UGC NET COMPUTERSCIENCE SOLVED QUESTIONS WITH EXPLANATION in Data Communication and Network

        1.       An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, then baud rate and bit rate of the signal are---------- and -------------

1.       4000 bauds\sec & 1000bps

2.       2000 bauds\sec & 1000bps

3.       1000 bauds\sec & 500bps

4.       1000 bauds\sec & 4000bps

                Ans: 1000 bauds\sec & 4000bps

                Explanation:

                                Bit rate is number of bits per second. Baud rate is the number of signals per second

                                So baud rate of the signal is 1000 bauds\sec

                                Bit rate                 of the signal is =1000*4=4000 bps

2. Using RSA algorithm, what is the value of cipher text c, if the plain text M=5 and p=3,q=11 & d=7?

1.       33

2.       5

3.       25

4.       26

Ans: 26

Explanation:

C=M^e       (mod n)

P=3

q=11

n=p*q=3*11=33

Φ(n)=(p-1)*(q-1)

=(3-1)*(11-1)

= 2*10

= 20

d=7

de mod Φ(n)=1 

7*e mod 20=1

So e=3

e  is relatively prime to Φ(n) and less than Φ(n)

gcd(Φ(n),e)=1;  1<e< Φ(n)          

C= 5 ³  ( mod 33)

=125 mod 33

C=26

3. The start and stop bits are used in serial communication for

a.       error detection

b.      error correction

c.       synchronization

d.      slowing down the communication

Ans: synchronization

4. In classful addressing an IP address 123.23.156.4 belongs to ------ class format

a.       A    b.  B    c. C     d.  D

Ans: A

5. Which of the following is a bit rate of an 8-PSK signal having 2500Hz bandwidth?

a.       2500bps

b.      5000bps

c.       7500bps

d.      20000bps

Ans:  7500bps

In 8 PSK the bit rate is 3 times the baud rate (for PSK baud rate is same as bandwidth)

So bit rate is 3*2500=7500bps