data communication -transmissionmedia

8/3/2019 data communication -TransmissionMedia

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Dat a Com m unic a t ion

Chapt er 4

Transm iss ion Media


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Overview Guided - wire

Unguided - wireless

Characteristics and quality of transmissiondetermined by medium and signal.

For guided, the medium is more important For unguided, the bandwidth produced by the

antenna is more important.

Key concerns in system design :data rate anddistance.


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Design Fac t ors Bandwidth

Higher channel bandwidth support higher data rates.

Transmission impairments

Attenuation: increase with distant and frequency.

Interference: EMC, crosstalk. Number of receivers

In guided media

More receivers (multi-point) introduce moreattenuation


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Elec t rom agnet i c Spec t rum


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Guided Transm iss ion Media Twisted Pair

Coaxial cable

Optical fiber


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Point -t o-Point Transm ission

Charac t er ist ic s o f Guided Media

Frequency

Range

Typical

Attenuation

Typical

Delay

Repeater

SpacingTwisted pair(with loading)

0 to 3.5 kHz 0.2 dB/km @1 kHz

50 s/km 2 km

Twisted pairs(multi-paircables)

0 to 1 MHz 0.7 dB/km @1 kHz

5 s/km 2 km

Coaxial cable 0 to 500 MHz 7 dB/km @ 10MHz

4 s/km 1 to 9 km

Optical fiber 186 to 370THz

0.2 to 0.5dB/km

5 s/km 40 km


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Tw ist ed Pa ir


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Tw is t ed Pai r - App l ic a t ions Most common medium for analog and digital.

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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Tw ist ed Pair - Pros and Cons Cheap

Easy to work with

Low data rate

Short range

Limited bandwidth.


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Tw ist ed Pai r - Transm iss ion

Charac te r i s t i cs Analog

Amplifiers every 5km to 6km

Digital

Use either analog or digital signals

repeater every 2km or 3km Limited distance

Limited bandwidth (1MHz)

Limited data rate (100Mbps)

Susceptible to interference and noise


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Near End Cross t a lk Coupling of signal from one pair to another

Coupling takes place when transmit signalentering the link couples back to receiving pair

i.e. near transmitted signal is picked up by near

receiving pair


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Unshie lded and Shie lded TP Unshielded Twisted Pair (UTP)

Ordinary telephone wire

Cheapest

Easiest to install

Suffers from external EMC interference

Shielded Twisted Pair (STP)

Metal braid or sheathing that reduces interference

More expensiveHarder to handle (thick, heavy)


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UTP Cat egor ies Cat 3 up to 16MHzVoice 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-up to 10GbPs ethernet. Cat 7-higher than 10GbPs applications, and CATV.


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Com par ison of Shie lded and

Unshie lded Tw is t ed Pai r Attenuation (dB per 100 m) Near-end Crosstalk (dB)

Frequency(MHz)

Category 3UTP

Category 5UTP

150-ohmSTP

Category 3UTP

Category 5UTP

150-ohmSTP

1 2.6 2.0 1.1 41 62 58

4 5.6 4.1 2.2 32 53 58

16 13.1 8.2 4.4 23 44 50.4

25 10.4 6.2 41 47.5

100 22.0 12.3 32 38.5

300 21.4 31.3


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Tw is t ed Pai r Cat egories and

ClassesCategory 3

Class CCategory 5

Class DCategory

5ECategory 6

Class ECategory 7

Class F

Bandwidth 16 MHz 100 MHz 100 MHz 200 MHz 600 MHz

Cable Type UTP UTP/FTP UTP/FTP UTP/FTP SSTP

Link Cost(Cat 5 =1)

0.7 1 1.2 1.5 2.2


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Coax ia l Cable


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Coax ia l Cable Appl ic at ions

Most versatile medium

Television distribution

Ariel to TV

Cable TV (few hundreds of TV channels)

Long distance telephone transmissionCan carry 10,000 voice calls simultaneously

Being replaced by fiber optic

Short distance computer systems links

Local area networks


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Coax ia l Cable - Transm ission

Charac te r i s t i cs

Analog

Amplifiers every few km (less than 10Km)

Closer if higher frequency

Up to 500MHz

DigitalRepeater every 1km

Closer for higher data rates


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Opt ic a l Fiber


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Opt ic a l f i ber c ont .

Core : consist of one or more glass or plasticfibers each has its own cladding.

Core diameter 8-50m.

Cladding: glass or plastic coating of different

optical properties from that of the core. cladding diameter 125m.

Jacket:plastic surround one or more fibers for

protection from crushing and moisture.


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Opt ic a l Fiber - Benef i t s

Greater capacity

Data rates of hundreds of Gbps up to 40km

Smaller size & weight

Lower attenuation (0.2 -0.5 dB/km)

Electromagnetic isolation Greater repeater spacing

10s of km at least.


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Opt ic a l Fiber - Appl ic at ions

Long-haul trunks (1500km and up to 60,000voice channels)

Metropolitan trunks(12km and up to 010,000voice channels)

Rural exchange trunks (40- 160km and up to5,000 voice channels)

Subscriber loops (fiber from central office to

subscriber home) LANs (up 100Tbps).


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Opt ic a l Fiber - Transm iss ion

Charac te r i s t i cs

Act as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrum

Light rays with angle of incidence grater than critical angle willsuffer total internal reflection at core-cladding interface.

Light Emitting Diode (LED)

Cheaper Wider operating temp range

Last longer

Injection Laser Diode (ILD) More efficient

Greater data rate

Wavelength Division Multiplexing (WDM,DWDM)


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Types o f op t ic a l f iber

Step-index multi-mode fiber

-multimode propagate.

-dispersion

-low data rates and short distance. Graded-index multimode fiber

-gradual change of index of refraction from coreto cladding.

-multimode.

-higher data rates.

Step-index single-mode fiber

-single mode propagate.-highest data rates and longest distant.


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Opt ic a l Fiber Transm iss ion

Modes


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Frequenc y Ut i l izat ion for Fiber

App l ica t ionsWavelength (invacuum) range

(nm)

Frequencyrange (THz)

Bandlabel

Fiber type Application

820 to 900 366 to 333 Multimode LAN

1280 to 1350 234 to 222 S Single mode Various

1528 to 1561 196 to 192 C Single mode WDM

1561 to 1620 185 to 192 L Single mode WDM


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At t enuat ion in Guided Media


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Wireless Transm iss ion

Frequencies

30MHz to 1GHzOmnidirectional

Broadcast radio 1GHz to 40GHz

Microwave

Highly directionalPoint to point

Satellite

3 x 1011 to 2 x 1014

Infrared

Local application


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Antennas

Electrical conductor used to radiate electromagneticenergy or collect electromagnetic energy

Transmission Radio frequency energy from transmitter Converted to electromagnetic energy

By antenna

Radiated into surrounding environment

Reception Electromagnetic energy impinging on antenna

Converted to radio frequency electrical energy Fed to receiver

Same antenna often used for transmission and reception


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Ant enna Radiat ion Pat t ern

Power radiated in all directions

Not same performance in all directions

Isotropic antenna is (theoretical) point in space

Radiates in all directions equally

Gives spherical radiation pattern


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Parabol ic Ref lec t ive Ant enna Used for terrestrial microwave and satellite communication. Parabola is locus of point equidistant from a line and a point

not on that line Fixed point is focus Line is directrix

Revolve parabola about axis to get paraboloid Cross section parallel to axis gives parabola Cross section perpendicular to axis gives circle

on transmission, Source placed at focus will produce wavesreflected from parabola in parallel to axis

On reception, signal is concentrated at focus, wheredetector is placed


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Parabol ic Ref lec t ive Ant enna


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Ant enna Gain Measure of directionality of antenna

Power output in particular direction compared with that

produced by isotropic antenna Measured in decibels (dB)

Results in loss in power in another direction

Effective area relates to size and shape Related to gain

2

2

2

4

, where 0.56 .56( )

7

e

e

A

G A A r

AG

= = =

=


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Ter rest r ia l Mic row ave

Parabolic dish antenna of 3m diameter.

Focused beam

Line of sight to receiving antenna.

Long haul telecommunications

Higher frequencies give higher data rates andsmaller antennas.


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Digi t a l m ic row ave perform anc e

Band (GHz) Bandwidth (MHz) Data rate (Mbps)

26

11

18

730

40

220

1290

135

274


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Sat e l l i t e M ic row ave

Satellite is microwave relay station

Satellite receives on one frequency band (uplink),

amplifies or repeats signal and transmits on anotherfrequency band (down link).

Requires geo-stationary orbit

Height of 35,784km Television

Long distance telephone

Private business networks GPS


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Sat e l l i t e Po in t t o Po in t L ink


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Sat e l l i t e Broadc ast L ink

Sat e l l i t e t ransm iss ion


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Sat e l l i t e t ransm iss ion

cha rac te r i s t i cs

Optimum frequency range 1-10 GHz.

Most popular satellite operate on

-4/6 GHz band

-12/14 GHz band

-20/30 GHz band

round trip Propagation delay about 0.25 secondaffects voice quality.


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Broadc ast Radio

Frequency range 30MHz-1GHz

Omnidirectional

FM radio

UHF and VHF television

Line of sight Suffers from multipath interference

Reflections


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In f rared

IR diodes to emit and detect infrared light

Line of sight (or reflection)

Blocked by walls.

e.g. TV remote control


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Wireless Propagat ion

Signal travels along three routes Ground wave

Follows contour of earth

Up to 2MHz

AM radio

Sky wave

3-30 MHz

Amateur radio, BBC world service, Voice of America

Signal reflected from ionosphere layer of upper atmosphere

(Actually refracted)

Line of sight

Above 30Mhz

May be further than optical line of sight due to refraction


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Ground Wave Propagat ion


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Sk y Wave Propagat ion


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Line of Sight Propagat ion


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Refrac t ion

Velocity of electromagnetic wave is a function of densityof material ~3 x 108 m/s in vacuum, less in anything else

As wave moves from one medium to another, its speedchanges Causes bending of direction of wave propagation at boundary

Towards more dense medium Index of refraction (refractive index) is

Sin(angle of incidence)/sin(angle of refraction)

Varies with wavelength May cause sudden change of direction at transitionbetween media

May cause gradual bending if medium density is varying Density of atmosphere decreases with height

Results in bending towards earth of radio waves


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Opt ic a l and Radio Hor izons

I i t f L i f Si h t


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Im pa i rm ent o f L ine o f Sigh t

Transmiss ion

Free space loss Signal attenuates over distance

Greater for lower frequencies (longer wavelengths) due toantenna gain.

Atmospheric Absorption Water vapour and oxygen absorb radio signals

Water greatest at 22GHz, less below 15GHz Oxygen greater at 60GHz, less below 30GHz

Rain and fog scatter radio waves

Multipath Better to get direct line of sight if possible.

Signal can be reflected causing multiple copies to be received

May be no direct signal at all

May reinforce or cancel direct signal Refraction

Free


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Free

SpaceLoss


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Mul t ipa t h In t e rferenc e

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