计算机网络和协议分析

For angels of incidence above a certain critical value
Hunan City University
Weihong Chen
光线在光纤中的折射
包层 纤 芯
折射角 包层 （低折射率的媒体）
入射角
纤芯 （高折射率的媒体）
包层 （低折射率的媒体）
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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8-10m和50m。 • 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。
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Weihong Chen
»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:
»…...
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Weihong Chen
双绞线的连接
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
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Example
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Gigabit Ethernet cabling.
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Weihong Chen
2.3 Wireless Transmission(无线传输)
• 无线传输的频率范围大致从无线电到可见光（104~1015Hz）
– 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。
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Weihong Chen
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.
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channel cannot transmit
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binary (I.e., two level) signals
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at a rate exceeding 6200 bps. 32
Max. data rate 6200 bps
12400 bps 18600 bps 24800 bps 31000 bps
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光纤的工作原理
低折射率 高折射率 (包层) (纤芯)
光线在纤芯中传输的方式是不断地全反射
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Weihong Chen
输入脉冲
多模光纤与单模光纤 多模光纤
输出脉冲
输入脉冲
单模光纤
输出脉冲
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Weihong Chen
S/R is given as 10log10S/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.
(2) Light trapped by total internal reflection.
Fig.3 The amount of refraction depends
on the properties of the two media (in particular, their indices of refraction)
An Example: •200GB, 1000, 24Hours, 19Gbps 1Hour , 400Gbps
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Weihong Chen
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
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Weihong Chen
• 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；
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)
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交叉线 EIA-568B
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直连线 EIA-568A
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Weihong Chen
环形光纤网络
A fiber optic ring with active repeaters.
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Weihong Chen
星形光纤网络
A passive star connection in a fiber optics network.
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• 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。
10log10S/N=30, S/N=1000 C =3000·log2 （1+1000） ≈ 30000bps
2. Guided Transmission Media
• Magnetic Media • Twisted Pair • Coaxial Cable • Fiber Optics
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propagate only in a straight line without bouncing – More expensive, used for longer distances
– The distance is 100km without the relay
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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
– Fiber’s core diameter is typically 8 to 10 microns – The fiber acts like a wave guide, and the light can
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Weihong Chen
• 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。
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Fiber Cables
Fig.4 (a) Side view of a single fiber. (b) End view of a sheath with three fibers.
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Weihong Chen
• 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
wenku.baidu.com
• Multimode fiber
– Fiber’s 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
C = 2H log2 V
C ——Data rate，bps H ——Bandwidth，Hz
V ——Signal discrete levels
Nyquist Theorem described an equation expressing
the maximum data rate.
V
e.g. A noiseless 3.1kHz
The maximum data rate of a channel
（2）Shannon Theorem:
for the channel subject to random noise.
C = H log2 （1+S/N）
C——Data rate，bps H——Bandwidth，Hz
S/N——Signal-to-noise ratio
• 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
计算机网络和协议分析
Weihong Chen Department of Computer Science
Hunan City University Sep 2010
Hunan City University
C B
Weihong Chen
Contents
1 The Basis for Data Communication
2.3 Coaxial Cable
50 - ohm, commonly used for digital transmission 75 - ohm, commonly used for analog transmission
Fig.2 A coaxial cable
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-tonoise ratio of the data signal. Modern cables have a bandwidth of close to 1 GHz.
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2.1 Magnetic Media
Features: •The bandwidth characteristics are excellent •The delay characteristics are poor. Transmission time is measured in minutes or hours.
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Transmission Media
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Public Switched Telephone Network
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Weihong Chen
1 The Basis for Data Communication
（1）Nyquist Theorem: for a finite bandwidth noiseless channel.