Download - 04-TransmissionMedia
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Transmission Media• Transmission is the process of transporting information
between end points of a system or network.• Transmission systems may use copper cable, optical cable, or
radio channels to interconnect far-end and near-end equipment.
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Transmission Media• Guided
– Twisted Pair– Cable– Fiber– Medium more important than the signal
• Unguided– Atmosphere– Outer Space– Signal more important than the medium
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Spectrum for telecommunications
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Twisted Pair• Two insulated copper wires in a spiral• Number of pairs are bundled together• Twisting decreases crosstalk• Most common form for analog and digital• Used in telephone system• Subscriber loops
– From a person’s home to the local office of the phone company
• LANS– 10Mbps with newer at 100Mbps
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Twisted pair• Long Distance
– 4 Mbps– ISDN – Integrated Services Digital Network
• Digital– Repeaters required every 2 –3 kilometers
• Analog– Amplifiers required every 5-6 kilometers– Bandwidth of 250KHz, carry a few voice channels
• Susceptible to noise, shielded (STP) and unshielded (UTP).
• Compared to optical and coax twisted pair is limited in bandwidth, distance, and data rate
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Parallel lines are often used as part of antenna arrays. Ordinary television twinlead, is an example of a parallel transmission line; Figure (a).Parallel lines can be shielded to reduce interference problems, as illustratedin Figure (b).
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Coaxial Cable• Hollow outer cylindrical conductor
surrounding a single view• Most versatile of mediums, used for TV, long
distance telephone,and LAN’S
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Coaxial Cable• Part of long distance telephone network• With FDM can carry over 10,000 voice channels• Transmits both analog and digital signals• Frequency characteristics superior to twisted pair
– Less susceptible to noise• For long distance
– Amplifiers needed every few kilometers– Repeaters needed every kilometer or so
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For coaxial cable, the characteristic impedance is given by:
Coaxial Cable Applications
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Optical fiber
• Thin, flexible light passing material made from glass or plastic
• Grouped into cables
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Optical fiber• Better than coaxial cable or twisted pair• Data rates of 2Gbps over 10’s of Km• Light weight – good for buildings• Lower attenuation than coax or twisted• Not effected by extreme electromagnetic fields
– Crosstalk and impulse• Very difficult to tap – good security• Applications
– Long Haul trunks, metro trunks, rural exchange trunks, more recently beginning to displace twisted pair in subscriber loops and in LANS
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Optical Fiber• Operates in range 1014 to 1015 Hz
– Infrared and visible spectrum• Multimode
– Variety of angles of light will reflect and propagate• Single Mode
– Radius of the core = order of a wavelength– Only single angle passes– Superior performance
• Two different light sources – both emit light when voltage applied– LED – Light Emitting Diode – less costly, longer life– ILD - Injection Laser Diode – greater data rate
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Wireless• Antennae
– Directional• Focused EM beam• The higher frequency the more focused
– Omnidirectional• Lower frequency• Spreads out to multiple receivers
• Three ranges of frequencies– 26GHz – 40GHz --- microwave– 30 MHz – 1GHz --- broadcast radio– 3x10**11 to 2x10**14 Hz --- infrared
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Terrestrial Microwave• Parabolic dish• Narrow beam – line of sight on towers to
avoid obstacles• Series of towers for long distance• Applications:
– Long haul telephone– Voice and TV– Short point to point between buildings
• Main Source of loss– Attentuation – especially with rainful– Repeaters or amplifiers 10 to 100km– Interference with overlapping bands
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Satellite Microwave• It is essentially a microwave relay
station• Uplink
– Receives transmission on one frequency
• Downlink– Transmits on a second frequency
• Operates on a number of frequency bands known as transponders
• Point to Point– Ground station to satellite to
ground station• Multipoint
– Ground station to satellite to multiple receiving stations
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Satellite Microwave• Satellite orbit
– 35,784 Km, to match earth rotation– Stays fixed above the
transmitter/receiver station as earth rotates
• Satellites need to be separated by distance– Avoid interference
ApplicationsTV, long distance telephone, private business networksOptimum frequency range 1 – 10 GHzBelow 1GHz results in noise, above 10GHz results in severe attenuation
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Broadcast Radio• Omnidirectional unlike satellite• Does not require dish like antennae• Frequency range
– Radio - 3kHz to 300Ghz– Broadcast radio – 30MHz to 1GHz
• Broadcast radio– Transmission limited to line of sight– Less sensitive to attenuation from rainfall than microwave– Prime source of interference is multipath
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Infrared
• Tranceivers must be within line of sight of each other or via reflection
• Does not penetrate walls like microwave• No frequency allocation or licensing