Computer Networks ISO/OSI Stack and SWP GATE Previous Year Solved Questions

Ques 1: Host A is sending data to host B over a full duplex link. A and B are using the sliding window protocol for flow control. The send and receive window sizes are 5 packets each. Data packets (sent only from A to B) are all 1000 bytes long and the transmission time for such a packet is 50 µs. Acknowledgement packets (sent only from B to A) are very small and require negligible transmission time. The propagation delay over the link is 200 us. What is the maximum achievable throughput in this communication?

GATE 2003

(A) 7.69 x 10⁶ bps

(B) 11.11 x 10⁶ bps

(C) 12.33 x 10⁶ bps

(D) 15.00 x 10⁶ bps

Ans: (B) 11.11 x 10⁶ bps

Solution:

Ques 2: Choose the best matching

GATE 2004

(P) Data Link Layer

(Q) Network layer

(R) Transport layer

1. Ensures reliable transport of data over a physical point- to-point link

2. Encodes/decodes data for physical transmission

3. Allows end-to-end communication between two  processes

4. Routes data from one network node to the next

P Q R

(A) 1 4 3

(B) 2 4 1

(C) 2 3 1

(D) 1 3 2

Ans: (A) P-1 Q-4 R-3

Ques 3: How many 8−bit characters can be transmitted per second over a 9600 baud serial communication link using asynchronous mode of transmission with one start bit, eight data bits, two stop bits and one parity bits?

GATE 2004

(A) 600

(B) 800

(C) 876

(D) 1200

Ans: (B) 800

Ques 4: A serial transmission T1 uses 8 information bits, 2 start bits, 1 stop bit and 1 parity bit for each character. A synchronous transmission T2 uses 3 eight-bit sync characters followed by 30 eight-bit information characters. If the bit rate is 1200 bits/second in both cases, what are the transfer rates of  T1 and T2?

GATE 2004

(A)  characters/sec, 153 characters/sec

(B) 80 characters/sec, 136 characters/sec

(C) 100 characters/sec, 136 characters/sec

(D) 80 characters/sec, 153 characters/sec

Ans: (C) 100 characters/sec, 136 characters/sec

Ques 5: In a data link protocol, the frame delimiter flag is given by 0111. Assuming that bit stuffing is employed, the transmitter sends the data sequence 01110110 as

GATE 2004

(A) 01101011

(B) 011010110

(C) 011101100

(D) 0110101100

Ans: (D) 01101011100

Ques 6: In a sliding window ARQ scheme, the transmitter’s window size is N and the receiver’s window size is M. The minimum number of distinct sequence numbers required to ensure correct operation of the ARQ scheme is

GATE 2004

(A) min (M,N)

(B) max (M,N)

(C) M + N

(D) MN

Ans: (C) M + N

Ques 7: A 20 Kbps satellite link has a propagation delay of 400 ms. The transmitter employs the “go back n ARQ” scheme with n set to 10. Assuming that each frame is 100 byte long, what is the maximum data rate possible?

GATE 2004

(A) 5 Kbps

(B) 10 Kbps

(C) 15 Kbps

(D) 20 Kbps

Ans: (B) 10 Kbps

Ques 8: Consider a parity check code with three data bits and four parity check bits. Three of the Code Words are 0101011,1001101 and 1110001. Which of the following are also code words?

1. 0010111
2. 0110110
3. 1011010
4. 0111010

GATE 2004

(A) 1 and 3

(B) 1, 2 and 3

(C) 2 and 4

(D) 1, 2, 3 and 4

Ans: (A) 1 and 3

Ques 9: The maximum window size for data transmission using the selective reject protocol with n-bit frame sequence numbers is:

GATE 2005

(A) 2ⁿ

(B) 2^n-1

(C) 2ⁿ-1

(D) 2^n-2

Ans: (B) 2^n-1

Ques 10: Consider the following message M=1010001101. The cyclic redundancy check (CRC) for this message using the divisor polynomial x⁵+x⁴+x²+1 is :

GATE 2005

(A) 01110

(B) 01011

(C) 10101

(D) 10110

Ans: (A) 01110