c transmissionmedia
DESCRIPTION
local networksTRANSCRIPT
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William StallingsData and Computer Communications7th Edition
Chapter 4Transmission Media
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Overview• Characteristics and quality determined by:
—Medium—Signal
• Medium—Guided - wire—Unguided - wireless
• For Guided Medium—The medium is more important
• For Unguided—The bandwidth produced by the antenna is more
important
• Key concerns are data rate and distance
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Guided Transmission Media• Twisted Pair• Coaxial Cable• Optical Fiber
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Twisted PairTwisted Pair
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Twisted Pair Architecture
Two Insulated copper wires
Issues:
(1) Interference due to unwanted electrical coupling of two copper
(2) Interference due to unwanted electrical coupling between the neighboring twisted pairs
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Twisted Pair Applications• Most commonly used medium• Telephone network
—Between house and local exchange (subscriber loop)
• Within buildings—To private branch exchange (PBX)
• For local area networks (LAN)—10Mbps or 100Mbps
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Twisted Pair - Pros and Cons• Advantages
—Less expensive —Easy to work with
• Disadvantages—Low data rate—Short range
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Twisted Pair (TP) Characteristics• Analog transmission
—Amplifiers every 5km to 6km• Digital transmission
—Use either analog or digital signals—repeater every 2km or 3km
• TP is Limited —Distance—Bandwidth —Data rate
• Susceptible to interference and noise—Easy coupling of electromagnetic fields
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Unshielded and Shielded TP• Unshielded Twisted Pair (UTP)
—Ordinary telephone wire—Less expensive—Weak immunity against noise and interference —Suffers from external EM interference
• Shielded Twisted Pair (STP)—An extra metallic sheath on each pair—Relatively more expensive—Provide better performance than UTP
• Increased Data rate • Increased Bandwidth
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UTP Categories• Cat 3
—up to 16MHz—Voice grade found in most offices—Twist length of 7.5 cm to 10 cm
• Cat 4—up to 20 MHz
• Cat 5—up to 100MHz—Commonly pre-installed in new office buildings—Twist length 0.6 cm to 0.85 cm
• Cat 5E (Enhanced) –see tables• Cat 6• Cat 7
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Coaxial CableCoaxial Cable
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Coaxial Layers
copper or aluminum conductor
insulating material
shield(braided wire)
outer jacket(polyethylene)
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Coaxial Cable Architecture
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Coaxial Cable Applications• Television (TV) signals 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 (LAN)—Metropolitan area network (MAN)
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Coaxial Cable Characteristics• Analog
—Amplifiers every few km—Closer if higher frequency—Up to 500MHz
• Digital—Repeater every 1km—Closer for higher data rates
• Problem—Inter-modulation noise —Thermal noise
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Optical Fiber
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plastic jacket glass or plasticcladding
fiber core
Fiber Optic Layers• consists of three concentric sections
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Optical Fiber Architecture
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Optical Fiber Benefits• Greater capacity
—Data rates of hundreds of Gbps
• Smaller size & weight—Made up of extremely thin fibers
• Lower attenuation—Electromagnetic isolation
• Greater repeater spacing—10s of km at least
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Wireless Examples• terrestrial or satellite microwave• broadcast radio• infrared
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Wireless Transmission Frequencies• 2GHz to 40GHz (Microwave
Frequency)—Highly directional—Point to point devices—Microwave communications
• 30MHz to 1GHz (Radio Frequency)—Omnidirectional—Broadcast radio
• 3 x 1011 to 2 x 1014 (Local Frequency)
—For Local applications
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Terrestrial Microwave• used for long-distance telephone service• uses radio frequency spectrum, from 2 to
40 Ghz• parabolic dish transmitter, mounted high• used by common carriers as well as
private networks• requires unobstructed line of sight
between source and receiver• curvature of the earth requires stations
(repeaters) ~30 miles apart
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Satellite Microwave Transmission• a microwave relay station in space• can relay signals over long distances• geostationary satellites
—remain above the equator at a height of 22,300 miles (geosynchronous orbit)
—travel around the earth in exactly the time the earth takes to rotate
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Satellite Microwave (SM)Satellite Microwave (SM)1. Satellite is relay station2. Satellite
• receives on one frequency • amplifies or repeats signal • transmits on another frequency
3. Requires geo-stationary orbit— Height of 35,784km
4. Applications• Television• Long distance telephone• Private business networks
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Satellite Point to Point LinkSatellite Point to Point Link
ground based microwave transmitter
ground based microwave receiver
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Satellite Broadcast LinkSatellite Broadcast Link
Is it really broadcast??
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Radio• radio is omnidirectional and microwave is
directional• Radio is a general term often used to
encompass frequencies in the range 3 kHz to 300 GHz.
• Mobile telephony occupies several frequency bands just under 1 GHz.
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Broadcast RadioBroadcast Radio1. Omnidirectional (travel in all directions)
2. Line of sight is over
3. Doesn’t need parabolic antenna
4. Example FM radio
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Ground Wave Propagation
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Line of Sight Propagation
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Infrared• Modulate noncoherent infrared light• Line of sight (or reflection)• Blocked by walls• e.g. TV remote control, IRD port