COMP3331/9331 — Computer Networks and Applications
Computer Networks and Applications
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COMP3331/9331 — Computer Networks and Applications
Mid-term Examination
Instructions:
1. TIME ALLOWED: 1 hours and 15 minutes.
2. TOTAL MARKS AVAILABLE: 20 marks worth 20% of the total marks for the course.
3. THERE ARE 21 QUESTIONS. ALL QUESTIONS MUST BE ANSWERED.
4. MARKS AVAILABLE FOR EACH QUESTION ARE SHOWN IN THE EXAM. THERE IS NO
NEGATIVE MARKING, IN THAT THE MINIMUM MARK FOR EACH QUESTION IS ZERO.
5. THE EXAM IS OPEN BOOK, OPEN NOTES. USE OF CALCULATORS IS PERMITTED.
6. STUDENTS ARE ADVISED TO READ THE EXAMINATION QUESTION BEFORE
ATTEMPTING TO ANSWER THE QUESTION.
7. THIS EXAM CANNOT BE COPIED, FORWARDED, OR SHARED IN ANY WAY.
8. STUDENTS ARE REMINDED OF THE UNSW RULES REGARDING ACADEMIC INTEGRITY
AND PLAGIARISM. STUDENTS CANNOT USE ANY GENERATIVE AI SOFTWARE FOR
ASSISTANCE.
9. YOUR WORK WILL BE SAVED PERIODICALLY THROUGHOUT THE EXAM AND WILL BE
AUTOMATICALLY SUBMITTED PROVIDED YOU ARE CONNECTED TO THE INTERNET.
Suppose a number of users share a 4 Mbps link. Also, suppose that each user
transmits continuously at 2 Mbps when transmitting, but each user transmits only
20% of the time.
Answer the 3 questions.
1 When circuit switching is used, how many users can be supported? No explanation is required.
Simply enter the numeric value in the space provided:
Maximum marks: 0.25
2 Now suppose packet switching is used. Why will there be essentially no queuing delay before the
link if two or fewer users transmit at the same time? Why will there be queuing delay if three users
transmit at the same time? Be brief (2 sentences at most for each question).
Fill in your answer here
Format Σ
Words: 0
Maximum marks: 0.75
3 Suppose with packet switching, there are three users. Find the probability that at any given time,
all three users are transmitting simultaneously. No explanation is required.Simply enter the
numeric value in the space provided:
Maximum marks: 0.5
Consider the network in the figure below. Host A can choose between two different
paths to communicate with host B. Host can choose to send packets via either
Router 1 or Router 2 to host B. The communication links are of two different types,
as indicated in the figure. The characteristics of these two types of links are:
Link type 1: Each link is of length 2000km, propagation speed is 2 x 10 m/s and
bandwidth is 100kbps.
Link type 2: Each link is of length 4000km, propagation speed is 2 x 10 m/s and
bandwidth is 50kbps.
Host A wishes to transmit a message of size 4Kbytes to host B. It breaks this
message into 4 packets of equal size. Neglect any packet headers. Remember that
routers work on the store-and-forward principle.
Assume that the processing delay and queuing delay in the routers are negligible.
You may also approximate file sizes to be an order of 10 (i.e. 4Kbytes = 4000 bytes
instead of 4096 bytes).
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4 If host A chooses to send the packets via Router 1, determine the time it takes to move the
packets from host A to host B, i.e., beginning from the time that host A starts to send the first bit of
the first packet till the time that host B receives the last bit of the last packet.
You are encouraged to draw a timing diagram to help you visualise the delays. However, you are
NOT required to upload such a diagram with your answers.
Do not simply write the final answer. Show us your work (just type it in the space provided).
Fill in your answer here
Format Σ
Words: 0
Maximum marks: 2
5 Now assume that host A chooses to send the packets via Router 2 to host B. Determine the time
it takes to move the packets from host A to host B, i.e., beginning from the time that host A starts
to send the first bit of the first packet till the time that host B receives the last bit of the last packet.
You are encouraged to draw a timing diagram to help you visualise the delays. However, you are
NOT required to upload such a diagram with your answers.
Do not simply write the final answer. Show us your work (just type it in the space provided).
Fill in your answer here
Format Σ
Words: 0
Maximum marks: 2
Assume a webpage comprised of 10 objects which includes the index.html file, 8
embedded images and one embedded audio clip. The 10 objects are so small that:
(i) their transmission time is negligible and (ii) each object can be completely
transmitted in one TCP segment. Consider a client wishing to download the
webpage.
You are asked to make the following assumptions:
the round trip time between the client and all servers is T
the time to set up and tear down a TCP connection is S and F, respectively.
You must account for both these times in your computations. Note
that, S includes the 3-way handshake (SYN, SYN-ACK, ACK) and F includes
the time for sending FINs and ACKs from both endpoints.
there are no packet losses.
the client knows the IP address of all servers (i.e. neglect DNS resolution
delay).
neither the client nor any of the servers support parallel TCP connections.
Answer the following 5 questions. No explanations are required. Simply write the
expression for each answer which should ONLY contain the
variables T, S and F (e.g., 20T+100S+50F) in the space provided.
6 Assume that the client uses non-persistent HTTP for downloading the web page. What is the time
required to complete the transfer of the web page (including the time for setting up and tearing
down each TCP connection involved)?
Fill in your answer here
Maximum marks: 0.6
7 Assume that the client uses persistent HTTP without pipelining for downloading the web page.
What is the time required to complete the transfer of the web page (including the time for setting
up and tearing down each TCP connection involved)?
Fill in your answer here
Maximum marks: 0.6
8 Assume that the client uses persistent HTTP with pipelining for downloading the web page. What
is the time required to complete the transfer of the web page (including the time for setting up and
tearing down each TCP connection involved)?
Fill in your answer here
Maximum marks: 0.6
9 Now assume that all 10 objects are located on 10 different servers (one object on each server).
The client can only have one active TCP connection at any given time. Assume that the round trip
time between the client and each of the 10 servers is T. Neglect DNS queries. Assume that the
client uses persistent HTTP with pipelining for downloading the web page. What is the time
required to complete the transfer of the web page (including the time for setting up and tearing
down each TCP connection involved)?