Transcript Chapter 4 - William Stallings, Data and Computer

Data and Computer
Communications
Chapter 4 –Transmission Media
Eighth & Ninth Editions
by William Stallings
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
Overview
media that are used to convey information can be
classified as guided or unguided
 guided - wire / optical fiber (provide Physical path)
 unguided – wireless employ an antenna for
transmitting through air, vacuum, or water.
 characteristics
and quality determined by
medium and signal


in unguided media - bandwidth produced by the
antenna is more important
in guided media - medium is more important
 key
concerns are data rate and distance
Directionality.
 One
key property of signals transmitted by
antenna is directionality.
 Signals at lower frequencies are
omnidirectional:the signal propagates in all directions
from the antenna.
At higher frequencies :it is possible to focus
the signal into a directional beam.
 In considering the design of data transmission
systems, key concerns are data rate and distance: the
greater the data rate and the less the distance
the better.

Design Factors
 bandwidth

higher bandwidth gives higher data rate
 transmission

impairments
eg. attenuation (limit the distance)
 Interference (competing signals in overlapping
frequency bands )
 number

of receivers in guided media
more receivers introduces more attenuation
and distortion
Electromagnetic Spectrum
frequencies at which various guided media and unguided transmission
techniques operate
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
Based on this chart a decision can be
made on what media to choose!
Guided Transmission Media
Twisted Pair
• Most common guided transmission medium for both analog and digital
signals
• Most commonly used medium in the telephone network (linking
residential telephones to the local telephone exchange, or office phones
to a PBX
• Much less expensive than the other commonly used guided transmission
media (coaxial cable, optical fiber) and is easier to work with.
• The twist length typically varies from 5 to 15 cm
Twisted Pair - Transmission
Characteristics

analog


digital



needs amplifiers every 5km to 6km
can use either analog or digital signals
needs a repeater every 2-3km
limited distance
 limited bandwidth (1MHz)
 limited data rate (100MHz)
 susceptible to interference and noise
Unshielded vs Shielded TP

unshielded Twisted Pair (UTP)






shielded Twisted Pair (STP)




ordinary telephone wire
cheapest
easiest to install
suffers from external EM interference
is ordinary telephone wire
metal braid or sheathing that reduces interference
more expensive
harder to handle (thick, heavy)
in a variety of categories - see EIA-568
UTP Categories
In response to the need to support higher speeds, Standard EIA-568-A
was issued in 1995.
The new standard reflects advances in cable and connector design and
test methods
C ate gory 3
Class C
C ate gory 5
Class D
C ate gory 5E
C ate gory 6
Class E
C ate gory 7
Class F
Ban dwi dth
16 MHz
100 MHz
100 MHz
200 MHz
600 MHz
C able Ty pe
UTP
UTP/FT P
UTP/FT P
UTP/FT P
SSTP
Li n k C ost
(C at 5 =1)
0.7
1
1.2
1.5
2.2
Comparison of Shielded and Unshielded
Twisted Pair
Table 4.2 summarizes the performance of Category 3 and 5 UTP, as well
as the STP specified in EIA-568-A
The strength of a signal
falls off with distance over
any transmission medium
Near-end crosstalk see next slide
Attenuation (dB per 100 m)
Frequency
(MHz)
C ate gory 3
UTP
C ate gory 5
UTP
1
2.6
4
Ne ar-en d Crosstalk (dB)
150-ohm STP
C ate gory 3
UTP
C ate gory 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
Coaxial Cable
like twisted pair, consists of two conductors, but is constructed
differently to permit it to operate over a wider range of frequencies
used in a wide variety of applications, including:
•
Television distribution - aerial to TV & CATV systems
•
Long-distance telephone transmission - traditionally used for inter-exchange
links, now being replaced by optical fiber/microwave/satellite
•
Short-run computer system links
•
Local area networks
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
An optical fiber is a thin flexible medium capable of guiding an optical ray.
Various glasses and plastics can be used to make optical fibers.
An optical fiber cable has a cylindrical shape and consists of three concentric sections:
1.
the core
2.
the cladding,
3.
the jacket
Optical Fiber
 The
core: is the innermost section and consists of one or more very thin
strands, or fibers, made of glass or plastic; the core has a diameter in the range of 8
to 50 µm.

The cladding: a glass or plastic coating that has optical properties
different from those of the core and a diameter of 125 µm. The interface between
the core and cladding acts as a reflector to confine light that would otherwise
escape the core.
 The

jacket: is composed of plastic and other material layered to protect
against moisture, abrasion, crushing, and other environmental dangers.
.
Optical Fiber - Benefits
 greater

capacity
data rates of hundreds of Gbps
 smaller
size & weight
 lower attenuation
 electromagnetic isolation


not vulnerable to interference, impulse noise, or crosstalk
high degree of security from eavesdropping
 greater

repeater spacing
10s of km at least
Where is Fiber used?
 long-distance telecommunications
 In
military applications is growing.
 local area networking.
 Long-haul trunks, Metropolitan trunks, Rural
exchange trunks, Subscriber loops & Local
area networks
Optical Fiber - Transmission
Characteristics
 uses

effectively acts as wave guide for 1014 to 1015 Hz
 can

total internal reflection to transmit light
use several different light sources
Light Emitting Diode (LED)
• cheaper, wider operating temp range, lasts longer

Injection Laser Diode (ILD)
• more efficient, has greater data rate
 relation
of wavelength, type & data rate
Both single mode and multimode can support several different
wavelengths of light and can employ laser or LED light sources.
Optical Fiber Transmission Modes
Optical Fiber Transmission Modes
(Step-index multimode)




Light from a source enters the cylindrical glass or plastic core.
Rays at shallow angles are reflected and propagated along the
fiber; other rays are absorbed by the surrounding material.
Multiple propagation paths exist, each with a different path
length and hence time to traverse the fiber. This causes signal
elements (light pulses) to spread out in time, which limits the
rate at which data can be accurately received.
This type of fiber is best suited for transmission over very
short distances.
Optical Fiber Transmission
Modes (Single-mode)





When the fiber core radius is reduced, fewer angles
will reflect.
By reducing the radius of the core to the order of a
wavelength, only a single angle or mode can pass.
provides superior performance Because there is a
single transmission path with single-mode
transmission.
the distortion found in multimode cannot occur.
used for long-distance applications, including
telephone and cable television.
Optical Fiber Transmission
Modes (Single-mode)
light rays moving down the axis advance more slowly
than those near the cladding.
 light in the core curves helically because of the
graded index, reducing its travel distance.
 often used in local area networks.

Attenuation in Guided Media
attenuation versus wavelength for the various types of wired media
attenuation for twisted pair is a
very strong function of
frequency
coaxial cable has frequency
characteristics that are superior
to those of twisted pair and can
be used effectively at higher
frequencies and data rates.
The unusual shape of the
curve is due to the
combination of a variety of
factors that contribute to
attenuation.
absorption and scattering, which
is the change in direction of light
rays after they strike small
particles or impurities in the
medium.
Wireless Transmission Frequencies



2GHz to 40GHz
 microwave
 highly directional
 point to point
 Used in satellite
30MHz to 1GHz
 Omnidirectional (radio range)
3 x 1011 to 2 x 1014
 Infrared
used for LAN
Antennas
For unguided media, transmission and reception are achieved by
means of an antenna
 electrical conductor used to radiate or collect
electromagnetic energy
 transmission antenna
 radio frequency energy from transmitter
 converted to electromagnetic energy by antenna
 radiated into surrounding environment
 reception antenna
 electromagnetic energy impinging on antenna
 converted to radio frequency electrical energy
 fed to receiver
 same antenna is often used for both purposes
Radiation Pattern
 power
radiated in all directions
 not same performance in all directions
 an isotropic antenna is a (theoretical) point
in space


radiates in all directions equally
with a spherical radiation pattern
Parabolic Reflective Antenna
Paraboloid
surfaces are used
in headlights,
optical and radio
telescopes, and
microwave
antennas
parabolic reflective antenna, used in terrestrial microwave and satellite applications.
A parabola is the locus of all points equidistant from a fixed line (the directrix) and a fixed point (the
focus) not on the line,
If a parabola is revolved about its axis, the surface generated is called a paraboloid.
Antenna Gain


measure of directionality of antenna
power output in particular direction verses that produced
by an isotropic antenna (a reference radiator with which other
sources are compared)


measured in decibels (dB)
Effective area relates to size and shape

Note: Antenna gain does not refer to obtaining
more output power than input power but rather to
directionality.
Terrestrial Microwave







used for long haul telecommunications
and short point-to-point links
requires fewer repeaters but line of sight
use a parabolic dish to focus a narrow beam
onto a receiver antenna
1-40GHz frequencies
higher frequencies give higher data rates
main source of loss is attenuation
 distance, rainfall also interference
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
typical uses




Television
long distance telephone
private business networks
global positioning
Satellite Point to Point Link
Figure 4.6 depicts in a general way two common configurations for satellite
communication. In the first, the satellite is being used to provide a point-to-point
link between two distant ground-based antennas.
Satellite Broadcast Link
the satellite provides communications between one ground-based
transmitter and a number of ground-based receivers.
Broadcast Radio





Radio is a general term used to encompass frequencies in the
range of 3 kHz to 300 GHz. We are using the informal term
broadcast radio to cover the VHF and part of the UHF band
use broadcast radio, 30MHz - 1GHz, for:
 FM radio
 Ultra high frequency (UHF) and VHF (Very high
frequency) television
is omnidirectional
still need line of sight
suffers from multipath interference
 reflections from land, water, other objects
Infrared Communications

Achieved using transmitters/receivers (transceivers)
that modulate noncoherent light(space time codes are a way of
transmitting data in wireless communications) infrared
 Transceivers
must be within the line of sigh
 are blocked by walls
 no licenses required
 typical uses


TV remote control
IRD port
Wireless Propagation
Ground Wave
A signal radiated from an antenna travels along one of three routes:
ground wave, sky wave, or line of sight
Ground wave propagation more or less follows the contour of the earth and can
propagate considerable distances, well over the visual horizon. This effect is found in
frequencies up to about 2 MHz.
The best-known
example of ground
wave communication
is AM radio.
Wireless Propagation
Sky Wave
Sky wave propagation is used for amateur radio, CB radio, and
international broadcasts such as BBC and Voice of America.
A sky wave signal can travel through a number of hops, bouncing back
and forth between the ionosphere and the earth's surface (Refraction).
Wireless Propagation
Line
of
Sight
• Neither ground wave nor sky wave propagation modes operate, and communication must be by
line of sight .
• The transmitting and receiving antennas must be within an effective line of sight of each other.
Refraction

velocity of electromagnetic wave is a function of
density of material
~3 x 108 m/s in vacuum, less in anything else

speed changes as move between media
 Index of refraction (refractive index) is



sin(incidence)/sin(refraction)
varies with wavelength
have gradual bending if medium density varies



density of atmosphere decreases with height
results in bending towards earth of radio waves
hence optical and radio horizons differ
Line of Sight Transmission
Some impairments specific to wireless 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
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 the antenna or, in
decibels, by taking 10
times the log of that
ratio.
Multipath Interference
Figure 4.11 illustrates in general terms the types of multipath
interference typical in terrestrial, fixed microwave and in mobile
communications.
For fixed microwave, in
addition to the direct line of
sight, the signal may follow a
curved path through the
atmosphere due to refraction
and the signal may also reflect
from the ground.
For mobile communications,
structures and topographic
features provide reflection
surfaces.
Summary
 looked
at data transmission issues
 frequency, spectrum & bandwidth
 analog vs digital signals
 transmission impairments