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IERG3310 COMPUTER NETWORKS DEPARTMENT OF INFORMATION ENGINEERING, THE CHINESE UNIVERSITY OF HONG KONG QUIZ ONE, SEP 2017, TIME ALLOWED: 20 MINUTES

STUDENT ID: (CLOSED BOOK & NOTES, ANSWER ALL QUESTIONS. TOTAL 20 MARKS.) 1. Name two examples of end-systems in a computer network. (2 marks) Ans: any computers, smartphones, IoT devices. (1 mark each)

2. In the Internet which protocol layers are implemented by end-systems? (2 marks) Ans: Application, transport, network, link, physical layers. (0.4 mark each) If student add session and/or presentation layers then those are simply ignored.

3. In the Internet which protocol layers are implemented by the network core? (2 marks) Ans: Network, link, and physical layers. (2/3 marks each) If student includes any upper layers then 2/3 marks will be deducted for each incorrect one.

4. Calculate the transmission delay for sending a 1000 bytes packet over a 1 Mbps network. (2 marks) Ans: Transmission delay = 1000 x 8 / 1000000 = 8 ms (2 marks) Note: Steps are not needed.

5. Calculate the propagation delay for Q4 provided that the distance to the destination is 1000 km and the signal propagation speed in the medium is 2×108 m/s. (2 marks) Ans: Propagation delay = 1000 x 1000 / 2x108 = 5 ms (2 marks) Note: Steps are not needed.

6. Repeat Q5 provided that the packet size is 10 KB and network bandwidth is 1 Gbps. (2 marks) Ans: Propagation delay = 1000 x 1000 / 2x108 = 5 ms (2 marks) Note: Propagation delay depends only on distance and propagation speed, not on packet size or bandwidth.

IERG3310 Computer Networks – Quiz #1

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7. In addition to transmission delay and propagation delay, a packet may also experience queueing delay at a router. Can queueing occur at a router with two 1 Gbps input links and one 1 Gbps output link? Why? (2 marks) Ans: Yes. (1 mark) The two input links together can generate up to 2 Gbps incoming traffic which exceeds the output link capacity, thereby could lead to queueing. (1 mark)

8. Can queueing occur at a router if it has one 1 Gbps input link and one 1 Gbps output link? (2 marks) Ans: No. (2 marks) Note: No explanation needed and if given, will be ignored.

9. Give two scenarios where a transmitted packet can be lost and failed to reach the receiver. (2 marks) Ans: Many possibilities, 1 mark each. Examples: - receiver host crashes, receiver application crash, network failure, operating system crash, etc.

10. Give one advantage and one disadvantage of packet switching compared to circuit switching. (2 marks) Ans: Advantage: more efficient use of network resources or allows statistical multiplexing (1 mark) Note: There could be other advantages. Disadvantage: does not guarantee performance (1 mark) Note: There could be other disadvantages.

IERG3310 Computer Networks – Quiz #1

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