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1 Part II Data Transmission Ch 4 Transmission Media
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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
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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
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4 Pin 1 2 3 4 5 6 7 8 Color 綠白 綠 橘白 藍 藍白 橘 棕白 棕 EIA/TIA-568A RJ-45 Transmission Media
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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)
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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
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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
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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
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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
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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
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11 Transmission Media Optical fiber (ctn.) Graded Index Multimode gradually less dense from the core Cladding with variable refractive index
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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
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13 Forms of Energy Used to Transmit Data (2) Directional electromagnetic waves Point-to-point satellite channel Limited broadcast (spot beam) Microwave Laser beam
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14 Types of Satellites Geosynchronous Earth Orbit (GEO) Low Earth Orbit (LEO) Array needed Earth 22300 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
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