Analog Transmission

1. ANALOG TRANSMISSION

2. MODULATION OF DIGITAL DATA
It is the process of changing one of
the characteristics of an analog
signal based on the information in
a digital signal.
e.g. To transmit digital data from one
PC to another using a phone-line.
Telephone line carries analog signal, so
digital data should be converted using
process of Modulation.

3. Most of us may be familiar with
MODEM.
MODEM stands for:
MODULATOR/ DEMODULATOR
MODEM uses Modulation process
to convert digital data to a form
suitable for transmission on
a telephone line.

4. WHY MODULATION ?
1-For allowing multiple signals to share
a single physical channel
2-Necessary for wireless communication
where the antenna diameter must be at
least equal to the wavelength of the
carrier signal.
This means, for a 3000 Hz signal
through space, the antenna diameter
must be at least 60 miles!
3-For a medium not suitable for digital
transmission.

5. Modulation process uses two types
of signals:
(a) Information Signal:
i- Analog or
ii- Digital
(b) Carrier Signal :A high frequency
Sine wave
One or more characteristics of
Carrier are varied in accordance to

6. MODULATION PROCESS PRODUCES
MODULATED SIGNAL, READY FOR
AMPLIFIATION and TRANSMISSION

7. Sine Wave as a CARRIER
• Think of a Sine Wave as a Carrier Signal,
i.e. the signal onto which the information
is loaded for sending to the end user
• A Carrier Signal is used as the basis for
sending e.m. signals between a transmitter
and a receiver, independently of the
frequency

8. Carrier signals
• A Carrier Signal may be considered to
travel at the speed of light, c, whether it is
in free space or in a metal wire
• Travels more slowly in most substances
• The velocity, frequency, and wavelength
of the carrier signal are uniquely
connected by
c = f λ
Wavelength
FrequencyVelocity of light

9. The receiver is tuned to
the frequency of the
carrier signal that is
expected from the
transmitter. See next slide

11. Basic Analog Modulation Techniques
• Parameter to be varied Analog Modulation
• Amplitude Amplitude Modulation AM
• Frequency Frequency Modulation FM
• Phase Phase Modulation PM
• RADIO transmission uses AM and FM
• TV broadcast uses FM

13. Basic Digital Modulation Techniques
• Parameter to be varied Digital Modulation
• Amplitude Amplitude Shift Keying
• Frequency Frequency Shift Keying
• Phase Phase Shift Keying

15. More on Digital Modulation
• Amplitude Modulation (AM) also known as
amplitude-shift keying. This method requires
changing the amplitude of the carrier phase
between 0 and 1 to encode the digital signal.
• Frequency Modulation (FM) also known as
frequency-shift keying. Must alter the
frequency of the carrier to correspond to 0 or
1.
• Phase Modulation (PM) also known as
phase-shift keying. At each phase shift, the
bit is flipped from 0 to 1 or vice versa.

16. Modulation Schematics

17. Coming slides further
highlight the process of
ASK
FSK
PSK

21. BAUD
• Baud refers to the number of
times a change of analog signal
occurs in the circuit.
• If one signal change carries per
bit, then baud and bps are the
same, e.g. baud = bps

22. Bit Rate vs. Baud Rate
• bit: a unit of information
• baud: a unit of signaling speed
• Bit rate (or data rate): b
– Number of bits transmitted per second
• Baud rate (or symbol rate): s
– number of symbols transmitted/sec
• General formula:
• Bit rate = Baud rate X Number of bits per symbol

23. Concept of Symbol
• Symbol: Each modification of the carrier
wave to encode information
e.g. Sending one bit (of information) at a time
– One bit encoded for each symbol (carrier wave
– change)  1 bit per symbol
e.g. Sending multiple bits simultaneously
– Multiple bits encoded for each symbol (carrier
wave change)  n bits per symbol, n > 1
– Need more complicated information coding
schemes

24. Sending Multiple Bits per Symbol
• Multiple bits per symbol might be
encoded using:
• amplitude, frequency,
and phase modulation
–e.g., in PSK:
–Phase shifts of 0o
, 90o
, 180o
, and 270o

25. Example: Two-bit ASK
4 symbols

26. EXAMPLE
An analog signal carries 4 bits in each
signal unit. If 1000 signal units are sent per
second, find the baud rate and bit rate.
Baud Rate = No. of Signal units/sec = 1000
Bit rate = Baud rate X No. of bits per signal Unit
= 1000 X 4
= 4000 bps

27. IMPORTANT NOTE
If fewer (less) signal units are
required to transmit more bits,
less BANDWIDTH will be
required on a medium.

28. BAND WIDTH IN ASK

29. BANDWIDTH IN ASK
As shown in the previous slide
If single bit is transmitted per
signal unit,
Bit Rate = Baud Rate
Bandwidth = Baud Rate

30. BANDWIDTH IN FSK

31. BANDWIDTH IN FSK
Fig shows the spectrum of FSK signal. Although one
bit is transmitted per signal unit, but because of
frequency shift if lowest frequency is fc0 and
highest frequency is fc1,
Bandwidth = Baud rate + fc1 – fc0

32. EXAMPLE
In FSK transmission, if two carriers fc0 and fc1
are separated by 3000 Hz, find the required
bandwidth at 2000 bps.
Solution:
fc1-fc0 = 3000 Hz
Bit Rate = 2000 bps
Baud Rate = Bit Rate = 2000
Bandwidth = fc1 – fc0 + Baud Rate
Bandwidth = 3000 + 2000
= 5000 Hz

33. BANDWIDTH IN PSK

34. BANDWIDTH IN PSK
As shown in the previous slide
If single bit is transmitted per
signal unit,
Bit Rate = Baud Rate
Bandwidth = Baud Rate

35. Constellation Diagrams or
Phase State Diagram for 2PSK
(2PSK because two phases and 1 bit per phase)

36. Example of 4-PSK: 4 Phases, 2 bits per phase

37. 4-PSK Constellation

38. Tribit using 8-PSK: 8 Phases
3 bits per phase

39. • Combined Modulation Techniques
• Combining ASK and PSK on the same
circuit. HOW?
• Vary Amplitude A and Phase P of the
Carrier simultaneously.
• Let x variations in A
• Let y variations in P
• will give total variations of x times y
• Thus corresponding number of bits per
variation

40. QUADERATURE AMPLITUDE MODULATION
Q A M
The modulation technique to vary A
and P simultaneously is called QAM
IMPORTANT NOTE
In QAM, number of variations in A is
kept minimum because A is affected
by noise. Number of variations in P
is limited only by the interpretation
of shifts in it. See next slide

41. QAM is a combination of ASK and PSK so
that a maximum contrast between each signal
unit (bit, dibit, tribit, and so on) is achieved.

42. QAM -
Is a widely used family
of encoding schemes.
A common form: 16-QAM
Uses:
• 8 different phase shifts
• 2 different amplitude levels,
• 16 possible symbols
• 4 bits/symbol

43. 16- QAM
Constellation

44. –TCM – Trellis-Coded
Modulation
•An enhancement of QAM
•Can transmit different
number of bits on each
symbol (6,7,8 or 10 bits
per symbol)

45. Transmission Modes
• Parallel mode
–Uses several wires, each wire
sending one bit at the same time as
the others
• A parallel printer cable sends 8 bits
together
• Computer’s processor and motherboard
also use parallel busses (8 bits, 16 bits,
32 bits) to move data around

46. Serial Mode
Sends bit by bit
over a single wire
Serial mode is
slower than
parallel mode

47. Parallel Transmission Example
Used for short distances (up to 6 meters)
(since bits sent in parallel mode tend to
spread out over long distances)
(8 separate copper wires)

48. Serial Transmission Example
Can be used over longer
distances (since bits stay in the
order they were sent)

49. DTE and DCE
• Data terminating equipments (DTEs)
are noncommunciations-oriented
components of a data communications
environment.
• Data communications devices (DCEs)
are communications-oriented
components of a networks, such as
telephone switching equipment, media,
modems, etc.

50. DTE and DTE

51. Modem
• A modem is a DCE device.
• Modems use amplitude,
frequency, or phase shift to
encode more that one bit per
baud.

52. More on Modems
• V-series of modem standards (by ITU-T)
– V.22
• An early standard, now obsolete
• Used FM, with 2400 symbols/sec
 2400 bps bit rate
– V.34
• Used 8.4 bits/symbol, with 3,428 symbols/sec
 multiple data rates(up to 28.8 kbps)
• Includes a handshaking sequence that tests the
circuit and determines the optimum data rate

53. V.90 and V.92 Modems
• Combines analog and digital transmission
• Uses a technique based on PCM concept
– Results in a max of 56 Kbps data rate
• V.90 Standard
– Based on V.34+ for Upstream transmissions (PC
to Switch)
– Max. upstream rate is 33.4 Kbps
• V.92 Standard (most recent)
– Uses PCM symbol recognition technique for both
ways
– Max. upstream rate is 48 kbps

54. 56k Modems
• 56k modems, the fastest possible on voice
grade lines, are based on the V.90 and V.92
standards.
• Downstream transmissions (from phone
switch to the user’s computer) use a
technique based on recognizing the 8-bit
digital symbol.
• With the V.90 standard, upstream lower data
rates. The max transmissions are still based
on the V.34+ standard.

55. Digital Modulation may not
mean what you think it means…
• The transmitted signal is a
continuous time signal (or
analog signal) regardless of
the modulation ‘analog’ or
‘digital’.