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A.Zahid-EE450 1
Transmission MediaEE450: Introduction to Computer Networks
Professor A. Zahid
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Classifications of Transmission Media• Guided media
– Twisted Pair– Coaxial cable– Optical fiber
• Un-Guided media– Radio/Wireless– Microwave– Satellite– Cellular– Infrared
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Unshielded & Shielded Twisted Pair• Unshielded Twisted Pair (UTP)
– Ordinary telephone wire– Cheapest– Easiest to install– Suffers from external EM interference
• Shielded Twisted Pair (STP)– Metal braid or sheathing that reduces interference– More expensive– Harder to handle (thick, heavy)
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Unshielded Twisted Pair
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Shielded Twisted Pair
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Why Twisting the Pair?
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UTP Categories• Category 3 (Cat 3)
– Support up to 16MHz– Voice-grade found in most offices– Twist length of 7.5 cm to 10 cm
• Cat 4– Support up to 20 MHz
• Cat 5– Support up to 100MHz– Commonly pre-installed in new office buildings– Twist length 0.6 cm to 0.85 cm
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Category 5 UTP Cable
RJ-45(Registered Jack)
Cat 5 cable
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Applications of Twisted Pair
• Most common medium• Telephone network
– Between house and local exchange (subscriber loop)
• Within buildings– To private branch exchange (PBX)
• For local area networks (LAN)– 10Mbps , 100Mbps, 1Gbps, 10 Gbps?
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Coaxial Cable
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Applications of Coaxial Cable• Television distribution
– Ariel to TV– Cable TV
• Long distance telephone transmission– Can carry 10,000 voice calls simultaneously– Being replaced by fiber optic
• Short distance computer systems links• Local area networks
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Optical Fiber
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Optical Fiber
• Long distance trunks• Metropolitan trunks• Rural exchange trunks• Subscriber loops• LANs
• Greater capacity (100’s of Gbps)• Smaller size & weight• Lower attenuation• Electromagnetic isolation• Greater repeater spacing 10s ofkm at least
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Single and Multimode Optical Fibers
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Wireless Transmission
• Unguided media• Transmission and reception via antenna• Directional
– Focused beam– Careful alignment required
• Omnidirectional– Signal spreads in all directions– Can be received by many antennae
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Electromagnetic Spectrum
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Broadcast Radio/TV
• Omni-directional Antenna• AM/FM radio• UHF and VHF television• Line of sight• Suffers from multipath
interference/reflections
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Satellite Communications
• Satellite is a relay station• Satellite receives on one frequency, amplifies/repeats
signal and transmits on another frequency• Television• Long distance telephone• Private business networks
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Terrestrial Microwave
• Parabolic dish• Focused beam• Line of sight• Long distance telecommunications• Higher frequencies give higher data rates
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Cellular System
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Design Considerations• Bandwidth
– Higher bandwidth allows for higher data rate• Transmission impairments
– Attenuation– Noise
• Interference• Number of receivers
– In guided media– More receivers (multi-point) ⇒ more attenuation