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