# Computer Networks - Physical Layer computer phone system by benbenzhou

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```									                                                                                              Physical Layer

Computer Networks
Physical Layer                                                        Provide the means to transmit bits from sender to receiver
involves a lot on how to use (analog) signals for digital information
Overview
Paolo Costa                                                                 Theoretical background: signal transmission and Fourier analysis
costa@cs.vu.nl                                                               Transmission media wires and no wires
http://www.cs.vu.nl/∼costa                                                          Modulation techniques: the actual encoding (multiplexing, and
switching)
Vrije Universiteit Amsterdam

(Version April 23, 2008)

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Analog vs. Digital                                                                            Transmitting Signals (1/2)
We’re living in a digital world, meaning that we’d preferably want to
send digital (i.e. two-valued) signals through wires.
Wires are pretty much physical, meaning that Mother Nature will
probably impose a few constraints here and there.
Observation: Signals are not entirely transmitted through a wire as you
would expect:

The continuous signal might represent speech
The discrete signal might represent binary 1s and 0s

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Transmitting Signals (2/2)                                                                            Sin Wave
Deﬁnition
Effect of frequency-dependent transmission delays:

s(t) = Asin(2πft + φ)
The sine wave is the fundamental periodic signal. A general sine wave
Effect of frequency-dependent attenuation:                                                            can be represented by three parameters:
peak amplitude A - the maximum value or strength of the signal
over time; typically measured in volts.
frequency f - the rate [in cycles per second, or Hertz (Hz)] at which
the signal repeats. An equivalent parameter is the period (T) of a
signal, so T = 1/f.
Overall effect including noise:
phase φ - measure of relative position in time within a single
period of a signal, illustrated subsequently

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Sin Wave                                                                                              Fourier Analysis
Example                                                                                               Deﬁnition

A periodic function with period T (and frequency f = 1/T ) g(t) can be
written as:
∞                             ∞
1
g(t) =     c+         an sin(2πnft) +               bn cos(2πnft)
2
n=1                           n=1

Example: g(t) = n =1 2k1 sin[(2k − 1)t] (n is the number of
k     −1
harmonics we take into account)

(a) f = 1Hz, A = 1, φ = 0                           (c) f = 2Hz, A = 1, φ = 0
(b) f = 1Hz, A = 0.5, φ = 0                         (d) f = 1Hz, A = 1, φ = π/4
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Fourier Analysis                                                                                  Fourier Analysis
Example                                                                                           Demo

The transmission of the
ASCII character b:
01100010
Let’s play:
(a) A binary signal and
its root-mean-square                                                                                 http://www.phy.ntnu.edu.tw/ntnujava/index.php?
Fourier amplitudes                                                                                   topic=17
2    2
an + bn

(b)-(e) Successive
approximations to the
original signal

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Bandwidth                                                                                         Bandwidth
Demo

What does this all mean?
Digital signal transmission can be thought of as being constructed
as an inﬁnite number of periodic analog signals.
The quality of transmission is frequency dependent                                           Let’s play:
not all parts of the digital signal get through the wire as you would
expect.                                                                                     http://www2.egr.uh.edu/~glover/applets/Sampling/
Sampling.html
Digital signal transmission is subject to attenuation, distortion, etc.
This is partly caused by disallowing high-frequency components to
pass through.
The range of frequencies transmitted without being strongly
attenuated is called bandwidth

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Bandwidth                                                                                               Bandwidth
Example                                                                                                 Example

We are trying to transmit a single byte:
The transmission of the                                                                                    With a bit rate of b bits/sec, it takes 8/b seconds to send a byte.
ASCII character b:                                                                                                   i.e., 8/b seconds is the period T of the sin wave
01100010                                                                                                             The frequency f1 of the ﬁrst harmonic is b/8 Hz;
Assume maximum supported frequency is 3000 Hz.
(a) the original signal
since all harmonics are multiple of the ﬁrst harmonic, we have
(b) the signal that                                                                                              (roughly) 3, 000/(b/8) or 24, 000/b harmonics
results from a channel                                                                                           e.g., b = 300bps ⇒ f1 = 37.5Hz with 80 harmonics
that allows only the ﬁrst                                                                                                              bps   T (ms)                 f1     # har.
harmonic f1 to pass                                                                                                                 300      26.67         37.5               80
through                                                                                                                             600      13.33         75.0               40
(c)-(e) reconstructed                                                                                                              1200       6.67        150.0               20
2400       3.33        300.0               10
signals fro higher                                                                                                                 4800       1.67        600.0                5
bandwidth channel                                                                                                                  9600       0.83       1200.0                2
19200       0.42       2400.0                1
38400       0.21       4800.0                0
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Bandwidth                                                                                               Bandwidth
Example (contd)                                                                                         Encoding

bps   T (ms)                 f1   # har.                                      Improvement: If there are four signal values available, we could
300   26.67         37.5             80                                       encode 2 bits at a time:
600   13.33         75.0             40                                                                        00 → 0 volt 01 → 2 volt
10 → 4 volt 11 → 6 volt
1200    6.67        150.0             20
The number changes in a signal per second is called the baud.
2400    3.33        300.0             10
Example 1: A 2400 bauds line (modem) can make a bit rate of 9600 bps provided it
4800    1.67        600.0              5
uses 16 (24 ) signal values, i.e., 4 bits per signal:
9600    0.83       1200.0              2
19200    0.42       2400.0              1
S   bits     S   bits       S           bits      S     bits
38400    0.21       4800.0              0
0   0000     4   0100       8           1000      12    1100
Most telephone carriers cut off the highest frequency at 3000 Hz                                                                1   0001     5   0101       9           1001      13    1101
we can never transmit at 9600 bps (or at a higher speed) on the                                                            2   0010     6   0110       10          1010      14    1110
telephone line                                                                                                             3   0011     7   0111       11          1011      15    1111

Question                                                                                                Example 2: A 2400 bauds line (modem) can make a bit rate of 33,600 bps provided it
uses 16,384 (214 ) signal values, i.e., 14 bits per signal.
You gotta be kidding. How about my modem at 56 Kbps ? We are
assuming a simple encoding technique based on the fact that the                                         56 kbps modems use a 8,000 baud line (4000 Hz) with 8 bits per sample (1 bit is
reserved for control purpose)
line supports only two signal values.
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Nyquist & Shannon                                                                                 Nyquist & Shannon
Why cannot we increase the number of samples per seconds ?
Nyquist showed that if the cut-off frequency is H Hz, the ﬁltered
signal can be reconstructed by making 2H samples. No more, no                                     Shannon showed that a noisy channel with a signal-to-noise ration
less.                                                                                             S/N, has a limit with respect to the bit rate:
maximum transmission rate = 2H log2 V bps                                                        maximum transmission rate = H log2 (1 + S/N) bps
(where V is the number of signal values)                                                         e.g., A telephone line with H = 3000 and
http://www2.egr.uh.edu/~glover/applets/Sampling/                                                 10 log10 (S/N) = 30 dB, can do no better than 30 kbps, no matter
Sampling.html                                                                                    how you do your encoding (excluding compression).
e.g., H = 3, 000Hz and V = 2 (binary transmission over a                                         56 kbps is still possible because with H = 4000 and little noise but
telephone line) data rate cannot exceed 6000 bps                                                 no more than 70 kbps
Group Quorum System Properties

Shannon showed that a noisy channel with a signal-to-noise ration                             Question
S/N, has a limit with respect to the bit rate:
No way. My brand new ADSL goes at 4 Mbps! ADSL uses up to 224
maximum transmission rate = H log2 (1 + S/N) bps
4-kHz channels. With 15 bits/baud and 4000 baud, the downstream
e.g., A telephone line with H = 3000 and
bandwidth would be 13.4 Mbps (more details later on)
10 log10 (S/N) = 30 dB, can do no better than 30 kbps, no matter
how you do your encoding (excluding compression).
56 kbps is still possible because with H = 4000 and little noise but
no more than 70 kbps 02 - Physical Layer
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Transmission Media                                                                                Throughput vs. Latency
Magnetic Tape

Never underestimate the bandwidth of a station wagon full of
tapes hurtling down the highway

Take a standard videotape that can carry about 7 gigabytes of                                    Throughput is the rate at which information can be moved
data.
A box of 50 × 50 × 50 cm can hold about 1000 tapes, which                                        Latency is the amount of time between request and response
corresponds to 7000 gigabytes.
Sending such a box can be done within 24 hours, worldwide.
We’ve got a transmission rate of 648 Mbps!

Question
What is overlooked in this reasoning? We’re overlooking latency.

Paolo Costa                            02 - Physical Layer   Transmission Media   19 / 94        Paolo Costa               02 - Physical Layer         Transmission Media    20 / 94
Twisted Pair (TP)                                                                      Twisted Pair
Characteristics

Twisted pair: Two insulated copper wires, twisted like a DNA string
(reduces electrical inference). Often, twisted pairs go by the bundle.
Comparable to telephone wiring at home.                                                      limited distance
needs a repeater every 2-3km
limited bandwidth
1MHz
limited data rate
For long-distance, data rates of up to a few Mbps are possible; for
(a) category 3:                                                                           very short distances, data rates of up to 10 Gbps have been
traditional phone wires,                                                                  achieved in commercially available products.
10 Mbps Ethernet                                                                    susceptible to interference and noise
(b) category 5:
100Mbps Ethernet

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Coaxial Cable                                                                          Coaxial Cable
Characteristics
Coaxial cable, like twisted pair, consists of two conductors, but is
constructed differently to permit it to operate over a wider range of
frequencies

superior frequency characteristics to TP

performance limited by attenuation and noise

Coax is better than twisted pair when you need more bandwidth, but is                        ampliﬁers every few km
now rapidly being replaced with ﬁber.
bandwidth: up to 500MHz

Like the one you use

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Fiber Optics                                                                                   Fiber Optics
Principle                                                                                      Medium

Rather than using electrical signals, we use optical ones that are
passed through optical ﬁber. Principal working is based on the
refraction property of light:

An optical ﬁber is a thin (2 to 125
When a light ray passes from one medium to another (e.g., silica
to air) the ray is refracted at the boundary.                                                micron), ﬂexible medium capable of
guiding an optical ray
e.g., the ray incident at an angle α1 emerges at an angle β
Various glasses and plastics can be
For angle of incidence above a certain critical value, the light is
used to make optical ﬁbers.
refracted back into the silica
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Fiber Optics                                                                                   Fiber Optics
Attenuation                                                                                    Bandwidth

As it turns out, attenuation is extremely well in optical ﬁber. This
means that they can be used for long distances. In addition, the
bandwidth is enormous.

c        df
c =λ·f ⇒f =   λ   ⇒    dλ
c
= − λ2 ⇒ ∆f = − c·∆λ
λ2
The wider the range, and the shorter the wavelength, the higher
the bandwidth.
Fiber optics often work at λ = 1.3 × 10−6 with ∆λ = 0.17 × 10−6
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Fiber Connections                                                                     Optical Fiber vs Copper Wire (1/2)
An interface consists of a receiver (photodiode) which transforms light
into electrical signals, and/or a transmitter (LED or laserdiode)
Passive interface: A computer is directly connected to the optical
ﬁber
Active interface: There’s an ordinary electrical repeater connected
to two ﬁber segments and the computer:

The optical ﬁber cable in the foreground has the equivalent
information-carrying capacity of the copper cable in the background.

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Optical Fiber vs Copper Wire (2/2)                                                    Guided Transmission: Recap

Bandwidth:
Fiber can support enormous bandwidths, exactly what we need
with upcoming image-based applications (video-on-demand).
Attenuation:
Because the attenuation in ﬁber is less than in copper (can you
imagine why?), we don’t need to boost the signal as often. In
practice, ﬁber requires an active repeater every 30 km, copper
every 5 km.
External inﬂuences:
That’s right, no more interference from other cables, radios, power
failures, etc. Crosstalk (you hearing another conversation) is out
of the question.
Weight:
Fiber simply doesn’t weigh as much. Good for backs, bones, and
the use of heavy maintenance equipment.
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Guided Transmission: Recap                                                              Wireless Transmission (1/5)
Attenuation

Wireless transmission is really great for all of us who can’t sit still,
or feel they have to be on-line all the time.
It’s also convenient when wiring is needed where it can’t be done,
or isn’t really worth the trouble (jungles, islands, mountains), or
because it’s just user-unfriendly (homes).

signal carried in electromagnetic spectrum
no physical “wire”
bidirectional
propagation environment effects:
reﬂection
obstruction by objects
interference

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Wireless Transmission (2/5)                                                             Wireless Transmission (2/5)
Naming
Wireless transmissions travel at the speed of light (c), uses a
frequency (f ) which has a wavelength (λ):
c =λ·f
The larger the wavelength is, the longer the distance it can travel
without attenuation. Also, the dispersion of higher frequencies is much
lower.

The terms LF, MF, and HF refer to low, medium, and high
frequency.
Nobody expected to go above 10 MHz
The higher bands were later named the Very, Ultra, Super,
Extremely, and Tremendously High Frequency bands.
Question Costa
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Wireless Transmission (3/5)                                                              Wireless Transmission (4/5)

The amount of information that an electromagnetic wave can carry
Frequency hopping:
is related to its bandwidth
Use a wide band, but let the transmitter hop from frequency to
c · ∆λ                                                  frequency (hundreds of times per second). Good for avoiding
∆f = −                                                       continuous interference and reducing the effect of reﬂected
λ2
The wider the range, and the shorter the wavelength, the higher                          signals (you won’t be listening to them).
the bandwidth.
Direct sequence:
wireless transmission will generally have a much lower bandwidth
(in practice: 50-100 Mbps maximum).
Simply spread the signal over a wide frequency band (and allow
several signals with different encoding/modulation techniques to
Fiber optics operate in the high frequency range, which explains                         be transmitted simultaneously).
the transmission rates of gigabits per second.

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Wireless Transmission (5/5)                                                              Microwave transmissions (100 MHz - 10 GHz)
Microwave transmission is also popular for telecommunication systems
Observation: Radio transmission (VLF–VHF) is extremely popular for                          It is good for long distances (repeater
its cheapness and range. Also, waves just go all over the place.                            every 10-100 km) but requires
line-of-sight transmission:
unlike radio waves, they don’t pass
through buildings
they don’t tolerate the curvature of the
earth (e.g., San Francisco - Amsterdam)
they propagate in straight lines
high signal-to-noise ration
high frequency
the curvature of the earth                                                               Problem
the density in the spectrum requires higher frequency (up to 10
(b) In the HF band, they bounce off the ionosphere
GHz)
hard for unguided transmissions
waves are few centimeters long and are absorbed by rain
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Observation: Satellites are attractive because they provide a relatively
simple model of communication: one signal up can be broadcast to
Taking Mother Nature into account (i.e., avoiding belts around the
earth consisting of highly-charged particles that would destroy a
satellite), there are three types of satellites:

Some waves may be refracted and may take slightly longer to arrive
The delayed waves may arrive out of phase with the direct wave and
thus cancel the signal
it is weather and frequency dependent
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Geostationary Orbit Satellites                                                            Medium-Earth Orbit Satellites
Feature: GEO satellites are placed at 35,800 km above the earth
where their rotational speed is the same as that of the earth. The
effect is that they appear to remain motionless in the sky.
VSATs: Very Small Aperture Terminals – simple systems that output 1
Watt at 19.2 kbps but can download as much as 512 kbps. To allow the
VSATs to communicate with each other, hubs are used:
Example: The Global Positioning System (GPS) orbit at 18,000 km. It
takes about 6 hours for a satellite to circle the earth. They are not used
for telecommunications.

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Low-Earth Orbit Satellites (1/2)                                                           Low-Earth Orbit Satellites (2/2)
Essence: We throw in a relatively large number of low-orbit satellites
which jointly cover the surface of the earth; when you are out of your                     Alternative: In Globalstar, much of the complexity is handled by ground
current satellite’s spot beam, you should be in that of the next satellite                 stations that pick up a connection from a satellite, and pass it on to the
(Iridium):                                                                                 one closest to the receiver:

Note: Iridium uses 66 satellites, each having a maximum of 48 cells
(i.e., spot beams), totaling 1628 cells.                                                   Observation: This scheme avoids much of the complexity for
Observation: This approach is virtually the same as that of cellular                       (managing) inter-satellite communication.
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Where (not) to use Satellites                                                              The Local Loop
When it comes the telephone system, from a networking perspective
the local loop (a.k.a. the last mile) is the most interesting to look at.
Bandwidth:                                                                             The general structure is as follows:
bandwidth. Satellites may make it easier to transfer data anyway
Mobility and remote locations:
Satellites win, although it isn’t clear whether simple cellular
techniques may do just ﬁne
Fast and reliable:
Give credits to ﬁber: satellites are pretty bad due to inherent high
latency (230 ms round-trip for geostationary satellites), and too
much Mother Nature (rain!)

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Modulation Techniques (1/3)                                                             Modulation techniques (2/3)

Problem: How can we encode our signals when we can effectively
use only a single frequency (or better: small frequency range)?
Change the amplitude (strength) of the signal: changing amplitude
means a binary 1, constant amplitude a binary 0.
Use different frequencies to encode your bits (these frequencies
can be put “on top” of your base frequency).
Change the phase of the wave (cf. sine and cosine) to do signal
encoding.

(a) A binary signal                                  (c) Frequency modulation
(b) Amplitude modulation                             (d) Phase modulation
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Modulation techniques (3/3)                                                             Modem
Observation: Modulation is strongly related to not being able to set a
Modem derives from
(wide-frequency-ranges) DC signal value on the wire as direct                              modulator-demodulator)
encoding of binary signals:                                                                     modulates an analog carrier
signal to encode digital
information
demodulates such a carrier
signal to decode the transmitted
information
Used for digital data
transmission through the
public telephone network
becomes                                                        replacing the analog
infrastructure would be too
costly

Two different frequencies are
used to enable trafﬁc in both
direction (full duplex)
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Increasing Transmission Rates                                                             Let’s Recap...

To get to higher and higher speeds, it is not possible to just
increase the sampling rate
Nyquist says that even with a perfect 3000 Hz line, there is no point                  Bandwidth
in sampling more than 6000 Hz                                                          The range of frequency that pass through a given medium with
=⇒ most modems sample 2400 times / sec and focus on getting more                          minimum attenuation (measured in Hz)
bits per sample                                                                        Baud Rate
The number of samples per second is measured in baud                                      The number of samples (symbols) / sec made and is constrained
an n-baud line transmits n symbols / sec                                             by the bandwidth (Nyquist’s theorem). The modulation technique
(e.g., QPSK) determines the number of bits per symbol.
Baud is NOT the modem’s speed (but they are related)
Bit (Data) Rate
e.g., if only two symbols (e.g., 0 volt and 1 volt) are sent on a 2400               The amount of information sent over the channel during a second
baud line, the bit rate is 2400 bps
(= number of symbols / sec × number of bits / symbol)
if the voltages 0, 1, 2, and 3 volts are used, each symbol consists of
2 bits and the bit rate becomes 4800 bps
instead of using 4 different voltages, it is possible to use 4 phase
shifts [Quadrature Phase Shift Keying (QPSK)]

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Modulation Techniques                                                                     Modulation Techniques
All modern modems use a combination of modulation techniques to
All modern modems use a combination of modulation techniques to
transmit multiple bits per baud.
transmit multiple bits per baud.

4 amplitudes and 4 phases
(a) Quadrature Phase Shift Keying (QPSK)                                                           16 (24 ) possible combinations leading to 4 bits per symbol
The phase of a dot is indicated by the angle                                              used to transmit 9600 bps over a 2400 baud line
The amplitude is the distance from the origin                                    (c) QAM-64
4 phases (45, 135, 225, and 315 degrees) with constant amplitude                          8 amplitudes and 8 phases
2 bits / symbol =⇒ 4800 bps (assuming 2400 baud line)                                     64 (26 ) possible combinations leading to 6 bits per symbol
used to transmit 14400 bps over a 2400 baud line
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Higher Speeds                                                                                     Higher Speeds
56K modem
With many dots, even a small amount of noise can result in an erroneous
decode of phase / amplitude                                                                      Standard modems stop at 33,600 because the Shannon limit for a telephone
Add extra bits to perform error correction (Trellis Code Modulation)                             system is about 35 Kbps

(a) V.32
V.32 modem uses 32 (25 ) dots to transmit 4 data bits and 1 parity bit
9,600 bps with error correction at 2400 baud                                              A call from “Computer” to “ISP 1” goes through two local loops
rotation was done only for engineering reason
(b) V.32 bis
“ISP 2”, instead, has a digital trunk
V.32 bis modem uses 128 (27 ) dots to transmit 6 data bits and 1 parity bit
less noise =⇒ maximum data rate now becomes 70 kbps
14,400 bps with error correction at 2400 baud (fax modems)
V.90 modems use 8 bits per symbol (7 data bit and 1 control bit) over a 8,000
Higher speeds (28,800 and 33,600 bps) are achieved using 12 and 14 bits /                        baud line (assuming 4,000 Hz band for phone cable)
symbol (V.34 and V.34 bis)
56,000 bps in downstream
Further improvements are made by also using compression techniques                                     33,000 bps in upstream (using V.34 to reduce noise)
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Beyond 56 K                                                                                       Digital Subscriber Lines
Traditional telephone cables are artiﬁcially limited to a 3000 Hz
bandwidth
good enough to carry human voice
a ﬁlter in the end ofﬁce attenuates all frequencies below 300 Hz
Cable TV industry was offering speeds up to 10 Mbps and satellite                                      and above 3400 Hz
companies were planning to offer 50 Mbps                                                         With xDSL, the incoming line of a customer is connected to a
Telephone companies realized that they need to offer higher                                      different kind of switch, that does not have any ﬁlter
speeds to their customers to remain competitive                                                        the limiting factor then becomes the physics of the local loop
56 kbps were not appealing any longer
The new service went under the name of xDSL (Digital Subscriber
Line)
the most popular is ADSL (Asymmetric DSL)

The service can be offered only within near proximity
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Asymmetric DSL                                                                                  Asymmetric DSL
Basics                                                                                          Data Rate

The spectrum of the local loop is about 1.1 Mhz spectrum
We can divide it into 256 channels of 4,000 Hz each (like in
each one plays a different role
Within each channel a modulation scheme similar to V.34 is used
15 bits / baud on 4000 baud line
=⇒ with 224 downstream channels, the downstream bandwidth is
13.44 Mbps !
In practice, the signal-to-noise ratio is never good enough
but 8 Mbps is possible on short-runs over high quality loops

channel 1-5 are not used to avoid interference
channels are allocated to downstream
this explains the “A” of Asymmetric
different combinations are possible.
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Asymmetric DSL                                                                                  Asymmetric DSL
Architecture                                                                                    Hardware

4:    CPU                   5:    JTAG interface
6:    8 Mb RAM              7:    Flash memory
13:   Ethernet port         16:   USB port
17:   Telephone port

The NID (Network Interface Device) is a small plastic box, marking the border                  DSLAM
between company’s and customer’s property
The splitter is a ﬁlter that separates the voice band (0-4000 Hz) from the rest
The ADSL modem can be seen as 250 QAM modems operating in parallel at
different frequencies
The DSLAM (DSL Access Multiplexer) recovers the signal into a bit stream and
sends off to the ISP
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Asymmetric DSL                                                                             Wireless Local Loop
Problem: Suppose you want to start an ISP but using the local
The bandwidth of the local loop has been expanded to 2.2 MHz                               loop is out of the question because it is owned by your competitor.
ADSL2+ allows to a maximum of 25 Mbps, with a 3–7 km distance to be
Solution: Set up a wireless direct connection between one of your
crossed by copper.                                                                  antennas and your subscribers (in a so-called sector):
Maximum upload speed is 1.2–3.5 Mbps                                                         e.g., WiMax (IEEE 802.16 standard)
Highly dependent on the cable length

A sector can operate at 36 Gbps downstream bandwidth and 1
Mbps upstream, to be shared by subscribers.
The range is about 2–5 km.
Practice: single users get 10 Mbps over 10 km.
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Multiplexing: FDM                                                                          Multiplexing: FDM
Example
Problem: Considering that the bandwidth of a channel can be
huge, wouldn’t it be possible to divide the channel into                                   Frequency Division Multiplexing is used by Radio and TV
Frequency Division Multiplexing: Divide the available bandwidth                                                 Station                        Channel   Frequency
Nederland 1                    22        479,25
into channels through frequency ﬁltering, and apply modulation                                                  Nederland 2                    25        503,25
techniques per channel:                                                                                         Nederland 3
RTL 4
26
28
511,25
527,25
RTL 5                          36        591,25
SBS 6                          37        599,25
RTL 7                          38        607,25
Veronica / Jetix               39        615,25
Net 5                          40        623,25
Tien                           41        631,25
Casema Service Kanaal          29        535,25
TV West                        44        655,25
Eén                            42        639,25
Ketnet / Canvas                43        647,25
Discovery Channel              46        671,25
National Geographic / CNBC     47        679,25
Animal Planet                  48        687,25
Eurosport                      54        735,25
MTV                            50        703,25
Nickelodeon / The BOX          51        711,25
TMF                            53        727,25
BBC 1                          55        743,25
BBC 2                          56        751,25

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Multiplexing: WDM                                                                                Multiplexing: TDM
Basics

Wavelength Division Multiplexing: Actually the same as FDM, but
used for ﬁber optics.                                                                           Time Division Multiplexing: Simply merge/split streams of digital
data into a new stream. Data is handled in frames – a ﬁxed series
of consecutive bits:

Light waves have their own frequency range; they are simply
combined and separated using standard (de)fraction properties                                   This is a full-digital solution in contrast to FDM and WDM

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Multiplexing: TDM                                                                                Multiplexing: TDM
PCM                                                                                              T1 system

Analog signals need to be digitalized before being eligible for TDM                        Example: The T1 system samples at 8000 Hz, and encodes each
PCM (Pulse Code Modulation)                                                                sample as a 7-bit number (i.e. 128 different values). With some extra
control bits, we merge samples into 193-bit frames, every 125 µsec:

The magnitude of the signal is sampled regularly at uniform
intervals, then quantized to a series of symbols in binary code
But because the quantized values are only approximations, it is
impossible to recover the original signal exactly
1
8 bits / sample are used for digitalized voice on the phone                           Observation: T1 supports a total of 193 × 125 × 106 = 1.544 Mbps
16 bits / sample are used for Audio CD
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Multiplexing: TDM                                                                                     Switching
QoS                                                                                                   Packet vs. Circuit

TDM also makes it easy to offer individual senders higher                                             Circuit switching: Make a true physical connection from sender to
bandwidth, by simply putting more data into a frame:                                                  receiver. This is what happens in traditional telephone systems.
Packet switching: (1) Split any data (i.e. message) into small
packets, (2) route those packets separately from sender to
receiver, and (3) assemble them again.

or to combine several trunks into higher-bandwidth trunks:

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Switching                                                                                             Switching
Message Switching                                                                                     Comparison

Message (Store-and-forward) switching: a message is completely
received at a router, stored, and then put into an outgoing queue
for further routing
analogous to packet switching but here a message must be fully

Note
Check also what discussed in the Introduction.
(a) circuit-switching;
Paolo Costa                  (b) store-and-forward; (c) packet-switchingSystem
02 - Physical Layer           Telephone           75 / 94            Paolo Costa           02 - Physical Layer      Telephone System   76 / 94
The Mobile Telephone System                                                                   Advanced Mobile Phone System (AMPS)
Overview                                                                                      Overview

We will discuss the following technologies:
ﬁrst generation of mobile phones
analog voice
a.k.a. (E)TACS
GSM (Global System for Mobile communication)
digital voice
digital data (EDGE and GPSR)
The whole idea is to break up an area into small regional cells,
UMTS (Universal Mobile Telecommunication System)
each has their own frequency range
third generation (3G)                                                                         no two adjacent cells have the same frequency.
high speed                                                                              Pretty good for handling different densities
digital voice and data
cells may be split in sub-cells
The problem is frequency allocation and energy emission
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Advanced Mobile Phone System (AMPS)                                                           GSM
Channels                                                                                      Overview

GSM: Global System for Mobile communications, is a full-blown
The AMPS uses 832 full-duplex channels, each consisting of a                                  digital cellular radio transmission system
pair of simplex channels                                                                      FDM is used with each mobile transmitting on one frequency and
832 simplex channels from 824 to 849 Mhz                                                receiving on a higher one
832 simplex channels from 869 to 894 Mhz                                                      also, a single frequency pair is split by time-division multiplexing
each channel is 30kHz wide                                                                    into time-slots shared by multiple mobiles
Thus, FDM is used to separate channels
Channels are used for different tasks
control (base to mobile) to manage the systems
paging (base to mobile) to notify calls
access (bidirectional) for call setup and channel assignment
data (bidirectional) for voice
Since the same frequencies cannot be reused in nearby cells, the
cell, each channel multiplexed by TDM (GSM-900):
actual number of voice channels per cell is about 45
this gives 8 × 124 = 992 full duplex channels.
a lot of them are not used to avoid interference with neighboring cells
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GSM                                                                                          GSM
Channels                                                                                     Data Transmission

EDGE (Enhanced Data rates for GSM Evolution) is GSM with
more bits per baud.
There are also separate channels for:                                                                    more bits per baud also mean more errors per baud
EDGE introduces nine different schemes for modulation and error
broadcasting cell info (so that a mobile station can see whether it                                 detection
has changed cells).
GPSR (General Packet Radio Service) is an overlay packet
cell maintenance (the base station has to know who’s in its cell).
network on top of GSM
call setup (incoming, and outgoing).
it allows mobile stations to send and receive IP packets
when GPSR is operating, some time slots on some frequencies are
reserved for packet transmission
the number of these slots are dynamically managed by the
base-station according to the trafﬁc conditions

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CDMA                                                                                         CDMA
Overview                                                                                     Math

Code Division Multiple Access (CDMA) allows transmissions to be                                Let’s indicate with S the m−chip vector for station S and S for its
interleaved, but avoids interference.                                                          negation
no message collisions                                                                    Key: chip sequences S and T belonging to two stations S and T
Principle: assign a chip sequence to a station, which is just an                               must be orthogonal
m-bit code (each bit in this code is called chip)                                                   i.e., the normalized inner product must be 0
to transmit a 1 bit, a station sends its chip sequence                                                                                      m
1
to transmit a 0 bit, it sends the one’s complement of the chip                                                            S•T=                   Si Ti = 0
sequence                                                                                                                              m
i=1
e.g., assuming m = 8, if A is assigned the chip sequence
note that if S • T = 0, then also S • T = 0
00011011, it sends 1 by sending 00011011 and 0 by sending
the normalized product of any chip sequence with itself is 1:
11100100
m                      m                     m
Let’s introduce bipolar notation:                                                                                     1                    1                     1
S•S=             Si Si =               (Si )2 =              (±1)2 = 1
rewrite a binary 0 as -1, and a binary 1 as +1                                                                  m                    m                     m
i=1                   i=1                   i=1
so, 1 bit for station A becomes (−1 − 1 − 1 + 1 + 1 − 1 + 1 + 1)
also note that S • S = −1
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CDMA                                                                                             CDMA
Simultaneous Transmissions                                                                       Example

When two stations transmit at the same time, their signals add
linearly
remember that no FDM or TDM is used
e.g., if three stations output +1 and one station outputs -1, the result
is +2
To recover the bit stream of an individual station, say C, the
sequence and the chip sequence of C
e.g., assume two stations A and C transmitting a 1 bit at the same
time that B transmits a 0 bit
the receiver sees the sum S = A + B + C and computes:

S • C = (A + B + C) • C = A • C + B • C + C • C = 0 + 0 + 1 = 1

the ﬁrst two terms vanish thanks to the orthogonality

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CDMA                                                                                             CDMA
Example                                                                                          Analogy

Let’s make an analogy:
TDM is comparable to all people being in the middle of the room
but talking at different times
Hidden Assumptions                                                                               FDM is comparable to people being in widely separated groups,
the receiver must known the chip sequence                                                each group holding separate conversations but at the same time
power levels of all stations are the same as perceived by the                            CDMA is comparable to everybody being in the middle of the
receiver                                                                                 room, talking at the same time but in different languages
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CDMA                                                                                          UMTS
Bandwidth

The amount of information grows from b bits / sec to mb bits / sec
it runs a 5 MHz bandwidth and is compatible with GSM
This can be done only if the available bandwidth is increased by a                                it offers 2 Mbps for indoor stationary basestation, 384 kbps for
factor m                                                                                          people walking, and 144 kbps for connections in cars
CDMA uses a form of spread spectrum communication                                           It is supposed to deliver the following services:
e.g., if we have 1-MHz band available for 100 stations                                     High-quality voice transmissions
with FDM, each one would have 10 kHz and could send at 10 kbps                             Messaging (e-mail, fax, SMS, chat, ...)
(assuming 1 bit per Hz)                                                                    Multimedia (playing music, viewing videos, ﬁlms, televisions, ...)
with CDMA, each station uses the full 1 MHz, so the chip rate is 1                         Internet access
megachips / second (chip is a bit in the chip sequence=
if we use less than 100 bit per chip sequence, the effective                                          on mobile phones ?
bandwidth for CDMA is higher than FDM and the channel allocation
problem is solved

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Wi-Fi: IEEE 802.11g                                                                           Cable Television
OFDM

Frequency division multiplexing (FDM) transmits multiple signals
simultaneously over a single transmission medium                                            TV cable companies are
each signal travels within its own unique frequency range (carrier)                  actively working to increase
Orthogonal FDM’s (OFDM) distributes the data over a large
their market
number of carriers that are spaced apart at precise frequencies
the sub-carrier frequencies are chosen so that the sub-carriers are                  They use a mix of ﬁber (for the
orthogonal to each other                                                             long-haul runs) and coaxial
cable (to the house)
=⇒ HFC (Hybrid Fiber Coax)
Issue: every cable is shared
among multiple users (a) while
in the telephone systems
At the center frequency of any particular subcarrier all other                              every house has its own
subcarriers attain a null point                                                             private cable (b)
no guardbands required
no special ﬁlter to demodulate each frequency carriers
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Cable Television                                                                                Cable Television
Principle                                                                                       Modem

There’s a lot of unused bandwidth that can be allocated to sending                            Internet access require a cable modem
bits over the wire:                                                                           The downstream data always comes from one source, which
makes it easier to handle.
Upstream data requires that subscribers contend for available
slots:

Because downstream (television) starts at 54 MHz, there is limited
bandwidth that can be used for upstream data
most trafﬁc, however, is likely to be downstream (like in ADSL)
QAM-64 (or QAM-256 if the cable quality is really high) is used on
each downstream 6-Mhz channel
36 Mbps (of which 27 Mbps for payload) data rate                                     Each modem is assigned a minislot to use to request upstream
For upstream QAM-64 does not work well (too noisy)                                            bandwidth
QPSK is used                                                                                this may be subject to contention ⇒ retry mechanism
2 bits / baud instead of 6 or 8 bits of QAM                                          Security is also a concern
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