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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Wireless Propagation
W l P
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Wireless Propagation
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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