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INTRODUCTION:Data Communication &

Networking

Mahan Institute of Technologies,New Delhi

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Organization

•Communication

• Signal Representation

•Analog Vs. Digital

•Digital Transmission

•

Channel Capacity•Data Communication System

•Networking Overview

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Simple Communication Model

Source Destination

Transmission

media

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Terminology

• Transmitter

• Receiver

• Medium

– Guided medium

• e.g. twisted pair, optical fiber

– Unguided medium

• e.g. air, water, vacuum

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Example

client program server

communications

channel

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Communication System

Source Modulator Demodulator Sink/Destination

Carrier

Noise

Transmission

media

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Signal Types

• Time domain concepts

– Continuous signal

• Various in a smooth way over time – Discrete signal

• Maintains a constant level then changes to another constant

level

– Periodic signal

• Pattern repeated over time

– Aperiodic signal

• Pattern not repeated over time

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Frequency, Spectrum and

BandwidthSignal bandwidth

(f upper -f lower )

Frequency

Amplitude

(or power)

f lower f upper

3 dB

Frequency

Attenuation (dB)

Signal bandwidth

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Bandwidth

3 dB

Frequency

Attenuation (dB)

Signal bandwidth

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Continuous & Discrete Signals

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Periodic Signals

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Sine Wave

• Peak Amplitude (A) – maximum strength of signal

– volts

• Frequency (f) – Rate of change of signal

– Hertz (Hz) or cycles per second

– Period = time for one repetition (T)

– T = 1/f

• Phase (φ ) – Relative position in time

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Varying Sine Waves

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Time domain Frequency domain

Vmax

f

T (1/f)

(1/T)

-V max

0

V max

AmplitudeAmplitude

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Wavelength

• Distance occupied by one cycle

• Distance between two points of corresponding

phase in two consecutive cycles∀ λ

• Assuming signal velocity v

λ

= vT λ f = v

– c = 3*108ms-1(speed of light in free space)

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Frequency Domain Concepts

• Signal usually made up of many

frequencies

• Components are sine waves

• Can be shown (Fourier analysis) that any

signal is made up of component sine waves

• Can plot frequency domain functions

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Addition of Frequency Components

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Frequency Domain

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Spectrum & Bandwidth

• Spectrum – range of frequencies contained in signal

• Absolute bandwidth – width of spectrum

• Effective bandwidth

– Often just bandwidth

– Narrow band of frequencies containing most of the energy• DC Component

– Component of zero frequency

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Signal with DC Component

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Data Rate and Bandwidth

• Any transmission system has a limited band

of frequencies

• This limits the data rate that can be carried

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Analog and Digital Data

Transmission

• Data – Entities that convey meaning

• Signals – Electric or electromagnetic representations of

data

•Transmission – Communication of data by propagation and

processing of signals

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Data

• Analog

– Continuous values within some interval

– e.g. sound, video

• Digital

– Discrete values

– e.g. text, integers

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Acoustic Spectrum (Analog)

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Signals

• Means by which data are propagated

• Analog –

Continuously variable – Various media• wire, fiber optic, space

– Speech bandwidth 100Hz to 7kHz

– Telephone bandwidth 300Hz to 3400Hz

– Video bandwidth 4MHz

• Digital – Use two DC components

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Data and Signals

• Usually use digital signals for digital data

and analog signals for analog data

• Can use analog signal to carry digital data

– Modem

• Can use digital signal to carry analog data

– Compact Disc audio

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Analog Signals Carrying Analog

and Digital Data

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Digital Signals Carrying Analog

and Digital Data

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Analog Transmission

• Analog signal transmitted without regard to

content

• May be analog or digital data

• Attenuated over distance

• Use amplifiers to boost signal

• Also amplifies noise

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Digital Transmission

• Concerned with content

• Integrity endangered by noise, attenuation etc.

• Repeaters used• Repeater receives signal

• Extracts bit pattern

• Retransmits• Attenuation is overcome

• Noise is not amplified

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Advantages of Digital Transmission• Digital technology

– Low cost LSI/VLSI technology

• Data integrity

– Longer distances over lower quality lines

• Capacity utilization

– High bandwidth links economical

– High degree of multiplexing easier with digital techniques

• Security & Privacy

– Encryption

• Integration

– Can treat analog and digital data similarly

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‘1’ ‘0’

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0 1 1 0 1 1 1 1 1 0 0 0 1

0 1 1 0 1 1 1 1 1 0 0 0 1

Bit streamto be

transmitted

Manchester

encoding

Extracted

receiver

clock

Receiver

bit stream

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noisedigital signal digital signal

+ noise

digitalthreshold

-digitalthreshold

+

comparator

+

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0 1 1 0 0 1 0 0 1 1 1 1

0V

+V

-V

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0 1 1 0 0 0 0 0 0 1 1 1

0V

+V

-V

No timing information

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1111111000100011000010101

Analogue

Signal

DAC output

PCM

Slope overload

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00000001001000110100010101100111

10001001101010111100110111101111

11101101

11000000

00010001

11101101

11000000

00010001

11101101

1100Encodeddata

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000

001

010

011

100

101

110

111

Digital code

000

001

010

011

101

110

111

Digital code

FSFS/2FS/4 3FS/4

FS - Full scale voltage

Voltage

Unipolar coding Bipolar coding

100

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Softspeech

Loudspeech

Quantization noise

Quantization noise

Quantization noise lessnoticeable because signalstrength swamps quantization

noise

Quantization noise noticeable

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Time domainFrequency domain

V max

f

T (1/f)

(1/T)

-V max

0V max

AmplitudeAmplitude

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1

0.3

0.2

1/T 3/T 5/T f

T

0.9

-0.9

0

Time domain Frequency domain

V

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Transmission Impairments

• Signal received may differ from signaltransmitted

• Analog - degradation of signal quality• Digital - bit errors

• Caused by

– Attenuation and attenuation distortion

– Delay distortion – Noise

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Attenuation

• Signal strength falls off with distance

• Depends on medium

• Received signal strength:

– must be enough to be detected

– must be sufficiently higher than noise to be received without error

• Attenuation is an increasing function of frequency

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Delay Distortion

• Only in guided media

• Propagation velocity varies with frequency

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Noise (1)

• Additional signals inserted between transmitter

and receiver

• Thermal – Due to thermal agitation of electrons

– Uniformly distributed

– White noise

• Intermodulation

– Signals that are the sum and difference of original

frequencies sharing a medium

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Noise (2)

• Crosstalk

– A signal from one line is picked up by another

• Impulse – Irregular pulses or spikes

– e.g. External electromagnetic interference

– Short duration

– High amplitude

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Channel Capacity

• Data rate

– In bits per second

– Rate at which data can be communicated

• Bandwidth

– In cycles per second of Hertz

– Constrained by transmitter and medium

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Data Communication Model

Modem

DCE

Modem

DCE

Computer

DTE

Computer

DTE

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Model vs. Example

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Key Communications Tasks

• Transmission System Utilization

• Interfacing

• Signal Generation

• Synchronization

• Exchange Management

• Error detection and correction

• Addressing and routing

• Recovery

• Message formatting

• Security

• Network Management

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Si lifi d D t C i ti

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Simplified Data Communications

Model

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DTE-DCE Interface

Transmit (T)

Control (C)

Receive (R)

Indication (I)

Signal timing

GND

Data Circuit

Termination

Equipment (DCE)

Data Termination

Equipment (DTE)

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Terminology

• Simplex

– One direction

• e.g. Television

• Half duplex

– Either direction, but only one way at a time

• e.g. police radio

• Full duplex – Both directions at the same time

• e.g. telephone

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Modes of Communication

Simplex

communications

Half-duplex (HDX)

communications

Full-duplex (FDX)

communications

or

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Communication Ports

9-pin D-type

male connector (COM1:)

9-pin or 25-pin D-type

male connector (COM2:)

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Transmitter Receiver

Receiver reads frombuffer

receiver buffer

transmittedcharacters

transmitter buffer

Receiver hasfailed to read thebuffer before another character has been

received

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Are you readyto receive ?

No

CTS

RTS

YesCTS

TD

Sendcharacter

Transmittingnode

CTS

RTS

RD

RTS

Receivingnode

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01100010 01001111 10100111 00100011

‘F’ ‘r’ ‘e’ ‘d’

First bit sent

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+12V

-12V

1 10 0010Inactive condition

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1 1 0 1 0

ASK

PSK

FSK

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Tx Character Tx Character Tx Character

Inactive condition

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0 b0 b1 P S1 S2 1

startbit

ASCIIcharacter

paritybit

stopbit(s)

0 1 0 0 0 0 0 1 1 1 1

‘A’ (100 0001)

b2 b3 b4 b5 b6

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B0

B1

B2

B3

Even/Odd ParityParity

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Startbit 0 1 1 0 1 0 1

Stop

bit

ASCII ‘V’ 101 0110Baud rate: 9600 baud

+12 V

-12 V

120 S

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+12V

-12V

1 10 0010Inactive condition

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113

14

23456789101112

1516171819202122232425

Pin Signal2 TxData3 RxData4 RTS5 CTS6 DSR7 GND20 DTR

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Transmission Media

• Wired

– Twisted pair, Coaxial cable, FO

• Wireless – Short range

• Infrared

– Long Range• Microwave

• Satellite

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Electromagnetic spectrum

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Electromagnetic spectrum(its uses for communication)

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Transmission Media

Metal sheathInner conductor

Insulating outer conductor

Coaxial

cable

Twisted-pair

cable

Inner cladding(glass)Inner fibre

(glass)

Outer cladding(PVC)

Fibreopticcable

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Terminology

• Direct link

– No intermediate devices

• Point-to-point – Direct link

– Only 2 devices share link

• Multi-point – More than two devices share the link

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Telephone Network

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Cable TV Network

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Suggested Reading

• William Stallings

Data and Computer Communications

chapter 3• B. Forouzan

Data Communication & Networks

chapter 2

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