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(COMP361)mid_sol_Spr2006.pdf
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COMP 361 Computer Communications Networks I
Spring Semester 2006
Midterm Examination
Date: April 8, 2006 Time: 2:00 pm C 3:30 pm

Name:_____________________ Student ID: ________________Email:__________________
Instructions
1.
Please write your name, student ID and email address on this page.

2.
Please answer all questions within the space provided on the examination paper. Please be concise, and this is NOT an essay contest.

3.
This paper consists of 4 questions and 8 pages.

4.
Please read each question very carefully and answer the questions clearly to the point. Make sure your answers are neatly written, legible, and readable.

5.
Show all the steps used in deriving your answer, wherever appropriate.


Question Points Score
1 20
2 20
3 15
4 10
5 10
6 25
Total 100

Dept. of Computer Science HKUST
1. (20 points) Answer the following true/false questions by circling either T or F.
a) UDP does not provide any flow control or congestion control T F
b) Cable modems offers dedicated access to the Internet. T F
c) HTTP and SMTP are called all in-band protocols T F
d) In a P2P system, the central difficulty is how to retrieve the content from a peer T F
e) SMTP and POP3 are both pull protocols T F
f) In a client/server model, a client contacts a server with well known port number T F
g) A persistent HTTP protocol always results in smaller delay than a non-persistent HTTP protocol. T F
h) A Go-back-N protocol is more complex than a Selective Repeat Protocol T F
i) Packet switching networks use statistical multiplexing T F
j) Root domain name server can resolve names to IP address mapping. T F

2. (20 points) Consider two nodes, node A and node B in the Figure below, are separated by two routers in between. At the beginning, suppose there are m packets in node A, n packets in Router 1 and k packets in Router 2. All the packets (in node A and in routers) are of equal size of p bits (including the packet header) and are on their way to the node B. The propagation delay between each hop is given by d seconds, and all link bandwidths are b (bits per second). Please calculate the total delay T when the last packet in node A arrives at node B under the following conditions.

a) If m=3, n=5 and k=2 and the propagation delay d is ignored? (3 points)
Answer: T= 3p/b + 5p/h + 2p/h
b) For arbitrary values of m, n and k, and d is ignored? Does it matter if m>n>k or m<n<k?

why? Please briefly justify your answer (5 points) Answer: T = (m+n+k)p/b c) If the propagation is sufficiently large, and d > max{mp/b, np/b, kp/b} (5 points) Answer: T = (m+2)p/b + 3d d) Consider the n packets in node A, what is the average waiting time of those n packets in
node A before it can be transmitted by node A? (7 points) Answer: the first packet waiting time is 0, the second packet waiting time is p/b, the last packet waiting time is (