2. Chapter Objectives
• Explain amplitude, frequency and
phase shift modulation
• Give an example of a modulation
technique used in modems
• Discuss modem standards
– Communication, compression etc.
Continued
3. Continuation of Chapter
Objectives
• Differentiate between bps and Baud
that are units used for measuring
communication speed
• Describe analog-to-digital modulation
• Explain digital-to-digital interface
• Summarize the different types of signal
conversions
– Digital-to-analog, analog-to-digital, analog-to-
analog and digital-to-digital
4. Chapter Modules
• Amplitude modulation
• Frequency and phase shift modulation
•Modems and modulation
• FM modulation in modems
• Speed of modulated signals
• Analog-to-digital modulation
• Digital-to-digital interfacing
9. Characteristics of Amplitude
Modulation
• Amplitude of the analog signal is
modulated
• One amplitude represents a 0
• Another amplitude represents a 1
• Frequency remains unchanged in both
cases
• Signals that are modulated at one end
are demodulated at the other end
10. Usage
• Amplitude is susceptible to interference
– This technique in not normally used in
modems
• A variation of this technique is used in
AM radio transmission
– Analog-to-analog modulation takes place
11. AM and Radio Transmission
Modulated Amplitude
Voice
Carrier
Wave
15. Characteristics of Frequency
Modulation
• Frequency is modulated
• Frequency f1
– Represents 1
• Frequency f2
– Represents 0
• The amplitude remains unaltered in
both cases
16. Usage
• Variations in frequency are easy to
detect
– They are less susceptible to interference
• FM and variations of this technique are
used in modems
• Easy to implement full duplex
transmission under FM
• A variation of the FM technique
described here is used in FM radio
transmission
17. Use of FM in Early Day Modems
F1
0
F2
1
F3
0
F4
1
Voice
Band-
Width
A B
Full-duplex Communication
18. Modulation in Modern Day
Modems
•Modern day modems may not use the
FM technique for modulation
• They may be using a technique known
as Phase Shift Modulation (or Phase
Shift Keying)
21. Phase of an Analog Signal
Y
Strength
X
0 90 180 270 360
Time Frame
22. The Concept of Phase Shift
90 degrees phase shift
180 degrees phase shift
0 90
0 180
23. Phase Modulation Technique
1
0
90 Degrees
0 Degree phase shift
phase shift
•This is also known as phase shift keying.
24. Characteristics of Phase Shift
Modulation
• Phase is modulated
• Phase shift of 0 represents a 0
• Phase shift of 90 degrees represents a 1
• Both amplitude and frequency remain
unaltered is both cases
• Also known as Phase Shift Keying, it is
used in a number of modern modems
as well
27. Module Objectives
• Explain the basic concept of modem
communication
• Provide an example of frequency
modulation used in modems
• Discuss the importance of call mode
setting
– Call mode and receive mode settings
28. Basic Concepts of Modem
Communication
F1
0
F2
1
F3
0
F4
1
Voice
Band-
Width
A B
Full-duplex Communication
29. FM Details
• Different frequencies are used for
transmission
• At node A
– F1 for 0
– F2 for 1
• At node B
– F3 for 0
– F4 for 1
30. Call and Receive Modes
• Setting for communication
– Set one side on call mode
– Set the other side on receive mode
– The above would ensures proper
assignment of frequencies
31. Mode Setting Rule
• Calling mainframes or on-line services
– Set the calling computer on call mode
• In general
– Set the home computer on the call mode
• Fortunately, in a number of cases, the
modems poll and set themselves
dynamically for communication
between the receiver and the sender
34. Overview
• In general, the terms used for
measuring speed are bps and Baud
• The former is being used more widely
than the latter
• bps is the accurate measure of the
speed of communication
• In the past, Baud was being used
interchangeable with bps
– Both are not interchangeable
• Only in certain circumstances they
amount to the same
35. Definition of bps and Baud
• bps represents the number of bits
transmitted per second
• Baud represents the number of times
the signal changes its state during a
given period of time
36. Example Where bps and Baud
Represent the Same
1
0
F1 F2
1 Second
bps = 1
Baud = 1
37. Example Where bps and Baud are
Different
00 01 10 11
bps = 2
Baud = 1
1 second
39. In Summary
• bps measures the speed of
communication correctly in bits per
second
• Baud indicates he number of times the
state of a signal changes in one second
42. Modem Standardization
• The International body that
standardizes the modulation technique
is known as the ITU
• ITU is also responsible for setting
standards pertaining to:
– Error correction
– Data compression
43. Sample ITU Specifications
• Modulation
– ITU V.34
• Error correction
– ITU V.42
– MNP 5
• Data compression
– ITU V.42 bis
– MNP 2 to 4
44. Bell Standard and its
Implications
• At 1200 bps and below there were two
standards
– CCITT (ITU at present)
– Bell
• A Bell modem cannot communicate
with a CCITT modem
• Bell standard at that time was used
predominantly in the US
• Today, all modems fall under the ITU
specifications
45. Sample Protocols and Speed
• V.92 for 56,000 bps
• V.90 for 56,000 bps
• V.34 for 28,800 bps
• V.32 bis for 14,400 bps
• V.32 for 9,600 bps
• A high speed modem could also operate at
the lower speed
– High speed modems can thus communicate with a
low speed modems
46. A Note on the Protocol Used in
the Faster 56K Modems
•When the 56K modems were first
introduced there were two competing
standards
• One was the X2 standard proposed by
US Robotics that is now part of 3Com
• The competing protocol was knows as
the Kflex56 standard
– A joint effort between Lucent and Rockwell
47. ITU Standard for 56K Modems
• Both standards have now been
superceded by the ITU V.90 standard
• The vendors now produce modems that
operate under the ITU V.90 protocol
• The vendors also offer upgrades to the
older X2 and Kflex modems so that they
could operate under V.90
48. In Summary
• ITU specified protocols with respect to
modems exist for the following.
– Modulation
– Error correction
– Data compression
• Different protocols apply to different
speeds of communication
• A high speed modem can communicate
with a low speed modem
51. An Overview of Analog-to-
Digital Modulation
• Representation of analog signals by
digital signals is known as analog-to-digital
modulation
• Often the digitized information is
further coded into binary form for
computer processing
• Sample applications include the
encoding of audio for computer
processing
52. Steps Involved in the Representation
of Analog Signals by Digital Signals
Analog Signals
PAM or PDM
PCM
* See earlier slides for
details on PAM and PCM
Computer
Processing
Digitize
Encode
54. Pulse Duration Modulation
110 001 101
6 1
5
Note: pulse duration is proportional to
The height of the analog wave
55. Salient Points of Pulse
Duration Modulation
• Sample the analog signal at
predetermined time intervals
– Sampling rate
• Generate digital pulses of duration
proportional to the amplitude of the
analog signal at the sampling point
• Encode the information into binary
form
59. Module Objectives
• Explain the difference between signal
modulation (conversion) and digital-to-digital
signal transformation
• Explain the concept of digital-to-digital
interfacing using ISDN as an example
• Provide a summary of the different
modulation processes
60. Overview
• Analog-to-Digital signal conversion requires
modulation
• Digital-to-Digital interfacing
– Requires conversion and not modulation
– In this case, digital signals are converted
from one digital format to another digital
format
– Hence, the need for an interface unit even
though the signals at both ends are
represented in digital form
• An example is the Computer-to-ISDN link
62. Summary of Modulation
• Digital-to-analog
– FM used in modems
• Analog-to-digital
– PAM and PCM used in the digitization of audio
• Analog-to-analog
– AM used in radio transmission
• Digital-to-digital
– This is not a modulation process
– Used by the ISDN interface to the computer
– Used in DSL communication
