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Page: 1© 2002 A.W. Krings 1CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
u Concepts and Terminology
– Transmission Terminology
» Transmission medium - the medium over which
communication occurs
» Guided Media - electromagnetic waves of transmission are
guided along a physical path (wires, cables.) (2x108m/s)
» Unguided Media - no physical guide (air, vacuum, seawater)
(Speed of light, i.e. 3x108m/s)
» Direct Link - a transmission path between two devices in which
signals propagate directly from transmitter of one to receiver
of the other with no intermediate devices (other thanamplifiers/repeaters)
Page: 2© 2002 A.W. Krings 2CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
» Point-to-point - if direct link is shared between only two
devices
» Multipoint - if direct link shared between multiple devices
» Simplex- one way transmission (commercial radio/TV)
» Half-duplex - one way transmission at a time, endpoints take
turns
» Full-Duplex - simultaneous two way transmission
» NOTE: These are US (ANSI) definitions, in Europe (CCITT)
simplex refers to half-duplex and duplex to full-duplex
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Page: 3© 2002 A.W. Krings 3CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
– Frequency, Spectrum and Bandwidth
» Time domain (examining the signal over time):
n Continuous signal - a signal that has no breaks or discontinuities
n Discrete signal - a signal that takes on only a finite number of
values
n Periodic signal - one which repeats after a period T
n Amplitude - the instantaneous value of a signal at any time, s(t) --
volts
n Frequency - inverse of the period (1/T) in cycles per second,
Hertz, Hz
n Phase - measure of relative position in time within a single period
Page: 4© 2002 A.W. Krings 4CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
» Frequency Domain (signal viewed as a function of frequency):
n if all components are integer multiples of one frequency, that
frequency is the fundamental frequency
n the period of total signal is period of fundamental frequency
n using Fourier analysis, any signal is made up of components at
various frequencies, each of which is a sinusoid, defined by S(f)
n Spectrum- the range of frequencies in a signal
n Absolute bandwidth - is the width of the spectrum (f n - f 1) where
f n is largest frequency in signal and f 1 is the smallest
n Effective bandwidth (bandwidth) - width of spectrum containing
most of the energy in the signal
n DC component - component of the signal of zero frequency
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Page: 5© 2002 A.W. Krings 5CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
u Basics
– periodic signal s(t + T) = s(t)
– general wave
» amplitude
» frequency
» phase
)2sin()( F+= ft At s p
Page: 6© 2002 A.W. Krings 6CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
u Periodic signals
Sta97 fig 2.3
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Page: 7© 2002 A.W. Krings 7CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
Sta97 fig 2.4
A sin(2pft + f)
Page: 8© 2002 A.W. Krings 8CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
u How does one represent a signal
– example
– add 2 frequencies
– frequency spectrum
)2sin(31)2sin()( 21 t f t f t s p p +=
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Page: 9© 2002 A.W. Krings 9CS420/520: Data Communications, Lecture 3
What does it take?What does it take?
u Want is a square wave?
– What frequency components are digital signalscomposed of?
– How many components do I need to recreate a squarewave?
– What is a realistic spectrum?
– Where is the main energy of the signal?
– This is a representation of a square wave withamplitude A:
s(t ) =A4
p
1
k sin(2p kft )
k odd ,k =1
•
Â
Page: 10© 2002 A.W. Krings 10CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
u Limited Bandwidth
– Fourier Analysis
v t a a n t b n t n
n
n
n
( ) cos sin= + +=
•
=
•
 Â01
01
0w w
aT
v t d t
aT
v t n t dt
b T v t n t dt
T
n
T
n
T
0
0
0
0
0
0
1
2
2
=
=
=
Ú
Ú
Ú
( )
( ) cos( )
( ) sin( )
w
w
v t
f
T f
( ) =
=
=
= =
voltage as a function of time
fundamental frequency component in radians / second
fundamental frequency in Hz
period in seconds
w 0
0
01
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Page: 11© 2002 A.W. Krings 11CS420/520: Data Communications, Lecture 3
Physical Aspects Physical Aspects
u Limited Bandwidth (cont.)
– Unipolar
– Bipolar
How much bandwidth do we need?What are the trade-offs if we compromise bandwidth?
v t V V
t t t ( ) {cos cos cos ...}= + - + -
2
2 1
33
1
550 0 0
p
w w w
v t V
t t t ( ) {cos cos cos ...}= - + -
4 1
33
1
550 0 0
p
w w w