network basic
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Chapter 2The Physical Layer
transmitter receiver
bits bits
transmission medium
electromagnetic waves
light
electric current
node 1 node 2
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2.1 The Theoretical Basis for Data Communication
Information can be transmitted on wires by varying some physical
property such as voltage or current. By representing the value ofthis voltage or current as a single-valued function of time,f(t), we
can model the behavior of the signal and analyze it mathematically.
2.1.1 Fourier Analysis
Any reasonably behaved periodic function, g(t), with period T
can be constructed by summing a (possibly infinite) number of
sines and cosines:
11)2cos()2sin(2
1)(
nn
nn nftbnftactg
wheref=1/Tis the fundamental frequency and an and bn are the
sine and cosine amplitudes of the nth harmonics.
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2.1 The Theoretical Basis for Data Communication
2.1.1 Fourier Analysis
T
n dtnfttgT
a0
)2sin()(2
T
n dttgT
c0
)(2
T
ndtnfttg
Tb
0
)2cos()(2
Root mean square amplitude
22nn ba
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2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
A binary
signal to be
transmitted
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2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
One harmonic
Two harmonics
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2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
Four harmonics
Eight harmonics
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2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
No transmission facility can transmit signals without losing some
power in the process. If all the Fourier components were equally
diminished, the resulting signal would be reduced in amplitude but
not distorted.
Unfortunately, all transmission facilities diminish different Fourier
components by different amounts, thus introducing distortions.
Usually, the amplitudes are transmitted undiminished from 0 up tosome frequencyfc [cycles/sec=Hertz(Hz)} with all frequencies
above the cutoff frequency strongly attenuated.
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2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
The time T required to transmit the character depends on both theencoding method and the signaling speed [the number of times
per second that the signal changes its value].
The number of changes per second is measured in baud.
Bit rate=(baud rate)*log2(# of signal levels)
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2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
Given a bit rate ofb bits/sec, the time required to send 8 bits is 8/b,
so the frequency of the first harmonic is b/8 Hz.
An ordinary telephone line, often called a voice-grade line, has an
artificially introduced cutoff frequency near 3000Hz. Thisrestriction means that the number of the highest harmonic passed
through is 3000/(b/8) or 24000/b roughly.
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2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
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2.1 The Theoretical Basis for Data Communication
2.1.3 The Maximum Data Rate of a Channel
Nyquists Theorem for noiseless channel
Maximum date rate=2Hlog2Vbits/sec
bandwidth number of signal levels
For example, a noiseless 3-kHz channel cannot transmit binary
signals at a rate exceeding 6000 bps.
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2.1 The Theoretical Basis for Data Communication
2.1.3 The Maximum Data Rate of a Channel
If random noise is present, the situation deteriorates rapidly. Theamount of thermal noise present is measured by the ratio of the
signal power to the noise power, called the signal-to-noise ratio
(S/N).
Usually, the ratio itself is not quoted; instead, the quantity 10
log10S/Nis given. These units are called decibels (dB).
Shannons Theorem
Maximum number of bits/sec=Hlog2(1+S/N)
For telephone line: 3000log2(1+30dB)30000bps.
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2. Physical Layer
2.2 Transmission Media
2.2.1 Magnetic Media
A tape can hold 7 gigabytes. A box can hold about 1000 tapes.
Assume a box can be delivered in 24 hours. The effective
bandwidth=7*1000*8/86400=648 Mbps
Cost of 1000 tapes=5000. If a tape can be reused 10 times and
the shipping cost is 200, we have a cost of 700 to ship 7000
gigabytes or 10 cents per gigabytes. No network carrier on
earth can compete with that.
Never underestimate the bandwidth of a station wagon full of
tapes hurtling down the highway.
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2.2 Transmission Media
2.2.2 Twisted Pair
Although the bandwidth characteristics of magnetic tape are
excellent, the delay characteristics are poor.
Twisted Pair: used in local loop in telephone systems
The purpose of twisting the wires is to reduced electrical
interference from similar pairs close by.
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2.2 Transmission Media
2.2.2 Twisted Pair
Unshielded Twisted Pair (UTP)
Category 3 and Category 5: 4 pairs grouped together in a
plastic sheath for protection and to keep the eight wires
together, used in high speed computer networks.
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2.2 Transmission Media
2.2.3 Baseband Coaxial Cable
Use digital transmission. For 1-km cables, a data rate of 1 to
2 Gbps is feasible.
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2.2 Transmission Media
2.2.4 Broadband Coaxial Cable
Any cable network using analog transmission is called broadband.
One key difference between baseband and broadband is thatbroadband systems typically cover a large area and therefore need
analog amplifiers to strength the signal periodically.
These amplifiers can only transmit signals in one direction, so a
computer outputting a packet will not be able to reach computers
upstream from it if an amplifier lies between them.
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2.2 Transmission Media
2.2.4 Broadband Coaxial Cable
Dual cable system, all computers
transmit on one cable and receiveon the other.
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2.2 Transmission Media
2.2.4 Broadband Coaxial Cable
Single cable system, use
frequency division multiplexing
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2.2 Transmission Media
2.2.4 Broadband Coaxial Cable
Technically, broadband cable is inferior to baseband (i.e. single
channel) cable for sending digital data but has the advantage
that a huge amount of it is already in place.
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2.2 Transmission Media
2.2.5 Fiber Optics
Computing speed: a factor of 10 improvement per decade
Communication speed: a gain of more than a factor of 100 per
decade
In the race between computing and communication,
communication won. The new conventional wisdom should be
that all computers are hopelessly slow, and networks should tryto avoid computation at all costs, no matter how much
bandwidth that wastes.
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2.2 Transmission Media
2.2.5 Fiber Optics
An optical transmission system has three components: the light
source, the transmission medium, and the detector.
Light source: LED (Light Emitting Diode) or Laser (LightAmplification by Simulated Emission of Radiation)
Transmission Media: ultra-thin fiber of glass
Detector: using light-electricity effect, generate an electrical
pulse when light falls on it
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2.2 Transmission Media
2.2.5 Fiber Optics
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2.2 Transmission Media
2.2.5 Fiber Optics
How to avoid light leaking when transmitting in glass?
Signal propagates in different material (air, cable, or fiber, etc.).
speed in dielectric is less that in vacuum
signal energy is absorbed in dielectric
propagation speed in dielectric material=c
speed of light in
vacuum
refraction index
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2.2 Transmission Media
2.2.5 Fiber Opticsrefraction and reflection
incident
rayreflected ray
refracted ray
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2.2 Transmission Media
2.2.5 Fiber Optics
perpendicular light partially reflected total reflection
critical angle
qa
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2.2 Transmission Media
2.2.5 Fiber Optics
d2
d1 A
BC
D
ab
In time t, one light goes fromA toB,
another from CtoD. Therefore,
ABtc
dCD
tc
d
AB CB CD CB
AB
CD
d
d
1 2
2
1
, .
cos , cos .
cos
cos.
Moreover,
a b
a
b
When = 0, cos =b a
d
d
2
1
.
refraction and reflection
Snell's LawTotal reflection
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2.2 Transmission Media
2.2.5 Fiber Optics
critical angle
qa
Total reflection
sin = cos =q ad
d21
.
Remember: Total reflection only occurs when light goes
from large index to small index.
Medium 1Medium 2
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2.2 Transmission Media
2.2.5 Fiber OpticsMultimode fiber
cross section
core
cladding
protective coatingtwo propagation modes
2
1
1 2>
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2.2 Transmission Media
2.2.5 Fiber OpticsMultimode fiber
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2.2 Transmission Media
2.2.5 Fiber Optics
Multimode fiber
As a pulse of light travels through the fiber, the pulse of
light spreads out This phenomenon is known as dispersion.
input pulse output pulse
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2.2 Transmission Media
2.2.5 Fiber Optics
Multimode fiber
Dispersion limits the achievable bit rate over a fiber of a
given length. Conversely, given a bit rate, dispersion limits
how long the link can be.
spread-out will cause interference.
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One second
2.2 Transmission Media
2.2.5 Fiber OpticsMultimode fiber
Why does fiber have more bandwidth than coaxial cable?
Bits are more
crowded, not
faster.
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2.2 Transmission Media
2.2.5 Fiber Optics
Multimode fiber
To avoid interference, one must either lengthen the intervalbetween bits (reducing the signaling rate) or shorten the
fiber by inserting some type of communication device that
restores a clean pulse.
more components more expensive andless reliable
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2.2 Transmission Media
2.2.5 Fiber Optics
Single-mode fiber
If the fibers diameter is reduced to a few wavelengths of light,
the fiber acts like a wave guide, and the light can only
propagate in a straight line, without bouncing, yielding a
single-mode fiber.
Single-mode fibers are more expensive but can be used forlonger distances and have larger data rates (since it has no
dispersion).
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2.2 Transmission Media
2.2.5 Fiber Optics
Attenuation in
decibels =
10log10(transmitt
ed_power/received_power)
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2 h i l
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2.2 Transmission Media
2.2.5 Fiber Optics
Three wavelength bands are used for communication. They are
centered at 0.85, 1.30, and 1.55 microns, respectively.
The latter two have good attenuation properties (less than 5percent loss per kilometer).
The 0.85 micron band has higher attenuation, but the nice property
that at that wavelength, the lasers and electronics can be made
from the same material (gallium arsenide).
All three bands are 25,000 to 30,000 GHz wide.
Ex1: Find out why.
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2 Ph i l L
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2.2 Transmission Media
2.2.5 Fiber Optics
Can have many nodes and the link can be kilometers long.
2. Physical Layer
2 1 Th Th i l B i f D C i i
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2.1 The Theoretical Basis for Data Communication
2.2 Transmission Media
2.2.5 Fiber Optics
A passive star fiber network
# of nodes limited
by the sensitivity
of the photodiodes
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2.2 Transmission Media
2.2.5 Fiber Optics
Comparison of fiber optics and copper wire
Fiber Copper
Higher bandwidth
30km per repeater 5km per repeater
less interference
thin and light weight
quite difficult to tap
a familiar technology
cheaper interface
bi-directional
advantages
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Electromagnetic Waves
speed=frequency wavelength
m/s=cycles/s m/cycles
one cycle
Hz(hertz)
speed of light (in vacuum)=
3 108 m / s
2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
cf
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2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
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2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
cf 22
cf
c
d
dfSince , we have
Thus given the width of a wavelength band, we can computethe corresponding frequency band, and from that the data rate
the band can produce. The wider the band, the higher the data
rate.
The amount of information that an electromagnetic wave can carry
is related to its bandwidth. With current technology, it is possible
to encode a few bits per hertz at low frequencies, but often as
many as 40 under certain conditions at high frequencies.
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2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
To prevent total chaos, there are national and international
agreements about who gets to use which frequencies. Since
everyone wants a higher data rate, everyone wants more
spectrum.
Therefore, we have to share.
FDMA: Frequency Division Multiple Access
TDMA: Time Division Multiple Access
CDMA: Code Division Multiple Access
(using spread spectrum technique)
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2.3 Wireless Transmission
2.3.2 Radio Transmission
Radio waves are easy to generate, can travel long distance, and
penetrate buildings easily, so they are widely used for
communication, both indoors and outdoors.
Radio waves are also omnidirectional, meaning that they travel
in all directions from the source, so that the transmitter and
receiver do not have to be carefully aligned physically.
Omnidirectional waves sometimes can have undesired side effects.
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2 Physical Layer
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2.3 Wireless Transmission
2.3.2 Radio Transmission
In the VLF, LF, and MF bands, radio waves follow the
curvature of the earth.
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2 Physical Layer
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2.3 Wireless Transmission
2.3.2 Radio Transmission
In the HF they bounce off the ionosphere.
At height 100 to 500km
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2 Physical Layer
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2.3 Wireless Transmission
2.3.3 Microwave Transmission
Above 100 MHz, the waves travel in straight lines and can
therefore be narrowly focused. Concentrating all the energy into a
small beam using a parabolic antenna gives a much higher signal
to noise ratio.
Since the microwaves travel in a straight line, if the towers are
too far apart, the earth will get in the way. Consequently,
repeaters are needed periodically.
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2 Physical Layer
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2.3 Wireless Transmission
2.3.3 Microwave Transmission
Disadvantages:
do not pass through buildings well
multipath fading problem (the delayed waves cancel the signal)
absorption by rain above 8 GHz
severe shortage of spectrum
Advantages:
no right way is needed (compared to wired media)relatively inexpensive
simple to install
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2.3 Wireless Transmission
2.3.3 Microwave Transmission
ISM (Industrial/Scientific/Medical) Band
Transmitters using these bands do not require government
licensing. One band is allocated worldwide: 2.400-2.484 GHz. Inaddition, in the US and Canada, bands also exist from 902-928
MHz and from 5.725-5.850 GHz. These bands are used for
cordless telephones, garage door openers, wireless hi-fi speakers,
security gates, etc.
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2 Physical Layer
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2.3 Wireless Transmission
2.3.4 Infrared and Millimeter Waves
Unguided infrared and millimeter waves are widely used for
short-range communication. The remote controls used on
televisions, VCRs, and stereos all use infrared communication.
They are relatively directional, cheap, and easy to build, but
have a major drawback: they do not pass through solid objects.
This property is also a plus. It means that an infrared system inone room will not interfere with a similar system in adjacent
room. It is more secure against eavesdropping.
2. Physical Layer
2 Physical Layer
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Affected by fog or rain
2.3 Wireless Transmission
2.3.5 Lightwave Transmission
2. Physical Layer
2 Physical Layer
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2.4 The Telephone System
PSTN (Public Switched Telephone Network)
POTS( Plain Old Telephone System)
LAN connection versus dial-up telephone line connection
speed 107~108 104
error-rate 10-13 10-5
Much time and effort have been devoted to trying to figure out
how to use it efficiently.
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2 Physical Layer
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2. Physical Layer
2.4 The Telephone System
2.4.1 Structure of the Telephone System
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2. Physical Layer
2.4 The Telephone System
2.4.1 Structure of the Telephone System
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2. Physical Layer
2.4 The Telephone System
2.4.1 Structure of the Telephone System
Digital transmission between offices becomes possible.
Advantages:
signal can be perfectly regeneratedall kinds of data can be interspersed
higher data rates
much cheaper
maintenance is easier
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2. Physical Layer
2.4 The Telephone System
2.4.1 Structure of the Telephone System
In summary, the telephone system consists of three major
components:
1. Local loops (twisted pairs, analog signaling)
2. Trunks (fiber optics or microwave, mostly digital)
3. Switching offices
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2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
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2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Transmission Impairments
Transmission lines suffer from three major problems:
attenuation: loss of energy as the signal propagates outward
delay distortion: caused by the fact that different Fourier
components travel at different speeds
noise: unwanted energy from sources other than thetransmitter
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2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Since both attenuation and propagation speed are frequency
dependent, it is undesirable to have a wide range of
frequencies in the signal.
Unfortunately, square waves, as in digital data, have a wide
spectrum and thus are subject to strong attenuation and delay
distortion. These effects make baseband signaling unsuitableexcept at slow speed and over short distances.
Modulation
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2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Amplitude modulation
Modulation
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2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Frequency Modulation
(frequency shift keying)
Modulation
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2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
Phase Modulation
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y y
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
A device that accepts a serial stream of bits as input and
produces a modulated carrier as output (or vice versa) is called
a modem (MODulator/Demodulator).
To go to higher and higher speeds, it is not possible to just
keep increasing the sampling rate (signaling rate). All research
on faster modems is focused on getting more bits per sample(i.e. per baud)
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y y
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
3 bits/baud modulation
(A combination of AM and PM)
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y y
2.4 The Telephone System
2.4.3 The Local Loop
ModulationQAM (Quadrature Amplitude Modulation)
Used to transmit 9600 bpsover a 2400-band line.
A 4 bits/baud modulation
(ITU V.32 9600 bps modem standard)
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y y
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
Diagrams, which show the legal combinations of amplitude and
phase, are called constellation patterns. Each high-speedmodem standard has its own constellation pattern and can talk
only to other modems that use the same one (although most
modems cam emulate all the slower ones).
The next step above 9600 bps is 14400 bps. It is called V.32 bis.After V.32 bis comes V.34, which runs at 28800 bps.
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y y
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
A complete different approach to high-speed transmission is
to divide the available 3000-Hz spectrum into 512 tiny bands
and transmit at, say, 20 bps in each one. This scheme requires
a substantial processor inside the modem, but has the
advantage ofbeing able to disable frequency bands that
are too noisy.
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y y
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
Many modems now have compression and error correction
built into them. The big advantage of this approach is that these
features improve the effective data rate without requiring any
changes to existing software.
One popular compression scheme is MNP 5, which uses run-
length encoding to squeeze out runs of identical bytes.Another scheme is V.42 bis, which uses a Ziv-Lempel
compression algorithm.
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y y
2.4 The Telephone System
2.4.3 The Local Loop
Echo
The effect ofecho is that a person speaking on the telephone
hears his own words after a short delay. Psychological studies
have shown that this is annoying to many people.
To eliminate the problem, echo suppressors are installed on
lines longer than 2000km. (On short lines the echoes come
back so fast that people are not bothered by them.)
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y y
2.4 The Telephone System
2.4.3 The Local Loop
Echo
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y y
2.4 The Telephone System
2.4.3 The Local Loop
Echo
The echo suppressors have several properties that are
undesirable for data communication.
1. Full-duplex becomes half-duplex.
2. Echo suppressors are designed to reverse upon detecting
human speech, not digital data.
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2.4 The Telephone System
2.4.3 The Local Loop
Echo
To alleviate these problems, an escape hatch has been
provided on telephone circuits with echo suppressors. Whenthe echo suppressors hear a pure tone at 2100 Hz, they shut
down and remain shut down as long as a carrier is present.
The arrangement is one of the many examples ofin-band
signaling, so called because the control signals that activate
and deactivate internal control functions lie within the band
accessible to the user.
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2.4 The Telephone System
2.4.3 The Local Loop
Echo
An alternative to echo suppressors are echo cancelers. These
are circuits that simulate the echo, estimate how much it is, and
subtract it from the signal delivered, without the need for
mechanical relays. When echo cancelers are used, full-duplex
operation is possible. For this reason, echo cancelers are
rapidly replacing echo suppressors in the US and other
countries.
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2.4 The Telephone System
2.4.3 The Local Loop
RS-232-C and RS-449
EIA's (Electronics Industries Association) standard RS-232-C
(Recommended Standard 232 revision C) (similar standard:
CCITT V.24)
DTE
Data
Terminal
Equipment
DCE
Data
Circuit-
terminating
Equipment
terminal or computer modem or printer or terminal
or network
handshaking
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2.4 The Telephone System
2.4.3 The Local Loop
RS-232-C and RS-449
25 pins
1 13
14 25
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2.4 The Telephone System
2.4.3 The Local Loop
Fiber in the Local Loop
For advanced future services, such as video on demand, the 3-
kHz channel currently used will not do. Discussions aboutwhat to do about this tend to focus on two solutions.
FTTH: Fiber To The Home (too expensive)
FTTC: Fiber To The Curb (or Community)
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2.4 The Telephone System
2.4.3 The Local Loop
Fiber in the Local LoopFTTC using the telephone network
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2.4 The Telephone System
2.4.3 The Local Loop
Fiber in the Local Loop FTTC using the cable TV network
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2.4 The Telephone System
2.4.4 Trunks and Multiplexing
Frequency Division
Multiplexing
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2.4 The Telephone System
2.4.4 Trunks and Multiplexing
Wavelength
Division
Multiplexing
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2.4 The Telephone System
2.4.4 Trunks and Multiplexing
Although FDM is still used over copper wires or microwave
channels, it requires analog circuitry and is not amenable to
being done by computer. In contrast, TDM (Time Division
Multiplexing) can be handled entirely by digital electronics, so ithas become far more widespread in recent years.
Unfortunately, it can only be used for digital data. Since the
local loops produce analog signals, a conversion is needed
from analog to digital in the end office, where all theindividual local loops come together to be combined onto
outgoing trunks.
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2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
sampling and quantization
Sampling is the periodic measurement of the signal everyTseconds. These periodic measurements are called samples.
Quantization is the approximation of the possible values of
the samples by afinite set of (binary) values.
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2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
Nyquist's sampling theorem
A signal with maximum frequencyfmax can be recovered
exactly from samples that are measured more frequently
than 2fmax every second.
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(1) telephone voice (~4000 Hz)
8000 samples per second, every sample 8 bits=64kbps
(DPCM: differential PCM, only encode the differences
between samples)
(Predictive encoding)
(Delta Modulation: use only 1 bit to mean a difference of +1 or -1)(2) compact discs (~20KHz)
41000 samples per second, encoded in 16 bits, two channels
=1.3Mbps
2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
digitization of audio:
2. Physical Layer
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2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
2. Physical Layer
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2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)T1 carrier
2. Physical Layer
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2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
Multiplexing T1 streams onto higher carriers
2. Physical Layer
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2.4 The Telephone System
2.4.5 Switching
Circuit Switching
2. Physical Layer
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2.4 The Telephone System
2.4.5 Switching
Packet Switching
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2.4 The Telephone System
2.4.5 Switching
2. Physical Layer
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2.4 The Telephone System
2.4.5 Switching
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2.7 Cellular Radio
2.7.1 Paging Systems
Beepers
(one way) Mobile phones
(two ways)
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2.7 Cellular Radio
2.7.2 Chordless telephones
Chordless telephones started as a way to allow people to walk
around the house while on the phone.
Standard
CT-1 (analog)
CT-2 (digital)
In 1992, a third generation, CT-3 or DECT (digital European
CT), was introduced, which supported roaming over basestations.
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2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Cell structure andfrequency reuse
Handoff
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2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Microcells to increasefrequency reuse and
cheaper handset
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2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
The AMPS system uses 832 full-duplex channels, each
consisting of a pair of simplex channels. There are 832 simplex
transmission channels from 824 to 849 MHz and 832 simplex
receive channels from 869 to 894 MHz. Each of these simplex
channels is 30 kHz wide. Thus AMPS uses FDM to separate the
channels.
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2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Call Management
Each mobile telephone has a 32-bit serial number and 10-digit telephone number. The telephone number is
represented as a 3-digit area code, in 10 bits, and a 7-digit
subscriber number, in 24 bits.
When a phone is switched on, it scans a preprogrammed list of
21 control channels to find the most powerful signal. From the
control channel, it learns the numbers of the paging and access
channels.
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2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Call Management
The phone then broadcasts its 32-bit serial number and 34-bittelephone number in digital form. When the base station hears
the announcement, it tells the MTSO (Mobile Telephone
Switching Office), which records the existence of its new
customer and also informs the customers home MTSO of hiscurrent location. During normal operation, the phone
reregisters about every 15 minutes.
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2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Call Management
To make a call, a mobile user switched on the phone, enters thenumber to be called and hits the SEND button. The phone then
sends the number to be called and its own identity on the
access channel. If a collision occurs, it tries again later. When
the base station gets the request, it informs the MTSO. The
MTSO looks for an idle channel. If one is found, the channel
number is sent back on the control channel. The mobile phone
then automatically switches to the selected voice channel.
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2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Call Management
Incoming calls work differently. All idle phonescontinuously listen to the paging channel to detect
messages directed at them. When a call is placed to a mobile
phone, a packet is sent to the callees home MTSO to find
out where it is. A packet is then sent to the base station in
its current cell, which then sends a broadcast on the paging
channel. When it is answered, the base station tells the phone
to switch to a channel for connecting to the incoming call.
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2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Security Issues
Message easily tapped
Use stolen telephone number for calls
Damages to antennas and base stations
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2.7 Cellular Radio
2.7.4 Digital Cellular Telephones
AMPS
(backward compatible) IS-54 and IS-135
IS-95 (direct sequence spread spectrum)
In Europe, GSM (Global Systems for Mobile Communications)
Use 1.8 GHz band and both FDM and TDM. The available
spectrum is broken up into 50 200-kHz bands. Within each
band TDM is used to multiplex multiple users.
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2.7 Cellular Radio
2.7.5 Personal Communications Services
PCS will use cellular technology, but with microcells, perhaps
50 to 100 meters wide. The allows very low power (1/4 watt),
which makes it possible to build very small, light phones. On
the other hand, it requires many more cells than the 20-kmAMPS cells. If we assume that a microcell is 1/200th the
diameter of an AMPS cell, 40,000 times as many cells are
required to cover the same area.
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2. Physical Layer
2 8 C i i S lli
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2.8 Communication Satellites
downlink channeluplink channel
Contain several transponders.
Properties:
1. Longer delay
2. Broadcast in nature
3. Bad security4. Deployment is fast
2. Physical Layer
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2.8 Communication Satellites
2.8.1 Geosynchronous Satellites
Keplers Law3
2
2
2
3
1
2
1
R
T
R
T
Near the surface of the earth, the period is about 90 minutes.Communication satellites at such low altitudes are problematic
because they are within sight of any given ground station for
only a short time interval.
Earth
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2.8 Communication Satellites
2.8.1 Geosynchronous Satellites
However, at an altitude of approximately 36,000 km above the
equator, the satellite period is 24 hours, so it revolves at the
same rate as the earth under it. Having the satellite fixed in the
sky is extremely desirable, because otherwise an expensivesteerable antenna would be needed to track it.
With current technology, it is unwise to have satellites spaced
much closer than 2 degrees in the 360-degree equatorial plane,to avoid interference. So there are only 180 geosynchronous
satellites in the sky at once.
2. Physical Layer
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2.8 Communication Satellites
2.8.1 Geosynchronous Satellites
Fortunately, satellites using different parts of the spectrum do
not compete, so each of the 180 possible satellites could have
several data streams going up and down simultaneously.
Alternately, two or more satellites could occupy one orbit slotif they operate at different frequencies.
To prevent total chaos in the sky, there have been international
agreements about who may use which orbit slots andfrequencies.
2. Physical Layer
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2.8 Communication Satellites
2.8.1 Geosynchronous Satellites
Commercial bands for satellites
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2.8 Communication Satellites
2.8.1 Geosynchronous Satellites
A new development in the communication satellite world is
the low-cost microstations, sometimes called VSATs (Very
Small Aperture Terminals). These tiny terminals have 1-
meter antennas and can put out about 1 watt of power. Theuplink is generally good for 19.2 kbps, but the downlink is
more, often 512 kbps.
2. Physical Layer
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2.8 Communication Satellites
Either the sender or the receiverhas a large antenna and a power
amplifier. The trade-off is a longer
delay in return for having cheaper
end-user stations.
Communication
between VSATs
2.8.1 Geosynchronous Satellites
2. Physical Layer
2 8 Comm nication Satellites
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2.8 Communication Satellites
2.8.2 Low-Orbit Satellites
Motorolas Iridium Project
(77 LOS original, later
revised to 66)
Filed for bankruptcy recently.
Operate in the L band, at 1.6 GHz,
making it possible to communicate
with a satellite using a small
battery-powered device.
2. Physical Layer
2 8 Communication Satellites
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2.8 Communication Satellites
2.8. Satellites versus Fiber
Niche for satellites
1. Bypass local loop
2. Mobile communications
3. Broadcasting4. Hostile terrain or a poorly developed terrestrial infrastructure
5. Obtaining the right of way for laying fiber is difficult
6. Rapid deployment
2. Physical Layer
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Exercises: 1, 3, 14, 49