The Physical Layer

1
Chapter 2 The Physical Layer
node 1
node 2
transmission medium
bits
bits
electromagnetic waves light
electric current
transmitter
receiver
2
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 of this
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
where f1/T is 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
Root mean square amplitude
4
2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
A binary signal to be transmitted
5
2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
One harmonic
Two harmonics
6
2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
Four harmonics
Eight harmonics
7
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 to some frequency fc
cycles/secHertz(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 the encoding 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 of b 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. This restriction
means that the number of the highest harmonic
passed through is 3000/(b/8) or 24000/b roughly.
10
2.1 The Theoretical Basis for Data Communication
2.1.2 Bandwidth-Limited Signals
11
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 rate2Hlog2V bits/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. The amount 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/N is given. These units
are called decibels (dB).
Shannons Theorem
Maximum number of bits/secHlog2(1S/N)
For telephone line 3000log2(130dB)?30000bps.
13
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 bandwidth710008/86400648
Mbps
Cost of 1000 tapes5000. 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.
14
2. Physical Layer
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.
15
2. Physical Layer
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.
16
2. Physical Layer
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. Physical Layer
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 that broadband 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.
18
2. Physical Layer
2.2 Transmission Media
2.2.4 Broadband Coaxial Cable
Dual cable system, all computers transmit on one
cable and receive on the other.
19
2. Physical Layer
2.2 Transmission Media
2.2.4 Broadband Coaxial Cable
Single cable system, use frequency division
multiplexing
20
2. Physical Layer
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. Physical Layer
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 try to avoid computation at
all costs, no matter how much bandwidth that
wastes.
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2. Physical Layer
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
(Light Amplification 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
23
2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
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2. Physical Layer
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

speed of light in vacuum
propagation speed in dielectric material
refraction index
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2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
refraction and reflection
incident ray
reflected ray
refracted ray
26
2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
perpendicular light partially reflected
total reflection
critical angle
q
a
27
2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
refraction and reflection
In time t, one light goes from A to B, another
from C to D. Therefore,
d2
D
C
b
B
a
d1
A
Snell's Law
Total reflection

28
2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
Total reflection
critical angle
q
Medium 1 Medium 2
a
Remember Total reflection only occurs when light
goes from large index to small index.
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2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
Multimode fiber
cross section
core
cladding
protective coating
two propagation modes
gt
30
2.2 Transmission Media
2.2.5 Fiber Optics
Multimode fiber
31
2. Physical Layer
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. Physical Layer
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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2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
Multimode fiber
Why does fiber have more bandwidth than coaxial
cable?
Bits are more crowded, not faster.
One second
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2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
Multimode fiber
To avoid interference, one must either lengthen
the interval between 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 and less reliable
35
2. Physical Layer
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 for longer distances and have larger data
rates (since it has no dispersion).
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2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
Attenuation in decibels 10log10(transmitted_powe
r/ received_power)
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2. Physical Layer
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 5
percent 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. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
Can have many nodes and the link can be
kilometers long.
39
2.1 The Theoretical Basis for Data Communication
2.2 Transmission Media
2.2.5 Fiber Optics
of nodes limited by the sensitivity of the
photodiodes
A passive star fiber network
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2. Physical Layer
2.2 Transmission Media
2.2.5 Fiber Optics
Comparison of fiber optics and copper wire
advantages
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
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2. Physical Layer
2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
Electromagnetic Waves
one cycle
speedfrequency
wavelength
m/scycles/s
m/cycles
Hz(hertz)
speed of light (in vacuum)
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2. Physical Layer
2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
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2. Physical Layer
2.3 Wireless Transmission
2.3.1 The Electromagnetic Spectrum
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.
, we have
Since
Thus given the width of a wavelength band, we can
compute the corresponding frequency band, and
from that the data rate the band can produce. The
wider the band, the higher the data rate.
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2. Physical Layer
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)
45
2. Physical Layer
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
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
2.3 Wireless Transmission
2.3.2 Radio Transmission
At height 100 to 500km
In the HF they bounce off the ionosphere.
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2. Physical Layer
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.
49
2. Physical Layer
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

50
2. Physical Layer
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. In
addition, 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.
51
2. Physical Layer
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 in one
room will not interfere with a similar system in
adjacent room. It is more secure against
eavesdropping.
52
2. Physical Layer
2.3 Wireless Transmission
2.3.5 Lightwave Transmission
Affected by fog or rain
53
2. Physical Layer
2.4 The Telephone System
PSTN (Public Switched Telephone Network) POTS(
Plain Old Telephone System)
LAN connection versus dial-up
telephone line connection speed 107108
104 error-rate
10-13 10-5
Much time and effort have been devoted to trying
to figure out how to use it efficiently.
54
2. Physical Layer
2.4 The Telephone System
2.4.1 Structure of the Telephone System
55
2. Physical Layer
2.4 The Telephone System
2.4.1 Structure of the Telephone System
56
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 regenerated
• all kinds of data can be interspersed
• higher data rates
• much cheaper
• maintenance is easier

57
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
58
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
59
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 the
transmitter
60
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
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
unsuitable except at slow speed and over short
distances.
61
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
To get around the problems associated with
digital signaling, analog signaling is used. A
continuous tone in the 1000 to 2000 Hz range,
called a sine wave carrier is introduced. We vary
the carrier to represent different signal (data).
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2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
Amplitude modulation
63
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
Frequency Modulation (frequency shift keying)
64
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
Phase Modulation
65
2. Physical Layer
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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2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
3 bits/baud modulation (A combination of AM and
PM)
67
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Modulation
QAM (Quadrature Amplitude Modulation)
Used to transmit 9600 bps over a 2400-band line.
A 4 bits/baud modulation (ITU V.32 9600 bps modem
standard)
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2. Physical Layer
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-speed modem 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.
69
2. Physical Layer
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 of being able to disable frequency
bands that are too noisy.
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2. Physical Layer
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.
71
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Echo
The effect of echo 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.)
72
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Echo
73
2. Physical Layer
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.
74
2. Physical Layer
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. When the 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 of
in-band signaling, so called because the control
signals that activate and deactivate internal
control functions lie within the band accessible
to the user.
75
2. Physical Layer
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.
76
2. Physical Layer
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
handshaking
terminal or computer
modem or printer or terminal or network
77
2. Physical Layer
25 pins
2.4 The Telephone System
13
1
2.4.3 The Local Loop
14
25
RS-232-C and RS-449
78
2. Physical Layer
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 about what to do about this tend
to focus on two solutions. FTTH Fiber To The
Home (too expensive) FTTC Fiber To The Curb (or
Community)
79
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
Fiber in the Local Loop
FTTC using the telephone network
80
2. Physical Layer
2.4 The Telephone System
2.4.3 The Local Loop
FTTC using the cable TV network
Fiber in the Local Loop
81
2. Physical Layer
2.4 The Telephone System
2.4.4 Trunks and Multiplexing
Frequency Division Multiplexing
82
2. Physical Layer
2.4 The Telephone System
2.4.4 Trunks and Multiplexing
Wavelength Division Multiplexing
83
2. Physical Layer
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 it
has 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 the
individual local loops come together to be
combined onto outgoing trunks.
84
2. Physical Layer
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 every T seconds. These periodic
measurements are called samples. Quantization is
the approximation of the possible values of the
samples by a finite set of (binary) values.
85
2. Physical Layer
2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
Nyquist's sampling theorem
A signal with maximum frequency fmax can be
recovered exactly from samples that are measured
more frequently than 2fmax every second.
86
2. Physical Layer
2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
digitization of audio
(1) telephone voice (4000 Hz) 8000 samples
per second, every sample 8 bits64kbps
(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
87
2. Physical Layer
2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
88
2. Physical Layer
2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
T1 carrier
89
2. Physical Layer
2.4 The Telephone System
2.4.4 Trunks and Multiplexing
PCM (Pulse Code Modulation)
Multiplexing T1 streams onto higher carriers
90
2. Physical Layer
2.4 The Telephone System
2.4.5 Switching
Circuit Switching
91
2. Physical Layer
2.4 The Telephone System
2.4.5 Switching
Packet Switching
92
2. Physical Layer
2.4 The Telephone System
2.4.5 Switching
93
2. Physical Layer
2.4 The Telephone System
2.4.5 Switching
94
2. Physical Layer
2.7 Cellular Radio
2.7.1 Paging Systems
Beepers (one way)
Mobile phones (two ways)
95
2. Physical Layer
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
base stations.
96
2. Physical Layer
2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Cell structure and frequency reuse
Handoff
97
2. Physical Layer
2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Microcells to increase frequency reuse and
cheaper handset
98
2. Physical Layer
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.
99
2. Physical Layer
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.
100
2. Physical Layer
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-bit telephone 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 his current location. During normal
operation, the phone reregisters about every 15
minutes.
101
2. Physical Layer
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 the number 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. Physical Layer
2.7 Cellular Radio
2.7.3 Analog Cellular Telephones
AMPS (Advanced Mobile Phone System)
Call Management
Incoming calls work differently. All idle phones
continuously 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.
103
2. Physical Layer
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
104
2. Physical Layer
2.7 Cellular Radio
2.7.4 Digital Cellular Telephones
(backward compatible) IS-54 and IS-135 IS-95
(direct sequence spread spectrum)
AMPS
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.
105
2. Physical Layer
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-km AMPS 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
2.7 Cellular Radio
2.7.5 Personal Communications Services
Some telephone companies have realized that their
telephone poles are excellent places to put the
toaster-sized base stations, since the poles and
wires already exist, thus greatly reducing the
installation costs. These small base stations are
sometimes called telepoints.
107
2. Physical Layer
2.8 Communication Satellites
Contain several transponders.
Properties 1. Longer delay 2. Broadcast in
nature 3. Bad security 4. Deployment is fast
downlink channel
uplink channel
108
2. Physical Layer
2.8 Communication Satellites
2.8.1 Geosynchronous Satellites
Keplers Law
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. Physical Layer
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
expensive steerable 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.
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2. Physical Layer
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 slot if they operate at different
frequencies.
To prevent total chaos in the sky, there have
been international agreements about who may use
which orbit slots and frequencies.
111
2. Physical Layer
2.8 Communication Satellites
2.8.1 Geosynchronous Satellites
Commercial bands for satellites
112
2. Physical Layer
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. The uplink is
generally good for 19.2 kbps, but the downlink is
more, often 512 kbps.
113
2. Physical Layer
2.8 Communication Satellites
2.8.1 Geosynchronous Satellites
Communication between VSATs
Either the sender or the receiver has a large
antenna and a power amplifier. The trade-off is a
longer delay in return for having cheaper
end-user stations.
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2. Physical Layer
2.8 Communication Satellites
2.8.2 Low-Orbit Satellites
Motorolas Iridium Project (77 LOS original,
later revised to 66)
Operate in the L band, at 1.6 GHz, making it
possible to communicate with a satellite using a
small battery-powered device.
Filed for bankruptcy recently.
115
2. Physical Layer
2.8 Communication Satellites
2.8. Satellites versus Fiber
Niche for satellites 1. Bypass local loop 2.
Mobile communications 3. Broadcasting 4. Hostile
terrain or a poorly developed terrestrial
infrastructure 5. Obtaining the right of way for
laying fiber is difficult 6. Rapid deployment
116
2. Physical Layer
Exercises 1, 3, 14, 49