# Data and Computer Communications

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```					 Data and Computer
Communications
Chapter 3 – Data Transmission

Ninth Edition
by William Stallings
Data Transmission

What we've got here is failure to communicate.

Paul Newman in Cool Hand
Luke
Data Transmission

•   quality of the signal
being transmitted

The successful
transmission of   •   characteristics of the
data depends on       transmission medium
two factors:
Transmission Terminology
Data transmission occurs between transmitter
and receiver over some transmission medium.

Communication                        Unguided
is in the form of   Guided
media
electromagnetic     media
(wireless)
waves.
twisted pair,
air, vacuum,
coaxial cable,
seawater
optical fiber
Transmission Terminology
Transmission Terminology
 Simplex

   signals transmitted in one direction
• eg. Television

 Half   duplex
   both stations transmit, but only one at a time

 Full   duplex
   simultaneous transmissions
• eg. telephone
Frequency, Spectrum and
Bandwidth
Time Domain Concepts

 analog signal
• signal intensity varies smoothly with no breaks
 digital signal
• signal intensity maintains a constant level and
then abruptly changes to another level
 periodic signal
• signal pattern repeats over time
 aperiodic signal
• pattern not repeated over time
Analog and Digital Signals
Periodic
Signals
Sine Wave
(periodic continuous signal)
   peak amplitude (A)
   maximum strength of signal
   typically measured in volts
   frequency (f)
   rate at which the signal repeats
   Hertz (Hz) or cycles per second
   period (T) is the amount of time for one repetition
   T = 1/f
   phase ()
   relative position in time within a single period of signal
Varying Sine Waves
s(t) = A sin(2ft +)
Wavelength ()
the wavelength of
a signal is the
distance occupied
by a single cycle

can also be stated as the   especially when v=c
distance between two
points of corresponding    • c = 3*108 ms-1
phase of two consecutive      (speed of light in
cycles              free space)

assuming signal               or
velocity v, then the
wavelength is related       equivalently
to the period as  = vT         f = v
Frequency Domain Concepts
 signals are made up of many frequencies
 components are sine waves
 Fourier analysis can show that any signal
is made up of components at various
frequencies, in which each component is a
sinusoid
 can plot frequency domain functions
Frequency
Components
(T=1/f)

c is sum of f & 3f
Frequency
Domain
Representations

 frequency domain
function of Fig 3.4c
 frequency domain
function of single
square pulse
Spectrum & Bandwidth
Signal with dc Component
Data Rate and Bandwidth
any transmission              this limits the data
system has a                  rate that can be
limited band of                 carried on the
frequencies              transmission medium

square waves
limiting
most energy in           have infinite
bandwidth
first few           components and
creates
components            hence an infinite
distortions
bandwidth

There is a direct relationship between
data rate and bandwidth.
Analog and Digital Data
Transmission
 data
   entities that convey information
 signals
   electric or electromagnetic representations of
data
 signaling
   physically propagates along a medium
 transmission
   communication of data by propagation and
processing of signals
Acoustic Spectrum (Analog)
Analog and Digital
Transmission
Digital Data

Examples:

IRA                  Text

Character
strings
of Digital Signals
Audio Signals
 frequency range of typical speech is 100Hz-7kHz
 easily converted into electromagnetic signals
 varying volume converted to varying voltage
 can limit frequency range for voice channel to
300-3400Hz
 to produce a video signal a TV
camera is used
 USA standard is 483 lines per
Video      frame, at a rate of 30 complete
Signals     frames per second
   actual standard is 525 lines but 42
lost during vertical retrace
 horizontal scanning frequency is
525 lines x 30 scans = 15750 lines
per second
 max frequency if line alternates
black and white
 max frequency of 4.2MHz
Conversion of PC Input to
Digital Signal
Analog Signals
Digital Signals
Analog and
Digital
Transmission
Transmission Impairments
 signal
transmitted causing:
   analog - degradation of signal quality
   digital - bit errors
 most   significant impairments are
   attenuation and attenuation distortion
   delay distortion
   noise
attenuation across             strength must be:
the band of                •strong enough to be
frequencies used               detected
coils or amplifiers.            without error

Strength can be
increased using
amplifiers or
repeaters.

ATTENUATION
 signal strength falls off with distance over any
transmission medium
 varies with frequency
Attenuation Distortion
Delay Distortion
 occurs  because propagation velocity of a
signal through a guided medium varies
with frequency
 various frequency components arrive at
different times resulting in phase shifts
between the frequencies
 particularly critical for digital data since
parts of one bit spill over into others
causing intersymbol interference
Noise
unwanted signals
inserted between
transmitter and

is the major limiting
factor in
communications
system performance
Categories of Noise

Intermodulation noise

• produced by nonlinearities in the
intervening transmission medium
• effect is to produce signals at a
frequency that is the sum or
difference of the two original
frequencies
Categories of Noise
Crosstalk:
   a signal from one line is
picked up by another
   can occur by electrical
coupling between nearby
twisted pairs or when
Impulse Noise:                              microwave antennas pick
up unwanted signals
   caused by external
electromagnetic interferences
   noncontinuous, consisting of
irregular pulses or spikes
   short duration and high
amplitude
   minor annoyance for analog
signals but a major source of
error in digital data
Channel Capacity
Nyquist Bandwidth
In the case of a channel that is noise free:
 if rate of signal transmission is 2B then can carry
signal with frequencies no greater than B
   given bandwidth B, highest signal rate is 2B
 for binary signals, 2B bps needs bandwidth B Hz
 can increase rate by using M signal levels
 Nyquist Formula is: C = 2B log2M
 data rate can be increased by increasing signals
   however this increases burden on receiver
   noise & other impairments limit the value of M
Shannon Capacity Formula
   considering the relation of data rate, noise and
error rate:
   faster data rate shortens each bit so bursts of noise
corrupts more bits
   given noise level, higher rates mean higher errors
 Shannon developed formula relating these to
signal to noise ratio (in decibels)
 SNRdb=10 log10 (signal/noise)
 capacity C = B log2(1+SNR)
   theoretical maximum capacity
   get much lower rates in practice
Summary
 transmission    concepts and terminology
   guided/unguided media
 frequency, spectrum and bandwidth
 analog vs. digital signals
 data rate and bandwidth relationship
 transmission impairments
   attenuation/delay distortion/noise
 channel   capacity
   Nyquist/Shannon
hannon developed formula relating these to
signal to noise ratio (in decibels)
 SNRdb=10 log10 (signal/noise)
 capacity C = B log2(1+SNR)
   theoretical maximum capacity
   get much lower rates in practice
Summary
 transmission    concepts and terminology
   guided/unguided media
 frequency, spectrum and bandwidth
 analog vs. digital signals
 data rate and bandwidth relationship
 transmission impairments
   attenuation/delay distortion/noise
 channel   capacity
   Nyquist/Shannon
enuation/delay distortion/noise
 channel   capacity
   Nyquist/Shannon

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