 5 up to 100MHz Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm Cat 5E (Enhanced) see tables Cat 6 Cat 7,26,UTP电缆的分类 （EIA/TIA-568标准中定义了前五类）,27,Comparison of Shielded and Unshielded Twisted Pair,28,Twisted Pair Categories and Classes,29,Avaya Gigaspeed UTP Wires,

11、10BASE-T 100BASE-TX/T4 1000BASE-T,30,UTP 连接器,RJ-11,RJ-45,RJ-12,31,Avaya RJ-45 Jackes,32,Avaya UTP Inside of RJ45 Jack,33,4对线的UTP与RJ-45的连接,RJ-45 UTP线对 引脚线 颜色 1 绿白 2 绿 3 橙白 4 蓝 5 蓝白 6 橙 7 棕白 8 棕,T R T R T R T R 1 2 3 4 5 6 7 8,对3,对4,对2,对1,34,4对线的UTP与RJ-45的另一种连接,T R T R T R T R 1 2 3 4 5 6 7 8,对2,对4,对3

12、,对1,RJ-45 UTP线对 引脚线 颜色 1 橙白 2 橙 3 绿白 4 蓝 5 蓝白 6 绿 7 棕白 8 棕,35,双绞线的网线制作,RJ-45接头 (水晶头),压线钳,36,37,双绞线接线标准,铰齐线头，插入插头,用压线钳夹紧，使用测试仪测试,接线的标准有EIATIA T568A和EIATIA T568B,38,RJ-45,制作,39,同轴电缆(Coaxial Cable) 由单股实心或多股绞合的铜质芯线（内导体）、绝缘层、网状编织的屏蔽层(外导体)以及保护外层所组成,40,Coaxial Cable,41,Coaxial Cable Applications,Most versa

13、tile medium Television distribution Ariel to TV Cable TV Long distance telephone transmission Can carry 10,000 voice calls simultaneously Being replaced by fiber optic Short distance computer systems links Local area networks,42,Coaxial Cable - Transmission Characteristics,Analog Amplifiers every fe

14、w km Closer if higher frequency Up to 500MHz Digital Repeater every 1km Closer for higher data rates,43,Categories of Coaxial Cables,44,粗同轴电缆 简称粗缆，用于传送基带数字信号.,细同轴电缆 简称细缆，也用于传送基带数字信号。 最大传输距离不及粗缆，但因比粗缆 便宜而被广泛使用。,TV电缆 用于模拟传输系统，它是社区天线电 视系统(CATV)中的标准传输电缆。 采用的是频分复用的宽带信号。,同轴电缆的种类,45,10BASE5 Media Specifica

15、tion,Thick Coaxial Cable,46,10BASE5,47,10BASE2,48,Optical Fiber （光纤）,光纤的基本知识 光及其自然特性 光纤的物理结构 光纤的导光原理 光纤的应用 光纤的优点 光纤的应用场合 光纤的传输特性 光纤传输模式 损耗特性与色散特性 最佳波长窗口,49,光纤的基本知识（一）,光 光是一种电磁能量形式 在真空中传播最快，300000km/s 光速与传输媒质密度成反比 光的自然特性 折射 全反射角 反射（全反射）,50,Bending of Light Ray,51,Refraction (折射),在均匀物质中光以直线形式传播。 当一束光线

16、由某种密度的媒质传播到不同密度的另一种媒质时，光线会在两种媒质的界面处向垂直的法线方向折射。 折射光线发生的角度变化取决于媒质发生的密度变化的大小（如斯涅尔定律所描述） 光线从密度较低的媒质穿入较高密度的媒质中时，折射角小于入射角 光线从密度较高的媒质穿入较低密度的媒质中时，折射角大于入射角,52,Refraction,折射角R比入射角I小 折射角R比入射角I大,53,Critical Angle (全反射角),从高密度媒质进入到低密度媒质的光线，随着入射角增大，折射角也增大，并逐渐远离垂直轴而靠近水平轴 入射角的变化使折射角增大到90度，折射光线完全成了沿媒质界面传播水平光线，此时的入射角称

17、为临界角，或称全反射角。,54,Reflection (反射),对于从高密度媒质到低密度媒质的光线，当入射角大于全反射角时，就产生了反射 光线再不会进入低密度媒质，此时反射光线与法线之间的夹角称为反射角 在此情况下，入射角总是等于反射角,55,斯涅尔反射定律斯涅尔折射定律,1=1 (1） n1sin1=n2sin2 (2) n1 、n2为不同媒质的折射率,电磁场理论中的斯涅尔定理描述了入射光线与反射光线、折射光线之间的关系,56,core、cladding、jacket,光纤的物理构成 光纤由三部分组成：纤芯、覆层和防护罩。,光纤的基本知识（二）,57,Fiber Construction,5

18、8,Optical Fiber,59,Typical Fiber Cable,60,Cable Structures,61,光纤的导光原理 光纤利用全反射将光线在信道内定向传输 光纤中心是密度较高的石英玻璃（二氧化硅）或塑料的纤芯，外面覆盖着密度较小的石英玻璃或塑料的包层 两种材料的密度差异必须保证能够使纤芯中的光线只能反射回来而不能折射入包层,光纤的基本知识（三）,62,光纤的纤芯和包层材料两者的折射率不同。设纤芯的折射率为n1，包层的折射率为n2，则要求有n1n2。纤芯和包层构成横截面很小的双层同心圆柱体，该圆柱体称为裸光纤。,63,64,光线A以入射角1A射到空气,在分界面被折射进纤芯。

19、折射角为2A，光线A折射进纤芯后继续传播，到达纤芯和包层的分界面时，因为n1n2且1A大于临界角，发生全反射，2 A等于1A。此后，入射到纤芯的光线将在包层界面上不断发生全反射，使光线在纤芯中以“Z”折线方式传播至另一端； 光线B的入射角1B 过大，使光线到达纤芯和包层的分界面时的入射角1B小于临界角，所以光线将折射进包层； 光线C是特殊情况，它平行于纤芯轴线射入光纤，入射1C为0, 光线沿着纤芯轴线传播。,65,渐变型光纤纤芯的折射率n1是半径r的函数，n1随r的增加而按一定规律减小，到达与包层的交界面为n2，而包层的折射率恒为n2，这种光纤的导光原理与上述不同。由于n1随r的增加按一定规律

20、减小，因此可根据n1（r）的不同，将纤芯分成若干层。,66,Optical Fiber - Benefits,Greater capacity Data rates of hundreds of Gbps Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing 10s（Tens） of km at least,67,Optical Fiber - Applications,Long-haul trunks Metropolitan trunks Rural exchan

21、ge trunks Subscriber loops LANs,68,光纤的传播模式,多模传播 多束光线从光源经由芯材通过不同的光路传播；按折射率分布的不同分为两种类型： 阶跃模式 n1、n2都为常数且n1n2； 渐变模式 n2为常数且n1随r增加而有规律减小。 单模传播 阶跃模式，因纤芯直径很小，入射角与反射角很大，几乎是沿轴线直线传播,69,光纤的种类,按照折射率分布的不同来分,70,如果光纤只能传输一个光线模式，这种光纤称为单模光纤（Single Mode fiber，简称SM）。 如果光纤允许光线中的多个模式传输，这种光纤称为多模光纤（Multi Mode fiber，简称MM）。 再

22、根据光纤的折射率分布的不同，多模光纤又分为多模阶跃型光纤和多模渐变型光纤。,按照传输模式的多少来分,71,Propagation Modes,72,Number of Modes,73,Optical Fiber Transmission Modes,74,Multimode Step-Index,光线以折线形状传输，脉冲信号畸变大，用于近距离传输。,75,Multimode Graded-Index,光线以曲线形状传输，脉冲信号畸变相对较小，比阶跃型传输距离更大，传输距离更长。,76,Single Mode,光线以直线传输，脉冲信号畸变小，用于长距离高速传输，是当前光纤发展的主流。,77,O

23、ptical Fiber Comparison,78,Optical Fiber - Transmission Characteristics,Act as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrum Light Emitting Diode (LED) Cheaper Wider operating temp range Last longer Injection Laser Diode (ILD) More efficient Greater data rate Wavelength Di

24、vision Multiplexing,79,Light Source,80,波分复用 Wavelength Division Multiplexing,不同频率的多路光线在同一光纤上传输，即在一根光纤上采用频分复用技术，承载若干不同频率点的光信号。 FDM的一种形式。 每一种颜色（波长）的光承载一个数据信道。,81,Dense Wavelength Division Multiplexing, DWDM密集波分复用,一般认为，信道间距大于1nm且信道总数低于8以下，称之为WDM系统, 反之，若波道间距小于1nm且波道总数大 于8以上者，即称之为DWDM系统。 DWDM能够在同一根光纤中，把不

25、同的波长同时进行组合和传输。为了保证有效，一根光纤转换为多个虚拟光纤。目前，由于采用了DWDM技术，单根光纤可以传输的数据流量达到400Gb/s以上。,Cernet2和中国下一代互联网示范工程CNGI也采用DWDM构建高速主干网。,82,光纤的传输特性,损耗特性 （类似于衰减） 吸收损耗（材料中杂质吸收） 散射损耗（材料、形状、折射率分布有缺陷或不均匀） 主要是瑞利散射：光纤材料折射率的随机性变化 色散特性（类似于时延失真） 材料色散（材料折射率随频率变化引起） 波导色散（几何结构、形状不完善使光进入包层传播） 模式色散（多模光纤的主要色散，单模光纤无）,83,波长,衰减,单模,多模,多模与单

26、模,84,Frequency Utilization for Fiber Applications,85,Avaya 12-fiber OpFib Bundle,86,Avaya OpFib Patch Wires,87,Attenuation in Guided Media,Attenuation in Guided Media,88,Overview,Characteristics and quality of a data communication 数据通信的特性与质量 determined by medium and signal 由媒体和信号确定 For guided(导向传输媒体

27、) the medium is more important 媒体本身更重要 For unguided the bandwidth of the signal produced by the antenna is more important 天线产生的信号带宽更重要 Key concerns data rate and distance 数据率、距离,89,Wireless Transmission Frequencies,2GHz to 40GHz Microwave Highly directional Point to point Satellite 30MHz to 1GHz Omn

28、idirectional （全向） Broadcast radio 3 x 1011 to 2 x 1014 Infrared Local,90,Antennas,Electrical conductor (or system of) used to radiate electromagnetic energy or collect electromagnetic energy Transmission Radio frequency energy from transmitter Converted to electromagnetic energy By antenna Radiated

29、into surrounding environment Reception Electromagnetic energy impinging on（接触） antenna Converted to radio frequency electrical energy Fed to receiver Same antenna often used for both,91,Radiation Pattern,Power radiated in all directions Not same performance in all directions Isotropic antenna （各向同性天

30、线）is (theoretical) point in space Radiates in all directions equally Gives spherical radiation pattern（球辐射）,92,Parabolic Reflective Antenna,Used for terrestrial and satellite microwave Parabola is locus of point equidistant from a line and a point not on that line Fixed point is focus Line is direct

31、rix（准线） Revolve parabola（抛物线） about axis to get paraboloid（抛物面） Cross section parallel to axis gives parabola Cross section perpendicular(垂直) to axis gives circle Source placed at focus will produce waves reflected from parabola in parallel to axis Creates (theoretical) parallel beam of light/sound/

32、radio On reception, signal is concentrated at focus, where detector is placed,93,Parabolic Reflective Antenna,94,Parabolic Dish Antenna (抛物面碟形天线),对称线,95,Antenna Gain,Measure of directionality (方向性)of antenna Power output in particular direction compared with that produced by isotropic antenna Measur

33、ed in decibels (dB) Results in loss in power in another direction Effective area relates to size and shape Related to gain,96,Terrestrial Microwave,Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates,97,Satellite Microwave,Satellite is rel

34、ay station Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Requires geo-stationary orbit Height of 35,784km Television Long distance telephone Private business networks,98,Satellite Point to Point Link,99,Satellite Broadcast Link,100,Satellite,101,

35、Geo-stationary Satellite,102,Broadcast Radio,Omnidirectional FM radio UHF and VHF television Line of sight Suffers from multipath（多径） interference Reflections,103,Infrared,Modulate noncoherent （非相干） infrared light Line of sight (or reflection) Blocked by walls e.g. TV remote control, IRD port,104,Wi

36、reless Propagation,Signal travels along three routes Ground wave Follows contour of earth Up to 2MHz AM radio Sky wave Amateur radio, BBC world service, Voice of America Signal reflected from ionosphere layer of upper atmosphere (Actually refracted) Line of sight Above 30Mhz May be further than opti

37、cal line of sight due to refraction More later,105,Ground Wave Propagation,106,Sky Wave Propagation,电离层ionosphere,107,Line of Sight Propagation,108,Refraction折射,Velocity of electromagnetic wave is a function of density of material 3 x 108 m/s in vacuum, less in anything else As wave moves from one m

38、edium to another, its speed changes Causes bending of direction of wave at boundary Towards more dense medium Index of refraction 折射系数(refractive index折射率) is Sin(angle of incidence)/sin(angle of refraction) Varies with wavelength May cause sudden change of direction at transition between media May

39、cause gradual bending if medium density is varying Density of atmosphere decreases with height Results in bending towards earth of radio waves,109,Optical and Radio Horizons 视觉地平线与无线电地平线,110,Line of Sight Transmission,Free space loss自由空间损耗 Signal disperses分散 with distance Greater for lower frequenci

40、es (longer wavelengths) Atmospheric Absorption Water vapour and oxygen absorb radio signals Water greatest at 22GHz, less below 15GHz Oxygen greater at 60GHz, less below 30GHz Rain and fog scatter radio waves Multipath多径 Better to get line of sight if possible Signal can be reflected causing multipl

41、e copies to be received May be no direct signal at all May reinforce or cancel direct signal Refraction折射 May result in partial or total loss of signal at receiver,111,Free Space Loss,112,Multipath Interference,113,Overview,Characteristics and quality of a data communication 数据通信的特性与质量 determined by

42、 medium and signal 由媒体和信号确定 For guided(导向传输媒体) the medium is more important 媒体本身更重要 For unguided the bandwidth of the signal produced by the antenna is more important 天线产生的信号带宽更重要 Key concerns data rate and distance 数据率、距离,114,Factors to select media,115,Assignments,习题 4.1 17 4.2 14 推荐阅读 Stallings C

43、hapter 4 洪恩在线 http:/ 打开网络之门通通透透看网线 http:/ 网络互联与建网技术 http:/ 复习 Lecture notes,116,4.1. Multiple Choice,1. Without repeater, twisted pair can transmit digital signals in（ ）. A. 100m B. 300m C. 2km D. 15km 2. Compare to UTP，the disadvantage of STP is： A. easy to cause interference B. short distance C. n

44、ot convenient for installation D. low data rate 3. The standard of UTP is（ ）. A. EIA-232-A B. EIA-449 C. EIA-530 D. EIA-568B,117,4.1. Multiple Choice,4. In LAN system, the interface of EIA-568B used by twisted pair is: A. RJ11 B. RJ12 C. RJ45 D. RJ46 5. The EIA-568B standard use colors to identify e

45、ach other. These colors are: Aorange、blue、purple、green Bpurple、black、blue、green Cblack、blue、brown、orange Dorange、green、blue、brown 6. The difference between EIA/TIA 568A and 568B is： Aswap of pair 1、2 and pair of 3、6 Bswap of pair 4、5 and pair of 7、8 Cswap of pair 1、2 and pair of 4、5 Dswap of pair 3、

46、6 and pair of 7、8,118,4.1. Multiple Choice,7. At a height of 35784 km, a communication satellite that remain stationary with respect to its position over the earth is called: AGeosynchronous Earth Orbit BHigh Earth Orbit CLow Earth Orbit DMedium Earth Orbit,119,4.2. Terms Definition,Twisted Pair UTP Optical Fiber Coaxial Cable,120,4.3 Fill in the blanks,1.There are two types of multimode fiber: A fiber with varying densities is called as _(1)_- index and another is called as _(2)_-index.,