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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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2.1.1 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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2.1.2 Bandwidth-Limited Signals

A binary

signal to be

transmitted


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One harmonic

Two harmonics


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Four harmonics

Eight harmonics


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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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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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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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2.1.3 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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2.1.3 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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2.2.2 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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2.2.2 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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2.2.3 Baseband Coaxial Cable

Use digital transmission. For 1-km cables, a data rate of 1 to

2 Gbps is feasible.

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2.2.4 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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Dual cable system, all computers

transmit on one cable and receiveon the other.

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Single cable system, use

frequency division multiplexing

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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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2.2.5 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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2.2.5 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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2.2.5 Fiber Optics

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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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2.2.5 Fiber Opticsrefraction and reflection

incident

rayreflected ray

refracted ray

2. Physical Layer


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2.2.5 Fiber Optics

perpendicular light partially reflected total reflection

critical angle

qa

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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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2.2.5 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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2.2.5 Fiber OpticsMultimode fiber

cross section

core

cladding

protective coatingtwo propagation modes

2

1

1 2>

2. Physical Layer


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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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2.2.5 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



Why does fiber have more bandwidth than coaxial cable?

Bits are more

crowded, not

faster.

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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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2.2.5 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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2.2.5 Fiber Optics

Attenuation in

decibels =

10log10(transmitt

ed_power/received_power)

2. Physical Layer

2 h i l


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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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2.2.5 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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2.2.5 Fiber Optics

A passive star fiber network

# of nodes limited

by the sensitivity

of the photodiodes

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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

2. Physical Layer

2 Ph i l L


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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

2. Physical Layer

2 Ph i l L


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2. Physical Layer

2 Ph i l L


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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.

2. Physical Layer

2 Ph i l L


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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)

2. Physical Layer

2 Ph i l L


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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.

2. Physical Layer

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In the VLF, LF, and MF bands, radio waves follow the

curvature of the earth.

2. Physical Layer

2 Physical Layer


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In the HF they bounce off the ionosphere.

At height 100 to 500km

2. Physical Layer

2 Physical Layer


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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.

2. Physical Layer

2 Physical Layer


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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

2. Physical Layer

2 Physical Layer


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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.

2. Physical Layer

2 Physical Layer


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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

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Affected by fog or rain


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.

2. Physical Layer

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2. Physical Layer


2.4.1 Structure of the Telephone System

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2. Physical Layer



2 Physical Layer


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2. Physical Layer



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



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


2.4.3 The Local Loop

2 Physical Layer


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2. Physical Layer



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



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


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2. Physical Layer



Amplitude modulation

Modulation

2. Physical Layer


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2. Physical Layer



Frequency Modulation

(frequency shift keying)

Modulation

2. Physical Layer


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2. Physical Layer



Modulation

Phase Modulation

2. Physical Layer


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y y



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)

2. Physical Layer


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y y



Modulation

3 bits/baud modulation

(A combination of AM and PM)

2. Physical Layer


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y y



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)

2. Physical Layer


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y y



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.

2. Physical Layer


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y y



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



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.

2. Physical Layer


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y y



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



Echo

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y y



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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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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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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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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RS-232-C and RS-449

25 pins

1 13

14 25

2. Physical Layer


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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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Fiber in the Local LoopFTTC using the telephone network

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Fiber in the Local Loop FTTC using the cable TV network

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2.4.4 Trunks and Multiplexing

Frequency Division

Multiplexing

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Wavelength

Division

Multiplexing

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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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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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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




digitization of audio:

2. Physical Layer


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2. Physical Layer


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PCM (Pulse Code Modulation)T1 carrier

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Multiplexing T1 streams onto higher carriers

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2.4.5 Switching

Circuit Switching

2. Physical Layer


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2.4.5 Switching

Packet Switching

2. Physical Layer


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2.4.5 Switching

2. Physical Layer


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2.4.5 Switching

2. Physical Layer


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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



Microcells to increasefrequency reuse and

cheaper handset

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2.7 Cellular Radio



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



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



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



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



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



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

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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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2.8.1 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

2. Physical Layer

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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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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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Commercial bands for satellites

2. Physical Layer

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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

2 8 C i ti S t llit


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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. Physical Layer

2 8 Comm nication Satellites


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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. 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

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Network Basic