This document contains a summary of key concepts in digital communication. It discusses digital communication, quantizing, encoding, advantages and disadvantages of digital communication, basic signal processing operations, common channels used, telephone channel specifications, adaptive equalization, waveform coding techniques including pulse modulation, analog pulse modulation types, digital pulse modulation types like PCM and DM, sampling, quantizing, uniform and non-uniform quantization, companding, applications of PCM, advantages and disadvantages of delta modulation and DPCM, and an introduction to digital modulation techniques.

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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.