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Digital Communication Viva Questions and Answers

Viva Questions and Answers on Digital Communication


1. What is Digital Communication?


The information-carrying digital signal is processed in a digital communication system so that it may be represented by a binary digit sequence (discrete messages). Then it's utilized for ON/OFF keying of a high-frequency sinusoidal carrier wave's amplitude, phase, or frequency, among other things.


If the message signal is analog, sampling, quantizing, and encoding are used to transform it to digital.


2. What is a digital communication system's primary feature?


A digital communication system's primary purpose is that it sends a signal waveform from a finite set of possible waveforms over a finite period. The receiver must detect which waveform from a finite collection of waveforms was delivered by the transmitter from a received noise-perturbed signal.


3. What is meant by Regeneration?


One of the techniques for correcting signal processing is regeneration. The received pulse is increased by a digital amplifier in Regeneration, which restores its original perfect form. The pulse is therefore renewed or recreated. Regenerative repeaters are circuits that execute this function at regular intervals across a transmission system.


4. Specify the benefits of digital communication


• To maintain a high level of communication confidentiality, very sophisticated encryption and decryption methods are available for digital data.

• Through error identification and repair, digital approaches may achieve extremely low error rates and great signal fidelity.

• Using 'Regenerative repeaters,' you can get a powerful, error-free signal at a reasonable power level.

• Digital circuits are less prone to distortion and interference than analog circuits, allowing for simple signal processing and image processing procedures such as speech and picture signal compression.


5. List down the major limitations of digital communication.


• As compared to analog communication systems, digital communication systems require higher bandwidth.

• At various levels, digital systems must allocate a large portion of their resources to synchronization.

• When the signal-to-noise ratio falls below a specific level, service quality might switch from excellent to bad in an instant.


6. Give a list of all available digital communication channels.


I.  Wireline channels using a physical media


1. Twisted wire pair (STP, UTP)

2. Coaxial Cable

3. Optical Fibre cable


II.  Wireless channels using free space


1. Microwave radio channel

2. Satellite channel

3. Wireless broadcast channel

4. Wireless mobile channel


7. What is a signal? Write the classification of signals


A signal is a single-valued time function that delivers information. The signals are mainly classified into four categories.


1. Deterministic and Random signals

2. Periodic and Non-periodic signals

3. Analog and Discrete signals

4. Energy and Power signals

5. The Unit Impulse Function


8. Define deterministic and random signals.


When there is no uncertainty about a signal's value at any given instant, it is said to be deterministic.

If there is any uncertainty before a signal occurs, it is said to as random.


9. Define periodic and non-periodic signals.


A signal x(t) can be said to be periodic in time, if there exists a constant To> 0 such that

x(t) = x (t+To) for - ∞ ≤ t ≤ ∞

where 't' indicates time.

A signal x(t) for which there is no value of To that satisfies the equation

x(t) = x (t+To) for - ∞ ≤ t ≤ ∞

is called a non-periodic signal


10. Define analog and discrete signals


An analog signal x(t) is a continuous function of time; that is, x(t) is unique for all t values.

x(kT) is a discrete signal that only exists at discrete times. It is defined as a sequence of integers kT, where k is an integer and t is a fixed time interval, for each time.


11. Define energy and power signal


A signal x(t) is said to be an energy signal if, and only if, it has non-zero but finite energy (0 < Ex<∞) for all time.


A signal x(t) is defined as a power signal if, and only if, it has finite but non-zero power (0 <px<∞) for all time.


12. Define Unit Impulse Function


An infinitely large amplitude pulse with zero pulse width and unity weight (area beneath the pulse) is defined as the impulse function, which is focused at the place where its argument is zero. The following relationships characterize the unit impulse.


∫ δ(t)dt = 1,  δ(t) = 0 for t ≠ 0


13. Define information capacity. What is Shannon’s limit for information capacity


The maximum rate at which data may be transferred through a channel without error is referred to as the information capacity. The Shannon channel capacity theorem establishes a basic restriction on the pace at which error-free data may be sent. It's as follows:


C = B log2(1+𝑆/𝑁), bits/s


where C = Channel capacity

B = Bandwidth

𝑆/𝑁 = Signal to noise power ratio


14. What is meant by data transmission? What are its types?


The transfer of data includes digitized voice, digital image and video, computer produced data, and so on. Data transmission may be divided into two categories. There are two types of transmission: 1) parallel transmission and 2) serial transmission. In serial transmission, there are two options. There are two types of transmission: 1) synchronous and 2) asynchronous.


15. What is the parallel transmission? What are its merits?


We transfer a word or byte at a time in parallel transmission. As a result, all of the bits of data are sent out at the same time on distinct communication channels. Parallel transmission mainly has three advantages. They are:


1. Fast data transmission

2. Less complexity

3. Easy to isolate any data line


16. What is the serial transmission? What are its merits?


Data is transmitted as a single bit at a time in serial transmission, with each bit having a predetermined time interval. As a result, serial transmission is simply only a single pair of cables or lines. Serial transmission has the following advantages:


1. It is the most preferred mode of communication for long distances.

2. The system's cost is reduced by using a single data line.


17. What are the applications of parallel and serial transmission?


Parallel Transmission


1. Parallel transmission is used by printers.

2. The data bus is used to transmit data inside the computer system, such as from the CPU to the memory and back.


Serial Transmission


1. Most long-distance communications employ serial transmission.


18. Differentiate asynchronous and synchronous transmission


The transmitter sends data with its time clock that is unknown to the recipient in asynchronous transmission. It transfers data one byte at a time. The transmitter and receiver are both synced to the same clock frequency in synchronous transmission. Data is transmitted in blocks of several bytes.

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