CS 6304 ANALOG AND DIGITAL COMMUNICATION

2 MARKS and 16 MARKS QUESTIONS AND ANSWERS

2 MARKS UNIT 1
1. Define noise
Noise is an unwanted electrical signal which gets added tom a transmitted signal when it is travelling
towards the receiver
2. Define noise figure
Noise figure is a figure of merit and used to indicate how much the signal to noise ratio gets degraded as a
signal passes through a series of circuits
3. What is meant by analog communication system?
The modulation systems or a technique in which one of the characteristics of the carrier is changed in
proportion with the instantaneous value of modulating signal is called analog communication system.
4. Define modulation
Modulation is defined as changing the characteristics of the carrier signal with respect to the instantaneous
change in message signal.
5. What are the needs for modulation?
In order to carry the low frequency message signal to a longer distance, the high frequency carrier signal
is combined with it.
a) Reduction in antenna height. b) Long distance communication. c) Ease of radiation.
d) Multiplexing. e) Improve the quality of reception. f) Avoid mixing up of other signals.
6. What are the advantages of Analog Communications?
a) Transmitters and Receivers are simple
b) Low bandwidth requirement
c) FDM can be used
7. What are the disadvantages of analog communication?
a) Noise affects the signal quality
b) It is not possible to separate noise and signal
c) Repeaters cant be used between transmitters and receivers
d) Coding is not possible
e) It is not suitable for the transmission of secret information
8. Define Amplitude modulation.
Amplitude Modulation is defined as changing the amplitude of the carrier signal with respect to the
instantaneous change in message signal.
9. Define Frequency modulation.
Frequency Modulation is defined as changing the frequency of the carrier signal with respect to the
instantaneous change in message signal.
10. Define Phase modulation.
Phase Modulation is defined as changing the phase of the carrier signal with respect to the instantaneous
change in message signal.
11. Define modulation index
It is defined as ratio of amplitude of the message signal to the amplitude of the carrier signal.
m=E /E
12. Define percentage modulation
It s defined as the percentage change in the amplitude of the output wave whn the carrier is acted on by a
modulating signal.
M=(E /E )*100
m

13. State Carsons rule.

Carsons rule states that the bandwidth required to transmit an angle modulated wave as twice the sum of
the peak frequency deviation and the highest modulating signal frequency. Mathematically carsons rule is

B = 2(f +fm) Hz.

14. Define Deviation ratio.
Deviation ratio is the worst case modulation index and is equal to the maximum peak frequency deviation divided
by the maximum modulating signal frequency. Mathematically, the deviation ratio is
DR= f (max)/fm(max)

UNIT 2
1. Define Digital modulation
Digital Modulation is defined as changing the amplitude of the carrier signal with respect to the binary
information or digital signal.
2. What are the advantages of Digital Communications?
a) It has a better noise immunity
b) Repeaters can be used between transmitters and receivers
c) It becomes simpler and cheaper as compared to the analog communication
3. What are the disadvantages of Digital Communications?
a) It requires a larger channel bandwidth
b) Delta modulation needs synchronization incase of synchronous modulation.
4. Define bit rate
Bit rate is defined as number of bits transmitted during one second between the transmitter and receiver.
5. Define baud rate.
Baud rate is defined as the rate of change of signal on transmission medium after encoding and
modulation have occurred.
6. Define Amplitude Shift Keying (ASK).
Amplitude Shift Keying is defined as changing the amplitude of the carrier signal with respect to the
binary information or digital signal.
7. Define Frequency Shift Keying (FSK).
Frequency Shift Keying is defined as changing the amplitude of the carrier signal with respect to the
binary information or digital signal.
8. Define Phase Shift Keying (PSK).
Phase Shift Keying is defined as changing the amplitude of the carrier signal with respect to the binary
information or digital signal.
9. Define Minimum Shift Keying (MSK).
The minimum frequency space that allows the 2 FSK representing symbols 0s and 1s. Thus CP
(Continuous Phase) FSK signal with a deviation ratio if one half is defined as MSK.
10. Define Quadrature Amplitude modulation (QAM).
QAM is defined as changing the amplitude as well as the frequency of the carrier signal with respect to
the binary information or digital signal.
11. Define bandwidth efficiency
Bandwidth efficiency is the ratio of the transmission bit rate to the minimum bandwidth required for a
particular modulation
12. What are the advantages of MSK?
MSK baseband waveform are smoother compared with QPSK MSK signals have continuous phase
It does not have any amplitude variation
13. What are the advantages of QPSK?
a) Very good noise immunity
b) Effective utilization of available bandwidth
c) Low error probability
d) Very high bit rate data transmission.

UNIT 3
1. What is meant by data communication?
Data communication can be defined as two personal computers connected through a Public
Telecommunication Network (PTN)
2. What are the applications of data communication?
a) Used in Automatic Teller Machine (ATM) Internet
b) Airline and Hotel reservation system
c) Mass media
d) NEWS network
3. What are the advantages and disadvantages of Parallel Communication?
Advantages:
a) Parallel transmission is speed
b) Used for short distance communication
Disadvantages:
a) Require more lines between source and destination
b) More cost
4. What are the advantages and disadvantages of series communication?
Advantages:
a) Number of transmission lines is less
b) Used for long distance communication
c) Low cost
Disadvantages: Speed is low
5. What is meant by point to point communication?
Point to point communication is the link between two stations A and B ie)., information is transferred
between a main frame computer and a remote computer terminal
6. What is multipoint communication?
A multipoint line configuration is one in which more than two specific devices share a single link
7. Define Morse code.
It is used to send messages. A key which turned the carrier of a transmitter ON and OFF to produce the
dots and dashes. These dots and dashes were detected at the receiver and it is converter back into letters and
numbers makes the original message
8. What are the different types of error detection techniques?
a. Redundancy
b. Echoplex
c. Exact count encoding
d. Parity
e. Check sum
f. Vertical Redundancy Check
g. Horizontal Redundancy Check
h. Cyclic Redundancy Check
9. What is meant by Forward Error Correction (FEC)?
FEC, a receiver can use an error correcting code, which automatically correct certain errors without any
retransmissions. In FEC, bits are added to the message before the transmission. Purpose of FEC code is to reduce
the wasted time of retransmission
10. Define Pulse Amplitude modulation
The amplitude of a carrier pulse is altered in accordance to that of amplitude of message signal to make it
accommodate the information signal.
11. Define Pulse width modulation
In PWM system, the message signals are used to vary the duration of carrier pulse. The message signal
may vary either the trailing edge or leading edge or both of the carrier pulses n order to accommodate the
intelligence of information system.

12. Define Pulse position modulation

The position of a carrier pulse is altered in accordance with information contained in sampled waveform.
13. Define Pulse code modulation
Pulse code modulation refers a form of source coding. It is a form of digital modulation techniques in
which the code refers a binary word that represent digital data.With PCM, the pulses are of fixed length and fixed
amplitude.
14. Define sampling rate
The sampling rate f must be atleast two times the highest frequency component of the original signal to be
accurately represented
f >=2f
s

UNIT 4
1. Define entropy
The entropy of a source is a measure of the average amount of information per source symbol in a long
message
2. What is meant by variable length coding?
The message occurring frequently can be assigned short code words, whereas message which occur rarely
are assigned long code word, such coding is called variable length coding
3. Define source encoder
The efficient representation of data generated by a discrete source is known as source encoding. This
device that performs this representation is called source encoder.
4. Define source encoding theorem
The discrete memory less source of entropy H(X), the average code word length (L) for any distortion less
source encoding is bounded.
5. Define code redundancy
It is the measure of redundancy bits in the encoded message sequence.
6. Mention the needs for encoding
a) To improve the efficiency of communication
b) To improve the transmission quality
7. What are the advantages of Shannon fano coding
a) Reduced bandwidth
b) Reduced noise
c) It can be used for error detection and correction
8. Define mutual information
It is defined as the amount of information transferred when X is transmitted and Y is received. It is
represented by I (X ,Y ). The average mutual information is defined as the amount of source information gain per
received symbol
9. Define channel capacity
Channel capacity is defined as the maximum of the mutual information that may be transmitted through
the channel
10. Define channel coding theorem
Channel coding theorem is applied for discrete memory less additive white Gaussian noise channels
11. What are the types of error control method?
a) Error detection and retransmission
b) Error detection and correction
12. Define linear block codes
A block code of length n and 2k code words is called linear (n, k) code if and only if its 2k code words
form a k-dimensional subspace of the vector space of all the n-tuples over the field GF(2).
13. What are the objectives of cyclic codes?
Encoding and syndrome calculations can be easily implemented by using simple shift register with
feedback connection. It is possible to design codes having useful error correction properties
i

UNIT 5
1. Define Advanced Mobile Phone Services (AMPS).
AMPS are a Standard Cellular Telephone Service (CTS). The AMPS system uses a seven cell reuse
pattern with provisions for sectoring and cell splitting to increase channel when needed. AMPS use frequency
modulation and frequency division duplex for radio transmission.
2. What is meant by super audio tone (SAT)?
SAT is superimposed on the voice signal on both the forward and reverse link and is barely audible to the
user
The particular frequency of the SAT denotes the particular base station location for a given channel and
is assigned by the MSC for each call.
3. What is meant by signaling tone?
The signaling tone is a 10 kbps data base which signals call termination by the subscriber. It is a special
end of call message consisting of alternating 1s and 0s which is sent on the RVC by the subscriber unit for 200ms.
The signaling tone alerts the base station that a subscriber has ended the call.
4. Define GSM.
Global system for mobile communication (GSM) is a globally accepted standard for digital cellular
communication. GSM is the name of a standardization group established in 1982 to create a common European
mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system
operating at 900MHz. It is estimated that many countries outside of Europe will join the GSM partnership.
5. What is meant by Telephone services in GSM?
Teleservices provides communication between two end user applications according to a standard protocol.
GSM mainly focuses on voice oriented tele services. This service includes emergency calling and facsimile. GSM
also supports video text and tele text.
6. Name the different types of GSM services
a. Telephone services
b. Data services
c. Supplementary services
7. Define Subscriber Identity Module (SIM).
It is a memory device that stores information such as the subscribers identification number, the networks
and countries where the subscriber is entitled to service, privacy tax and other user specific information. A
subscriber uses the SIM with a four digit personal ID number to activate service from GSM phone.
8. Define HLR.
Home location register (HLR)The HLR is a database used for storage and management of
subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers,
including a subscriber's service profile, location information, and activity status. When an individual buys a
subscription from one of the PCS operators, he or she is registered in the HLR of that operator.
9. Define VLR.
Visitor location register (VLR)The VLR is a database that contains temporary information about
subscribers that is needed bythe MSC in order to service visiting subscribers. The VLR is always integrated with
the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data
about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the
information needed for call setup without having to interrogate the HLR each time.
10. Define MSC.
Mobile services switching center (MSC)The MSC performs the telephony switching functions of the
system. It controls calls to and from other telephone and data systems. It also performs such functions as toll
ticketing, network interfacing, common channel signaling, and others.
11. What is Bluetooth?
Bluetooth is a standard developed by a group of electronics manufacturers that allows any sort of
electronic equipment from computers and cell phones to keyboards and headphones to make its own connections,
without wires, cables or any direct action from a user. Bluetooth is intended to be a standard that works at two
levels.

12. What are the three orbits of satellite?

a) Low Earth orbit
b) Medium Earth orbit
c) Geosynchronous Earth orbit
13. State Keplers laws of planetary motion
1. A satellite will orbit a primary body following an elliptical path
2. For equal intervals of time a satellite will sweep out equal areas in orbital plane
3. The square of the periodic time of orbit is proportional to the cube of the mean distance between the
primary and the satellite
14. What are the links in satellite communication?
i) Uplink
ii) Downlink
iii) Crosslink
15. Define apogee
The point in an orbit that is located farthest from earth
16. Define perigee
The point in an orbit that is located closest to earth
17. Define Handoff.
When a user/call moves to a new cell, then a new base station and new channel should be assigned
(handoff)
Handoffs should be transparent to users, while their number should be kept to minimum
A threshold in the received power (Pr, handoff) should be determined to trigger the handoff process. This
threshold value should be larger than the minimum acceptable received power (Pr, acceptable)
Define: =Pr,handoff - Pr,acceptable
If is large then too many handoffs
If is small then insufficient time to complete a handoff.