Telecom lect 6

1. Digital Signal as a Composite Analog
Signal
It should be clear that:
• A digital signal, with all its sudden
changes, is actually a composite
signal
having an infinite number of
frequencies.
• In other words, the bandwidth
of a digital signal is infinite.

2. Digital Transmission
A digital signal theoretically
needs a bandwidth between 0
and infinity. The lower limit (0)
is fixed; the upper limit
(infinity) can be decreased if
we accept a limited number of
harmonics. This means a
bandwidth between 0 and f for
a low-pass signal.

3. TRANSMISSION IMPAIRMENT
• Signals travel through transmission
media, which are not perfect.
• The imperfections cause impairment in
the signal.
• So the signals at the beginning and end
of the medium are not the same. What is
sent is not what is received.
• Three types of impairment may occur:
attenuation, distortion, and noise (see
Fig.)

5. Attenuation
Attenuation means loss of
energy.
•When a signal, travels through a
medium, it loses some of its
energy to overcome the
resistance of the medium.
•That is why a wire carrying
electrical signals gets warm,
after a while.

6. •Some of the electrical
energy in the signal is
converted to heat.
•Amplifiers compensate this
loss
•Next Fig shows the effect of
attenuation and
amplification.

8. Distortion
• The signal changes its form or
shape.
• Distortion occurs in a composite
signal.
• Each signal component has its own
propagation speed through a
medium and, therefore, its own
delay in arriving at the destination.
• Figure shows the effect of distortion
on a composite signal.

10. Noise may corrupt the signal
• Types:
• Thermal noise: Random motion of
electrons in a wire which creates an
extra signal not originally sent by the
transmitter.
• Induced noise comes from sources
such as motors and appliances.
These devices act as a sending
antenna and the transmission
medium acts as the receiving
antenna.

11. Crosstalk
• The effect of one wire on the other.
• One wire acts as a sending antenna
and the other as the receiving
antenna.
• Impulse noise is a spike (a signal
with high energy in a very short
period of time) that comes from
power lines, lightning, and so on.
• Fig shows the effect of noise on a

13. ANALOG/ DIGITAL
TRANSMISSION
Transmission of information may
be in
• 1- Analog form
• 2- Digital form
• Analog signal is highly affected
by
noise
• so digital is better
• Need to convert analog into

14. STEPS REQUIRED:
• A- SAMPLING
(i) Measuring amplitude of analog
signal
at equal intervals of time and
(ii) Holding the taken sample for some
time
• B- Produce a PULSE corresponding to
each sample
• C- QUANTIZATION
Assign a +Ve/ -Ve value to each
sampled amplitude

15. • D- BINARY ENCODING
Convert each value into 8 bit binary data
7 bits for magnitude, 8th
bit for sign
• E- LINE CODING
Convert binary data into a digital signal
suitable for transmission
• NOTE 1: STEPS A to B give rise to a process
called PULSE AMPLITUDE
MODULATION or PAM
• NOTE 2: STEPS A to E give rise to a process
called PULSE CODE
MODULATION or PCM

16. NYQUIST THEOREM
How many samples for digital reproduction
of an
analog signal ?
According to Nyquist theorem, for accurate
reproduction of analog signal using PAM,
sampling rate must be twice the highest
frequency contained in original signal
E.g. Telephone voice has max frequency of
4KHz. We need sampling rate of
8KHz.

17. HOW MANY BITS TO ENCODE A SAMPLE ?
More bits more precise value of sampled
amplitude. For voice, 8bits are used for
each
sample
For telephone voice,
Bit rate = Sampling rate X No. of bits/
sample
= 8000 x 8 = 64Kbps

18. PCM - Pulse CodePCM - Pulse Code
ModulationModulation
local loop
phone switch
(DIGITAL)
Central
Office
(Telco)
Analog
transmission
To other
switches
trunk
Digital
transmission
convert analog signals to digital data
using PCM (similar to PAM)
• 8000 samples per second and 8 bits
per sample (7 bits for sample+ 1 bit
for control)
 64 Kb/s (DS-0 rate)
• DS-0:
• Basic digital
communications
unit used by phone
network
• Corresponds to 1
digital voice signal