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: (4+4) =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, but inappropriate explanation in one of the cases. o 2 marks – average explanation/s, with at least two correct PDUs and layers and diagram, but incorrect explanation in other cases. o 1 marks – the answer is largely incorrect, however some minor parts of the explanations/s are correct with at least one PDUs correct in the diagram;
o 0 marks – no parts of the answer are correct 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 error /inaccuracies in one of the top four layers;
o 2 marks – average explanation/s, with clear correct and incorrect aspects and at least two correct layers;
o 1 marks – the answer is largely incorrect or at least one correct steps of decapsulation;
o 0 marks – no parts of the answer are correct and/or there has been no attempt to address any of the requirements of the question.
Question-2: (6+3) = 9 marks
1) In many networks, redundant links and devices are often added to ensure the network remains 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, broadcasts 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 broadcasts. In your diagram show the MAC table updates for switch-1 and switch-2,
location of frames in both LANs. (6)
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 incorrect explanation mostly in one case. o 2 marks – average explanation/s, with incorrect aspects in two cases.
o 1 marks – the answer is largely incorrect, with incorrect aspects in three cases, partly correct in one cases;
o 0 marks – no parts of the answer are correct 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 correct diagram
o 2 Marks: Your diagrams are correct, but has error at least one cases o 1 Marks: Most of your diagram, is incorrect
o 0 Marks: You did not attempt or all your diagram is incorrect
Question-3: (6+3) = 9 marks
In the following network, two hosts Alice and Bob are connected by three links and two routers. 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, routing decision. o 4 marks- Explained but has incorrect explanation mostly in one case error (in ARP and routing). o 3 marks – average explanation/s, with incorrect aspects in two cases, partial correct in ARP or routing.
o 2 marks-poor explanation/s, with incorrect aspects in two cases and missing ARP and routing explanation. o 1 marks – the answer is largely incorrect, with incorrect aspects in three cases, partly correct in one cases;
o 0 marks – no parts of the answer are correct 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 correct (0.5) each
o 2 Marks: the source and destination link layer addresses of two frames are correct (0.5) each
o 1 Marks: the source and destination link layer addresses of one frame are correct (0.5) each
o 0 Marks: You did not attempt or all your diagram is incorrect
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. (5 Marks)
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 incorrect explanation mostly in one case. o 3 marks – average explanation/s, with incorrect aspects in two cases. o 2 marks – with incorrect aspects in three cases. o 1 marks --the answer is largely incorrect, with incorrect aspects in four cases. o 0 marks – no parts of the answer are correct 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 correct and incorrect aspects; o 1 marks – the answer is largely incorrect, however some minor parts of the explanations/s are correct;
o 0 marks – no parts of the answer are correct 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, carrier frequency amplitude change, analog to digital converter at receiver. o 2 marks – average explanation/s, with clear correct and incorrect aspects; missed one of the three components in Fullmark category in above.
o 1 marks – the answer is largely incorrect, however some minor parts of the explanations/s are correct and missed to mention two of three components in Fullmark category in above ;
o 0 marks – no parts of the answer are correct and/or there has been no attempt to address any of the requirements of the question.
Question-6: (5+2) = 7 marks
In the following network diagram. Different network devices have been used including switches, hub, bridges and router.
How many collision domains are found in the above network diagram? Explain your answer.
Use correct devices with their label for each collision domain in your explanation.
Marking:
Identified correct number of collision domains =2 marks
Marking:
Explanations: (5 marks):
o 5 marks – mostly good explanation/s, explained for correct five devices/device group for domains. o 4 marks –good explanation/s, but have error in any of five devices/device group. o 3 marks –average explanation, but have error in any three of five devices/device group.
o 2 marks – poor explanation/s, with incorrect aspects in three cases.
o 1 marks – largely incorrect explanation/s, with incorrect aspects in four cases. o 0 marks – no parts of the answer are correct and/or there has been no attempt to address any of the requirements of the question.