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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