Data and Computer Communications Chapter 4 Transmission Media

Overview Guided - wire Unguided - wireless Characteristics and quality determined by medium and signal For guided, the medium is more important For unguided, the bandwidth produced by the antenna is more important Key concerns are data rate and distance

Design Factors Bandwidth Transmission impairments Interference Higher bandwidth gives higher data rate Transmission impairments Attenuation Interference Number of receivers In guided media More receivers (multi-point) introduce more attenuation

Electromagnetic Spectrum

Guided Transmission Media Twisted Pair Coaxial cable Optical fiber

传输介质 金属导体 双绞线、 同轴电缆 光纤 无线介质 无线电、短波、微波、卫星、光波

传输介质

Electromagnetic Spectrum for Telecommunications Frequency (Hertz) Power and telephone Rotating generators Musical instruments Voice microphones Radio Radios and televisions Electronic tubes Integrated circuits Twisted pair Coaxial cable AM radio FM radio and TV Terrestrial and satellite transmission Microwave Radar Microwave antennas Magnetrons Infrared Lasers Guided missiles Rangefinders Visible light Optical fiber P109 Figure 4.1 缺少数量单位标记 Wavelength In space (meters) ELF=Extremely low frequency MF=Medium frequency UHF=Ultrahigh frequency VF=Voice frequency HF=High frequency SHF=Superhigh frequency VLF=Very low frequency VHF=Very high frequency EHF=Extremely high frequency LF=Low frequency

Twisted Pair

Twisted Pair - Applications Most common medium Telephone network Between house and local exchange (subscriber loop) Within buildings To private branch exchange (PBX) For local area networks (LAN) 10Mbps or 100Mbps

Twisted Pair Cheap Easy to work with Low data rate Short range

Twisted Pair - Transmission Characteristics Analog Amplifiers every 5km to 6km Digital Use either analog or digital signals repeater every 2km or 3km Limited distance Limited bandwidth (1MHz) Limited data rate (100MHz) Susceptible to interference and noise

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 interference More expensive Harder to handle (thick, heavy)

UTP Categories Cat 3 Cat 4 Cat 5 up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cm Cat 4 up to 20 MHz Cat 5 up to 100MHz Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm

Near End Crosstalk Coupling of signal from one pair to another Coupling takes place when transmit signal entering the link couples back to receiving pair i.e. near transmitted signal is picked up by near receiving pair

双绞线 内导体芯线 绝缘 箔屏蔽 铜屏蔽 外套

传输介质 非屏蔽双绞线（UTP）、屏蔽双绞线（STP）

屏蔽双绞线 (STP) 非屏蔽双绞线 (UTP) 以箔屏蔽以减少 干扰和串音 3类、5类双绞线外没有任何附加屏蔽

传输介质 同轴电缆

Coaxial Cable

Coaxial Cable Applications Most versatile 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

Coaxial Cable - Transmission Characteristics Analog Amplifiers every few km Closer if higher frequency Up to 500MHz Digital Repeater every 1km Closer for higher data rates

同轴电缆 基带 宽带 一条电缆同时传输不同频率的几路模拟信号，75 ，用于模拟传输，300—450MHz，100km，需要放大器 一条电缆只用于一个信道，50，用于数字传输 宽带 一条电缆同时传输不同频率的几路模拟信号，75 ，用于模拟传输，300—450MHz，100km，需要放大器 铜芯 绝缘层 外导体屏蔽层 保护套

宽带同轴电缆系统 单电缆系统 双电缆系统 头端 头端 放大器 放大器 计算机 计算机 低频用于入境（PC--->头端） 入境电缆 出境电缆 单电缆系统 双电缆系统

Optical Fiber

Optical Fiber - Benefits Greater capacity Data rates of hundreds of Gbps Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing 10s of km at least

Optical Fiber - Applications Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs

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 Division Multiplexing

Optical Fiber Transmission Modes

光纤 依靠光波承载信息 高传送速率，通信容量大 传输损耗小，适合长距离传输 抗干扰性能好，保密性好 轻便

光纤 光纤

光纤传送模式 多模 波长 : 850,1300 nm 波长: 1300,1550 nm 单模 h2 h1 h2 h1 芯/封套特性 输入电信号 输出电信号 多模 波长 : 850,1300 nm h2 h1 单模 光纤的直径减小到 一个光波波长 波长: 1300,1550 nm

光纤传输原理

三种类型光纤的比较

典型的光缆 单芯光缆 玻璃封套 塑料外套 玻璃内芯 塑料外套 外壳 多芯光缆 玻璃内芯 玻璃封套

剖面结构 芯 封套 外套 加强芯 光纤 外鞘 光纤束

光接收器 光信号 检测器 前置放大器 均衡器 后置放大器 滤波 判决电路 时钟恢复 数据

红外线和毫米波 光波传输 用于短距离通信，如电视、录象机等的遥控,也可用于无线LAN 缺点：不能穿透固体 缺点：不能穿透雨和浓雾，易受天气影响

Wireless Transmission Unguided media Transmission and reception via antenna Directional Focused beam Careful alignment required Omnidirectional Signal spreads in all directions Can be received by many antennae

Frequencies 2GHz to 40GHz 30MHz to 1GHz 3 x 1011 to 2 x 1014 Microwave Highly directional Point to point Satellite 30MHz to 1GHz Omnidirectional Broadcast radio 3 x 1011 to 2 x 1014 Infrared Local

Terrestrial Microwave Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates

Satellite Microwave Satellite is relay 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

Broadcast Radio Omnidirectional FM radio UHF and VHF television Line of sight Suffers from multipath interference Reflections

Infrared Modulate noncoherent infrared light Line of sight (or reflection) Blocked by walls e.g. TV remote control, IRD port

无线介质 使用电磁波 无需物理连接 适用于长距离

几种媒体的传输频带 红外线 无线电 紫外线 可见光 微波 双绞线 电话业务 同轴电缆 AM 无线电 FM 无线电和 TV 地面微波接力 光纤 卫星 102 Hz 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 无线电 红外线 紫外线 可见光 微波

Digital Data Characteristics of Unguided Communications Bands 0.1 to 100 bps To 750Mbps To 100Mbps To 10Mbps To 100 kbps 10 to 3000 bps 10 to 1000 bps Data rate To 1GHz To 500MHz To 20 MHz 5kHz to 5MHz To 4 KHz To 4 kHz Bandwidth Experimental short point-to-point PSK FM EHF(extremely high frequency) 30-300GHz Terrestrial microwave,Satellite microwave SHF(super high frequency) 3-30GHz UHF television, terrestrial microwave FM,SSB UHF(ultra high frequency) 300-3000MHz VHF television,FMRadio FSK,PSK AM,SSBFM VHF(very high frequency) 30-300MHz Shortwave radio ASK,FSK ,MSK AM,SSB HF(high frequency) 3-30MHz Commercial AM radio ASK,FSK,MSK AM MF(high frequency) 300-3000kHz Navigation ASK,FSK MSK Generally not practical LF(low frequency) 30-300kHz Modulation Principal Applications Digital Data Analog Data Name Frequency Band P120.Table 4.3

Characteristics of Unguided Communications Bands P120.Table 4.3

无线电 F1, F2, F3 = 使用的频率 基站覆盖的无线电区域 固定终端点（基站）和终端之间是无线链路 基站 用户计算机和终端 F2 BS 基站 用户计算机和终端 基站覆盖的无线电区域 固定终端点（基站）和终端之间是无线链路 F2 F3 F1 F1, F2, F3 = 使用的频率

地面微波接力 d h 地球 两个地面站之间传送： 这里,h是天线的高度(m)；根据微波沿地表传输路线稍有弯曲,故而传输距离远于光线直线传播距离,K是一调整因子,较好的取值为K=4/3。故二架高100米的天线可相距： 一般距离为50 -100 km 地球 地面站之间的直视线路 微波传送塔 d h

对任一传输系统,微波的一个主要损耗源是辐射对微波或无线电频率,该损耗可表示为: 这里d是距离,是波长,单位相同。 因此损耗值是距离的平方。这与双绞线和同轴电缆不同,它们的损耗与距离成对数关系(取分贝则为线性)。这样,对微波系统,中继器或放大器可以相隔的更远,典型地为10-100公里。

卫星 C波段 4/6 GHz KU波段 12/14 GHz 上行14 - 14.5 GHz 地球 使用微波 使用转发器接收和转发 地面站 C波段 4/6 GHz 上行5.925 - 6.425 GHz 下行3.7 - 4.2 GHz KU波段 12/14 GHz 上行14 - 14.5 GHz 下行11.7 - 12.2 GHz

地球同步卫星 22,300 公里 地球 与地面站相对固定位置 使用3个卫星覆盖全球

常用传输媒体的比较