Chapter 4:Transmission Media

COE 341: Data & Computer Communications (T061)Dr. Radwan E. Abdel-Aal

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Remaining five chapters:

Physical Layer

Transmission Medium

Data Link

Chapter 4: Transmission Media

Chapter 3: Signals, their representations, their

transmission over media, Impairments

Chapter 5: Encoding: From data to signals

Chapter 7: Data Link: Flow and Error control,

Link management

Chapter 6: Data Communication: Synchronization,

Error detection and correction

Chapter 8: Improved utilization: Multiplexing

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Agenda Overview Guided Transmission Media

Twisted Pair Coaxial Cable Optical Fiber

Unguided (open space, wireless) Transmission Antennas Terrestrial Microwave Satellite Microwave Broadcast Radio Infrared

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Overview Media:

Guided - wire Unguided - wireless

Transmission characteristics and quality determined by: Signal Medium

For guided, the medium is important For unguided, the antenna is important

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Design Issues Key communication objectives are:

High data rate Low error rate Long distance Bandwidth: Tradeoff - Larger for higher data rates

- But smaller for low link cost Transmission impairments

Attenuation: Twisted Pair > Cable > Fiber (best) Interference and Cross talk: Twisted Pair > Cable > Fiber (best)

Worse with unguided… (the medium is shared!) Number of receivers

In multi-point links of guided media:Attenuation increases with increased number of connected receivers

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Part of the Electromagnetic Spectrum1 KHz 1 MHz 1 GHz 1 THz

Guided

Unguided

f

V = f

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Study of Transmission Media

Physical description Main transmission characteristics Main applications

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Guided Transmission Media

Twisted Pair Coaxial cable Optical fiber

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Transmission Characteristics of Guided Media: Overview  

  Frequency Range

Typical Attenuatio

n

Typical Delay

Repeater Spacing

Twisted pair (with loading)

0 to 3.5 kHz 0.2 dB/km @ 1 kHz

50 µs/km 2 km

Twisted pairs (multi-pair cables)

0 to 1 MHz 3 dB/km @ 1 kHz

5 µs/km 2 km

Coaxial cable

0 to 500 MHz

7 dB/km @ 10 MHz

4 µs/km 1 to 9 km

Optical fiber 186 to 370 THz

0.2 to 0.5 dB/km

5 µs/km 40 km

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Twisted Pair (TP)

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UTP Cablesunshielded

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Twisted Pair - Applications The most commonly used guided medium Telephone network (Analog Signaling)

Analog Data (original purpose) : Between houses and the local exchange, e.g. 5 km (subscriber loop)

Digital Data: Transmitted using modems, low data rates Within buildings (short distances) (Digital Signaling)

To private branch exchange (PBX) (64 Kbps) For local area networks (LAN) (10-100Mbps)

Example, Ethernet: 10BaseT: Unshielded Twisted Pair, 10 Mbps,100m range

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Twisted Pair - Pros and ConsCompared to other guided mediaPros:

Low cost Easy to work with (pull, terminate, etc.)Cons: Limited bandwidth

Limited data rate Limited distance range (due to large attenuation) Susceptible to interference and noise

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Twisted Pair - Transmission Characteristics Analog Transmission Mode For analog signals only Amplifiers every 5km to 6km Bandwidth up to 1 MHz (several voice channels): ADSL (Ch 8)

Digital Transmission Mode Using either analog or digital signals Repeaters every 2km or 3km Data rates up to few Mbps

(1Gbps: over very short distances) Impairments:

Attenuation: A strong function in frequency, can be modified with loading coils

EM Interference: Crosstalk, Impulse noise, Mains interference

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Attenuation in Guided Media

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Attenuation in Twisted pairs (unloaded)

Thinner Wires

1 KHz 1 MHz

Wire DiameterWire Gauge

300 3400Telephone Voice

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Ways to reduce EM interference

Shielding the TP with a metallic braid or sheathing Twisting reduces low frequency interference

Tighter twisting Better performance Using different twisting lengths for adjacent pairs

to reduce crosstalk

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STP: Metal Shield

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Unshielded (UTP) and Shielded (STP) Unshielded Twisted Pair (UTP)

Ordinary telephone wire: Abundantly available in buildings Cheapest Easiest to install Suffers from external EM interference

Shielded Twisted Pair (STP) Shielded with metal braid or sheathing:

Reduces interference Reduces attenuation at higher frequencies

Better Performance: Increases data rates used Increases distances covered

But becomes: More expensive Harder to handle (thicker, heavier)

Transmit faster and go further!

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UTP Categories: EIA-568-A Standard (1995) (cabling of

commercial buildings for data) Cat 3 Up to 16MHz Voice grade In most office buildings Twist length: 7.5 cm to 10 cm

Cat 4 Up to 20 MHz

Cat 5 Up to 100MHz Data grade Pre-installed in many new office buildings Twist length: 0.6 cm to 0.85 cm

(Tighter twisting increases cost but improves performance)

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Near End CrossTalk (NEXT) Coupling of signal from a wire pair to an adjacent

pair Coupling takes place when a transmitted signal

entering a pair couples (leaks) into an adjacent receiving pair on the same (near) end of the link

Disturbing pair

Disturbed pair

Transmitted Power, P1

Coupled Received Power, P2

“NEXT” Attenuation = 10 log P1/P2 dBs The larger … the smaller the crosstalk (The better the performance)

NEXT attenuation is a desirable attenuation- The larger the better!

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Transmission Properties for Shielded & Unshielded TP

Signal Attenuation (dB per 100 m) Near-end Crosstalk Attenuation (dB)

Frequency (MHz)

Category 3 UTP

Category 5 UTP

150-ohm STP

Category 3 UTP

Category 5 UTP

150-ohm STP

1 2.6 2.0 1.1 41 62 68?

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

Not Usable Not Usable

Undesirable Attenuation- Smaller is better Desirable Attenuation- Larger is better!

BetterBetter

BetterBetter

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Newer Twisted Pair Categories and Classes   Category

3 Class CCategory 5 Class D

Category 5E

Category 6 Class E

Category 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

UTP: Unshielded Twisted Pair FTP: Foil Twisted Pair SSTP: Shielded-Screen Twisted Pair

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Coaxial CablePhysical Description:

Designed for operation over a wider frequency rage

1 - 2.5 cm

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Physical Description

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Coaxial Cable Applications Most versatile medium Television distribution (FDM Broadband)

Cable TV (CATV): 100’s of TV channels over 10’s Kms Long distance telephone transmission

Can carry 10s of thousands of voice channels simultaneously (though FDM multiplexing) (Broadband)

Now facing competition from optical fibers and terrestrial microwave links

Local area networks, e.g. Thickwire Ethernet cable(10Base5): 10 Mbps, Baseband signal, 500m segment

Frequency Division Multiplexing

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Coaxial Cable - Transmission CharacteristicsImprovements over TP Extended frequency range

Up to 500 MHz Reduced EM interference and crosstalk

Due to enclosed concentric construction EM fields terminate within cable and do not stray out

Remaining limitations: Attenuation Thermal and noise Inter modulation noise (especially for broadband

operation)

Because of FDM

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Attenuation in Guided Media

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Coaxial Cable - Transmission Characteristics Analog Transmission

Amplifiers every few kms Closer spacing for higher frequency

Digital Transmission Repeater every 1km Closer repeater spacing for higher data rates

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Optical Fiber A thin (2-125 m) flexible strand

of glass or plastic Light entering at one end travels

confined within the fiber until it leaves at the other end

As fiber bends around corners, the light stays within the fiber

Lowest losses (attenuation) with ultra pure fused silica glass… but difficult to manufacture

Reasonable losses and performance with multi-component glass and with plastic Pure

GlassMulti-component Glass

Plastic

Cost, Difficulty of Handling Attenuation (Loss)

Best Performance

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Optical Fiber: Construction An optical fiber consists of three main parts

Core A narrow cylindrical strand of glass/plastic, with refractive index n1

Cladding A tube surrounding each core, with refractive index n2 The core must have a higher refractive index than the cladding to

keep the light beam trapped inside: n1 > n2

Protective outer jacket Protects against moisture, abrasion, and crushing

Individual Fibers:(Each having core & Cladding)

Multiple Fiber Cable(Note multiple cladding)

Single Fiber Cable

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Reflection and refraction of light At a boundary between a denser (n1) and a rarer (n2)

medium, n1 > n2 (e.g. water-air, optical fiber core-cladding) a ray of light will be refracted or reflected depending on the incidence angle

Total internal Total internal refreflelectionction

Critical angle Critical angle refrefraractionction

RefRefraractionction

denser

rarer

1

2

n1

n2

2

1

1

2

)()(nn

SinSin

)(sin

)()90(

1

21

2

1

nnnn

SinSin

critical

critical

critical

90

1 2 21

n1 > n2

Increasing Incidence angle, 1

critical 1 critical 1 critical 1

v1 = c/n1

v2 = c/n2

ShallowerIncidence

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Optical Fiber

n1

n1 > n2

DenserDenserRarer

Rarer

n1

n2

i

Total Internal Reflection at boundary for i > critical

Refraction at boundary for . Escaping light is absorbed in jacketi < critical

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Attenuation in Guided MediaWhich side is the IR?

Compare attenuation ranges!

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Optical Fiber - Benefits Greater channel capacities over larger distances

Fiber: 100’s of Gbps over 10’s of Kms Cable: 100’s of Mbps over 1’s of Kms Twisted pair: 100’s of Mbps over 10’s of meters

Lower/more uniform attenuation (Fig. 4.3c) An order of magnitude lower Relatively constant over a larger

frequency interval Electromagnetic isolation

Fiber is not affected by external EM fields: No interference, impulse noise, crosstalk

Fiber does not radiate (light ray trapped inside): Not a source of interference Difficult to tap (data security)

But what could happen at the repeater?

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Optical Fiber – Benefits, Contd. Greater repeater spacing: Fewer Units, Lower cost

Fiber: 10-100’s of Kms Cable, Twisted pair: 1’s Kms

Smaller size and weight: An order of magnitude thinner for same channel

capacity Useful in cramped places Reduced cost of digging in populated areas Reduced cost of cable support structures

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Optical Fiber - Applications Long-haul trunks between cities

Telephone traffic over long routes between cities, and undersea:

Fiber & Microwave now replacing coaxial cable 1500 km, Up to 60,000 voice channels

Metropolitan trunks Joining exchanges inside large cities:

12 km, Up to 100,000 voice channels Rural exchange trunks

Joining exchanges of towns and villages: 40-160 km, Up to 5,000 voice channels

Subscriber loops Joining subscribers to exchange:

Fiber replacing TP, allowing all types of traffic LANs, Example:

10BaseF 10 Mbps, 2000 meter segment

City

City

Exchange

MainExchange

Compare segment length with twisted pair and coaxial!

Competition:Fiber, Coaxial, Waves

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Optical Fiber - Transmission Characteristics Acts as a wave guide for light (1014 to 1015 Hz)

Covers portions of infrared and visible spectrum Transmission Modes:

Single Mode Multimode

Step Index Graded IndexSingle ray

Multiple rays

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Optical Fiber Transmission Modes

CoreCladding

n 1n 2

n 1n 2

Shallow reflectionDeep reflection

Dispersion: Spread in ray arrival time

Large

Smallest

Smaller

i < critical

Refraction

2 ways to reduce dispersion:

• v = c/n• Make n1 lower away from center…this speeds up deeper rays and compensates for their larger distances, arrive together with shallower rays

n1 > n2

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Optical Fiber – Transmission modes Spread of received light pulse in time (dispersion) is bad:

Causes inter-symbol interference bit errors Limits usable data rate and usable distance

Caused by propagation through multiple reflections at different angles of incidence

Dispersion increases with: Larger distance traveled Thicker fibers with step index

Dispersion can be reduced by: Limiting the distance Thinner fibers and a highly focused light source

In the limit: Single mode: High data rates, very long distances Graded-index thicker fibers: The half-way solution

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The transmission system is not just the medium (fiber)! We have also light Sources and detectors…Light Sources

Light Emitting Diode (LED) Lower cost, longer life Wider operating temp range

Injection Laser Diode (ILD) More efficient (more light power for same electric

power input) Faster switching Higher data rate

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Wavelength Division Multiplexing (WDM)

A form of FDM (Channels sharing the medium by occupying different frequency bands)

Multiple light beams at different frequencies (wavelengths) transmitted on the same fiber

Each beam forms a separate communication channel

Example: 256 channels @ 40 Gbps each 10 Tbps total data rate

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Optical Fiber – Four Transmission bands (windows) in the Infrared (IR) region Selection based on:

Attenuation of the fiber Properties of the light sources Properties of the light receivers

L S

C

Note: in fiber = v/f = (c/n)/f = (c/f)/n = in vacuum / ni.e. in fiber < in vacuum

values shown are in vacuum

Bandwidth, THz

3312 4 7

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Wireless Transmission

Free-space is the transmission medium Need efficient radiators, called antennas / aerials

Signal fed from transmission line (wireline) and radiated into free-space (wireless)

Popular applications Radio & TV broadcast Cellular Communications Microwave Links Wireless Networks

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Wireless Transmission Frequency Ranges Radio: 30 MHz to 1 GHz

Omni directional Broadcast radio

Microwaves: 1 GHz to 40 GHz Highly directional beams

Point to point (Terrestrial) Satellite

Infrared Light: 0.3 THz to 20 THz (just below visible light) Localized communications (confined spaces)

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Antennas Electrical conductor (or system of conductors) used to

radiate / collect electromagnetic energy into/from the environment (TX/RX operation)

Transmission Radio frequency electrical energy obtained

from transmitter Converted into electromagnetic energy Radiated into surrounding environment

Reception Electromagnetic energy impinging on antenna Converted to radio frequency electrical energy Fed to receiver

Same antenna often used for both TX and RX in 2-way communication systems

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Radiation Pattern Power radiated in different directions, usually

not with the same efficiency: Isotropic (Omni-directional) antenna

A hypothetical point source in space Radiates equally in all directions

– A spherical radiation pattern Used as a reference for other antennae

Directional Antenna Concentrates radiation in a given desired direction

– hence point-to-point, line of sight

communications Gives ‘gain’ in that direction

relative to isotropic

Radiation Patterns

Isotropic

Directional

Point Source

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Parabolic Reflective Antenna

Axis

Focus Parabola

(The Dish!)

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Parabolic Reflective Antenna (The Dish!) Used for terrestrial and satellite microwave

On Transmission: Source placed at the focus will produce waves that get reflected from parabola parallel to the parabola axis Creates a (theoretically) parallel beam to the parabola axis that

does not spread (disperse) in space ( Zero radiation off axis) In practice, some divergence (dispersion) occurs, because source

at focus has a finite size (not exactly a point!) On reception: Only signal from the axis direction is

concentrated at focus, where detector is placed. Signals from other directions miss the focus ( Zero o/p off axis)

i.e. Directionality in both TX, RX operation The larger the antenna (in wavelengths) the better the directionality

High frequency is advantageous

Focus Parabola

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Antenna Gain, G A measure of directionality of the antenna Power output in a given direction compared to that produced

by a perfect isotropic antenna

Can be expressed in decibels (dB, dBi) (i = relative to isotropic)

Increased power radiated in one direction causes less power radiated in other directions (Total power is fixed)

Gain G depends on the effective area (Ae) of the antenna: Depends on size and shape of the antenna

The Radiation pattern

)()()(

IsotropicPAntennapAntennaG

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Antenna Gain, G in the direction of maximum radiation

An isotropic antenna has a gain G = 1 (0 dBi) i.e.

A parabolic antenna has:

Substituting we get:

Gain in dBi = 10 log G Important: Gains apply to both TX and RX antennas

)Source"Point ' a -GHz 30at cm 0.1 ( 4

22

eA

2

2 2

4 4e eA f AGc

AAe 56.0A = Actual Area of antenna circle = r2

22

7)56.0(4

AAG Higher

frequencies higher

gains

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Terrestrial Microwave Parabolic dish i.e. Focused beam Line of sight requirement:

Beam should not be obstructed Curvature of earth limits maximum range Use relays to increase

range (multi-hop link) Link performance affected by antenna alignment (effect of wind!)

Applications: Long haul telecommunications

Many channels over long distances between large cities, possibly through intermediate relays:Competes with coaxial cable and fiber

Short links between buildings: CCTV links Links between LANs in different buildings Cellular Telephony

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Terrestrial Microwave: Transmission Properties 1 - 40 GHz: f range used depends on application

Advantages of higher frequencies : Larger bandwidth, B higher data rate (Table 4.6) Smaller smaller (lighter, cheaper) antenna for a given

antenna gain (see gain eqn) But Higher f larger attenuation due absorption by rain So,

Long-haul links (long distances) operate at lower frequencies (4-6 GHz,11 GHz) to avoid large attenuation

Short links between close-by buildings operate at higher frequencies (22 GHz) (Attenuation is not a big problem for the short distances, reduces antenna size)

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Terrestrial Microwave: Propagation Attenuation

2

10410logdBdL

2

1 d

Pd

As signal propagates in space, its power drops with distance according to the inverse square law

i.e. loss in signal power over distance traveled, d

2 dL

While with a guided medium, signal drops exponentially with distance… giving larger attenuation and lower repeater spacing

• Show that L increases by 6 dBs for every doubling of distance d.• For guided medium, corresponding attenuation = a d dBs, a = dBs/km

d’ = distance in ’s

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Satellite Microwave Satellite is used as a relay station for the link to avoid

limitation on distance due to earth curvature Satellite receives on one frequency (uplink), amplifies or

repeats signal and re-transmits it on another frequency (downlink)

Spatial angular separation (e.g. 3) to avoid interference from neighboring TXs

Require a geo-stationary orbit (satellite rotates at the same speed of earth rotation, so appears stationary): Height: 35,784km (long link, large transmission delays)

Applications: Television direct broadcasting Long distance telephony Private business networks linking multiple company sites

worldwide

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Satellite Point to Point Link

Earth curvature (overcome)

Relay

Uplink Downlink

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Satellite Broadcast LinkDirect Broadcasting Satellite

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Transmission Characteristics 1-10 GHz Frequency Trade offs:

Lower frequencies: Noise and interference Higher frequencies: Smaller antenna, but

rain attenuation, Downlink/Uplink frequencies recently going higher:

4/6 GHz 12/14 20/30 (due to better receivers)

Delay 0.25 s noticeable for telephony Inherently a broadcasting facility

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Broadcast Radio (30 MHz -1 GHz) Omni directional (you want to reach everybody)

(so, no need for antenna directionality) No dishes No line of sight requirement No antenna alignment problems

Applications: FM radio UHF and VHF television

Choice of frequency range:Reflections from ionosphere < 30 MHz -1 GHz < Rain attenuation

Propagation attenuation:Lower than Microwaves (as is larger)

Problems of omni directionality: Interference due to multi-path reflections e.g. TV ghost images

2

10410logdBdL

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Multi-Path effects due to omni-directionality

Omni-Directional BroadcastingAntenna

TV ghost images

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Infrared

Non coherent infrared light used as carrier Relies on line of sight (or reflections through

walls or ceiling) Blocked by walls (unlike microwaves) No licensing required for frequency allocation Applications:

TV remote control Wireless LAN within a room