Data Communications &
Computer Networks
Chapter 4
Transmission Media
1
Fall 2008
Agenda
•
•
•
•
•
•
Overview
Guided Transmission Media
Wireless Transmission
Wireless Propagation
Line of Sight Transmission
Home Exercises
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Overview
3
0. Overview
• Characteristics and quality determined by
medium and signal
• Guided media
—wire (solid medium)
—the medium is more important
• Unguided media
—wireless (uses air)
—the bandwidth produced by the antenna is more
important
—Key concerns: data rate and distance
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Design Factors
• Bandwidth
—Higher bandwidth gives higher data rate
• Transmission impairments
—Attenuation limits distance
• Interference
—May result in signal distortion for both guided and
unguided media
• Number of receivers
—In guided media
—More receivers (multi-point) introduce more
attenuation
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Electromagnetic Spectrum
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Guided Transmission Media
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1. Guided Transmission Media
• Media connecting network components
— NIC cards transmitting on the cable
— LAN cables only carry one signal at a time
— WAN cables can carry multiple signals simultaneously
• Electromagnetic waves are guided along a solid
medium. Three primary types of cabling:
— Twisted Pair (or copper)
— Coaxial cable
— Fiber-Optic cable
• For guided transmission media, capacity depends on
— Distance
— Whether the medium is point-to-point or multipoint
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Transmission Characteristics of
Guided Media
Frequency
Range
Typical
Attenuation
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
1 to 9 km
Optical fiber
186 to 370 THz
0.2 to 0.5 dB/km
5 µs/km
40 km
Note:
1
1
1
1
kHz (kilo Herz) = 103 Hz
MHz (Mega Herz) = 106 Hz
GHz (Giga Hetz) = 109 Hz
THz (Tera Herz) = 1012 Hz
1
1
1
1
ms (milli second) = 10-3 s
µs (micro second) = 10-6 s
ns (nano second) = 10-9 s
ps (pico second) = 10-12 s
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Twisted Pair
•
Twisting reduces crosstalk
interference between adjacent
pairs in a cable
•
Neighbouring pairs in a bundle
typically have different twist
lengths to reduce crosstalk
•
Wire thickness 0.4mm –
0.9mm
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Twisted Pair –
Applications
• Most common medium
• Telephone network
—Between house and local telephone exchange
(subscriber loop)
• Within buildings
—For digital signaling to private branch exchange (PBX)
• For local area networks (LAN)
—10Mbps or 100Mbps
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Twisted Pair –
Advantages/Disadvantages
• Advantages
—Cheap
—Easy to work with
• Disadvantages
—Low data rate (usually up to 100Mbps, although
some 1Gbps networks have been developed using
multiple twisted pair cabling)
—Short range (up to a few km)
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Twisted Pair –
Transmission Characteristics
• Analog
—Amplifiers every 5km to 6km
• Digital
—Use either analog or digital signals
—repeater every 2km or 3km
•
•
•
•
Limited distance
Limited bandwidth (1MHz)
Limited data rate (100Mbps)
Susceptible to interference and noise
—Because of easy coupling with electromagnetic fields
—Eg. impulse noise, 50Hz pick-up energy from AC power
lines
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Near-End Crosstalk
• What is Near-End Crosstalk?
—It is the coupling of unwanted signals from one pair
to another
• Coupling takes place when transmit signal
entering the link couples back to receiving pair
—i.e. near transmitted signal is picked up by near
receiving pair
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Unshielded and Shielded TP
• Unshielded Twisted Pair (UTP)
—Ordinary telephone wire
—Cheapest
—Easiest to install
—Suffers from external ElectroMagnetic Interference
(EMI)
• Shielded Twisted Pair (STP)
—Metal braid or sheathing that reduces interference
—Better performance at higher data rates
—More expensive
—Harder to handle (thick, heavy)
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Twisted-Pair (UTP and STP)
STP only:
Twisted-Pair
Shielded Insulation
to Reduce EMI
Outer Jacket
Speed and throughput:
10/100 Mbps
Relative cost:
Least costly
Color-Coded
Plastic Insulation
RJ-45
Connector
Media and connector size: Small
Maximum cable length:
ACOE312 Transmission Media
100 m
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UTP Categories
• Cat 3
— up to 16MHz
— Voice grade found in most offices
— Twist length of 7.5 cm to 10 cm
• Cat 4
— up to 20 MHz
• Cat 5
— up to 100MHz
— Commonly pre-installed in new office buildings
— Twist length 0.6 cm to 0.85 cm
— More expensive but better performance than Cat 3 UTP cables
• Cat 5E (Enhanced) – see tables
• Cat 6
• Cat 7
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Comparison of Shielded and
Unshielded Twisted Pair
Attenuation
(dB per 100 m)
Frequency
(MHz)
Category 3
UTP
Category 5
UTP
150-ohm
STP
1
2.6
2.0
1.1
4
5.6
4.1
2.2
16
13.1
8.2
4.4
25
—
10.4
6.2
100
—
22.0
12.3
300
—
—
21.4
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Twisted Pair Categories and
Classes
Category 3
Class C
Category 5
Class D
Bandwidth
16 MHz
100 MHz
Cable Type
UTP
Link Cost
(Cat 5 =1)
0.7
Category
5E
Category 6
Class E
Category 7
Class F
100 MHz
200 MHz
600 MHz
UTP/FTP
UTP/FTP
UTP/FTP
SSTP
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 Cable
Outer sheath
Insulation
Inner conductor
Braid
•
•
•
•
Outer conductor
Outer conductor is braided shield
Inner conductor is solid metal
Separated by insulating material
Covered by padding
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Coaxial Cable
OuterJacket
Braided Copper Shielding
Plastic Insulation
Copper Conductor
Speed and throughput:
10/100 Mbps
Relative cost:
More than UTP, but still low
Media and connector size:
Medium
Maximum cable length:
200/500 m
BNC Connector
Coaxial Cable - Applications
• Most versatile medium
• Television distribution
—Arial to TV
—Cable, Satellite TV
• Long distance telephone transmission
—Can carry 10,000 voice calls simultaneously
—Now being replaced by fiber optic
• Short distance computer systems links
• Local Area Networks
—Not used any more
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Coaxial Cable - Transmission
Characteristics
• Better performance than twisted pair
—Superior frequency characteristics
—Much less susceptibility to interference and crosstalk
• For Analogue signals
—Amplifiers needed every few km
—Much less distance for higher frequencies
—Up to 1GHz of bandwidth
• For Digital signals
—Repeater needed every about 1 km
—Less distance for higher data rates
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Fiber-optic cables
Glass
Core
Glass
Cladding
Plastic
Cover
Bundled Optical Fibers
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Plastic
Jacket
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Fiber-Optic Cable
Outer Jacket
Kevlar Reinforcing
Material
Plastic
Shield
Glass Fiber
and Cladding
Single mode: One stream of laser-generated light (100 km)
Multimode:
Multiple streams of LED-generated light (2 km)
Speed and throughput:
100+ Mbps
Average cost per node:
Most expensive
Media and connector size:
Small
Maximum cable length:
Up to 2 km
Multimode
Connector
Fiber Optic - Applications
• Long-haul trunks in telephone networks
— Circuit lengths of about 1500 km
— 20.000 to 60.000 voice channels
• Metropolitan trunks
— Circuit lengths of about 12km
— May have 100.000 voice channels in a trunk group
• Rural exchange trunks
— Circuit lengths of 40 – 160 km
— Typically fewer than 5.000 voice channels
• Subscriber loops
— Fiber to the business, fiber to the home in the near future
• LANs
— Support 100s and 1000s of stations at rates of about 10Gbps
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Fiber Optic - Transmission
Characteristics
• Act as wave guide for 1014 to 1015 Hz
—Portions of infrared and visible spectrum
• Types of light sources in fiber optic systems
—Light Emitting Diode (LED)
• Cheaper
• Wider operating temperature range
• Last longer
—Injection Laser Diode (ILD)
• More efficient
• Greater data rate
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Fiber optic – operation (1)
• How light travels in a fiber optic cable
— The source of light is usually a Light Emitting Diode (LED) or
a LASER. The light source is placed at one end of the optical
fiber
— Light that hits the glass core of the fiber at a certain angle,
known as the critical angle, is transmitted down through it by
total internal reflection.
— The detector, which is placed at the other end of the fiber, is
usually a Photo Diode and it generates an electrical pulse
when light falls on it.
Glass Core
Critical Angle
Diagram of Total Internal Reflection
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Fiber optic – operation (2)
—Hence by attaching a light source on one end of an
optical fiber and a detector at the other end, we have
a unidirectional data transmission system (Simplex)
—The light source would accepts an electrical signal,
converts and transmits it as light pulses
—The detector at the far end reconverts the light
pulses into an electrical signal to be then interpreted
as 1 or a 0
—The typical response time of the photodiode when
light falls on it is 1 nanosecond. This limits the data
rate to 1Gbps
—Higher data rates and longest distances require
LASER sources
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Fiber optic Transmission Modes
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Advantages of Fiber Optic over
Copper Cable
• Greater capacity
— Data rates of hundreds of Gbps
• Smaller size & weight
— 1000 twisted pair cables 1 km long = 800kg
— 2 optical fiber cables 1km approx = 100kg allows transfer of more data
• Lower attenuation and greater repeater spacing
— repeaters are needed every 100km rather than every 5km for copper
• Electromagnetic isolation
— Photons of light in a fiber do not affect each other as they have no
electrical charge and they are not affected by stray photons outside the
fiber
— In the case of copper, electrons move through the cable and these are
affected by each other and by electrons outside the cable
• Better security
— Difficult to tap
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Disadvantages of Fiber Optic over
Copper Cable
• Fiber technology is relatively new and certain new skills
are required in handling it
• Optical transmission in a fiber is one way only (Simplex)
— if you want bidirectional (two-way) communication, then you
must use two fibers or else use two frequency bands on the
one fiber
• Fiber optic cables and network interface cards to
connect a computer to the fiber are an order of
magnitude more expensive than their corresponding
copper cable equivalents
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Attenuation in Guided Media
Optical fiber
Twisted Pair cable
twisted
pair
coaxial
Co-axial cable
Composite graph
Optical
fiber
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Wireless Transmission
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2. Wireless Transmission
• Three frequency ranges are of interest
—30 MHz to 1 GHz
• Radio range
• Omnidirectional applications
• Broadcast radio and TV
—2 GHz to 40 GHz
•
•
•
•
Microwave range
Highly directional
Point to point
Satellite communications
—300 GHz – 200 THz
• Infrared range
• Point-to-point and multipoint applications within confined
areas (rooms)
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Antennas
• Electrical conductor (or a system of conductors) used to
radiate or collect electromagnetic energy
• For transmission of a signal
— Radio frequency energy from transmitter is converted to
electromagnetic energy by the antenna and radiated into
surrounding environment
• For reception of a signal
— Electromagnetic energy collected by the antenna is converted
into electrical energy and fed into the receiver
• In two-way communication the same antenna is often
used for both transmission and reception
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Radiation Pattern
• An antenna radiates power in all directions but
does not perform equally well in all directions
• Performance of an antenna is characterized by
its radiation pattern
—graphical representation of its radiation properties as
a function of space coordinates
• Isotropic antenna is (theoretical) point in space
—Radiates in all directions equally
—Gives spherical radiation pattern
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Parabolic Reflective Antenna
• Used for terrestrial and satellite microwave
• A source placed at the focus of the parabola will
produce waves reflected from parabola in
parallel to axis
—Creates (theoretical) parallel beam of
light/sound/radio
• Very directional
—On reception, signal is concentrated at focus, where
detector is placed
—Requires rigid mounting for precise alignment
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Dish diameter
Parabolic Reflective Antenna
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Antenna Gain (1)
• Antenna Gain is a measure of directionality of antenna
• Defined as the power output (in a particular direction)
compared with that produced by an isotropic antenna
• Measured in decibels (dB)
• Results in loss in power in another direction
• Effective area relates to antenna size and shape
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Antenna Gain (2)
• Relationship between antenna gain and effective area
G = (4πΑe)/λ2 = (4π f 2 Αe)/c2
where
G = antenna gain,
f = carrier frequency,
λ = carrier wavelength
Ae = effective area
c = speed of light (~3x108 m/s)
•
Example 1:
— effective area of an ideal isotropic antenna is λ2/4π, with a power gain of 1
— effective area of a parabolic antenna with a face area of A is 0.56A, with a
power gain of 7A/λ2
•
Example 2:
— For a parabolic reflective antenna with a diameter of 2m, operating at 12
GHz, what is the effective area and the antenna gain?
— Solution: The area of the antenna is A = πr2=π m2, since r=diameter/2, so
Ae=0.56π. Wavelength λ=c/f = (3·108)/(12·109) = 0.025m
G=(7A)/λ2 = 7π/(0.025)2 = 35,186 Hence, GdB=10log10(35,186)= 45.46 dB
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Terrestrial Microwave
• Parabolic dish
• Focused beam
• Tranceivers must be within line-of-sight
• Applications
—Long-haul telecommunications as an alternative to
coaxial or optical fiber
—Short point-to-point links
—Cellular systems
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Satellite Microwave
• Satellite is a relay station
• Satellite receives on one frequency, amplifies or repeats
signal and transmits on another frequency
• Requires geo-stationary orbit
— Stationary with respect to the satellite’s position over the Earth
— A height of 35,863 km above Equator
• Applications
— Television
— Long-distance telephone
— Private business networks
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Satellite Point to Point Link
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Satellite Broadcast Link
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Broadcast Radio
• Omni-directional
• FM radio
• UHF and VHF television
• Line of sight
• Suffers from multipath interference
—Reflections
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Infrared
• Modulate non-coherent infrared light
• Tranceivers must be within line of sight (or
reflection from a light-colored surface)
• Blocked by walls
—Security and interference problems encountered in
microwave systems are not present
• e.g. TV remote control, IRD port
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Wireless Propagation
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3. Wireless Propagation
• Signal travels along three routes
— Ground wave
• Follows contour of earth
• Up to 2MHz
• AM radio
— Sky wave
• Amateur radio, CB radio and international broadcasts (BBC world
service, Voice of America)
• Signal reflected from ionosphere layer of upper atmosphere
• (Actually refracted)
— Line of sight
• Above 30Mhz
• May be further than optical line of sight due to refraction
• More in section 4
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Ground Wave Propagation
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Sky Wave Propagation
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Line of Sight Propagation
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Refraction
• Velocity of electromagnetic wave is a function of density
of material
— ~3 x 108 m/s in vacuum, less in anything else
• As wave moves from one medium to another, its speed
changes
— Causes bending of direction of wave at boundary
— Towards more dense medium
• Index of refraction (refractive index) is
— Sin(angle of incidence)/sin(angle of refraction)
— Varies with wavelength
• May cause sudden change of direction at transition
between media
• May cause gradual bending if medium density is varying
— Density of atmosphere decreases with height
— Results in bending towards earth of radio waves
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Optical and Radio Horizons
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Line of Sight Transmission
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4. Line of Sight Transmission
• Free space loss
— Signal disperses with distance
— Greater for lower frequencies (longer wavelengths)
• Atmospheric Absorption
— Water vapour and oxygen absorb radio signals
— Water greatest at 22GHz, less below 15GHz
— Oxygen greater at 60GHz, less below 30GHz
— Rain and fog scatter radio waves
• Multipath
— Better to get line of sight if possible
— Signal can be reflected causing multiple copies to be received
— May be no direct signal at all
— May reinforce or cancel direct signal
• Refraction
— May result in partial or total loss of signal at receiver
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Free space loss
• For any type of wireless communication the signal
disperses with distance
— An antenna with a fixed area will receive less power farther it is
from the transmitting antenna
— This form of attenuation is Free space loss
• Free space loss can be expressed in terms of the ratio of
the radiated power, Pt to the power Pr received by an
antenna
Pt/Pr = (4πd)2/λ2 = (4π f d)2/c2
where
Pt = signal power at tx antenna, λ = carrier wavelength
Pr = signal power at rx antenna, f = carrier frequency,
c = speed of light (~3x108 m/s)
d = propagation distance between antennas
In decibels: LdB = 10 log10 (Pt/Pr) = -20log(λ) + 20log(d) + 21.98 dB
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Free
Space
Loss
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Multipath Interference
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Required Reading
• W. Stallings Chapter 4
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Home Exercises
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Exercises (1)
1.
A telephone line is known to have a loss of 20dB. The input signal
power is measured as 0.5W and the output noise level is
measured as 4.5µW. Using this information calculate the output
signal-to-noise ratio in dB.
2.
Suppose that data are stored on 1.4MByte floppy disks that weigh
25g each. Suppose than an airliner carries 10 tons of these disks
at a speed of 1000km/h over a distance of 5000 km. What is the
data transmission rate in bits per second of this system?
3.
Show that doubling the transmission frequency or doubling the
distance between transmitting and receiving antennas attenuates
the power received by 6dB.
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Exercises (2)
4.
A microwave transmitter has an output of 0.1W at 2GHz. Assume
that this transmitter is used in a microwave communication
system where the transmitting and receiving antennas are
parabolas, each 1.2m in diameter.
a) What is the effective area of each antenna?
b) What is the gain of each antenna in decibels?
5.
Some people may receive radio signals in metal fillings in their
teeth. Suppose you have one metal filling that is 2.5mm long that
acts as a radio antenna. That is, it is equal in length to one-half
the wavelength. What frequency do you receive?
6.
What is the length of an antenna one-half wavelength long for
sending radio at 300 Hz?
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