Chapter 4:

Transmission Media

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

2

Agenda Overview Guided Transmission Media

Twisted Pair Coaxial Cable Optical Fiber

Wireless Transmission Antennas Terrestrial Microwave Satellite Microwave Broadcast Radio Infrared

3

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

6

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)

12

UTP Cablesunshielded

13

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)

15

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

60

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