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24071-DIGITAL COMMUNICATION QUESTION BANK

RANGANATHAN POLYTECHNIC COLLEGE, COIMBATORE.

Prepared by A.Indhumathi, Lecturer/ECE

UNIT-I BASICS OF DIGITAL COMMUNICATION

1. What is digital communication?

The message signal to be transmitted may be analog or digital form. In digital

signal, both the amplitude & time are in discrete form. Computer data &

telegraph signals are examples of digital signal.

In digital communication, the analog signal should be converted into digital

form by using the processes of sampling, quantizing & encoding.

2. What is quantizing?

Quantizing is a process of mapping (dividing) an analog (modulating) signal

into several equivalent discrete levels (range or steps). That means, it is the process of

converting continuous input values to discrete output levels.

3. Define encoding.

In encoding process, the selected level is represented by a code word that

consists of prescribed number of code elements.

4. What are the advantages of digital communication?

Relatively inexpensive digital circuits can be used.

Privacy is preserved by using data encryption

Greater dynamic range

In long distance systems, noise does not accumulate from repeater to repeater.

5. What are the disadvantages of digital communication?

More bandwidth is required.

Synchronization is required.

6. What are the basic signal processing operations of digital communication?

Source coding, Channel coding & Modulation

7. What are the important channels used in digital communication?

Telephone channels, Coaxial cables, Optical fibers, Microwave radio, Satellite

channels, Computers.

8. Mention the frequency range & signal to noise ratio of a telephone channel.

The frequency range of telephone channel is if 300 to 3400Hz and the signal to

noise ratio is about 30dB.

9. What is adaptive equalization?

The equalizer automatically varies its delay characteristics, while sampling the

return signal to determine when the channel delay plus equalizer delay reaches proper

tolerances. This type of equalization is called adaptive equalization.

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10. Mention the frequency range of a HF radio & submarine cables.

The frequency range of a HF radio & submarine cables is 300-2800Hz & for

industries about 4KHz.

11. Define speed.

The transmission speed of a communication channel is described in „baud rate‟.

Baud rate is the unit of signaling speed.

12. What is dibit?

Each of the 2-bit elements is called a dibit. Each of the 2bit pairs is converted to

a phase values of in the data set.

13. Define nyquist rate.

The effect of the noise on the data channel can be reduced by increasing the

signal to noise ratio for an ideal channel the nyquist rate twice the bandwidth.

14. Define baud rate.

Baud rate is the unit of signaling speed generally the information may transfer

at a rate equal to or different from the baud rate.

15. Define crosstalk.

Any transmission system which conveys more than one signal simultaneously

can experience cross talk.

16. What is the use of echo compressor or echo suppressor?

Echo compressors or echo suppressors are used on long distance circuits to

overcome echoes caused by circuit imbalances.

17. Define information capacity.

Information capacity is a measure of how much information can be propagated

through a communication system and is a function of bandwidth & transmission time.

IαBt

I-Information capacity, B-Bandwidth, t-Transmission time

18. Give the expression for Shannon limit information capacity.

I=B log2 (1+𝐒

𝐍);

I = 3.32 log10 (1+𝐒

𝐍)

I-Information capacity, B-Bandwidth, S/N- signal to noise ratio.

19. What is distortion?

Signals of different frequencies can be passed by a channel with different values

of amplitude attenuation & at different propagation speeds. This results in distortion.

20. Define equalizers.

Phase distortion & attenuation can be reduced by using equalizer on the

channel.

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21. Define bit rate.

Bit rate is simply the number of bits transmitted during one second & is

expressed in bits per second.

22. Define data transmission.

Data transmission, digital transmission or digital communication is the physical

transfer of data (a data bit stream) over a point to point or point to point

communication channel.

23. What are the advantages of parallel transmission?

Due to parallel connecting wires, the data transfer is very fast.

Complexity of parallel transmission is less, since all line transmits separately.

It is possible to isolate any one line for testing or other purpose, without any

intelligent hardware or software.

24. What are the disadvantages of parallel transmission?

Cost of parallel transmission is more, since more hardware connections are

required & it is not suitable for longer distances

25. What are the applications of parallel transmission?

Printers & Processors to processors communications use parallel transmission

Most of the short distance communications

26. What is serial transmission?

In serial transmission, it has only one transmission line, since it can transmit

only one bit at a time. It requires 4 clock pulses to transmit four bits.

27. What is parallel transmission?

In parallel transmission, it has many data lines and can transmit many bits at a

time with a single clock pulse. This transmission is also called as parallel by bit or

serial by character.

28. Compare synchronous & asynchronous transmission?

S.No Synchronous Transmission Asynchronous Transmission

1 Does not transmit & receive data at

same clock pulse.

Transmit & receive data at same clock

pulse.

2 It has Start & stop bits It do not have Start & stop bits

3 More timing errors Less timing errors

29. Mention the disadvantage of asynchronous transmission.

More timing errors in asynchronous transmission & to determine the time

period for sampling is difficult.

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UNIT-II WAVEFORM CODING TECHNIQUES

1. Define pulse modulation?

Pulse modulation is used to transmit analog information such as continuous

speech or data. In this system, the continuous waveforms are sampled at regular

intervals along with the synchronizing pulses. The original signals reconstructed at te

receiver.

2. What are the types of modulation?

Analog pulse modulation

Digital pulse modulation

3. What are the types of analog pulse modulation?

Pulse Amplitude modulation

Pulse time modulation

Pulse width modulation

Pulse position modulation

Pulse frequency modulation

4. What are the types of digital pulse modulation?

Pulse code modulation

Delta modulation

Differential pulse code modulation

Adaptive delta modulation

5. Define PCM.

In PCM, the total amplitude range occupied by the signal is divided into a

number of standard levels. The actual number of levels is a power of 2. In PCM the

signal are continuously sample, quantized, coded and sent. It is converted into nearest

standard amplitude & corresponds back-to-front binary number.

6. Define sampling.

The information (modulating) signal is sampled with a train of narrow

rectangular pulses. The sampling range must be greater than twice the highest

frequency component of information to recover the original signal with minimum

distortion at the receiver. fs≥2fc

7. Define quantizing.

It is a process of mapping (dividing) an analog (modulating) signal into several

equivalent discrete levels (range or steps). That means, it is the process of converting

continuous input values to discrete output levels.

8. Define uniform quantization.

In uniform quantization, the quantization step difference between two

quantization levels remains constant over the complete amplitude range.

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9. Expand DPCM & DM.

DPCM- Differential Pulse Code Modulation

DM – Delta Modulation

10. What are the types of uniform quantizer or linear quantizer?

Mid tread quantizer

Mid riser quantizer

Biased quantizer

11. Define non-uniform quantizer.

In non-uniform quantization the step size is not fixed. It varies according to

certain law or as per input signal amplitude. The quantization error is low at small step

size level. Hence signal to noise power ratio remains constant throughout the dynamic

range of quantizer.

12. Define companding.

Companding is the process of signal compression & expansion. It is used to

overcome the problems of quantizing error & noise. It improves the quality of signal.

13. What are the applications of PCM?

PCM is used in multichannel telephone communications

It is used in space communications

14. What are the advantages of delta modulation?

DM transmitter & receiver require very simple & inexpensive hardware.

Smaller signaling rate & transmission channel bandwidth.

No need of ADC in delta modulation.

15. What are the disadvantages of delta modulation?

Severe slope overload distortion is produced.

Granular noise is produced.

Transmission channel is produced.

16. What are the advantages of adaptive delta modulation?

Signal to noise ratio is better than DM

The dynamic range of ADM is high

Better utilization of bandwidth.

17. Define slope overhead.

The slope of the analog signal is greater than the delta modulator can maintain

slope overload. The probability of slope overload is reduced by increasing the clock

frequency or by increasing the magnitude of the minimum step size.

18. Define granular noise.

When the original analog input has relatively constant amplitude, the

reconstructed signal has variations that were not present in the original signal. This is

called granular noise.

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19. What are the disadvantages of DPCM?

DPCM requires less bandwidth compared to PCM.

Its signal to noise ratio is better than DM & ADM.

20. What are the limitations of DPCM?

Implementation of DPCM is complex compared to PCM.

Slope overhead distortion & quantization noise is present in DPCM.

More transmission error will be generated.

More sensitive to noise.

UNIT 3- DIGITAL MODULATION TECHNIQUES

1. Define Digital Modulation techniques.

In band pass transmission, the digital signal modulates high frequency

sinusoidal carrier. They are called digital modulation techniques. With these

techniques it is possible to transmit data over long distances. In base band

transmission, the data is transmitted without modulation.

2. Define ASK.

In Amplitude Shift Keying (ASK) modulation, the digital signal is switched

between amplitude levels of digital signal. In this system the carrier signal is ON &

OFF, accordance with the binary input signal. The carrier signal is transmitted during

the high level (1) input; conversely the carrier signal is also called ON-OFF keying.

3. Define FSK.

Frequency Shift Keying (FSK) is that which shifts carrier frequency to indicate

a mark or a space for a common digital signal. In this, the unmodulated carrier

frequency corresponds to mark condition & space by downward frequency shift.

4. Define PSK.

PSK is a system in which the carrier may be phase shifted by +90° for a mark &

-90° for a space. In this, the unmodulated carrier frequency corresponds to mark

condition & space by downward frequency shift.

5. Define QPSK.

QPSK is a Quaternary Phase Shift keying or Quadrature PSK which is an angle

modulated constant amplitude digital modulation. It transmits higher bit rate. QPSK is

widely used in satellite links.

6. Define MSK.

In Minimum Shift Keying (MSK), the output waveform is continuous in phase

hence there are no abrupt changes in amplitude. The side lobe of MSK are very small

hence band pass filtering is not required to avoid inter channel interference.

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7. What is TDM?

Time Division Multiplexing (TDM) is a type of digital multiplexing. In TDM

each signal can occupy the entire bandwidth of the channel. i.e. the multiple signal

take turns transmitting over the single channel. Each signal is allowed to use the

channel for a fixed period of time one after the other.

8. What is frame?

One byte of data may be transmitted during the time interval assigned to a

particular channel. On transmission of each channel completes one cycle of operation

called a frame.

9. What is the function of a commutator?

To take a narrow sample of each of the N input message at the rate fs, that is

slightly higher than 2𝜔, where 𝜔 is the cut off frequency of the prealias filter

To sequentially interleave these N samples inside a sampling interval Ts=1/ fs

10. What are the applications of TDMA?

TDMA is used in the digital 2G cellular systems such as Global System for

Mobile communication (GSM), Personal Digital Cellular (PDC) & in the Digital

Enhanced Cordless Telecommunications (DCET) standard for portable phones.

11. What is ASCII framing?

In ASCII mode, messages start with a “colon (:)” character (ASCII 3A hex) &

end with a “carriage return-line feed” (CRLF) pair (ASCII 0D and 0A hex). The

allowable characters transmitted for all other fields are hexadecimal 0-9, A-F.

12. What is E1 framing?

E1 circuits are very common in most telephone exchanges & are used to

connect medium and large companies, to remote exchanges & in many cases between

exchanges. An E1 link operates over two separate sets of wires, usually coaxial cable.

13. What is T1 Framing?

T1 is a digital carrier. It transmits DS-1 signal. T1 operates at a base band bit

rate of 1.544Mbps.

14. Describe the ASCII frame format.

SYNC: it contains one character & used for starting synchronization.

Message length: It contains 3 characters between „006‟ & „252‟.

Slave address: it contains 2 characters from „00‟ & „99‟.

Message type: it consists of one character representing the type of host request.

They are case sensitive.

15. Describe the message type in ASCII frame format.

The message type in ASCII frame format is given below:

0 – read the basic data registers

1 – read basic setup

2 – write basic set up

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4 – reset/clear functions

J – read pulse counter set up

J – write pulse counter set up

S – real time clocks

16. Mention the bit rate of T1 framing.

T1 operates at a base band bit rate of 1.544Mbps. Only 1.536 Mbps is

accessible for payload data & the rest 8Kbps is reserved for framing.

17. Mention the purposes of E1 time slots.

TS0: Used for synchronization alarms & messages & reserved for framing

purposes

TS1-TS15: These time slots are used for user data.

TS16: E1 Signaling data is carried on TS16. This includes control, call setup &

teardown.

TS17-TS31: E1 frame time‟s slots are used for carrying user data.

18. Define ASCII code.

ASCII stands for American Standard Code for Information Interchange. It is a

seven bit code. The seven bits are formed, based on a standard binary propagation.

19. Draw T1 framing Format.

20. Draw E1 framing format.

21. Draw ASCII frame format.

Field no. 1 2 3 4 5 6 7

Contents SYNC(!) Message

Length

Slave

Address

Message

Type

Message

Body

Check

Sum

Trailer

(CRLF)

Length

char 1 3 2 1 0 to 246 1 2

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UNIT-4- DATA COMMUNICATION CODES

1. What are the different types of codes?

Baudot code

Binary code

ASCII code

EBCDIC code

2. What is ASCII code?

ASCII stands for American Standard Code for Information Interchange. It is a

seven bit code. The seven bits are formed, based on a standard binary propagation.

3. What are the advantages of ASCII code?

Error detection can be achieved by increasing the total numbers of bits to

8.The parity bit is added as the 8th bit, usually the MSB.

It can be easily used in a computer. A modern computer uses hexadecimal

code for internal computations. Since ASCII is an 8 bit code with parity bit, it

can be easily accommodated in computer as 8 bit data.

4. What is EBCDIC code?

EBCDIC code stands for Extended Binary Coded Decimal Interchange Code.

It is an 8 bit fixed length code. Here all bits are used for representing the

information.

5. What are the advantages of EBCDIC Code?

It is similar to ASCII code. It can be readily used in computer.

Total number of combinations is higher.

6. What are the disadvantages of EBCDIC code?

Here all the 8 bits are used for data encoding. There is no provision for parity

bit.

Here error correction is not possible.

7. What is baudot code?

The baudot code is a 5 bit code which has been used in telegraphy & paper

tape systems. There are totally 32 different code combinations (25=32).

This is not sufficient to represent the alphabets (26), numerals (10) and other

characters (+,-,>, <etc.) hence we increase the total number of combination by

incorporating the shift codes.

8. Mention the drawbacks of baudot code.

It does not provide extra combination of bits to code punctuation & various

codes.

All the five bits are used for coding the data. Hence error correction using

parity technique is not possible.

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9. What are the uses of baudot code?

Used in teletypewriter machines incorporating a paper to be punched & reader

mechanism.

10. What are the limitations of baudot code?

Only 5bits are available.

The numbers of combinations are limited only up to 25=32 codes.

This code is not sequential & it is not used for error detection & correction.

11. Mention the main methods for error control coding.

Forward acting error correction & Error detection with transmission

12. Define error detection.

Error detection is the process of monitoring data transmission & determining

when errors have occurred. They indicate only when an error has occurred.

13. Define hamming code.

Hamming code is an interesting error detection code. This code adds several

parity check bits to a data word. The hamming code is capable of locating a single

error but it fails if multiple errors occur in a single data block.

14. What are the common error detecting techniques?

Redundancy checking

Cyclic redundancy checking.

15. Define linear block code.

A code is said to be linear if the sum (modulo-2 sum of corresponding bits) of

any two of its code vectors results again in a code vector. i.e. the code words of the

code should obey the closure property with respect to modulo - 2 addition of

corresponding bits.

16. What is systematic code?

In the systematic block code, the message bit appears at the beginning of the

code word. The message bits appear first & then check bits are transmitted in a

block. This type of code is called systematic code.

17. Define parity code.

The most widely used approach for detecting errors that arise in storing &

moving is the use of a parity check bit added to each character code group. Code of

this type is called parity check codes. A parity bit (0 or 1) is added to the end of the

character code block according to some logical process.

18. Explain the types of parity.

Even parity means adding an extra bit to the group of bits to make the

group of bits to make the whole number of 1‟s as even.

In case of odd parity, add one more bit on the end of character, to make

the whole word as odd number of 1‟s.

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19. Define redundancy codes.

Most error detection system use some form of redundancy to check whether

the received data contains errors. That means, additional data is sent with the

basic data. The redundancy takes the form of transmitting the information

twice & comparing the two sets of data to see that are same.

If a discrepancy is noted between the two sets of data, an error is assumed and

the data is caused to be retransmitted. When the two sets of data are same

error free transmission is assumed.

20. Define CRC.

The cyclic redundancy check is a more reliable error detection scheme. It is a

mathematical technique which is applied to the transmitted data. It is effective in

catching 99.9% or more of transmission errors.

21. Mention the types of error correction methods.

Retransmission

Forward error correcting code

Hamming code.

22. What is CRC character?

The entire strings of bits in a block of data are considered to be one giant

binary number which is divided by some preselected constant. The quotient resulting

from the division is discarded and any remainder is retained. This reminder is known

as the character.

23. What is convolution coding?

A convolutional coding is done by combining the fixed number of input bits.

The input bits are stored in the fixed length shift register & they are combined with

the help of mod-2 adders. This operation is equivalent to being convolution & hence

it is called convolutional coding.

24. Define error correction codes.

Error correction is an important aspect of data transmission. This process

involved with error correction normally results in an increase in the number of bits

per second which are transmitted, & naturally this increases the cost of transmission.

25. What is retransmission?

Retransmission is the method of error correction. The information which has

errors is transmitted again. Then it checks for parity and if there is no error then it

transmits the information to the next block else the process is repeated.

26. What is error control?

In a data communication circuit, there occur errors during transmission of

information. The errors are reduced by using error control coding techniques. It is

used for error detection & error correction purpose.

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27. Expand CRC.

Cyclic Redundancy Code

28. Expand EBCDIC code.

Extended Binary Coded Decimal Interchange Code.

29. What is forward error correcting code?

Forward error correcting code is a code, which permits correction of errors by

the receive station, without retransmission. Error detection is done by including

sufficient redundancy in the transmitted data.

UNIT – 5- DATA NETWORK

1. What is a network?

A data network is a structure that enables a data user at one location to have

access to some data processing function or service available at another location.

2. What are the applications of data network?

They are used to provide access to a remote computer system for computational

tasks.

They are used for the remote updating of data bases in addition to accessing the

data; as in airline systems.

They are used for accessing remote data bases such as financial services.

3. What is simplex, half duplex & full duplex transmission?

Simplex - signals are transmitted in one direction only, with one acting as the

transmitter & the other station as the receiver.

Half duplex – both stations transmits, but only one at a time.

Full duplex – both transmits simultaneously.

4. What are the characteristics of a subnet?

The connections between stations & nodes are provided by point to point links.

The network may provide more than one possible path between any pair of

stations.

5. Mention the transmission mediums used in communication network.

Copper wires, optical fibers, radio links or satellites.

6. What is circuit switching?

Circuit switching provides a dedicated communication path or circuit between

two stations. The circuit consists of a connected sequence of links from source to

destination. The links may have time slots in form of TDM or FDM system.

7. What are the types of switching?

Circuit switching

Store & Forward switching

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8. What is ring topology?

In ring topology, the various nodes (stations) are connected together in the form

of a closed loop through ring interface units. All the nodes are connected to the nearest

adjacent nodes in a notational ring. In this topology, data travels around the ring in a

unidirectional manner.

9. Mention the 2 basic types of store & forward switching.

Message switching

Packed switching

10. What is multiple access communication?

In multiple access technique or multiple access communication networks, the

various nodes of the network share a multiple access medium

(e.g. satellite, radio broadcast, and multi-tap bus). Here the received signal at

any node depends on the transmitted signals at two or more modes.

11. Mention the layers of OSI model.

Physical layer, Data link control layer

Network layer, Transport layer

Session layer, Presentation layer

Application layer

12. Mention the different types of packet network.

Virtual circuit service & Datagram service

13. What is the function of network layer protocol?

Handle the routing of packets across the network is a major function of the

network layer protocol.

14. What are the advantages of packet switched network?

Speed & flexibility with which data are transmitted across the network are the

advantages of packet switched network.

15. Mention the different LAN topologies.

Bus, Ring, Star

16. What is LAN?

A local area network is a resource sharing data communication network with 3

general characteristics mentioned as follows:

It is limited in geographic scope to the range of 0.1 – 10km.

It provides high data rates over relatively inexpensive transmission media.

It is usually privately owned.

17. Expand TDMA & FDMA.

TDMA- Time Division Multiple Access

FDMA- Frequency Division Multiple Access

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18. Expand CSMA/CD.

CSMA/CD- Carrier Sense Multiple Access with Collision Detection

19. What are the features of CSMA/CD?

A sender continuously listens to the network so as to detect collisions

To transmits only when there is no activity detected in the medium.

If a collision is detected, the sender aborts transmission & then reschedules it.

20. What is star topology?

The star topology employs a central node, to establish a path between any pairs

of stations wishing to communicate with each other. The failure of node affects the

entire network

21. What are the functions of congestion control method?

The prevention of throughout degradation & loss of efficiency due to overload

The prevention of deadlocks.

A fair allocation of the resources among competing users of network.

22. What are the four classes of routing algorithm?

Isolated routing algorithm & Distributed routing algorithm

Centralized routing algorithms & Mixed routing algorithms

23. Mention any three principles of a layer.

Each layer performs well defined functions.

A boundary is created at a point where description of services offered is small.

A layer is created from easily localized function. So that modification can be

done without affecting other layers.

24. What are the features of routing algorithm?

A rapid delivery of packets when the current path ability to detect low delay

paths.

Find an alternative path when the current path becomes unusable due to link or

node failure

Detect an alternate path when the current path becomes congested.

25. What are the reasons for efficient use of a packet switched network?

No need for a call setup or disconnect procedure.

No provision for error control & used for real time applications.

26. Define OSI model.

The Open Systems Interconnection model (OSI) is a conceptual model that

characterizes and standardizes the internal functions of a communication system by

partitioning it into abstraction layers.