ts lecture09-11 transmission medium

8/6/2019 TS Lecture09-11 Transmission Medium

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Telecom Systems

Lecture09 Transmission Media


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Transmission Medium

The following terms will be used interchangeably

transmission medium,

Transmission system, and

transmission facility

Need of a physical transmission medium

The conveyance, or transmission, of information across a distancenecessarily involves some form of transmission medium

Selection of a physical transmission medium Every transmission medium has some pros and cons which makes it

suitable or unsuitable for a certain environment


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Categorization of Transmission medium

Types of Transmission media fall into two distinctive

categories,

the first of which includes all wired media

also referred as conducted, guided, or bounded media

The second category includes all traditional wireless

media

also referred as radiated, unguided, or unbounded media


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Frequency Spectrum


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Transmission Characteristics of a

transmission medium

Basic transmission characteristics

Bandwidth

Error performance

Distance between network elements

The attractiveness of any given transmission system increases with:

greater bandwidth

fewer errors

greater maximum distance between various network elements (e.g.,amplifiers and repeaters).


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Interrelation of Bandwidth, error

performance, and distance

Example

In a twisted pair network, bandwidth can be increased by using

higher frequencies

Unfortunately, higher frequencies attenuate (loose power) morerapidly than do lower frequencies. This fact results in more errors

in transmission, unless the amplifiers/repeaters are spaced more

closely together


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Propagation Delay

It refers to the time required for a signal to travel from transmitter toreceiver across a transmission system

Every transmission system has a certain value of propagation delay,which makes it suitable or unsuitable to be selected for transmission

Main factors of propagation delay nature of the transmission system

total length of the circuit

number of network elements (devices) in the network


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Transmission media

Different transmission media are used for transmission

The three most important media are:

copper

which is used in two main types of cable: paired cable andcoaxial cable;

glass fiber

which is used in optical fiber cable

radio waves which are used in terrestrial point-to-point systems or area

coverage systems (such as mobile telephony), and

for point-to-point or area coverage communication via satellite


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Wired Medium

Open wires

Twisted pair

C

oaxial Optical fiber


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Open wires

Open wire is usually made of steel, coated with copper Steel is used for the strength necessary to withstand the

suspension weight of the wire between poles

Frequency range up to 160kHz

Disadvantages: Bulky

Affected by weather conditions

i.e. large leakage with wet insulators

Severe cross-talk


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Twisted Pair

Insulated Pairs of copper wirebundled together

Individual pairs of wires twistedtogether to minimize cross-talk

Cables can contain severalhundreds of twisted pairs indifferent gages.

Laid in cities underground


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AWG


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Wire gauge Ohms per 1000

feet

19 9.5

22 19

24 32

26 48


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Twisted Pair

Suffer from cross-talk because of pairs being bound closely.

Due to the small diameter of the wires, resistance contributessignificantly to signal loss

Repeaters required every 3 to 6.5 km Frequency range up to 1MHz

Example

Used in the Access network

Also used in the core network, where there are small distances tocover


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Twisted Pairs

UTP

Ordinary Telephone wires

Cheapest and easy toinstall

Subject to external

electromagnetic

interference


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UTP

Distance

As the distance between network elements increases, attenuation (signalloss) increases

Even low-speed (voice grade) analog voice transmissions requireamplifiers spaced at least every 2-4 miles (10,000 to 18,000 feet)

In case of digital transmission (1.544 Mbps), repeaters are required atintervals of approximately 6,000 feet.

Cost

Very cheap for inside wire applications

Not suitable for long haul trunks


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Twisted Pairs

STP

Better performance at

higher data rates More expensive

Harder to handle and

work with

Suitable for high-noise

environments


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Coaxial cable

The center conductor is much thicker than a twisted pair conductor,

It is surrounded by an outer shield/conductor that serves to greatlyimprove signal strength and integrity.

Frequency Range (~1000MHz) Radiation losses and adjacent channel interference are virtually eliminated

by coaxial shielding


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History

Invented by AT&T Bell Telephone Laboratoriesin 1934,

first coaxial cable was placed into service inNew York City in 1936.

The Bell System's L5 coaxial carrier A long-haul trunk that includes 22 coaxial

tubes bound together to form a single cable.

Total of 108,000 simultaneous two-way voiceconversations can be carried by the cable.

Overall system frequency 58Mhz


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Transmission Characteristics of

Guided Media

FrequencyRange

TypicalAttenuation

TypicalDelay

RepeaterSpacing

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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Optical Fiber

An optical fibre is a glass orplastic fibre that carries lightalong its length

It is as thin as a human hair

The use of fiber-optics wasgenerally not available forcommunication until 1970 whenCorning Glass Works was ableto produce a fiber with a loss of17 dB/km

Today's optical fiber attenuationcan be as low as 0.2 dB/km


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Advantages of Optical Fiber

Light has a greater information-carrying capacitythan the highest radio frequencies

Greater repeater spacing

low error rates Immunity to electrical interference

Secure media

Can not be tapped

light weight

Longer life


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Operation windows

Specific regions in the optical spectrum whereoptical attenuation is low

the first window for silica-based optical fiber; systems

were developed to operate around 850 nmwavelength

second window (S band), at 1310 nm, soon proved tobe superior because of its lower attenuation

followed by a third window (C band) at 1550 nm withan even lower optical loss

Today, a fourth window (L band) near 1625 nm is underdevelopment and early deployment


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Wavelengths used in Fiber Optic

communication


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Single optical fiber

Core - Thin glass center of thefiber where the light travels

Cladding - Outer opticalmaterial surrounding the corethat reflects the light back intothe core

Buffer coating - Plasticcoating that protects the fiberfrom damage and moisture


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Bending of light ray


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Total internal reflection


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Optical fiber


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Fiber types


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Multimode fiber

Multi-mode fibers have larger cores (about 2.5 x 10-3

inches or 62.5 microns in diameter) and transmit infrared

light (wavelength = 850 to 1,300 nm) from light-emitting

diodes (LEDs)

Multimode fiber is best designed for short transmission

distances, and is suited for use in LAN systems and video

surveillance


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Multimode step index fiber

The principle of total internal reflection applies to multimodestep-index fiber

The cores index of refraction is higher than the claddings indexof refraction, the light that enters at less than the critical angle is

guided along the fiber. The disparity between arrival times of the different light rays is

known as dispersion, and the result is a muddied signal at thereceiving end


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Multimode Graded Index

25 times increase in bandwidth over step index

More bandwidth could have been achieved

but core size is kept large for convenient termination and use of

lower cost diodes Popular standard for use in medium distance (2-15km) data

communication links


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Single mode fiber

Single-mode fibers have small cores (about 3.5 x 10-4 inches or9 microns in diameter) and transmit infrared laser light(wavelength = 1,300 to 1,550 nanometers).

Single-mode fiber is best designed for longer transmission

distances, making it suitable for long-distance telephony andmultichannel television broadcast systems.


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Single mode fiber

Single-mode fiber gives you a higher transmission rate andup to 50 times more distance than multimode

More costly

The small core and single light-wave virtually eliminate anydistortion that could result from overlapping light pulses,providing the least signal attenuation and the highesttransmission speeds of any fiber cable type.


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Optical fiber modes


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Optical fiber Applications

Long-distance trunks:

Average about 1500 km with high capacity (20 - 60,000voice channels).

Undersea optical fiber being used.

Metropolitan trunks:Average about 12 km with 100,000 voice channels

Join telephone exchanges.

Rural exchange trunks:

Ranging from 40 to 160 km with fewer than 5000 voicechannels.

LANs: capacity of 100 Mbps to 1 Gbps.


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Optical transmission- analog and

digital


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Wavelength division multiplexing

(WDM)

Analogous to FDM

WDM technique is used to transmit multiple signals

at the same time, thus increase in bandwidth is

achieved many folds

Each signal is distinguished with a different

wavelength


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Single fiber unidirectional transmission

Unidirectional WDM Transmission


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Single fiber bi-directional transmission

Bi-directional WDM Transmission


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Development ofDWDM

Technology


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Advantages of WDM/DWDM

Enormous increase in bandwidth using the same

cable

Only needs to replace the equipment capable of WDM


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Light sources

Light Emitting Diode (LED)

relatively slow devices, suitable for use at speeds of less

than 1 Gbps

cheaper, wider operating temp range, lasts longer

Suited for multimode fibers

Injection Laser Diode (ILD)

more efficient, has greater data rate


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Other equipment

Light detectors

Amplifiers

OEO amplifiers

The attenuated signal needs to be converted to electricalsignal, and then to a fresh optical signal

Optical amplifiers The OA has made it possible to amplify the optical signal

without optical-electrical-optical (OEO) conversion

Add/Drop multiplexers

remove or insert one or more wavelengths at somepoint along this path


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Attenuation in Guided Media


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Wireless medium

Broadcast radio range

Frequencies of upto 1GHz

Microwave radio

Terrestrial

Satellite

Infra red


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Broadcast Radio

Frequency of upto 1GHz is known as broadcast

radio range

Applications

Radio stations

UHF and VHF television

Cellular transmission

Omnidirectional antennas are used mostly need line of sight above 30 MHz


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Wireless Propagation

Ground wave propagation

Sky wave propagation

Line of sight propagation


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W l P


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Line of Sight

Above 30 MHz, neither ground wave nor sky wave propagation modes operate,

and communication must be by line ofsight


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Microwave radio

A form of radio transmission which uses frequencies of

1GHz 100GHz is known as microwave transmission

Developed by Harold T. Friis and his associates at Bell

Telephone Laboratories in 1945 Advantage is high bandwidth

Problem

High frequency means high attenuation, and less distance covered


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Microwave radio

The radio beams are highlyfocused, in order tomaximize the strength ofsuch a high-frequencysignal Much as a light bulb in a

flashlight is centered in amirror which serves to focusthe light beam


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Antenna shape

Parabolic antenna (most commonly used antenna type)

A parabolic antenna is an antenna that uses a parabolic reflector, asurface with the cross-sectional shape of a parabola, to direct the radiowaves. The most common form is shaped like a dish and is popularlycalled a dish antenna or parabolic dish.


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LOS calculation

The coverage area for LOS propagation is limited

by the curvature of the earth.

max LOS for a transceiver at height h is

approximately (assuming no physical obstructions such asmountains)

d= 3.57(Kh)1/2

where K=4/3


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Measurement of Distance and Loss

Loss L due to attenuation over distance dat wavelength isexpressed as


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Microwave Links

Advantages Fewer repeaters are necessary for amplifying

signals Underground facilities are not necessary

High bandwidth

Minimal delay times

Fewer repeaters mean increased reliability and lessmaintenance


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Satellite Radio

A microwave transmission system utilizing a

nonterrestrial relay station positioned in space

First satellite Intelsat I (called Early Bird) was

designed to handle 240 voice channels (in 1965)


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Satellite Communications

A device called a transponder is used in the satellite to

receive the weak microwave signal, amplify and

condition it, and retransmit the signal back to another

earth station in a different location on earth

Most commercial satellite links separate, transmit, and

receive carrier frequencies by about 2 GHz

Earth stations typically transmit their signals to satellites on

carrier frequencies in the 6-GHz band (the up-link frequency)

The satellite's transponder down-converts these signals to a 4-

GHz band (the down-link frequency)


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The Altitude and Velocity of Satellites


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SATELLITEALTITUDES


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SATELLITEALTITUDES


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Satellite orbits

Geostationary Earth Orbit (GEO)

altitude: 36 000 km

TV/radio broadcast, research, weather, backbone, navigation,

Medium Earth Orbit (MEO)

altitude: 5000-12 000 km

remote access, navigation,

(MEOs not common yet)

Low Earth Orbit (LEO)

altitude: 500-1500 km satellite phones, remote access,

~same apps as MEO


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Satellite Communications

At an altitude of 22,300 miles, 40% of the Earth is exposed. Thesatellite's antenna is designed to emit a radiation pattern that coversthis entire exposed portion

Three Satellites positioned in geo-synchronous orbit, 120 apart fromeach other, can cover the entire surface of the earth

Subject to long delays signals must travel approximately 22,300 miles up to the satellite;

the resulting delay is approximately .25 seconds

Adding the satellite processing time and the return path, it makesaround 0.64 seconds

Hence, highly interactive voice, data, and video applications are not

effectively supported via two-way satellite communication Much lower cost per channel than submarine cable for transatlantic

communications


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GEO

GEO pros

huge coverage (large footprints) only 3 sat. to cover earth (populated areas)

fixed antenna position simple adjustment/tuning of earth stations/terminals

long system lifetime (~15-18 years) GEO cons

poor coverage at north/south poles (low elevation, need high positioned antennas)

large footprints

bad for point-to-point links (good for broadcast) long delays (long distance, ~0.3 s, critical for voice)

high transmission power (~10W) (excludes battery-powered devices)

expensive to launch/transfer into orbit


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LEO

LEO pros

shorter delays: 5-10 ms (shorter distance)

lower transmission power (1W)

(handheld devices, omni-directional antennas) cheaper to launch

LEO cons

global coverage = many satellites

complex system (moving satellites,) routing between sat.

short systems lifetime (5-8 years)


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MEO

Medium Earth Orbit (MEO) circular rotation (in arbitrary plane)

rotation period: ~6 h,

visibility period: ~2-3 h

Pros and Cons (between LEO, GEO) coverage (footprint)

Satellites for global coverage (~12)

delays (~45 ms)

transmission power (3-5W)

system complexity, (


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Broadcast


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Applications

Mapping

Ice and sand movement

Locating environmental situations (such as disappearing rainforests)

Locating mineral deposits

Finding crop problems

Researching plants and animals

Earth science, such as monitoring volcanoes

Tracking wildlife

Astronomy

Global Positioning System, or GPS

Press agency news feeds

Stock market, business and other financial information

International radio broadcasters moving from short-wave to (or supplementing their short-wavebroadcasts with) satellite feeds using microwave uplink feeds

Global television

Digital radio for CD-quality audio


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Infrared

Infrared light transmissions have existed for many years with their uselimited to remote controls for TV sets, slide projectors, etc.

Infrared systems use the infrared light spectrum (TeraHertz, or THz,range) to send a focused light beam to a receiver, much as would amicrowave system,


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Infrared

No licensing requirements.

Require line-of-sight and suffer from environmental

interference

Limited to distances of two miles Good for building to building connectivity


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Reference

Chp4 Transmission Medium

Wireless Communication & Networks

William Stallings

Chp2 Fundamentals of Transmission Systems:Technologies & Applications

Communication Systems & Networks

Ray Horak

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