Q-1. Distinguish
between data and signal.
Ans: Data is an
entity, which conveys some meaning. On the other hand, the signal is a
representation of data in some electric,
electromagnetic or optical form. So, whenever data needs to be sent, it has to
be converted into signal of some
form for transmission over a suitable medium.
Q-2. What do you mean by a “Periodic Signal”? And what
are the three parameters that
characterize it?
Ans: A signal is periodic
signal if it completes a pattern within a measurable timeframe. A periodic
signal is characterized by the
following three parameters.
Amplitude: It is the value
of the signal at different instants of time. It is measured in volts.
Frequency: It is inverse
of the time period, i.e. f=1/T. The unit of frequency is Hertz (Hz) or cycles
per
second.
Phase: It gives a
measure of the relative position in time of two signals within a single period.
Q-3. Distinguish
between time domain and frequency domain representation of a signal.
Ans: Time Domain
Representation: Whenever
a signal is represented as a function of time, it is called
time domain representation. An
electromagnetic signal can be either continuous or discrete. It is
represented as s (t).
Frequency Domain
Representation: Whenever
a signal is represented as a function of frequency, it is
called frequency domain
representation. It is expressed in terms of different frequency components and represented as s (f).
Q-4. What
equipments are used to visualize electrical signals in time domain and
frequency
domain?
Ans: Cathode Ray
Oscilloscope is used to visualize electrical signals in time domain and
Spectrum
Analyzer used to visualize
electrical signals in frequency domain.
Q-5. What do you
mean by the Bit Interval and Bit rate in a digital signal?
Ans: The bit
interval is the time required to send one single bit. The bit
rate is the number of bit intervals per second. This mean that the
bit rate is the number of bits send in one second, usually expressed in bits per second (bps).
Q-6. On what
parameters the quality of transmission depends in case of guided transmission
media?
Ans: It is mainly
decided by the frequency of transmission and the characteristics of the
transmission
media.
Q-7. Why wires
are twisted in case of twisted pair of transmission medium?
Ans: It minimizes
electromagnetic interferences between the pairs of wires, which are bundled
together,so that the cross talk is
minimum
Q-8. Give a
popular example where co-axial cables are used for broadband signaling.
Ans: Use of co-axial
cable for broadband signaling is cable TV (CATV) application.
Q-9. What
devices are used as source and detector in case of single mode of fiber?
Ans: LASER is used as
source and photodiode is used as detector in case of single mode of fiber.
Q-10. In what
way multi-mode and single-mode fibers differ?
Ans: The core
diameter of single-mode fiber is much smaller than that of multi-mode fiber.
For example,
For multi-mode
fiber:
· Core diameter
lies in the range of 50-200μm
· Cladding
diameter lies in the range of 125-400μm
· Repeater spacing
is 2Km.
For single-mode
fiber:
· Core diameter
lies in the range of 8-12μm
· Cladding diameter
125μm
· Repeater spacing
is 20Km.
Q-11. Why does
single-mode fibres are used for large distance communications rather than
multimode
fibres?
Ans: In a multi-mode
fiber, the quality of signal-encoded light deteriorates more rapidly than
singlemode fiber, because of interference of
many light rays. As a consequence, single-mode fiber allows longer distances without
repeater. For multi-mode fiber, the typical maximum length of the cable without a repeater is 2km, whereas for
single-mode fiber it is 20km.
Q-12. What is
crosstalk? How is it minimized in case of twisted-pair of wire?
Ans:
(a) Crosstalk refers to the
picking up of electromagnetic signals from other adjacent wires by
electromagnetic induction.
(b) When a pair of wires is
twisted together, the electromagnetic signals generated by the two wires
cancel each other as these are of
opposite polarity. This helps to reduce the susceptibility of
interference to the adjacent
wires.
Q-13. What are
the factors responsible for attenuation in case of terrestrial microwave
communication?
Ans: Attenuation due
to distance is 10 log (4πd/λ)2. Factors responsible for attenuation are given
below:
· Distance – Attenuation is
more if distance increases.
· Wavelength – Attenuation is
less if wavelength is longer. (i.e. high frequency components are
attenuated more than the low
frequency component)
· Rainfall – Attenuation is
less if there is no rain.
Q-14. What
parameters decide the spacing of repeaters in case of terrestrial microwave
communication?
Ans: Parameters are
the height of the antenna ‘h’ and adjustment factor ‘k’ based on the relation
d=7.14√kh, where d is the
distance in Km between two the two antennas.
Q-15. Why two
separate frequencies are used for uplink and downlink transmission in case of
satellite
communication?
Ans: Two separate
frequencies are used so that one cannot interfere with the other and full
duplex
communication is possible. And
other reason is that the Power required to transmit a signal is
proportional to the frequency of
the signal. And more power requirement more would be the weight of
the system. As there are
constraints on the load that can be carried with the satellite, mainly down
linking frequency is lower than the up
linking one.
Q-16. Why uplink
frequencies are higher than downlink frequencies in case of satellite
communication?
Ans: The satellite
gets power from solar cell. So, the transmitter is not being of higher power.
On the
other hand the ground station can
have much higher power. As we want less attenuation and better signalto-
noise ratio, lower frequency is
more suitable for downlink and higher frequency is commonly used for
uplink.
