CN |Gate-2006| Previous Year Questions| Set-15

Set-15 GATE-2006 CN

CN |Gate-2006| Computer Networks

1. For which one of the following reasons does Internet Protocol (IP) use the time-to- live (TTL) field in the IP datagram header? [GATE – 2006]

a. Ensure packets reach destination within that time
b. Discard packets that reach later than that time
c. Prevent packets from looping indefinitely
d. Limit the time for which a packet gets queued in intermediate routers

Answer : c)


2. Consider the diagram shown below where a number of LANs are connected by (transparent) bridges. In order to avoid packets looping through circuits in the graph, the bridges organize themselves in a spanning tree. First, the root bridge is identified as the bridge with the least serial number. Next, the root sends out (one or more) data units to enable the setting up of the spanning tree of shortest paths from the root bridge to each bridge. Each bridge identifies a port (the root port) through which it will forward frames to the root bridge. Port conflicts are always resolved in favour of the port with the lower index value. When there is a possibility of multiple bridges forwarding to the same LAN (but not through the root port), ties are broken as follows: bridges closest to the root get preference and between such bridges, the one with the lowest serial number is preferred. 

 For the given connection of LANs by bridges, which one of the following choices represents the depth first traversal of the spanning tree of bridges? [GATE – 2006]

a. B1, B5, B2, B3, B4
b. B1, B3, B5, B2, B4
c. B1, B5, B3, B4, B2
d. B1, B3, B4, B5, B2

Answer : a)


3. Station A needs to send a message consisting of 9 packets to Station B using a sliding window (window size 3) and go-back-n error control strategy. All packets are ready and immediately available for transmission. If every 5th packet that A transmits gets lost (but no acks from B ever get lost), then what is the number of packets that A will transmit for sending the message to B? [GATE – 2006]

a. 12
b. 14
c. 16
d. 18

Answer : c)


4. Station A uses 32 byte packets to transmit messages to Station B using a sliding window protocol. The round trip delay between A and B is 80 milliseconds and the bottleneck bandwidth on the path between A and B is 128 kbps. What is the optimal window size that A should use? [GATE – 2006]

a. 20
b. 40
c. 160
d. 320

Answer : b)


5. Two computers C1 and C2 are configured as follows. C1 has IP address 203.197.2.53 and netmask 255.255.128.0. C2 has IP address 203.197.75.201 and netmask 255.255.192.0. Which one of the following statements is true? [GATE – 2006]

a. C1 and C2 both assume they are on the same network
b. C2 assumes C1 is on same network, but C1 assumes C2 is on a different network
c. C1 and C2 both assume they are on different networks
d. C1 assumes C2 is on same network, but C2 assumes C1 is on a different network

Answer : d)

6. HELO and PORT, respectively, are commands from the protocols : [GATE – 2006]

a. SMTP and FTP
b. HTTP and TELNET
c. TELNET and POP3
d. FTP and HTTP

Answer : a)


7. Which of the following statements is TRUE? [GATE – 2006]

a. Both Ethernet frame and IP packet include checksum fields
b. Ethernet frame includes a checksum field and IP packet includes a CRC field
c. Ethernet frame includes a CRC field and IP packet includes a checksum field
d. Both Ethernet frame and IP packet include CRC fields

Answer : c)


8.  Which of the following statement(s) is TRUE? [GATE – 2006]

1. A hash function takes a message of arbitrary length and generates a fixed length code.
2. A hash function takes a message of fixed length and generates a code of variable length.
3. A hash function may give the same hash value for distinct messages.

a. 1 only
b. 2 only
c. 2 and 3 only
d. 1 and 3 only

Answer : d)


9. Suppose that it takes 1 unit of time to transmit a packet (of fixed size) on a communication link. The link layer uses a window flow control protocol with a window size of N packets. Each packet causes an ack or a nak to be generated by the receiver, and ack/nak transmission times are negligible. Further, the round trip time on the link is equal to N units. Consider time i > N. If only acks have been received till time i(no naks), then the goodput evaluated at the transmitter at time i(in packets per unit time) is : [GATE – 2006]

a. 1 – N/i
b. i/(N + i)
c. 1
d. 1 – e(i/N)

Answer : a)


10. In the 4B/5B encoding scheme, every 4 bits of data are encoded in a 5-bit codeword. It is required that the codewords have at most 1 leading and at most 1 trailing zero. How many such codewords are possible? [GATE – 2006]

a. 14
b. 16
c. 18
d. 20

Answer : c)


11. A router has two full-duplex Ethernet interfaces each operating at 100 Mb/s. Ethernet frames are at least 84 bytes long (including the Preamble and the Inter-Packet-Gap). The maximum packet processing time at the router for wirespeed forwarding to be possible is (in micro­seconds) [GATE – 2006]

a. 0.01
b. 3.36
c. 6.72
d. 8

Answer : b)
CN |Gate-2006|


12. A link of capacity 100 Mbps is carrying traffic from a number of sources. Each source generates an on-off traffic stream; when the source is on, the rate of traffic is 10 Mbps, and when the source is off, the rate of traffic is zero. The duty cycle, which is the ratio of on-time to off-time, is 1:2. When there is no buffer at the link, the minimum number of sources that can be multiplexed on the link so that link capacity is not wasted and no data loss occurs is S1. Assuming that all sources are synchronized and that the link is provided with a large buffer, the maximum number of sources that can be multiplexed so that no data loss occurs is S2. The values of S1 and S2 are, respectively, [GATE – 2006]

a. 10 and 30
b. 12 and 25
c. 5 and 33
d. 15 and 22

Answer : a)
CN |Gate-2006|


13. On a wireless link, the probability of packet error is 0.2. A stop-and-wait protocol is used to transfer data across the link. The channel condition is assumed to be independent from transmission to transmission. What is the average number of transmission attempts required to transfer 100 packets? [GATE – 2006]

a. 100
b. 125
c. 150
d. 200

Answer : b)
CN |Gate-2006|


14. A program on machine X attempts to open a UDP connection to port 5376 on a machine Y, and a TCP connection to port 8632 on machine Z. However, there are no applications listening at the corresponding ports on Y and Z. An ICMP Port Unreachable error will be generated by : [GATE – 2006]

a. Y but not Z
b. Z but not Y
c. Neither Y nor Z
d. Both Y and Z

Answer : d)
CN |Gate-2006|


15. A subnetted Class B network has the following broadcase address : 144.16.95.255. Its subnet mask  : [GATE – 2006]

a. could be any one of 255.255.224.0, 255.255.240.0, 255.255.248.0
b. is necessarily 255.255.248.0
c. is necessarily 255.255.240.0
d. is necessarily 255.255.224.0

Answer : a)


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