网络通信_ch4_传输媒体课件.ppt

Bilingual Course Network Communications网络通信(For Master Students in the Department of Electronic Engineering)Chapter 4 Transmission Media;本课程课件的FTP地址;Outline of Chapter 4;Outline of Chapter 4 (Cont’s);Overview;Electromagnetic Spectrum;电磁波频段;Transmission Characteristics of Guided Media;Names of frequency bands;§4-1 Guided Transmission Media;双绞线抗干扰示意图;Comparison between UTP and STP (Table 4.2, P.114); (2) STP 屏蔽双绞线 特点：每一对双绞线外均有屏蔽层 有助于屏蔽外界干扰 有助于消除串扰（来自另一线路的） 2. Coaxial Cable （同轴电缆） ? 频率特性： 100kHz—500MHz ? 对于模拟信号传输，几km需要一个放大器，对于数字信号传输，1km需要一个转发器。 ? 应用：电视、视频信号传输；长途电话；计算机连接；局域网 ;3. Optical Fiber ? 光纤是传导光的媒体，由玻璃或塑料制成； ? 特点： - 容量大，以2Gbps的速率，可以传输几十km，可以容纳20000— 60000个电话信道； - 体积小，重量轻； - 衰减小； - 隔离电磁场 ? 波分复用技术：当不同频率的多条光线在同一根光纤中传播时，实际 上是一种FDM，但是通常称为WDM（Wavelength division multiplexing）， 1997年贝尔实验室实现100路WDM，每路10Gbps，总容量达到1Tbps。 ? 传播模式 - 多模传播：多束光线从光源经由芯材通过不同光路传播； - 单模传播：只有一个角度入射 （参见Figure 4.4 in P.118）;光的折射和全反射;Optical Fiber;光纤传播模式;Step-Index Multimode Light from a source enters the cylindrical glass or plastic core. Rays at shallow（平缓）angles are absorbed by the surrounding material. This form of propagation is called step-index multimode, （多模突变）referring to the variety of angles that will reflect. With multimode transmission, multiple propagation paths exist, each with a different path length and hence time to traverse（穿过）the fiber. This causes signal elements (light pulses) to spread out in time, which limits the rate at which data can be accurately received. Put another way（另一方面）, the need to leave spacing between the pulses limits data rate. This type of fiber is best suited for transmission over short distances. When the fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the core to the order of a wavelength, only a single angle or mode can pass the axial ray. ;Single-mode This single-mode pr