Computer Networks

1. 5.1
Analog Transmission
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2. 5.2
5-1 DIGITAL-TO-ANALOG CONVERSION
Digital-to-analog modulation is the process of
changing one of the characteristics of an analog
signal based on the information in digital data.

3.  A sine wave is defined by three characteristics:
amplitude, frequency, and phase.
 When we vary anyone of these characteristics,
we create a different version of that wave.
 Any of the three characteristics can be altered in
this way, giving us at least four mechanisms for
modulating digital data into an analog signal:
 Amplitude shift keying (ASK), frequency shift
keying (FSK), and phase shift keying (PSK) ,
quadrature amplitude modulation (QAM).
5.3

4. 5.4
Figure 5.2 Types of digital-to-analog conversion

5. 5.5
Figure 5.1 Digital-to-analog conversion

6. Aspects of Digital-to-Analog Conversion
 Before we discuss specific methods of
digital-to-analog modulation, two basic
issues must be reviewed: bit and baud
rates and the carrier signal.
5.6

7. Bit rate and Baud Rate
 Bit rate: the number of bits transmitted in
a second
 Baud rate: the number of signal units per
second that is required to represent those
bits
 A signal unit is composed of one or more
bits
 The fewer the signals, the more efficient
the system and less bandwidth required
5.7

8.  Baud rate determines the bandwidth
required to send a signal
 Bit rate = baud rate x number of bits
represented by each signal unit
 Baud is always less than or equal to bit
rate
5.8

9.  A baud is analogous to a car and the bit to
a passenger
 A car can carry one or more passengers
 If 1000 cars travel, carrying one
passenger each, 1000 passengers are
transported
 BUT: A car can carry 4 pple; so 4000 pple
will be transported
 NOTE: the number of cars NOT
passengers determine the traffic
5.9

10. 5.10
Bit rate is the number of bits per second.
Baud rate is the number of signal
elements per second.
In the analog transmission of digital
data, the baud rate is less than
or equal to the bit rate.
Note

11. Example
 A signal carries 8 bits in each signal unit. If 2000
signal units are sent per seconds, find the baud
and bit rates
 Given that signal’s bit rate is 3000 and that each
signal unit carries 6 bits, what is the baud rate
 An analog signal has a bit rate of 8000 bps and
a baud rate of 1000 baud. How many data
elements are carried by each signal element?
How many signal elements do we need?
5.11

12. Carrier signal
 Sending device produces high frequency
signals.
 The base signal is called the carrier signal
or carrier frequency
 The receiving device must be tuned to the
frequency that it expects from the sender
 Digital info then modulates the carrier
signal.
 This process is called modulation
5.12

13.  This kind of modification is called
modulating the signal OR shift keying
5.13

14. Amplitude Shift Keying
 In amplitude shift keying, the amplitude of
the carrier signal is varied to create signal
elements.
 Both frequency and phase remain
constant while the amplitude changes.
5.14

15. 5.15
Figure 5.3 Binary amplitude shift keying

16. Bandwidth for ASK
 Bandwidth of a signal is the total range of
frequencies occupied by a signal
 BW = (1 + d)xNbaud
 d = factor related to the modulation process
(minimum value is 0)
 Nbaud = the baud rate
 In ASK, baud rate and bit rate are always the
same
5.16

17. Example
 Find the minimum bandwidth for and ASK
signal transmitting 2000bps. The
transmission is half duplex
5.17

18. Frequency shift keying
 The frequency of the carrier signal is
varied to represent data.
 The frequency of the signal is constant for
the duration of one signal element, but
changes for the next signal element if the
data element changes.
 Both peak amplitude and phase remain
constant for all signal elements.
5.18

19. 5.19
Figure 5.6 Binary frequency shift keying

20. Phase shift keying
 In phase shift keying, the phase of the
carrier is varied to represent two or more
different signal elements.
 Both peak amplitude and frequency
remain constant as the phase changes
5.20

21. 5.21
Figure 5.9 Binary phase shift keying

22. Bandwidth for PSK
 Is the same as ASK
 BW = (1 + d) x Nbaud
 D = factor related to the modulation
process
 Nbaud = the baud rate
 In ASK, baud rate and bit rate are always
the same
5.22

23. 5.23
Analog-to-analog modulation
Analog-to-analog modulation is the representation of
analog information by an analog signal.
This can be done in three ways:
•Amplitude modulation
•Frequency modulation
•Phase modulation

24. 5.24
Figure 5.15 Types of analog-to-analog modulation

25. Amplitude modulation (AM)
 the carrier signal is modulated so that its
amplitude varies with the changing
amplitudes of the modulating signal.
 The frequency and phase of the carrier
remain the same; only the amplitude
changes to follow variations in the
information.
5.25

26.  The bandwidth of an audio signal (speech
and music) is usually 5 kHz.
 Therefore, an AM radio station needs a
bandwidth of 10kHz.
5.26

27. 5.27
Figure 5.16 Amplitude modulation

28. AM Bandwidth
 The modulation creates a bandwidththat is
twice the bandwidth of the modulating
signal
5.28

29. Example
 We have and audio signal with bandwidth
of 4KHz. What is the bandwidth needed if
we use AM modulation?
5.29

30. 5.30

The total bandwidth required for AM
can be determined
 from the bandwidth of the audio
signal: BAM = 2B.

31. Frequency Modulation
 the frequency of the carrier signal is
modulated to follow the changing voltage
level (amplitude) of the modulating signal.
 The peak amplitude and phase of the
carrier signal remain constant, but as the
amplitude of the information signal
changes, the frequency of the carrier
changes correspondingly.
5.31

32. 5.32
Figure 5.18 Frequency modulation

33. FM Bandwidth
 The actual bandwidth is difficult to
determine exactly, but it can be shown
empirically that it is several times that of
the analog signal or 2(1 + β)B where β is
a factor depends on modulation technique
with a common value of 4.
5.33

34. 5.34
The total bandwidth required for FM can
be determined from the bandwidth
of the audio signal: BFM = 2(1 + β)B.
Beta is usually 4
Therefore the BW = 10B
Note

35. Standard bandwidth allocation for FM radios
 The bandwidth of an audio signal (speech and
music) broadcast in stereo is almost 15 kHz.
 The FCC allows 200 kHz (0.2 MHz) for each
station. This mean β = 4 with some extra guard
band.
 FM stations are allowed carrier frequencies
anywhere between 88 and 108 MHz. Stations
must be separated by at least 200 kHz to keep
their bandwidths from overlapping.
5.35

36.  If we have an audio signal of 4Mhz. What
is the Bw needed if we are to modulate
the signal?
5.36

37. Phase Modulation
 the phase of the carrier signal is
modulated to follow the changing voltage
level (amplitude) of the modulating signal.
 The peak amplitude and frequency of the
carrier signal remain constant, but as the
amplitude of the information signal
changes, the phase of the carrier changes
correspondingly.
5.37

38. 5.38
Figure 5.20 Phase modulation

39. 5.39
The total bandwidth required for PM can
be determined from the bandwidth
and maximum amplitude of the
modulating signal:
BPM = 2(1 + β)B.
The value of beta is usually 1 or 3
Note

40. THANKS FOR YOUR TIME!!!
5.40
Ntuthu Ernest Mbili
+27 73 627 4141