SIT202 Computer Networks Trimester 2, 2018 Problem Solving Report 1 (due 11:59 pm Friday August 3rd 2018) Total Marks =45 This will be converted to 15% of total marks Question-1: XXXXXXXXXX) =8 marks...

SIT202 Computer Networks Trimester 2, 2018
Problem Solving Report 1 (due 11:59 pm Friday August 3rd 2018)
Total Marks =45
This will be converted to 15% of total marks
Question-1: XXXXXXXXXX) =8 marks
1) Consider two processes communicating over a TCP/IP network using the TCP protocol
on an Ethernet network. As data from a sending process moves through the protocol
stack each layer will encapsulate the payload and generate a Protocol Data Unit (PDU)
which is then passed down to the next lower layer. Prepare a diagram illustrating the
layers, direction of data flow, peer layer communication, and identify the PDU name
and the structure of components (structure) of the PDU and explain encapsulation steps
for each layer.
Marking:
▪ Encapsulation part: (4 marks):
o 4 marks – mostly good explanation/s with appropriate names, components of
each PDU, peer layer communications, direction of data flows with appropriate
diagram
o 3 marks –good explanation/s with appropriate names, components of each PDU,
peer layer communications, direction of data flows with appropriate diagram,
ut inappropriate explanation in one of the cases.
o 2 marks – average explanation/s, with at least two co
ect PDUs and layers and
diagram, but inco
ect explanation in other cases.
o 1 marks – the answer is largely inco
ect, however some minor parts of
the explanations/s are co
ect with at least one PDUs co
ect in the
diagram;
o 0 marks – no parts of the answer are co
ect and/or there has been no
attempt to address any of the requirements of the question.
2) At the receiving end, data is reconstructed form the digital signal received at physical
layer then progresses up through the protocol stack to reach the process receiving the
application layer protocol. This whole process is known as decapsulation. Explain the
steps of decapsulation at the receiving end and show decapsulation steps jointly with
the encapsulation diagram of above.
Marking:
▪ Decapsulation part (4 marks):
o 4 marks – mostly good explanation, explained all steps of decapsulation/s with
very minor problems/oversights/inaccuracies for top four layers;
o 3 marks –good explanation, explained all steps of decapsulation/s with some
e
or /inaccuracies in one of the top four layers;
o 2 marks – average explanation/s, with clear co
ect and inco
ect aspects and at
least two co
ect layers;
o 1 marks – the answer is largely inco
ect or at least one co
ect steps of
decapsulation;
o 0 marks – no parts of the answer are co
ect and/or there has been no
attempt to address any of the requirements of the question.
Question-2: XXXXXXXXXX) = 9 marks
1) In many networks, redundant links and devices are often added to ensure the network
emains available in the event of network failures. If one link stops working, the other
link provides the connection paths. If this approach was used without a loop avoidance
scheme however,
oadcasts would loop and flood endlessly in the network, potentially
disabling the network. In the network illustrated below, host F in LAN2 sends a frame to
host A in LAN1. Explain how the MAC address table in the switches would be updated
and suffer thrashing, and why the frame would be forwarded endlessly. Recreate
necessary diagrams using following diagram required for your explanation to show the
oadcasts. In your diagram show the MAC table updates for switch-1 and switch-2,
location of frames in both LANs XXXXXXXXXX)

Marking:
Explanation of the steps of loop and MAC table thrashing (6 marks):
o 6 marks – mostly good explanation/s, explained all steps including how the
switches MAC table update, how it can result loop and MAC table
thrashing.
o 4 marks- Explained but has inco
ect explanation mostly in one case.
o 2 marks – average explanation/s, with inco
ect aspects in two cases.
o 1 marks – the answer is largely inco
ect, with inco
ect aspects in three
cases, partly co
ect in one cases;
o 0 marks – no parts of the answer are co
ect and/or there has been no
attempt to address any of the requirements of the question.
Included diagram (3 marks)
o 3 Marks: Included diagrams of three cases showing the networks,
frames, MAC table updates (3 marks), 1 mark for each co
ect diagram
o 2 Marks: Your diagrams are co
ect, but has e
or at least one cases
o 1 Marks: Most of your diagram, is inco
ect
o 0 Marks: You did not attempt or all your diagram is inco
ect

Question-3: XXXXXXXXXX) = 9 marks
In the following network, two hosts Alice and Bob are connected by three links and two
outers. There are two addresses (N: IP address and L: Link layer address) for each
interface of hosts and routers. Host Alice sends a datagram to host Bob. The datagram
passes through the routers and links to host Bob. Before sending, Alice makes a frame
from the datagram, then sends to the router-R1.
Explain how host Alice encapsulates the frame from the datagram, and then explain how
Router-R1 and Router-R2 prepare and forward those frames on link-2 and link-3 towards
host Bob. Mention how Alice gets R1’s link layer address, how routers get next
destination and link layer addresses in your explanation. Mention all link layer addresses
(source and destination) for all three frames in the links.
Marking:
Explanation of Alice and two routers’ frame preparation and forwarding (6 marks):
o 6 marks – mostly good explanation/s, explained all steps including how the
encapsulation happens for frame, ARP, routing decision of hosts or router
whichever applicable in each case.
o 5 marks – mostly good explanation/s, explained the steps including how the
encapsulation happens for frame, however did not mention either ARP or,
outing decision.
o 4 marks- Explained but has inco
ect explanation mostly in one case e
or
(in ARP and routing).
o 3 marks – average explanation/s, with inco
ect aspects in two cases, partial
co
ect in ARP or routing.
o 2 marks-poor explanation/s, with inco
ect aspects in two cases and missing
ARP and routing explanation.
o 1 marks – the answer is largely inco
ect, with inco
ect aspects in three
cases, partly co
ect in one cases;
o 0 marks – no parts of the answer are co
ect and/or there has been no
attempt to address any of the requirements of the question.
Link layer addresses (3 marks)
o 3 Marks: the source and destination link layer addresses of three frames
are co
ect (0.5) each
o 2 Marks: the source and destination link layer addresses of two frames
are co
ect (0.5) each
o 1 Marks: the source and destination link layer addresses of one frame
are co
ect (0.5) each
o 0 Marks: You did not attempt or all your diagram is inco
ect


Question 4 (1+5) = 6 marks
1) Explain the importance of PPP with respect to wide area networks (WANs)? (1)
Marking:
Importance of PPP (1 marks):
2) Explain how PPP enables users to exchange data in WAN by using sub-protocols
such as LCP, PAP, CHAP, NCP, IPCP XXXXXXXXXXMarks)
Marking:
PPP data exchange process(5 marks):
o 5 marks – mostly good explanation/s, explained all sub protocols in PPP.
o 4 marks- Explained well but has inco
ect explanation mostly in one case.
o 3 marks – average explanation/s, with inco
ect aspects in two cases.
o 2 marks – with inco
ect aspects in three cases.
o 1 marks --the answer is largely inco
ect, with inco
ect aspects in four
cases.
o 0 marks – no parts of the answer are co
ect and/or there has been no
attempt to address any of the requirements of the question.

Question 5 (3+3) = 6 marks
1) Explain why modulation of a digital signal is required for transmission in digital
cellular telephony?
Marking:
digital cellular telephony modulation(3 marks):
o 3 marks – mostly good explanation/s with at most minor
problems/oversights/inaccuracies;
o 2 marks – average explanation/s, with clear co
ect and inco
ect aspects;
o 1 marks – the answer is largely inco
ect, however some minor parts of
the explanations/s are co
ect;
o 0 marks – no parts of the answer are co
ect and/or there has been no
attempt to address any of the requirements of the question.
2) Briefly explain digital signal transmission on a bandpass channel.
Marking
digital signal transmission on a bandpass channel (3 marks):
o Fullmark: 3 marks – mostly good explanation/s with at most minor,
described digital to analog conversion, ca
ier frequency amplitude change,
analog to digital converter at receiver.
o 2 marks – average explanation/s, with clear co
ect and inco
ect aspects;
missed one of the three components in Fullmark category in above.
o 1 marks – the answer is largely inco
ect, however some minor parts
of the explanations/s are co
ect and missed to mention two of three
components in Fullmark category in above ;
o 0 marks – no parts of the answer are co
ect and/or there has been no
attempt to address any of the requirements of the question.
Question-6: XXXXXXXXXX) = 7 marks
In the following network diagram. Different network devices have been used including
switches, hub,
idges and router.
How many collision domains are found in the above network diagram? Explain your answer.
Use co
ect devices with their label for each collision domain in your explanation.
Marking:
Identified co
ect number of collision domains =2 marks
Marking:
Explanations: (5 marks):
o 5 marks – mostly good explanation/s, explained for co
ect five
devices/device group for domains.
o 4 marks –good explanation/s, but have e
or in any of five devices/device
group.
o 3 marks –average explanation, but have e
or in any three of five
devices/device group.
o 2 marks – poor explanation/s, with inco
ect aspects in three cases.
o 1 marks – largely inco
ect explanation/s, with inco
ect aspects in four
cases.
o 0 marks – no parts of the answer are co
ect and/or there has been no
attempt to address