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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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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