Student Name: ____________________ Student Number: ___________________ Seat: ______
CS310: Computer Networks
School of Information Technology, Engineering, Mathematics, and Physics
Final Examination
Semester 1 2023
F2F Only
Duration of Exam: 3 hours + 10 minutes
Reading Time: 10 minutes
Writing Time: 3 hours
Total marks: 100
Instructions:
1. This exam is a closed book exam.
2. This exam has three sections:
• Section A: 30 Marks
• Section B: 40 Marks
• Section C: 30 Marks
3. This exam has 3 main questions (with sub questions) and ALL are compulsory.
4. There are 14 pages in this paper including this cover page.
5. Show all necessary working. Partial marks will be awarded for partially correct
answers.
6. Only non-programmable calculators are allowed.
7. This exam is worth 40 % of your overall mark.
8. The minimum passing mark is 40/100.
Section A (Questions 1) - Short and Long Answer Questions
(30 Marks)
a) What is the difference between a host and an end system? List several different
types of end systems. Is a Web server an end system?
(3 marks)
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b) Consider the Figure 1 below, that draws the analogy between store-and-forward
link transmission as well as propagation of bits in packet along a link, and cars in a
caravan being serviced at a toll booth and then driving along a road to the next
tollbooth.
Figure 1: An analogy between store-and-forward link.
Suppose the caravan has 5 cars, and that the tollbooth services (that is, transmits)
a car at a rate of one car per 2 seconds. Once receiving serving a car proceeds
to the next tool both, which is 500 kilo-meters away at a rate of 20 kilo-meters per
second. Also assume that whenever the first car of the caravan arrives at a
tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in
its caravan have arrived, and lined up behind it before being serviced at the toll
booth. (That is, the entire caravan must be stored at the tollbooth before the first
car in the caravan can pay its toll and begin driving towards the next tollbooth).
i.
Once a car enters service at the tollbooth, how long does it take until it
leaves service?
(1 mark)
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ii.
How long does it take for the entire caravan to receive service at the
tollbooth (that is the time from when the first car enters service until the last
car leaves the tollbooth)?
(2 marks)
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iii.
Once the first car leaves the tollbooth, how long does it take until it enters
service at the next tollbooth?
(3 marks)
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iv.
Are there ever two cars in service at the same time, one at the first toll booth
and one at the second toll booth? Answer Yes or No. Explain your selected
choice.
(3 marks)
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c) Explain what an Internet Protocol version 4 (IPv4) address is?
(2 marks)
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d) The Selective Repeat protocol usually provides for sending multiple frames
depending upon the availability of frames in the sending window. However, this
depends on the maximum number of frames to be sent and the size of the sending
window. Figure 2, shows an example of SR operation in the presence of lost
packets. After the thorough observation fill in the gaps from (a) to (f).
(6 marks)
Hint: An example on the receiver’s side is completed for you.
Figure 2: The SR receiver events and actions.
e) Consider a store and the forward packet switched network in Figure 3, in which
the application data is segmented into packets, with each packet carrying up to
1800 bits of data, and additional 200 bits of header. Host A and Host B are
interconnected by the 2 routers as shown below. All the 3 links are exactly the
same, with each link having a length of 300km and the transmission rate of 100x103
bps. Furthermore, the propagation speed of the signal on the links is 3 x 106 km/s.
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Figure 3: A delay concepts.
Suppose Host A (PC A) sends a file of the size 500 x 103 bits. From start of transmission at
A, how long does B wait to receive the entire contents of the file?
Hint: If you are to ignore queuing and processing delays.
(4 marks)
f) HTTP with Non-Persistent Connections.
In many Internet applications, the client and server communicate for an extended
period of time, with the client making a series of requests and the server
responding to each of the requests. Let’s suppose the page consists of a base
HTML file and 10 JPEG images, and that all 11 of these objects reside on the same
server.
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Further suppose the URL for the base HTML file is:
g) Decribe in 6 steps thoroughly what happens when the above URL is requeted.
[HTTP with Non-Persistent Connections].
(6 marks)
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Section B (Questions 2) - Short and Long Answer Questions
(40 Marks)
a) In the scenario below, imagine that you're sending an http request to another
machine somewhere on the network. You are asked to state what layer in the IP
stack best corresponds to the phrase from 1-15.
(7.5 marks)
Figure 4: A http request scenario.
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Hint: Identify the layers involved at each stage from the Figure 4 above to fill in the table
below.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
b) Consider the 7-node network shown in Figure 5 below, with the given link costs.
Figure 5: Portrayal of the 7-node network.
Using Bellman-Ford Algorithm, find the shortest path.
Ensure to show the detailed working as learnt in CS310 course.
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(12 marks)
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c) Consider the figure below, which shows the arrival of 6 messages for transmission
at different multiple access wireless nodes at times:
t = 0.6, 1.3, 1.6, 2.3, 3.8, 4.5 and each transmission requires exactly one-time unit.
Figure 6: Nodes implementing the Slotted Aloha protocol.
i.
Suppose all nodes are implementing the Slotted Aloha protocol. For each
message, indicate the time at which each transmission begins. Separate each
value with a comma and no spaces.
(3 marks)
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ii.
Which messages transmit successfully? Write your answer as a comma separated
list with no spaces using the messages' numbers.
(2 marks)
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d) TCP Retransmissions (Reliable Data Transmission with Ack Loss)
Consider the figure below in which a TCP sender and receiver communicate over a
connection in which the segments can be lost. The TCP sender wants to send a total of
10 segments to the receiver and sends an initial window of 5 segments at t = 1, 2, 3, 4,
and 5, respectively.
Suppose the initial value of the sequence number is 90 and every segment sent to the
receiver each contains 636 bytes. The delay between the sender and receiver is 7-time
units, and so the first segment arrives at the receiver at t = 8, and an ACK for this segment
arrives at t = 15.
As shown in the figure, 1 of the 5 segments is lost between the sender and the receiver,
but one of the ACKs is lost. Assume there are no timeouts and any out of order segments
received are thrown out.
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Figure 7: Portrayal of the TCP Retransmissions.
i.
What is the sequence number of the segment sent at t=5?
(2 marks)
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ii.
What is the value of the ACK sent at t=8? (If segment lost, write 'x').
(2.5 marks)
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iii.
What is the value of the ACK sent at t=12? (If segment lost, write 'x').
(2.5 marks)
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e) First describe how the client and the server interact? To support the claim, draw
a diagram to show their interaction flow.
(4.5 marks)
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f) Briefly describe what are the functions of Transport, Network and Data Link layer.
(4 marks)
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Section C (Questions 3) - Short and Long Answer Questions
(30 Marks)
The Vision Limited is building a retail outlet in Port Villa, Vanuatu. There will be 2
departments required in the retail outlet. The number of IP addresses required on a per
department basis is as follows:
•
•
Customer Wi-Fi – 100 IP Addresses
Retail Devices – 50 IP Addresses
Part A
(19 marks)
a. Subnet the major network 172.16.0.0/16 using Variable Length Subnet Mask to
design the network for the Retail outlet.
Fill the table below. All the calculations should be clearly stated.
Table 1: Carries the subnet information’s.
Department
Network
Usable IP Range
Customer
Wi-Fi
Retail
Devices
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Broadcast Address
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Part B
b. Draw the topology diagram. Use the last Usable IP address as for the Default
gateway.
(5 marks)
c. What is the difference between access and trunk ports?
(2 marks)
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d. What is a VLAN?
(2 marks)
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e. Give two benefits of using VLAN.
(2 marks)
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THE END
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Student Name: ____________________ Student Number: ___________________ Seat: ______
THE END
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