Download 3.Data Transmission

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3. Data Transmission
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Contents
 Concept and Terminology
 Analog and Digital Data Transmission
 Transmission Impairments
 Asynchronous & Synchronous Transmission
Prof. Sang-Jo Yoo
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Concept and Terminology
 Transmission medium
 Guided - twisted pair, coaxial cable, optical fiber
 Unguided - air, vacuum, sea water
 Direct link
 the transmission path in which signals propagate directly from the
transmitter to the receiver with no intermediate devices
 Point-to-Point vs Multipoint transmission
 Point-to-point - Only two devices share the medium
 Multipoint - more than two devices share the same medium
Prof. Sang-Jo Yoo
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Concept and Terminology
 Communication Modes
 Simplex
 Data is transmitted in one direction only.
 Ex) pager, TV
A
B
A
Send and receive
Only one way at a time B
 Half-duplex
 Data is exchanged alternatively.
 Ex) Walkie-Talkie
 Duplex(or Full-duplex)
 data is exchanged in both directions simultaneously
 Ex) telephone
A
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Send
receive
simultaneously
B
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Frequency, Spectrum and Bandwidth
 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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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 ()
 Relative position in time
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Varying Sine Waves
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Wavelength
 Distance occupied by one cycle
 Distance between two points of corresponding phase in
two consecutive cycles

 Assuming signal velocity v
  = vT
 f = v
 c = 3*108 ms-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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All of the frequency
components are integer
multiple of one frequency
(fundamental frequency)
The period of the total signal
is equal to he period of the
fundamental frequency.
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Frequency
Domain
1.0
-X/2
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X/2
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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
 Signal bandwidth is limited by
the channel bandwidth
 Effect of channel bandwidth on
a digital signal
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Analog and Digital Data Transmission
 The terms analog and digital are used in three contexts
 Data
 entities that convey meaning
 Signal
 electric or electromagnetic encoding of data
 Transmission
 the communication of data by the propagation and processing of
signals
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Analog and Digital Transmission
 Data
 Analog Data take on continuous values
 Ex) acoustic data, video data
 Digital Data take on discrete values
 Ex) text or character string
 Signal
 Analog Signal is a continuously varying electromagnetic wave
 Ex) acoustic signal, video signal
 Digital Signal is a sequence of voltage pulses
 Ex) horizontal sync/blanking pulse in video signal, binary digital
signal
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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 and Digital Transmission
 Analog and Digital Data vs Signal
Analog Signal
Digital Signal
Two Alternatives : (1) signal Analog data are encoded using
occupies the same spectrum as a codec to produce a digital bit
the analog data; (2) analog data stream
Analog Data are encoded to occupy a different
portion of spectrum
Ex) Telephone
Digital Data
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Ex) CODEC
Digital data are encoded using a Two alternatives: (1) signal
modem to produce analog signal consists of two voltage levels
to represent the two binary
values; (2) digital data are
encoded to produce a digital
signal with desired properties.
Ex) Modem
Ex) Digital transmitter
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Analog and Digital Transmission
 Analog and Digital Signal vs Transmission
Analog Transmission
Is propagated through amplifiers;
same treatment whether signal is
used to represent analog data or
Analog Signal digital data.
Not Used
Digital Signal
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Digital Transmission
Assumes that the analog signal
represents digital data. Signal is
propagated through repeaters; at
each repeater, digital data are
recovered from inbound signal and
used to generate a new analog
outbound signal
Digital signal represents a stream
of 1s and 0s, which may represent
digital data or may be an encoding
of analog data. Signal
is
propagated through repeaters; at
each repeater, stream of 1s and 0s
is recovered from inbound signal
and used to generate a new digital
outbound signal
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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
 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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Transmission Impairments
 Generally, the received signal differs from the transmitted
signal due to various transmission impairments
 Kinds of impairments
 Attenuation and attenuation distortion
 Delay distortion
 Noise
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Attenuation
 The strength of a signal falls off with distance and this
reduction is generally logarithmic.
 Attenuation
introduces three
transmission
considerations
for
 a received signal must have sufficient strength so that the receiver
can detect and interpret the signal
 the signal must maintain a level sufficiently higher than noise
 Attenuation is an increasing function of frequency
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Delay Distortion
 It is caused by the fact that the velocity of propagation of a
signal through a guided medium varies with frequency.
 Various frequency components of a signal will arrive at the
receiver at different times.
 Delay distortion particularly corrupts digital signal by
causing intersymbol interference.
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Noise
 Four categories of noise
 Thermal noise
 caused by thermal agitation of electrons in a conductor
 cannot be eliminated
 Intermodulation noise
 caused by nonlinearity in the transmitter, receiver, or by intervening
transmission system
 Crosstalk
 caused by coupling between signal paths
 Impulse noise
 generated from various sources including external electromagnetic
disturbances, such as lightning and faults and flaws in the
communications system.
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Noise
 Effect of noise on a digital signal
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Channel Capacity
 Various impairments limit maximum data rate.
 Channel capacity - the rate at which data can be transmitted
over a given communication path under given condition
 Shannon’s formula
S

C  W log2 1  
 N
, where C : the capacity of the channel (bps)
W : the bandwidth of the channel (Hz)
S : the signal power
N : the noise power
Prof. Sang-Jo Yoo