Achyut S.Godbole, “ Data Communications and Networks”, Tata McGraw-Hill Publishing Company Limited, Ninth reprint, 2007

1. Data Communications and Networks
Analog and Digital Transmission
R.Anushiya
Assstiant Professor,
P.K.R Arts College for Women, Gobi
Author: Achyut S.Godbole, “ Data Communications and Networks”, Tata McGraw-Hill
Publishing Company Limited, Ninth reprint, 2007

2. Analog and Digital Transmission Modes

3. A signal is an electrical or electromagnetic current
that is used for carrying data from one device
or network to another …
Transmission mode refers to the mechanism of
transferring of data between two devices connected over
a network. It is also called Communication Mode.
• Digital Transmission: data transmission using square waves
• Analog Transmission: data transmission using all sine waves.

4. Four possibilities to consider:
Analog Signal, Analog transmission → “as is”
(e.g., Telephone, radio)
Analog Signal, Digital transmission → sampling
(e.g., voice, audio, video)
Digital Signal, Analog transmission → broadband &
wireless
Digital Signal, Digital transmission → baseband (e.g.,
Ethernet)

5. Analog Signal, Analog Transmission
• Telephone is the best example
• Human voice generates an analog signal
which is transmitted as analog signal over
the medium.
• When the signal suffers attenuation ,
amplifiers are used to overcome the
problem

6. Digital Signal, Digital Transmission
• Digital-to-digital encoding is the representation of digital
information by a digital signal.
• When binary 1s and 0s generated by the computer are
translated into a sequence of voltage pulses that can be
propagated over a wire, this process is known as digital-to-
digital encoding
• Binary signals have two basic parameters: amplitude and
duration.
• As the number of bits sent per unit of time increases, the bit
duration decreases

7. Digital Signal, Analog Transmission
• Converts digital signal to analog signal,
such that it could be over by telephone
network at the other end it again converted
to digital signal
• To make his process easy we use modem
for conversion
– Modulation
– Demodulation

9. Modulation Techniques

10. Amplitude Shift Keying (ASK)
• To represent binary signals, the amplitude is varied by1 or 0.
• Keying means turning a transmitter on and off.
• Highly susceptible to noise interference.
• Noise -- random electrical signals (voltages) that tend to
generate errors in transmission; introduced into a line by
heat from circuit components, or natural disturbances .

11. Frequency Shift Keying (FSK)
• Frequency is varied to represent binary 1 or 0.
• Noise interference not a problem because it's looking
for frequency changes and doesn't care about voltage
spikes.

12. Phase Shift Keying (PSK)
• Phase is varied to represent binary 1 or 0.
• Limited by the ability of the equipment to
detect small differences in phase. This limits
its potential bit rate.

13. Bit Rate and Baud Rate
Used in transmitting data from one computer to another across a
public access phone line
Bit Rate and Baud Rate
•Bit rate is the number of bits transmitted per second.
•Bit rate is always >/= to the baud rate
•Baud rate is the number of signal units per second required to
send those bits.
Carrier signal
•A high-frequency signal that acts as a basis for the information
signal - by sender
•Digital information is encoded by modulating the signal's:
amplitude, frequency, or phase .

14. Analog to Digital Transmission

16. Nyquist Theorem
• The Nyquist Theorem, also known as the
sampling theorem, is a principle that
engineers follow in the digitization of
analog signals
Nyquist sampling (f) = d/2,
where d=the smallest object, or highest
frequency, you wish to record.

17. • The Nyquist Theorem states that in order
to adequately reproduce a signal it should
be periodically sampled at a rate that is 2X
the highest frequency you wish to record.
• With images, frequency is related to
structure size.
• Small structures are said to have a high
frequency. Thus, the imaging sample rate
(or pixel) size should be 1/2 the size of the
smallest object you wish to record

18. A High Sampling Rate = much greater than 2X the
highest frequency.
This is 'Oversampling' that, while not "bad" will take time
and create a large digital file.

19. • Nyquist Sampling Rate = The minimum
sample rate that captures the "essence" of
the analog information.
• Note that while Nyquist is appropriate for
sampling, it may not capture nuances in
information.

21. Differences between Analog and Digital Signals
Analog Signal
• An analog signal signifies a continuous signal
that keeps changes with a time period.
• Analog signals are continuous sine waves.
• Analog signals describe the behavior of the
wave with respect to amplitude, time
period, & phase of the signal.
• Analog signal range will not be set.
• Analog signal is further horizontal toward
distortion during the response to noise.
• An analog signal broadcasts the information
in the signal form.
• The example of an analog signal is the
human voice
Digital Signal
• A digital signal signifies a discrete signal that
carries binary data and has discrete values.
• Digital signal is square waves.
• Digital signals describe the behavior of the
signal with respect to the rate of a bit as well
as bit interval.
• Digital signal is limited as well as ranges
from 0 to 1.
• A digital signal has resistance in response
toward the noise, therefore, it does not often
face distortion.
• A digital signal broadcasts the information in
the form of binary that is bits.
• The example of a digital signal is the data
transmission in a computer.