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(comp361)[2011](s)midterm~548^_93280.pdf
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COMP 361 Computer Networks
Spring Semester 2011
Midterm Examination
Date: April 4, 2011 Time: 7:00 pm C 8:30 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. You may use back of the pages for your rough work.

3. Please be concise, and this is NOT an essay contest.

4. This paper consists of 4 questions and 7 pages.

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

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




Question
Points
Score

1
20


2
20


3
30


4
30






Total
100






1. (20 points) Answer the following true/false questions by circling either T or F.

a) The server must always be available in a client-server model T F

b) SMTP protocol is the only protocol that e-mail services utilize T F

c) All transport protocols provide in-order segment delivery T F

d) Transport protocol is an end-to-end protocol. T F

e) UDP provides bandwidth guarantee, thus is often used by real-time applications. T F

f) There must be a connection establishment in a circuit-switching network. T F

g) FTP protocol requires only one TCP connection. T F

h) A Go-back-N protocol can achieve better performance than a Selective Repeat protocol, so TCP uses the go-back-N protocol not the Selective Repeat protocol. T F

i) HTTP protocol is a stateless protocol. T F

j) Root domain name server can resolve all names to IP address mapping. T F


2. (20 points) Suppose that a sender with a files of size M bits to be transmitted to a receiver across K-1 routers (i.e., K-hops, see diagram below) in a packet-switched network in a series of packets. Each packet contains p data bits and h header bits, with M >> p+h and m=M/p (an integer). All the link speed (bit rate) is b bits per second.













k-1 routers


a) Assume that the propagation delay between each hop is d, there is no error and ignore all processing and queuing delay, what is the total end-to-end delay T in order to transmit the file (10 points)


Answer:
Since m = M/p, There are total m packet and each has p+h bits. The last packet waits for (m-1)(p+h)/b time before it can be transmitted, and it takes (n+h)K/b+Kd to reach the receiver, so the total end-to-end delay is
T = (m-1)(p+h)/b + Kd + (p+h)K/b
b) Now suppose we ignore the propagation delay, what is the value of p (data packet size) that minimizes the total end-to-end delay (10 points)


Answer:
So the total end-to-end delay is
. 1()()11(1)()(1)[(1)]()phphMMMhKKhfpmKKphppbbbpbbbb......