Computer Networks

Q21. There are n stations in a slotted LAN. Each station attempts to transmit with a probability p in each time slot. What is the probability that ONLY one station transmits in a given time slot? (Gate 2007)

1. np(1 − p)ⁿ⁻¹
2. (1 − p)ⁿ⁻¹
3. p(1 − p)ⁿ⁻¹
4. 1 − (1 − p)ⁿ⁻¹

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Q22. Which one of the following uses UDP as the transport protocol?(Gate 2007)

1. HTTP
2. Telnet
3. DNS
4. SMTP

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Q23. In Ethernet when Manchester encoding is used, the bit rate is(Gate 2007)

1. Half the baud rate
2. Twice the baud rate
3. Same as the baud rate
4. None of these

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Q24. 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)

1. 12
2. 14
3. 16
4. 18

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Q25. 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 favor 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)

1. B1,B5,B3,B4,B2
2. B1,B3,B5,B2,B4
3. B1,B5,B2,B3,B4
4. B1.B3.B4.B5.B2

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Q26.Consider the correct spanning tree for the previous question. Let host H1 send out a broadcast ping packet. Which of the following options represents the correct for forwarding table on B3? (Gate 2006)

1. H1,H2,H3,H4	3
   H5,H6,H9,H10	1
   H7,H8,H11,H12	2

2. H1, H2	4
   H3, H4	3
   H5, H6	1
   H7, H8, H9, H10,H11,H12	2

3. H3, H4	3
   H5, H6, H9, H10	1
   H1, H2	4
   H7, H8, H11, H12	2

4. H1, H2, H3, H4	3
   H5, H7, H9, H10	1
   H7, H8, H11, H12	4

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Q27. Two computers C1 and C2 are configured as follows. C1 has an IP address of 203. 197.2.53 and netmask 255.255. 128.0. C2 has an IP address of 203.197.75.201 and a netmask 255.255.192.0. Which one of the following statements is true? (Gate 2006)

1. C1 and C2 both assume they are on the same network
2. C2 assumes C1 is on the same network, but C1 assumes C2 is on a different network
3. C1 assumes C2 is on the same network, but C2 assumes C1 is on a different network
4. C1 and C2 both assume they are on different networks

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Q28. 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 A and B is 128 kbps. What is the optimal window size that A should use? (Gate 2006)

1. 20
2. 40
3. 160
4. 320

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Q29. For which one of the following reason: does Internet Protocol (IP) use
the time-to-live (TTL) field in the IP datagram header? (Gate 2006)

1. Ensure packets reach the destination within that time
2. Discard packets that reach later than that time
3. Prevent packets from looping indefinitely
4. Limit the time for which a packet gets queued in the intermediate
   routers

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Q30. Suppose the round trip propagation delay for a 10 Mbps Ethernet having a 48-bit jamming signal is 46.4 μs. The minimum frame size is :(Gate 2005)

1. 94
2. 416
3. 464
4. 512

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