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ELEC 4120: Computer Communications Networks
Prof. Tsang, Danny H. K.
(Fall 2012)

Department of Electronic and Computer Engineering
The Hong Kong University of Science and Technology

Mid-Term Examination

Date: Oct. 25, 2012, Thursday
Time: 10:30 am C 11:50 am (1 hour and 20 minutes)
Venue: Room 1505

Total Problems: 4
Possible Points: 100





1. This is a closed book exam.

2. This examination paper consists of 4 questions.

3. Please write your name, student ID and Email on this page.

4. You may use a simple calculator.

5. Please answer all the questions on the answer book provided by the examiner, except the questions giving specific instructions.

6. Please read each question very carefully and answer the question clearly and to the point. Make sure that your answers are neatly written, readable and legible.

7. Show all the steps you use in deriving your answer, wherever appropriate.

8. Return this question paper along with your answer book. You are not allowed to leave the exam room with this question paper.

Good Luck!

PROBLEM 1 Multiple Choice (30 pts)

For each of the following questions there may be one or multiple correct answers. Please write down the correct answers in your answer book. You will receive 3 points for each correctly answered question, 1 point if you select less but correct choices for each question, and 0 points otherwise.

1. We are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 x 108 meters/s, and the distance between source and destination is 10,000 km. Initially suppose there is only one link between source and destination. Also suppose that the entire MP3 file is sent as one packet. The transmission delay is: a. 50 milliseconds

b. 3 seconds

c. 3.05 seconds

d. none of the above.

2. Referring to the above Question 1, the end-to-end delay (transmission delay plus propagation delay) is a. 3.05 seconds

b. 3 seconds

c. 6 seconds

d. none of the above

3. Now suppose there are two links between source and destination, with one router connecting the two links in the middle (each link is 5,000 km long). Again suppose the MP3 file is sent as 3 packets, and each packet is 10Mbits long (ignore headers added to the packets). Also ignore router processing delays. Assume store and forward packet switching at the router and suppose there is no congestion so that the packet is transmitted onto the second link as soon as the router receives the entire packet. The end-to-end delay is a. 3.05 seconds

b. 4.10 seconds

c. 4.05 seconds

d. 6.05 seconds

e. none of the above