Post on 19-Dec-2015
transcript
Transmission Media
Guided Transmission Media
Transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint
Examples twisted pair wires coaxial cables optical fiber
Design FactorsBandwidth
Higher bandwidth gives higher data rate
Transmission impairments Attenuation
InterferenceNumber of receivers
In guided media More receivers (multi-point) introduce more
attenuation
Electromagnetic Spectrum
Guided Transmission MediaTwisted PairCoaxial cableOptical fiber
Twisted Pair
Twisted Pair - ApplicationsMost common mediumTelephone network
Between house and local exchange (subscriber loop)
Within buildings To private branch exchange (PBX)
For local area networks (LAN) 10Mbps or 100Mbps
Twisted Pair WiresConsists of two insulated copper wires
arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs
Often used at customer facilities and also over distances to carry voice as well as data communications
Low frequency transmission medium
Types of Twisted PairSTP (shielded twisted pair)
the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference
UTP (unshielded twisted pair) each wire is insulated with plastic wrap, but
the pair is encased in an outer covering
Ratings of Twisted PairCategory 3 UTP
data rates of up to 16mbps are achievable
Category 5 UTP data rates of up to 100mbps are achievable more tightly twisted than Category 3 cables more expensive, but better performance
Category 6, 6E, 7 STP (250, 550, 1Ghz More expensive, harder to work with
Twisted Pair AdvantagesInexpensive and readily availableFlexible and light weight Easy to work with and install
Twisted Pair DisadvantagesSusceptibility to interference and noiseAttenuation problem
For analog, repeaters needed every 5-6km For digital, repeaters needed every 2-3km
Relatively low bandwidth (3000Hz)
Twisted Pair - Pros and ConsCheapEasy to work withLow data rateShort range
Twisted Pair - Transmission CharacteristicsAnalog
Amplifiers every 5km to 6km
Digital Use either analog or digital signals repeater every 2km or 3km
Limited distanceLimited bandwidth (1MHz)Limited data rate (100MHz)Susceptible to interference and noise
Unshielded and Shielded TPUnshielded 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)
UTP CategoriesCat 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
Near End CrosstalkCoupling of signal from one pair to
anotherCoupling takes place when transmit signal
entering the link couples back to receiving pair
i.e. near transmitted signal is picked up by near receiving pair
Coaxial Cable
Coaxial Cable ApplicationsMost versatile mediumTelevision 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 linksLocal area networks
Coaxial Cable - Transmission CharacteristicsAnalog
Amplifiers every few km Closer if higher frequency Up to 500MHz
Digital Repeater every 1km Closer for higher data rates
Coaxial Cable (or Coax)Used for cable television, LANs, telephonyHas an inner conductor surrounded by a
braided meshBoth conductors share a common center
axial, hence the term “co-axial”
Coax Layers
copper or aluminum conductor
insulating material
shield(braided wire)
outer jacket(polyethylene)
Coax AdvantagesHigher bandwidth
400 to 600Mhz up to 10,800 voice conversations
Can be tapped easily (pros and cons)Much less susceptible to interference than
twisted pair
Coax DisadvantagesHigh attenuation rate makes it expensive
over long distanceBulky
Evolution of Fiber1880 – Alexander Graham Bell1930 – Patents on tubing1950 – Patent for two-layer glass wave-
guide1960 – Laser first used as light source1965 – High loss of light discovered1970s – Refining of manufacturing process1980s – OF technology becomes backbone
of long distance telephone networks in NA.
Advantages of Optical Fibre
ThinnerLess ExpensiveHigher Carrying CapacityLess Signal Degradation& Digital SignalsLight SignalsNon-FlammableLight Weight
Fiber Optic Disadvantagesexpensive over short distancerequires highly skilled installersadding additional nodes is difficult
Type of Fibers
Optical fibers come in two types:Single-mode fibers – used to transmit one
signal per fiber (used in telephone and cable TV). They have small cores(9 microns in diameter) and transmit infra-red light from laser.
Multi-mode fibers – used to transmit many signals per fiber (used in computer networks). They have larger cores(62.5 microns in diameter) and transmit infra-red light from LED.
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Fiber Types
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Fiber Attenuation
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Fiber Optic Applications
Outside Plant vs Premises
Fiber Optic CableRelatively new transmission medium used
by telephone companies in place of long-distance trunk lines
Also used by private companies in implementing local data communications networks
Require a light source with injection laser diode (ILD) or light-emitting diodes (LED)
plastic jacket glass or plasticcladding
fiber core
Fiber Optic Layersconsists of three concentric sections
Fiber Optic Typesmultimode step-index fiber
the reflective walls of the fiber move the light pulses to the receiver
multimode graded-index fiber acts to refract the light toward the center of
the fiber by variations in the density
single mode fiber the light is guided down the center of an
extremely narrow core
fiber optic multimodestep-index
fiber optic multimodegraded-index
fiber optic single mode
Fiber Optic Signals
Optical Fiber
Optical Fiber - BenefitsGreater capacity
Data rates of hundreds of Gbps
Smaller size & weightLower attenuationElectromagnetic isolationGreater repeater spacing
10s of km at least
Optical Fiber - ApplicationsLong-haul trunksMetropolitan trunksRural exchange trunksSubscriber loopsLANs
Optical Fiber - Transmission CharacteristicsAct as wave guide for 1014 to 1015 Hz
Portions of infrared and visible spectrum
Light Emitting Diode (LED) Cheaper Wider operating temp range Last longer
Injection Laser Diode (ILD) More efficient Greater data rate
Wavelength Division Multiplexing
Optical Fiber Transmission Modes
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Fiber Optic Link Power Budget
Wireless TransmissionUnguided mediaTransmission and reception via antennaDirectional
Focused beam Careful alignment required
Omnidirectional Signal spreads in all directions Can be received by many antennae
Frequencies2GHz to 40GHz
Microwave Highly directional Point to point Satellite
30MHz to 1GHz Omnidirectional Broadcast radio
3 x 1011 to 2 x 1014
Infrared Local
Wireless Examplesterrestrial microwavesatellite microwavebroadcast radioinfrared
Terrestrial Microwaveused for long-distance telephone serviceuses radio frequency spectrum, from 2 to
40 Ghzparabolic dish transmitter, mounted highused by common carriers as well as
private networksrequires unobstructed line of sight
between source and receivercurvature of the earth requires stations
(repeaters) ~30 miles apart
Terrestrial MicrowaveParabolic dishFocused beamLine of sightLong haul telecommunicationsHigher frequencies give higher data rates
Satellite MicrowaveApplications
Television distributionLong-distance telephone transmissionPrivate business networks
Microwave Transmission Disadvantagesline of sight requirementexpensive towers and repeaterssubject to interference such as passing
airplanes and rain
Satellite Microwave Transmissiona microwave relay station in spacecan relay signals over long distancesgeostationary 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
Satellite Transmission Linksearth stations communicate by sending
signals to the satellite on an uplinkthe satellite then repeats those signals on
a downlinkthe broadcast nature of the downlink
makes it attractive for services such as the distribution of television programming
dish dish
uplink station downlink station
satellitetransponder
22,300 miles
Satellite Transmission Process
Satellite Transmission Applicationstelevision distribution
a network provides programming from a central location
direct broadcast satellite (DBS)
long-distance telephone transmission high-usage international trunks
private business networks
Principal Satellite Transmission BandsC band: 4(downlink) - 6(uplink) GHz
the first to be designated
Ku band: 12(downlink) -14(uplink) GHz rain interference is the major problem
Ka band: 19(downlink) - 29(uplink) GHz equipment needed to use the band is still very
expensive
Fiber vs Satellite
Broadcast RadioOmnidirectionalFM radioUHF and VHF televisionLine of sightSuffers from multipath interference
Reflections
Radioradio is omnidirectional and microwave is
directionalRadio 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.
InfraredUses transmitters/receivers (transceivers)
that modulate noncoherent infrared light. Transceivers must be within line of sight
of each other (directly or via reflection ). Unlike microwaves, infrared does not
penetrate walls.
Satellite MicrowaveSatellite is relay stationSatellite receives on one frequency,
amplifies or repeats signal and transmits on another frequency
Requires geo-stationary orbit Height of 35,784km/22235 miles
TelevisionLong distance telephonePrivate business networks