计算机网络与互联网(英语)chap.ppt

Electromagnetic Spectrum Forms of Energy Used To Transmit Data Electric current Audible sounds Omnidirectional electromagnetic waves Radio Frequency (RF) Infrared Directional electromagnetic waves Point-to-point satellite channel Limited broadcast (spot beam) Microwave Laser beam Figure Electromagnetic spectrum for wireless communication Figure Propagation methods Types of Satellites Geosynchronous Earth Orbit (GEO) Kepler’s third law : P2=K D3 P : time to rotate around another planetary (period) D : distance between satellite and the planet’s center K : constant depending on gravitational forces Geosynchronous orbit P= 24-hour ?D=(Radius+22300) miles 22,300 kilometre earth Types of Satellites Low Earth Orbit (LEO) Array needed Site x A earth Site y (a) Relaying in space. (b) Relaying on the ground. Globalstar Terrestrial Microwave Parabolic dish(抛物柱面反射器) Focused beam(聚焦光束) Line of sight(瞄准线) Long haul(持久) 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 Infrared Modulate no coherent infrared light Line of sight (or reflection) Blocked by walls e.g. TV remote control, IRD port Chapter 4 Signals, Media, And Data Transmission Transmission of Information Well-understood basics From physics Energy Electromagnetic wave propagation From mathematics Coding theory Transmission Media Transmission Media Copper wire Need two wires Possibilities Twisted pair Coaxial cable Optical fiber Flexible Light “stays in” Air / space Used for electromagnetic transmission Twisted pair Two insulated copper wires are twisted around each other Twisted pairs often are bundled together and wrapped in a protective coating that allows the bundled cable to be buried Twisting minimizes electromagnetic interference betw