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Transcript of Chapter 4: Transmission Media COE 341: Data & Computer Communications (T061) Dr. Radwan E....
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Chapter 4:
Transmission Media
COE 341: Data & Computer Communications (T061)Dr. Radwan E. Abdel-Aal
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Agenda Overview Guided Transmission Media
Twisted Pair Coaxial Cable Optical Fiber
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 more important For unguided, the bandwidth provided by the
antenna is more important
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Design Issues Key communication objectives are:
High data rate Low error rate Long distance Bandwidth economy: Tradeoff - Larger for higher data rates
- But smaller for economy Transmission impairments
Attenuation: Twisted Pair > Cable > Fiber (best) Interference:
Worse with unguided… (the medium is shared!)
Number of receivers In multi-point links of guided media:
More connected receivers introduce more attenuation
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The Electromagnetic Spectrum
10 KHz 100 MHz
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Standard Multiplier Prefixes 1-18 to 10+18
exa- E 1018 = 1,000,000,000,000,000,000 peta- P 1015 = 1,000,000,000,000,000 tera- T 1012 = 1,000,000,000,000 giga- G 109 = 1,000,000,000 mega- M 106 = 1,000,000 kilo- K 103 = 1,000
milli- m 10-3 = 0.001 micro- 10-6 = 0.000,001 nano- n 10-9 = 0.000,000,001 pico- p 10-12 = 0.000,000,000,001 femto- f 10-15 = 0.000,000,000,000,001 atto- a 10-18 = 0.000,000,000,000,000,001
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Electromagnetic Spectrum Ultra violet,X-Rays,Gamma-Rays Used for Communications
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Study of Transmission Media
Physical description Main applications Main transmission characteristics
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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 0.7 dB/km @ 1 kHz
5 µs/km 2 km
Coaxial cable
0 to 500 MHz
7 dB/km @ 10 MHz
4 µs/km Up to 9 km
Optical fiber 186 to 370 THz
0.2 to 0.5 dB/km
5 µs/km 40 km
Larger OperatingFrequencies
Lower Attenuation
Same attenuation(except with loading)
Fewer Repeaters
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Twisted Pair (TP)
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UTP Cablesunshielded
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Twisted Pair - Applications Most commonly used guided medium Telephone network (Analog Signaling)
Between houses and the local exchange (subscriber loop)
Originally designed for analog signaling. Digital data transmitted using modems at low data rates
Within buildings (short distances): (Digital Signaling) To private branch exchange (PBX) (64 Kbps) For local area networks (LAN) (10-100Mbps)
Example: 10BaseT: Unshielded Twisted Pair, 10 Mbps,100m range
Digital signal travels in its base band i.e. without modulating a carrier(short distances)
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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 Large Attenuation
Limited distance range Susceptible to interference and noise
(exposed construction)
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Twisted Pair - Transmission Characteristics Analog Transmission For analog signals only Amplifiers every 5km to 6km Bandwidth up to 1 MHz (several voice channels): ADSL
Digital Transmission For either analog or digital signals (carrying digital data) Repeaters every 2km or 3km Data rates up to few Mbps (1Gbps: very short distance)
Impairments: Attenuation: A strong function in frequency (
Distortion) EM Interference: Crosstalk, Impulse noise, Mains
interference, etc.
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Attenuation in Guided Media
Thinner Wires
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Ways to reduce EM interference
Shielding the TP with a metallic braid or sheathing Twisting reduces low frequency interference Different twisting lengths for adjacent pairs help
reduce crosstalk
WK 7
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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 foil, metal braid or sheathing:
Reduces interference Reduces attenuation at higher frequencies (increases BW)
Better Performance: Increased data rates used Increased distances covered
But becomes: More expensive Harder to handle (thicker, heavier)
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TP Categories: EIA-568-A Standard (1995) (cabling of
commercial buildings for data) Cat 3: Unshielded (UTP) Up to 16MHz Voice grade In most office buildings Twist length of 7.5 cm to 10 cm
Cat 5: Unshielded (UTP) Up to 100MHz Data grade Pre-installed now in many new office buildings Twist length 0.6 cm to 0.85 cm
(Tighter twisting increases cost but improves performance) Newer, shielded twisted pair: (150 STP)
Up to 300MHz
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Near End Crosstalk (NEXT) Coupling of signal from one wire pair to another Coupling takes place when a transmitted signal
entering a pair couples back to an adjacent receiving pair at the same end
i.e. near transmitted signal is picked up by near receiving pair
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
Undesirable Attenuation- Smaller is better Desirable Attenuation- Larger is better!
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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 ApplicationsMost 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
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Coaxial Cable - Transmission Characteristics:Improvements 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
outside Remaining limitations:
Attenuation Thermal and inter modulation noise
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Attenuation in Guided Media
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Coaxial Cable - Transmission Characteristics Analog Transmission
Amplifiers every few kms Closer amplifier 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 remains within the fiber through multiple reflections
Lowest losses (attenuation) with ultra pure fused silica glass… but difficult to manufacture
Reasonable losses with multi-component glass and with plastic Pure
GlassMulti-component Glass
Plastic
Cost, Difficulty
of Handling Attenuation (Loss)
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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 in: n1 > n2
Protective outer jacket Protects against moisture, abrasion, and crushing
Individual Fibers:(Each having its core & Cladding)
Multiple Fiber CableSingle Fiber Cable
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Reflection and Refraction 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 reflectionreflection
Critical angle Critical angle refractionrefraction
RefractionRefraction
denser
rarer
1
2
n1
n2
2
1
1
2
)(
)(
n
n
Sin
Sin
)(sin
)(
)90(
1
21
2
1
n
n
n
n
Sin
Sin
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
AnglesWith the Normal
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Optical Fiber
n1
n1 > n2
DenserDenser
Rarer
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 Media
Larger Frequency
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Optical Fiber - Benefits Greater capacity
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 range of frequencies
Electromagnetic isolation Not affected by external EM fields:
No interference, impulse noise, crosstalk Does not radiate:
Not a source of interference Difficult to tap (data security)
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Optical Fiber – Benefits, Contd. Greater repeater spacing: Lower cost, Fewer Units
Fiber: 10-100’s of Kms Cable, Twisted pair: 1’s Kms
Smaller size and weight: An order of magnitude thinner for same capacity
Useful in cramped places Reduced cost of digging in populated areas Reduced cost of support structures
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Optical Fiber - Applications Long-haul trunks
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 data LANs, Example:
10BaseF 10 Mbps, 2000 meter segment
City
City
Exchange
MainExchange
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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 Index
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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:
• v = c/n• n1 is made lower away from center…this speeds up deeper rays and compensates for their larger distances, arrive together with shallower rays
Curved path: n is not uniform- decreasing
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Optical Fiber – Transmission modes Spread of received light pulse in time (dispersion) is bad:
Causes inter-symbol interference bit errors (similar to delay distortion)
Limits usable data rate and usable transmission distance Caused by propagation through multiple reflections at
different angles of incidence Dispersion increases with:
Larger distance traveled Thicker fibers with step index
Can be reduced by: Limiting the distance Thinner fibers and a highly focused light source
Single mode (in the limit): High data rates, very long distances Or Graded-index multimode thicker fibers: The half-way (lower
cost) solution
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Optical Fiber – Light Sources
Light Emitting Diode (LED) Incoherent light- More dispersion Lower data rate Low cost Wider operating temp range Longer life
Injection Laser Diode (ILD) Coherent light- Less dispersion More efficient Faster switching Higher data rate
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Optical Fiber – 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 Separated at destination by filters
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 Band selection is a system
decision 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
Bandwidth, THz
3312 4 7
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Wireless Transmission
Free-space is the transmission medium Need efficient radiators, called antennas
Signal fed from transmission line (wireline) and radiated it 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 Localized communications (confined spaces)
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Antennas Electrical conductor (or system of conductors) used to
radiate / collect electromagnetic energy into/from surrounding space
Transmission Radio frequency electrical energy from
transmitter Converted into electromagnetic energy Radiated into surrounding space
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 all directions, but usually not with the
same efficiency Isotropic 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
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Parabolic Reflective Antenna Used for terrestrial and satellite microwave Source placed at the focal point will produce waves that
get reflected from parabola parallel to the parabola axis Creates a (theoretically) parallel beam of light/sound/radio that
does not spread (disperse) in space 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.
The larger the antenna (in wavelengths) the better the directionality so, using
Higher frequency is advantageous
Focus Parabola
WK 8
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Parabolic Reflective Antenna
Axis
WK 8
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Antenna Gain, G A measure of antenna directionality Power output in a particular 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 another direction (Total power is fixed) Effective area Ae:
Related to size and shape of antenna Determines the antenna gain,
Ae is the effective area
2
2 2
4 4e eA f AG
c
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Antenna Gain, G: Effective Areas 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 AG
c
AAe 56.0A = Actual Area = r2
22
7)56.0(4
AA
G
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Terrestrial Microwave Parabolic dish 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 sensitive to antenna alignment
Applications: Long haul telecommunications
Many voice/data channels over long distances between large cities, possibly through intermediate relays: Competes with cable and fiber
Short wireless links between buildings: CCTV links Links between LANs in different buildings
Cellular Telephony
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Terrestrial Microwave: Transmission Properties 1 - 40 GHz
Higher f Advantages: 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, smaller antenna size)
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Terrestrial Microwave: Propagation Attenuation
2
10
410logdB
dL
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 = d dBs, in dBs/km
d’ = distance in ’s
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Satellite Microwave Satellite is used as a relay station for the link 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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a. Satellite Point to Point Link
Earth curvatureObstructs line of sightfor large distances
Relay
Uplink Downlink
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b. Satellite Broadcast LinkDirect Broadcasting Satellite
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Transmission Characteristics 1-10 GHz Frequency Trade offs:
Lower frequencies: More noise and interference Higher frequencies: Larger rain attenuation, but smaller
antennas Downlink/Uplink frequencies recently going higher:
4/6 GHz 12/14 20/30 (better receivers becoming available)
Delay 0.25 s noticeable for telephony Inherently a broadcasting facility
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Broadcast Radio Omni directional (no need for antenna directionality horizontally)
No dishes No line of sight requirement No antenna alignment
Applications: FM radio UHF and VHF television
Choice of frequency range:Reflections from ionosphere < 30 MHz -1 GHz < Rain
Propagation attenuation:Lower than for Microwaves (as is larger)
Problems caused by omni directionality: Interference due to multi-path reflections
e.g. TV ghost images
2
10
410logdB
dL
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Multi-Path effects due to omni-directionality
Omni-Directional TV BroadcastingAntenna
TV ghost images
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Infrared
Data Modulates a non coherent infrared light 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