Transcript Chapter 4 - William Stallings, Data and Computer Communications

Data and Computer
Communications
Chapter 4 –Transmission Media
Eighth Edition
by William Stallings
Lecture slides by Lawrie Brown
Transmission Media
Communication channels in the animal world include
touch, sound, sight, and scent. Electric eels even use
electric pulses. Ravens also are very expressive. By a
combination voice, patterns of feather erection and
body posture ravens communicate so clearly that an
experienced observer can identify anger, affection,
hunger, curiosity, playfulness, fright, boldness, and
depression. —Mind of the Raven, Bernd Heinrich
Transmission Media - Overview
 Transmission

Physical path between transmitter and receiver
 Guided

Medium
Media
Waves are guided along a solid medium
• e.g., copper twisted pair, copper coaxial cable,
optical fiber
 Unguided


Media
Provides means of transmission but does not
guide electromagnetic signals
Employ an antenna for transmission
• e.g., atmosphere, outer space
3
Transmission Media - Overview
 Characteristics
and quality determined by
medium and signal
 For guided

Medium is more important
 For

Bandwidth produced by the antenna is more
important
 Key


unguided
concerns are
Data rate and Distance
The greater the BETTER!
4
Design Factors
 Bandwidth

higher bandwidth gives higher data rate
 Transmission

impairments
Attenuation limits distance.
 Interference


Overlapping frequency bands in unguided
media.
Nearby cables.
 Number


of receivers
In guided media
More receivers (multi-point) introduce more
attenuation
5
Electromagnetic Spectrum
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)
Coaxial cable
0 to 1 MHz
0.7 dB/km @
1 kHz
5 µs/km
2 km
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
Transmission Media
 Guided



media
Twisted-pair
Coaxial Cable
Optical Fiber
 Unguided



media
Satellites
Terrestrial Microwave
Broadcast Radio
8
Guided Transmission Media
 Transmission

Capacity
Either in terms of
• Bandwidth, or
• Data Rate

Depends critically on
• Distance
• Type of medium


Point-to-point
Mutipoint
9
Twisted Pair
 Most
common medium
 Two insulated wires twisted together in a
helical manner (like DNA)
 Advantages


Cheap
Easy to work with
 Disadvantages


Low data rate
Short range
10
Twisted Pair
Twisted Pair - Applications
 Telephone

Between house and local exchange
 Within

buildings
To private branch exchange (PBX)
 For

network
local area networks (LAN)
10 Mbps or 100 Mbps
12
Twisted Pair - Transmission
Characteristics
 Analog

Amplifiers every 5 km to 6 km
 Digital


Use either analog or digital signals
Repeater every 2 km or 3 km
 Limited



in
Distance
Bandwidth (1 MHz)
Data rate (100 Mbps)
 Susceptible
to interference and noise
13
Unshielded vs Shielded TP

unshielded Twisted Pair (UTP)





shielded Twisted Pair (STP)




ordinary telephone wire
cheapest
easiest to install
suffers from external EM interference
metal braid or sheathing that reduces interference
more expensive
harder to handle (thick, heavy)
in a variety of categories - see EIA-568
Unshielded vs Shielded TP

unshielded Twisted Pair (UTP)





shielded Twisted Pair (STP)




ordinary telephone wire
cheapest
easiest to install
suffers from external EM interference
metal braid or sheathing that reduces interference
more expensive
harder to handle (thick, heavy)
in a variety of categories - see EIA-568
UTP Categories
 Cat



Up to 16 MHz
Voice grade found in most offices
Twist length of 7.5 cm to 10 cm
 Cat



4
Up to 20 MHz
 Cat

3
5
Up to 100 MHz
Commonly pre-installed in new office
buildings
Twist length 0.6 cm to 0.85 cm
16
UTP Categories
Category 3
Class C
Category 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
Comparison of Shielded and
Unshielded Twisted Pair
Attenuation (dB per 100 m )
Frequency
(MHz)
Category 3
UTP
Category 5
UTP
1
2.6
4
Near-end Cros stalk (dB)
150-ohm STP
Category 3
UTP
Category 5
UTP
150-ohm STP
2.0
1.1
41
62
58
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
Near End Crosstalk

coupling of signal from one pair to another
 occurs when transmit signal entering the link
couples back to receiving pair
 ie. near transmitted signal is picked up by near
receiving pair
 The tighter the twist in the cable, the more
effective the cancellation

Twisting is used to balance the noise.
http://www.cabletesting.com
Coaxial Cable
 Most
versatile medium
*Braided shield is also referred to as the “outer conductor”
20
Coaxial Cable Applications
 Television

Cable TV
 Long


distribution
distance telephone transmission
Can carry 10,000 voice calls simultaneously
Being replaced by fiber optic
 Short
distance computer systems links
 LANs
21
Coaxial Cable - Transmission
Characteristics
 superior
frequency characteristics to TP
 performance limited by attenuation & noise
 analog signals



amplifiers every few km
closer if higher frequency
up to 500MHz
 digital


signals
repeater every 1km
closer for higher data rates
Optical Fiber
 Greater

capacity
Data rates of hundreds of Gbps
 Smaller
size & weight
 Lower attenuation
 Electromagnetic isolation
 Greater repeater spacing

10s of km at least
23
Optical Fiber
 System



components:
Transmission medium - fiber optic cable
Light source - LED or laser diode
Detector - photodiode
24
Optical Fiber - Applications
 Telephone

Long-haul, metropolitan, rural, and subscriber
loop circuits
 Local



Network Applications
Area Networks
Optical fiber networks
Data rates from 100 Mbps to 1 Gbps
Support hundreds (or even thousands) of
stations
25
Optical Fiber - Transmission
Characteristics
 Light

Sources
Light Emitting Diode (LED)
• Cheaper
• Wider operating temp range
• Last longer

Injection Laser Diode (ILD)
• More efficient
• Greater data rate
 Wavelength
Division Multiplexing
26
Optical Fiber
Optical Fiber Transmission Modes

Three types of transmission modes:
– Step-index multimode:
• Rays are reflected, absorbed and propagated along the fiber.
• Signal elements (light pulses) spread out in time.
• Suited for short distance transmission.
– Single mode:
• Fiber core diameter is reduced to a single wavelength (3-10 µm).
• Single transmission path.
• Long distance application (telephone and cable TV).
– Graded-index multimode:
• Intermediate between single mode and step-index multimode.
• Used in LAN.
28
Optical Fiber Transmission
Modes
Optical Fiber Transmission Modes
30
Frequency Utilization for
Fiber Applications
Fiber Type
Appli cation
Multim ode
LAN
S
Single mode
Various
196 to 192
C
Single mode
WDM
192 to 185
L
Single mode
WDM
Wave length (in
vacuum) range
(nm)
Frequency
Range (THz)
820 to 900
366 to 333
1280 to 1350
234 to 222
1528 to 1561
1561 to 1620
Band
Label
Attenuation in Guided Media
Wireless Transmission
 Unguided media (transmission and reception via antenna).

– Transmission: the antenna radiates electromagnetic energy
into the medium (usually air).
– Reception: the antenna pick up electromagnetic waves from
the surrounding medium.
Two basic configurations:
– Directional:


Focused electromagnetic beam.
Careful alignment required.
– Omnidirectional:


Signal spreads in all directions.
Can be received by many antennas.
33
Wireless Transmission
Frequencies

>1 GHz to 40GHz





30MHz to 1GHz (radio)



microwave
highly directional
point to point
satellite
omnidirectional
broadcast radio
3 x 1011 to 2 x 1014


infrared
local
Antennas

electrical conductor used to radiate or collect
electromagnetic energy
 transmission antenna




reception antenna




radio frequency energy from transmitter
converted to electromagnetic energy by antenna
radiated into surrounding environment
electromagnetic energy impinging on antenna
converted to radio frequency electrical energy
fed to receiver
same antenna is often used for both purposes
Radiation Pattern
 Away
to characterize the performance of an
antenna
 A graphical representation of the radiation as a
function of space coordinates
 Power radiated in all directions
 Not same performance in all directions
 Isotropic antenna is (theoretical) point in space


Radiates in all directions equally
Gives spherical radiation pattern
36
Radiation Pattern
Parabolic Reflective Antenna


Used for terrestrial and satellite microwave
Parabola is locus of point equidistant from a line and a
point not on that line



Revolve parabola about axis to get paraboloid



Cross section parallel to axis gives parabola
Cross section perpendicular to axis gives circle
Source placed at focus will produce waves reflected
from parabola in parallel to axis


Fixed point is focus
Line is directrix
Creates (theoretical) parallel beam of light/sound/radio
On reception, signal is concentrated at focus, where
detector is placed
38
Parabolic Reflective Antenna
Parabolic Reflective Antenna
Antenna Gain
 measure
of directionality of antenna
 power output in particular direction verses
that produced by an isotropic antenna
 measured in decibels (dB)
 results in loss in power in another direction
 effective area relates to size and shape

related to gain
Antenna Gain
G


4Ae

2
4f Ae

2
c
2
G= Antenna gain
Ae = effective area (related to the physical size of the antenna and to its shape)
F = carrier frequency
C = speed of light
the effective area of an ideal isotropic antenna
is
with a power gain of 1
 effective area of a parabolic antenna with a face
area of A is 0.56A
Example
Terrestrial Microwave
 used

alternative to coaxial cable or optical fiber
 and

for long haul telecommunications
short point-to-point links
closed-circuit TV or as a data link between
local area networks
 requires
fewer repeaters but line of sight
 use a parabolic dish to focus a narrow
beam onto a receiver antenna
Terrestrial Microwave
 Characteristics
 1-40GHz frequencies
 higher frequencies give higher data rates
 main source of loss is attenuation
distance, rainfall (above 10 GHz)


also interference
 4d 
L  10 log 
 dB
  
2
d and lamda have the same unit
Satellite Microwave

satellite is relay station
 receives on one frequency, amplifies or repeats
signal and transmits on another frequency


typically requires geo-stationary orbit



eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz
height of 35,784km
spaced at least 3-4° apart (angular displacement as
measured from the earth)
typical uses




television
long distance telephone
private business networks
global positioning
Satellite Point to Point Link
Satellite Broadcast Link
Broadcast Radio
 radio
is 3kHz to 300GHz
 use broadcast radio, 30MHz - 1GHz, for:


FM radio
UHF and VHF television
 is
omnidirectional
 the ionosphere is transparent to radio
waves above 30 MHz

still need line of sight
 suffers

from multipath interference
reflections from land, water, other objects
Infrared
 modulate
noncoherent infrared light
 end line of sight (or reflection)
 are blocked by walls
 no licenses required
 typical uses


TV remote control
IRD port
Wireless Propagation-Ground wave
 Follows
contour of earth
 Up to 2MHz
 AM radio
 Reasons


the electromagnetic wave induces a current in
the earth’s surface
 slow the wavefront near the earth
 tilt downward and hence follow the earth’s
curvature
diffraction, the behavior of electromagnetic
waves in the presence of obstacles
51
Wireless Propagation-Sky wave
 Amateur
radio, BBC world service, Voice of
America
 Signal reflected from ionosphere layer of upper
atmosphere
 (Actually refracted)
 sky wave signal can travel through a number
of hops, bouncing back and forth between
the ionosphere and the earth’s surface
52
Wireless Propagation- Line of sight
 Above
30Mhz
 neither ground wave nor sky wave
propagation modes operate

communication must be by line of sight
 satellite
communication, signal is not
reflected by the ionosphere
 ground-based communication, the
transmitting and receiving antennas must be
within an effective line of sight of each other
53
Wireless Propagation
Ground Wave
Wireless Propagation
Sky Wave
Wireless Propagation
Line of Sight
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
Moving from a less dense to a more dense medium
• Towards more dense medium
57
Refraction
 Index
of refraction (refractive index) of one
medium relative to another 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
58
Optical and Radio Horizons
• no intervening obstacles, the optical line of sight
d  3.75 h
d = the distance between an antenna and the horizon
•For radio
d  3.75 Kh
K is adjustment factor to account for the refraction =4/3
59
Optical and Radio Horizons
 the
maximum distance between two
antennas for LOS

3.75
 where
kh1 
kh2

h1 and h2 are the heights of the two
Optical and Radio Horizons
Line of Sight Transmission
 Free

space loss
loss of signal with distance
 Atmospheric

Absorption
from water vapour and oxygen absorption
 Multipath

multiple interfering signals from reflections
 Refraction

bending signal away from receiver
Line of Sight Transmission

Free space loss


Signal disperses with distance
Greater for lower frequencies (longer wavelengths)

Free space loss (ideal isotropic)
Pt
(4d ) 2
(4fd ) 2


2
Pr

c2

For other antenna
pt 4  (d ) (d )
(cd )


 2
2
pr
Gr Gt 
Ar At
f Ar At
2
2
2
2
63
Free Space Loss
64
Line of Sight Transmission

Atmospheric Absorption





Multipath





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
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
65
Multipath Interference
66
Free Space Loss
Summary
 looked
at data transmission issues
 frequency, spectrum & bandwidth
 analog vs digital signals
 transmission impairments
Problem Assignments
 Solve
all the review questions
 Try the following problems:
 4.1, 4.3, 4.6, 4.7, 4.14, 4.15, 4.16, 4.18