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Contents

1. The CommunicationsProcess

2. PrimaryCommunicationResources

3. Sources of Information

4. CommunicationNetworks

5. CommunicationChannels

6. Modulation Process

7. Analog and DigitalTypes ofCommunication

8. Shannons Information

Capacity Theorem

9. A DigitalCommunicationProblem

10. Historical Notes


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The Communications Process

1. The generation of a message signal: voice, music,picture, or computer data

2. The description of that message signal with a certainmeasure of precision, by a set of symbols: electrical,

aural, or visual.3. The encoding of these symbols in a form that is suitablefor transmission over a physical medium of interest

4. The transmission of the encoded symbols to the desireddestination

5. The decoding and reproduction of the original symbols6. The re-creation of the original mesage signal, with a

definable degradation in quality: the degradation iscaused by imperfections in the system


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Two Basic modes of communication

1. Broadcasting, which involves the usedof single powerful transmitter andnumerous receivers that are relatively

inexpensive to build.

2. Point-to-point communications, inwhich the communication process

takes places over a link between asingle transmitter and a receiver.


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Elements of communication system

Source of

information

Transmitter

Channel

ReceiverUser of

information

Communication System

Messagessignal

Transmittedsignal

Receivedsignal

Estimate ofmessage

signal


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Primary Communication Resources

1. Transmitted power, is the average power of thetransmitted signal.

2. Channel bandwidth, is the band of frequenciesallocated for the transmission of the message signal

3. The average voice spectrum extend beyond 10 kHz,though most of the average power is concentratedin the range of 100 to 600 Hz, and a band from 300to 3100 Hz gives good articulation

4. Noise refers to unwanted waves that tend to disturbthe transmission and processing of messagesignals in a comunications system

5. Signal-to-noise ratio (SNR) is the ratio of theaverage signal power to the average noise power,measured in the same point, express in dBs.


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Sources of Information

1. A source of information (Speech,Music, Pictures, Computer data) maybe characterized in terms of the signal

that cariers the information.2. A signal is defined as a single-valued

function of time that plays the role ofthe independent variable; at everyinstant of time, the function has aunique value.


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Speech

Is the primary method of humancommunication. Specifically, the processinvolves the transfer of information from a

speaker to a listener, in three successivestages:

1. Production, in the speakers mind: speech

signal.

2. Propagation, through the air at a speed300m/s

3. Perception, incoming sound: received

message


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Music

1. Originates from instruments such as thepiano, violin, and flute.

2. A melodie structure consisting of a time

sequence of sounds3. A harmonic structure consisting of a set of

simultaneous sounds

4. Musical signal demand a much widerchannel bandwidth than speech signal fortheir transmission


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Pictures

1. Relies on human visual system for itsperception, can be dynamic (TV) or static(facsimile)

2. The pictures in motion are converted intoelectrical signals to facilitate their transportfrom the transmitter to the receiver. To doso, each complete picture is sequentiallyscanned, carried out in a TV camera.

3. In color TV, the perception of color is basedon the three types of color receptor in thehumans eye: red (570 nm), green (535 nm),and blue (445 nm).


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Computer data

The text transmitted by PC is usually encodedusing the American Standard Code forInformation Interchange (ASCII). Each

character is represented by seven data bits(binary digit) constituing a unique binarypattern made up of 0s and 1s.

0

b1

b2

b3

b4

b5

b6

b7

b8

Startbit

Paritybit

Stopbit

Data bit

High

Low

Idle

1


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Data transmission

1. Asynchronous transmission, the text prepared on aPC and then transmitted over a communicationchannel with a single character being sent at a time.

2. Synchronous transmission, in which a wholesequence of encoded characters is sent over thechannel in one long transmission.

3. The multiplexed stream of data so formed is thenapplied to a modem (modulator-demodulator) for

the purpose of transmission over the channel.


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Data compression

Another way in which we use computer is todownload compressed forms of text, audio,and video data from a service provider at a

remote location.1. Lossless compression (data compaction),

operates by removing the redundantinformation.

2. Lossy compression involves the loss ofinformation in a controlled manner, acompression ratio higher than that attainablewith lossless compression.


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Standard compression algorithms

JPEG (Joint Photographic Experts Groups) image codingstandard is designed to compress full-color or grayscale imagesof natural, real-world scenes by exploiting known limitations ofthe human visual system

MPEG (Motion Photographic Experts Groups) -1/ video coding

standard is designed to compress video signal at 30 frames persecond (fps) into bit streams running at the rate of 1.5 megabitsper second

MPEG-1/audio coding standard is based on perceptual coding,which is waveform-preserving process; that is the amplitude-time wave form of the decoded audio signal closelyapproximates that of the original audio signal.


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The design of a data network

1. A network in which the hosts are all made up ofcomputers and terminals, may proceed in an orderlyway by looking at the network in terms of a layeredarchitecture, regarded as a hierarchy of nested layers.

2. Layer refers to a process or device inside a computersystem, design to performed a specific function.

3. At the system level, a user views the layer as a blackbox that is described in terms of inputs, outputs, and thefunctional relation between outputs and inputs

4. In layer architecture, each layer regards the next lowerlayer as one or more black boxes with some givenfunctional specification to be used by the given higherlayer


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OSI (Open System Interconnection) 1/3International Organization for Standardization

1. The design of data network, may proceed in anorderly way by looking at the network in terms oflayers architecture, regarded as hierarchy of nestedlayers. Each layer regards the next lower layer asone or more black boxes with some given

functional specification to be used by the the givenhigher layer.

2. Layers refers to a process or device inside acomputer systems, designed to performed a specificfunction.

3. At the system level, a user views the layer as ablack box that is described in terms of inputs,outputs, and the functional relation between outputsand inputs.


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1. The term Open, refers to the ability of any twosystems conforming to the reference model and itsassociated standards to interconnect

2. In OSI reference model, the communications andrelated-connection functions are organized as aseries of layers, or levels, with well-definedinterfaces, and with each layers built on itpredecessors.

3. In particular, each layer performs a related subset ofprimitive functions, and it relies on the next lowerlayer to perform additional primitive functions


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Layer End-user X End-user Y

Layer 7 protocol7

5

4

6

3

2

1

Application

Presentation

Session

Transport

Network

Data linkcontrol

Physical

DLC

Physical

DLC

Physical

Application

Presentation

Session

Transport

Network

Data linkcontrol

Physical

Network

Physical link Physical link

Subnet node System BSystem A

Layer 3protocol

Layer 3protocol

Layer 2protocol

Layer 2protocol

Mechanical, electrical,functional, and procedural

Provision of access to the OSI

Error control for the reliable

transfer of information

Transformation of the input datato provide service selected

Provision of the control

End to end control of themessage exchange

Routing of the packets andflow control design

Virtual communication

Physical communication


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Communication Channels

1. Telephone network

2. A coaxial cable (inner and outer conductor)

3. An optical fiber (dielectric wave guide)

4. Wireless broadcast channel (radio and TV)5. A mobile radio channel (encompass

terrestrial situation where a radio transmitteror receiver is capable of being moved)

6. A satellite channel (broad-area coverage)


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Modulation Process

1. To deliver a message signal, the transmittermodifies into a form suitable for transmissionover the channel, known as modulation, whichinvolves varying some parameter of carrier

wave in accordance with the message signal.2. The receiver re-creates the original message

signal from a degraded version of thetransmitted signal after propagation through the

channel, known as demodulation, which is thereverse of the modulation process used intransmitter.


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Continuous-wave modulation

A sinusoidal wave is used as the carrier:

1. Amplitude modulation (AM), the amplitude ofthe carrier is varied in accordance with the

message2. Angle modulation, the angle of the carrier is

varied

3. Frequency modulation (FM), the

instantaneous frequency of the carrier isvaried

4. Phase modulation (PM), the instantaneous

phase of the carrier is varied


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Pulse modulation

The carrier consists of a periodic sequence ofrectangular pulses.

1. In analog pulse modulation, the amplitude,

duration, or position of a pulse is varied inaccoerdance with sample values of themessage signal

2. PAM (Pulse-amplitude modulation), PDM

(Pulseduration modulation) , and PPM(Pulse-position modulation)


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Pulse-code modulation (PCM)

1. The standard digital forme of pulsemodulation.

2. PCM starts out essentially as PAM:

The amplitude of each, modulatedpulse (sample of the original messagesignal) is quantized or roundedd off tothe nearest value in a prescribed set ofdiscrete amplitude level and thencoded into a corresponding sequenceof binary symbols


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Digital communication system

Source ofinformation

Sourceencoder

Channelencoder

Modulator

Channel

Sourcedecoder

Demodulator

User ofinformation

Message signal

Estimate of

message signal

Transmitter Receiver

Source code word

Channel code word Estimate of channel code word

Received signal

Waveform

Channeldecoder

Estimate of source code word


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Analog and Digital Types of

Communication1. Encode/modulate the message signal generated by

the source of information, transmit it over thechannel, and produce an estimate of it at thereceiver output that satisfies the requirments of theend user

2. The design of a digital communication system israther complex in conceptual terms but easy tobuild, moreover, the system is robust, offeringgreater tolerance of physical effects than its analogcounterpart.

3. The design of an analog communication system issimple in conceptual term but difficult to buildbecause of stringent requirements on linearity andsystem adjusment.


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Other advantages of digital

communication 1/2

1. Relatively inexpensive digital circuits

2. Privacy is preserved by using data encryption

3. Greater dynamic range (the diference between

the largest and smallest values) is possible4. Data from voice, video, and data source may

be merged and transmitted over a commondigital transmission system

5. In long distance system, noise does notaccumulate from repeater to repeater


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Other advantages of digital

communication 2/2

6. Error in detected data may be small, even whenthere is large amount of noise on the receivedsignal

7. Error may often be corrected by the use ofcoding

Disadvantages of digital

communication:

1. Generally, more bandwidth is required than thatfr analog systems

2. Synchronize is required


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Shannons Information Capacity

Theorem

1. The mesage signal is delivered to the userboth efficiently and reliably, subject tocertain design constraints: allowabletransmit power, available channel

bandwidth, and affordable cost of buildingthe system.

2. In the case of digital comunication system,reliability is commonly expressed in terms ofbit error rate (BER) or probability of bit errormeasured at the receiver output.


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The information capacity theorem

The maximum rate at which information can betransmitted across the channel without error;its measured in bits per second (b/s)

C = B log2 (1 + SNR)B: the channel bandwidth

C: the information capacity of the channel


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Elements of a digital Communication

system (transmitter, channel, receiver)

XMessagesignal m(t)

Carrier waveAc cos (2 fc t)

Transmittedsignal s(t)

(a)

Noise w(t)

Channel output(received signal) x(t)

Transmittedsignal s(t)

+

+

(b)

XReceivedsignal x(t)

Local carriercos (2 fc t)

dtDecisionmakingdevice

Say 1 if yT > 0

Otherwise, say 0

Threshold = 0

Correlator

yT

(c)


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A Digital Communication Problem 1/4

1. The issue of the receiver determining whether abinary symbol sent over a noisy channel is decodedin error or not is of fundamental importance to thedesign of digital communication systems

2. Suppose a random signal, m(t), consisting ofsymbols 1 and 0. Symbol 1 is represented by aconstant level +1, and symbol 0 is represented by aconstant level -1, each of which lasts for duration T.

Such a signal may represent the output of digitalcomputer or digitized version of speech signal.

3. To facilitate the transmision, we employ a simplemodulation scheme known as phase-shift keying.


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4. Specifically, the information bearing signal m(t) ismultiplied by sinusoidal carrier wave Ac cos(2 fct)where Ac is the carrier amplitude, fc (multiple of 1/T)is the carrier frequency and t is time with 0 t T

5. The output of the transmitter (figure: Elements ...) isdefined by:

s(t) = Ac cos(2 fct) for symbol 1

-Ac cos(2 fct) for symbol 0

where Ac is the carrier amplitude, fc (multiple of 1/T)is frequency and t is time with 0 t T


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XMessage

signal m(t)

Carrier waveAc cos (2 fc t)

Transmitted

signal s(t)

(a)

Noise w(t)

Channel output

(received signal) x(t)

Transmitted

signal s(t)+

+

(b)

XReceivedsignal x(t)

Local carrier

cos (2 fc t)

dt

Decisionmakingdevice

Say 1 if yT > 0

Otherwise, say 0

Threshold = 0

Correlator

yT

(c)

6. Elements of a digital Communication system(Block diagram of transmitter, channel, and receiver)


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7. The channel is assumed to be distorsionless butnoisy, the receive signal is defined by:

x(t) = s(t) + w(t)

where w(t) is the additive channel noise

8. The receiver concist of a correlator followed by adecision-making device. The correlator output is:

yT = + Ac/2 + wT for symbol 1

- Ac/2 + wT for symbol 0, where

wT is the contribution of the correlator outputdue to the channel noise w(t), the correlatoroutput yT is compared against a threshold of zerovolts by the DMC (decision-making device)


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Some basic issues

From the Fourier analysis we find that the time-bandwidthproduct of a pulse signal is constant. This means thatthe bandwidth of a rectangular pulse of duration T isinversely proportional to T. The transmitted signal infigure consists of the product of this rectangular signaland the sinusoidal carrier Ac cos (2 fc t). Themultiplication of signal by a sinusoid has the effect ofshifting the Fourier transform of the signal to the rightby fc and to the left by an equal amount, except for thescaling factor of . It follows therefore that the

bandwidth of the transmitted signal m(t), and thereforethe required channel bandwidth, is inverselyproportional to the reciprocal of the symbol duration T.For the problem at hand, the reciprocal of T is also thesignaling rate of the system in b/s


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Theoritical & practical considerations

Theoritical issues:1. What is the

justification for thereceiver structur?

2. How do we relate thestatistics of therandom variable W tothe statistical

characteristic of thechannel noise?

3. What is theprobability ofdecision errors?

Practical issues:1. How do we choose a

modulation scheme?

2. How do we design

the channel to comevery close toShannonsinformation capacity

theorem?3. How do we

synchronize thereceiver to thetransmitter?


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Propagation of radio frequencies

1. Ground wavePropagation (Below2 MHz)

2. Sky-wavePropagation (2-30MHz)

3. Line of Sight (LOS)Propagation (Above30 MHz)


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Frequency Bands 1/4

FrequencyBand

Designation PropagationCharacteristics

Typical Uses

3-30 kHz Very lowfrequency(VLF)

Ground wave; lowattenuation day n night;high atmospheric noise

level

Long-range navigation;submarinecommunication

30-300 kHz Low F (LF) Similar to VLF, slightly lessreliable; absorption indaytime

Long-range navigationand marinecommunication radiobeacons

300-3000kHz Medium F(MF) Ground wave and night skywave; attenuation low atnight and high in day;atmospheric noise

Maritime radio,direction finding, andAM broadcasting


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Frequency Bands 3/4

FrequencyBand

Designation Propagation Characteristics Typical Uses

1.0-2.0

2.0-4.0

3-30GHz

L

S

Superhigh

frequency(SHF)

LOS propagation; rainfall attenuationabove 10 GHz, atmosphericattenuation because of oxygen andwater vapor, high water vaporabsorption at 22.2 GHz

Satellitecommunication, radar

microwavelinks

4.0-8.0

8.0-12.0

12.0-18.0

18.0-27.027.0-40.0

26.5-40

C

X

Ku

KKa

R

30-300GHz

Extremelyhigh freq

(EHF)

Same; high water-vapor absorption at183 GHz and oxygen absorption at 60

ang 119 GHz

Radar,satellite,

experimental


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Frequency Bands 4/4

Frequency Band Designation PropagationCharacteristics

Typical Uses

27.0-40.0

26.5-40

33.0-50.040.0-75.0

75.0-110.0

110.0-300.0

103-107 GHz

Extremelyhigh

frequency(EHF)

LOS propagation; Opticalcommunications

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