Reg. No. Name: FIFTH SEMESTER B.TECH DEGREE …studoob.in/app/q/cse/s5/cs307/CS307_Model_2.pdf · QUESTION PAPER NOVEMBER 2017 ... Twisted-pair can be used for both analog and digital

Reg. No.______________

Name:________________________

FIFTH SEMESTER B.TECH DEGREE EXAMINATION MODEL TEST

QUESTION PAPER NOVEMBER 2017

CS 307 DATA COMMUNICATION

Time: 3 hour Max marks: 100

Part A

(Answer all questions ) 4 X 3=12

1. Discuss about simplex, half duplex and full duplex communication

2. Write a short note on frequency domain concepts

3. Why does single-mode fibres are used for large distance communications rather than

multi-mode fibres

4.

Explain satellite communication

Part B

(Answer any Two) 2 X 9=18

5.

6

7

8.

9.

10.

11.

Elaborate transmission Impairments in data communication

The digital signal is to be designed to permit 160 kbps for a bandwidth of 20

KHz. Determine (a) number of levels and (b) S/N ratio

Discuss about guided transmission media

Part C

(Answer all questions) 4 X 3=12

How does Manchester encoding differ from differential Manchester encoding

What is quantization error? How can it be reduced

In what situation Multiplexing is used. explain

compare FDM and WDM techniques

Part D

(Answer any Two) 2 X 9=18

12. Discuss Digital data Digital signal encoding with an example

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13.

14.

Elaborate Digital data Analog signal encoding methods

Explain the working of FDM

Part E

(Answer any Four) 4 X 10=40

15. What are the different error detection methods? Explain

16. Discuss about asynchronous and synchronous transmission

17. How Hamming code works .Explain with example

18. Describe the transmitter and receiver functions in DSSS

19. Compare packet switching and circuit switching network with neat diagrams

20. Elaborate the working of FHSS


CS 307 DATA COMMUNICATION ANSWER KEY

1. 1) Simplex (1 Mark)

A simplex communication channel only sends information in one direction.

For example, a radio station usually sends signals to the audience but never

receives signals from them, thus a radio station is a simplex channel.

It is also common to use simplex channel in fiber optic communication.

2) Half duplex ( 1Mark)

In half duplex mode, data can be transmitted in both directions on a signal carrier

except not at the same time.

At a certain point, it is actually a simplex channel whose transmission direction

can be switched. Walkie-talkie is a typical half duplex device

3) Full duplex ( 1Mark)

A full duplex communication channel is able to transmit data in both directions on

a signal carrier at the same time.

It is constructed as a pair of simplex links that allows bidirectional simultaneous

transmission. Take telephone as an example

2.

The frequency domain refers to the analysis of mathematical functions or signals

with respect to frequency, rather than time.

The inverse Fourier transform converts the frequency domain function back to a

time function. ( 2Mark)



( 1Mark)

3.

In a multi-mode fiber, the quality of signal-encoded light deteriorates more rapidly than

single-mode fiber, because of interference of many light rays. ( 1Mark)

Single-mode fiber allows longer distances without repeater. ( 1Mark)

Multi-mode fiber, the typical maximum length of the cable without a repeater is 2km,

whereas for single-mode fiber it is 20km. ( 1Mark)

4. Satellite communication is based on ideas similar to those used for line-of-sight. A

communication satellite is essentially a big microwave repeater or relay station in the sky.

( 1Mark)



( 2Mark)

5. The impairment can be broadly categorised into the following three types:

• Attenuation and attenuation distortion (3 Marks)

• Delay distortion (3 Marks)

• Noise (3 Marks)

Attenuation and attenuation distortion :

Irrespective of whether a medium is guided or unguided, the strength of a signal falls off

with distance.

In case of guided media, the attenuation is logarithmic, whereas in case of unguided

media it is a more complex function of the distance and the material that constitutes the

medium.

Delay distortion :

The velocity of propagation of different frequency components of a signal are different in

guided media. This leads to delay distortion in the signal.

For a bandlimited signal, the velocity of propagation has been found to be maximum near

the center frequency and lower on both sides of the edges of the frequency band.

In case of analog signals, the received signal is distorted because of variable delay of

different components

Noise

Signal is transmitted through a channel, undesired signal in the form of noise gets

mixed up with the signal, along with the distortion introduced by the transmission

media. Noise can be categorised into the following four types:

o Thermal Noise

o Intermodulation Noise



o Cross talk

o Impulse Noise

6.

(a) Apply Nyquist Bit Rate to determine number of levels. (4 Marks)

C = 2B log2 (M), or 160×103 = 2×20×103 log2 (M),

or

M = 24 , which means 4bits/baud.

(b) Apply Shannon capacity to determine the S/N ratio (5 Marks)

C = B log2 (1+S/N), or 160×103 = 20×103 log2 (1+S/N) ×103 log2 (M) ,

Or

S/N = 28 - 1, or S/N = 255, or S/N = 24.07 dB

7. Most commonly used guided transmission media such as twisted-pair of cable, coaxial

cable and optical fiber.

Twisted Pair (3 Marks)

Twisted-pair can be used for both analog and digital communication.

The data rate that can be supported over a twisted-pair is inversely proportional to

the square of the line length.

Maximum transmission distance of 1 Km can be achieved for data rates up to 1

Mb/s.

To reduce interference, the twisted pair can be shielded with metallic braid. This

type of wire is known as Shielded Twisted-Pair (STP) and the other form is

known as Unshielded Twisted-Pair (UTP).



Base band Coaxial With ``coax'', the medium consists of a copper core surrounded by

insulating material and a braided outer conductor. The term base band indicates digital

transmission (as opposed to broadband analog). (3 Marks)

Fiber Optics (3 Marks)

In fiber optic technology, the medium consists of a hair-width strand of silicon or

glass, and the signal consists of pulses of light.

A pulse of light means ``1'', lack of pulse means ``0''.

It has a cylindrical shape and consists of three concentric sections: the core, the

cladding, and the jacket

8.

In the Manchester encoding, a low-to-high transition represents a 1, and a high-to low

transition represents a 0.

There is a transition at the middle of each bit period, which serves the purpose of

synchronization and encoding of data.

In Differential Manchester, the encoding ofa 0 is represented by the presence of a

transition at the beginning of a bit period, and a 1 is represented by the absence of a

transition at the beginning of a bit period.

The midbit transition is only used for synchronization.

Any 3 differences( 3x1 marks)



9. (3 Marks)

To convert analog signal to digital signal, the analog signal is first sampled and

each of these analog samples must be assigned a binary code.

Each sample is approximated by being quantized into some binary codes.

The quantized values are only approximations, it is impossible to recover the

original signal exactly.

Error due to this quantization is known as quantization error.

Quantization error can be minimized by using non-linear encoding.

10. Multiplexing is used in situations where the transmitting media is having high

bandwidth, but the signals have lower bandwidth. Multiplexing can be used to achieve

• To send a large number of signals simultaneously. (3 Marks)

• To reduce the cost of transmission

• To make effective use of the available bandwidth

11. Wave-division multiplexing (WDM) is conceptually the same as FDM, except that the

multiplexing and demultiplexing involves light signals transmitted through fiber optic channels.

The idea is the same: we are combining different frequency signals. However, the difference is

that the frequencies are very high. Any 3 differences – 3x1 Marks

12.

If the signal elements all have the same algebraic sign, that is, all positive or

negative, then the signal is unipolar.

In polar signaling, one logic state is represented by a positive voltage level, and

the other by a negative voltage level.

The data signaling rate, or just data rate, of a signal is the rate, in bits per second,

that data are transmitted.

The duration or length of a bit is the amount of time it takes for the transmitter to

emit the bit; for a data rate R,the bit duration is 1/R.The modulation rate

( 3 Marks)



( 3 Marks)

( 3 Marks)

13. Amplitude Shift Keying (3 Marks)

In ASK, the two binary values are represented by two different amplitudes of the carrier

frequency. Commonly, one of the amplitudes is zero; that is, one binary digit is represented by

the presence, at constant amplitude, of the carrier, the other by the absence of carrier



Frequency Shift Keying (3 Marks)

The most common form of FSK is binary FSK (BFSK), in which the two binary values

are represented by two different frequencies near the carrier frequency

Phase Shift Keying (3 Marks)

In PSK, the phase of the carrier signal is shifted to represent data.

Two-Level PSK The simplest scheme uses two phases to represent the two binary

digits and is known as binary phase shift keying.

The resulting transmitted signal for one bit time is

14.

In frequency division multiplexing, the available bandwidth of a single physical

medium is subdivided into several independent frequency channels.



Independent message signals are translated into different frequency bands using

modulation techniques, which are combined by a linear summing circuit in the

multiplexer, to a composite signal. (4 Marks)

(3 Marks)

(2 Marks)

15. Basic approach used for error detection is the use of redundancy, where additional bits

are added to facilitate detection and correction of errors. Popular techniques are: (1 Marks)

• Simple Parity check (3 Marks)

• Two-dimensional Parity check (3 Marks)



• Cyclic redundancy check (3 Marks)



16. Asynchronous Transmission (5 Marks)

Two approaches are common for achieving the desired synchronization.

The first is called, oddly enough, asynchronous transmission.The strategy with this

scheme is to avoid the timing problem by not sending long, uninterrupted streams of bits.

Data are transmitted one character at a time, where each character is five to eight bits in

length.

Timing or synchronization must only be maintained within each character; the receiver

has the opportunity to resynchronize at the beginning of each new character



Synchronous Transmission (5 Marks)

A block of bits is transmitted in a steady stream without start and stop codes.

The block may be many bits in length.

To prevent timing drift between transmitter and receiver, their clocks must somehow be

synchronized

For digital signals, this can be accomplished with Manchester or differential Manchester

encoding.

For analog signals, a number of techniques can be used; for example, the carrier

frequency itself can be used to synchronize the receiver based on the phase of the carrier.

With synchronous transmission, there is another level of synchronization required, to

allow the receiver to determine the beginning and end of a block of data. To achieve this,

each block begins with a preamble bit pattern and generally ends with a postamble bit

pattern.

In addition, other bits are added to the block that convey control information used in the

data link control.

The data plus preamble, postamble, and control information are called a frame.

The exact format of the frame depends on which data link control procedure is being

used.



17. Error Correction can be handled in two ways.

One is when an error is discovered; the receiver can have the sender retransmit the entire data

unit. This is known as backward error correction.

In the other, receiver can use an error-correcting code, which automatically corrects certain

errors. This is known as forward error correction.

Basic approach for error detection by using Hamming code is as follows:

• To each group of m information bits k parity bits are added to form (m+k) bit code

• Location of each of the (m+k) digits is assigned a decimal value.

• The k parity bits are placed in positions 1, 2, …, 2k-1positions.–K parity checks are

performed on selected digits of each codeword.

• At the receiving end the parity bits are recalculated. The decimal value of the k parity bits

provides the bit-position in error, if any. (2 Marks)



(4 Marks)

Example:

Consider an example for 5-bit data. Here 4 parity bits are required. Assume that during

transmission bit 5 has been changed from 1 to 0. The receiver receives the code word and

recalculates the four new parity bits using the same set of bits used by the sender plus the

relevant parity (r) bit for each set. Then it assembles the new parity values into a binary number

in order of r positions (r8, r4, r2, r1). (4 Marks)



18. Direct sequence spread spectrum (DSSS), each bit in the original signal is represented by

multiple bits in the transmitted signal, using a spreading code. The spreading code spreads the

signal across a wider frequency band in direct proportion to the number of bits used. Therefore, a

10-bit spreading code spreads the signal across a frequency band that is 10 times greater than a

1-bit spreading code. One technique with direct sequence spread spectrum is to combine the

digital information stream with the spreading code bit stream using an exclusive-OR (XOR).





19. Circuit Switching (2 Marks)

In circuit switching network dedicated channel has to be established before the

call is made between users.

The channel is reserved between the users till the connection is active. For half

duplex communication, one channel is allocated and for full duplex

communication, two channels are allocated.

It is mainly used for voice communication requiring real time services without

any much delay.

If user-A wants to use the network; it need to first ask for the request to obtain the

one and then user-A can communicate with user-C. During the connection phase if

user-B tries to call/communicate with user-D or any other user it will get busy signal

from the network.

Packet Switching (2 Marks)

In packet switching network unlike CS network, it is not required to establish the

connection initially.

The connection/channel is available to use by many users. But when capacity or

number of users increases then it will lead to congestion in the network.

Packet switched networks are mainly used for data and voice applications

requiring non-real time scenarios.



If user-A wants to send data/information to user-C and if user-B wants to send

data to user-D, it is simultaneously possible.

The information is padded with header which contains addresses of source and

destination. The header is sniffed by intermediate switching nodes to determine

their route and destination.

(6x1 Marks)

20.

A number of channels are allocated for the FH signal. Typically, there are carrier

frequencies forming channels.

The spacing between carrier frequencies and hence the width of each channel

usually corresponds to the bandwidth of the input signal.

The transmitter operates in one channel at a time for a fixed interval; for example,

the IEEE 802.11 standard uses a 300-ms interval.

During that interval, some number of bits is transmitted using some encoding

scheme.

A spreading code dictates the sequence of channels used. Both transmitter and

receiver use the same code to tune into a sequence of channels in synchronization.

(3x1 Marks)



(2 Marks)

(Fig: 3 Marks)

Explain slow and fast FHSS also— ( 2 Marks)


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Reg. No. Name: FIFTH SEMESTER B.TECH DEGREE …studoob.in/app/q/cse/s5/cs307/CS307_Model_2.pdf · QUESTION PAPER NOVEMBER 2017 ... Twisted-pair can be used for both analog and digital