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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