1 Part II Data Transmission Ch 4 Transmission Media

2 Transmission of Information  All computer comm. Involves encoding data in a form of energy, and sending the energy across a transmission media Electric current  wire Laser  fiber Electromagnetic wave  air  From physics Energy Electromagnetic wave propagation  From mathematics Coding theory

3 Transmission Media  Copper ( 銅 ) wire Twisted pair ( 雙絞線 )(to minimize interference)  UTP (Unshielded Twisted Pair)  STP (Shielded Twisted Pair) Coaxial cable ( 同軸線 ) Copper conductor Insulator Metal shield Outer cover

4 Pin Color 綠白 綠 橘白 藍 藍白 橘 棕白 棕 EIA/TIA-568A RJ-45 Transmission Media

5  Optical fiber Flexible Light “stays in”  Air / space Used for electromagnetic transmission 光纖 (Core) 外圍材料 (Cladding) 保護外皮 (Protective Sheath)

6 Transmission Media  Twisted pair Two insulated copper wires are twisted around each other Twisting minimizes electromagnetic interference between pairs Digital signal data rates vary with distance  100 Mbps over a segment length of 100m  2400 bps for up to 10 miles

7 Transmission Media  Coaxial cable Bandwidth of about 500 Mbps More expensive than twisted pair Transmits information  baseband mode >bandwidth devoted to a single stream of data  broadband mode >bandwidth is divided into ranges >each range typically carries separate coded information >allows the transmission of multiple data streams over the same cable simultaneously >eg. Cable TV

8 Transmission Media  Optical fiber Use optical signals instead of electrical signals Light sources :  LED (Light Emitting Diode, 發光二極體）  Laser Diode Detector: Photoelectric Diodes

9 Transmission Media  Optical fiber (ctn.) Advantages over wires  No electrical interference  A fiber can carry a pulse of light much farther  Light can carry more information than electrical signal Disadvantages  Installing a fiber requires special equipment  More expensive

10 Transmission Media  Optical fiber (ctn.) Stepped Index Multimode  Multiple propagation paths exist  Modal dispersion phenomenon >different path length (time) to traverse the fiber A B C C A B 可接受入射角度  Very pure glass core cladding Protective cover

11 Transmission Media  Optical fiber (ctn.) Graded Index Multimode  gradually less dense from the core Cladding with variable refractive index

12 Forms of Energy Used To Transmit Data (1)  Audible ( 聽得見的 ) sounds  Electric current  Omni-directional electromagnetic waves Radio Frequency (RF) Infrared X

13 Forms of Energy Used to Transmit Data (2)  Directional electromagnetic waves Point-to-point satellite channel Limited broadcast (spot beam) Microwave Laser beam

14 Types of Satellites  Geosynchronous Earth Orbit (GEO)  Low Earth Orbit (LEO) Array needed Earth miles Kepler’s third law : P 2 =K D 3 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 Site x A earth Site y 200 – 400 miles At least 66 LEOs