SIT202 Computer Networks

Trimester 2, 2020

Problem Based Learning Task 1

Due Date: 8pm Sunday August 30th, 2020

This assessment is marked out of 100 marks and is worth 35% of your final unit mark.

This assessment task must be completed individually,

group work and/or collaboration with other students is prohibited.

All work completed/submitted as part of this assessment task must be your own, individual work. 

Any content drawn from other materials, including unit materials, must be clearly quoted where 

appropriate, and/or clearly referenced. All students should review and be familiar with the content 

provided by the University regarding how to reference other materials:

https://www.deakin.edu.au/students/studying/study-support/referencing

And in particular the information provided regarding Academy Integrity:

https://www.deakin.edu.au/students/studying/academic-integrity

Use of Illustrations

Note that several questions below may require you to include an illustration in your answer. This 

must be your own illustration, not a graphic copied from the Internet or any other source 

(textbook, class slides, etc.). Illustrations from the Internet (whether directly copied or 

reproduced by you) will not be awarded any marks.

There are many illustration programs that you may use for this purpose, including several free 

applications accessible via the Web. If you are unable to use these programs however, you may 

include a scanned/photographed hand-drawn illustration in as long as that illustration is clearly 

legible.

HD Tasks

Note that there are one or more “HD Tasks” identified in the requirements below. These tasks are 

more difficult to answer and total 20% of the marks available for this assessment. A HD result 

reflects achievement in the unit of a deep understanding of the unit content and ability to extend 

your knowledge beyond that covered in the unit directly. You can expect these tasks will be 

assessed harder than other tasks reflecting demonstration of this level of achievement.

Question 1 / Week 1 (4 + 6 + 6 + 8 = 24 marks)

Computer networks are constructed using a modular architecture known as layering.

a) Explain how layering simplifies the protocols used in networks and intermediary devices.

b) Explain how encapsulation and decapsulation works and how these mechanisms are used 

to achieve logical communication between peer layers.

c) A key advantage of layering is that a protocol in one layer can be replaced with another 

with little or no impact. Discuss how this works, including any influence/expectations on 

adjacent layers (layers above/below where the protocol is replaced).

d) (HD Task) Consider a home network where a computer connected via Ethernet to the 

router is printing to a printer that is connected by Wi-Fi to the same router using the 

Internet Printing Protocol (IPP). Assuming that the Wi-Fi network is just an extension of 

the existing wired LAN, i.e., they are the same subnet, illustrate the communication 

protocol stacks of all devices in the communication path, indicating both the layer name 

and actual protocol (see Week 1 Slide #32 for an example without the actual protocols

named).

Question 2 / Week 2 (15 marks)

Consider the differences between datagram switching and virtual-circuit switching networks. 

Explain how the following aspects of communication are impacted (or why they are not impacted) 

by these different switching techniques:

i. Delays encountered by data travelling from sender to receiver;

ii. The order packets would arrive at the receiver; and

iii. The reliability of packets being delivered to the receiver.

Question 3 / Week 3 (10 + 13 = 23 marks)

Given the following network topology:

a) Illustrate the use of network layer addresses and link-layer addresses as a packet is sent 

from the PC on the left to the PC on the right.

b) Explain what is happening in your illustration, including why some addresses are changing 

and others not, where encapsulation/decapsulation is occurring and at what layers.

Question 4 / Week 4 (10 + 7 = 17 marks)

a) Modern Ethernet networks make use of switches which, when only one device is 

connected to a switch port, enables CSMA/CD to be disabled and full-duplex 

communication to be used instead. Explain why this same technique is not possible for 

WiFi networks, which require use of CSMA/CA.

b) (HD Task) The CSMA/CA protocol, used in WiFi networks is able to reduce the likelihood of 

collisions occurring, but is unable to eliminate them. Given that the RTS/CTS flow control 

mechanism ensures only one device transmits at any one time, explain why collisions can 

still occur. In your answer, use a scenario/example to aid in your explanation (an 

illustration may work well for this).

Note: assume that only one WiFi network is in use and there are no other devices are using 

the same frequencies.

Question 5 / Week 5 (8 + 8 + 5 = 21 marks)

The IP protocol includes error control in the form of a checksum that validates only the content of 

the IP header. As noted in the Week 5 slides, this design decision was based on the inefficiency of 

providing error control at the network layer once fragmentation had been considered.

a) Draw an illustration that shows a packet arriving at a router, which is then fragmented into 

two or more fragments for transmission to the receiver. Indicate any relevant IP header 

fields in your illustration.

b) Explain why fragmentation may occur and how it works (both fragmentation and 

reconstruction at the destination).

c) (HD Task) Explain why the presence of fragmentation would influence the decision to 

validate only the IP header instead of the entire datagram.