# 计算机网络代写 | SIT202 Computer Networks Problem Solving Report 1

SIT202 Computer Networks Trimester 2, 2019
Problem Solving Report 1 (due 11:59 pm Friday August 2nd 2019) Total Marks = 45
This will be converted to 15% of total marks
General Marking Policies (if no specific policies are given):
▪ A full mark – mostly good explanation/s with appropriate names, components of each PDU, peer layer communications, direction of data flows with appropriate diagram
▪ 3⁄4 of full marks – good explanation, explained all steps of decapsulation/s with some error /inaccuracies
▪ a half mark – average explanation/s, with clear correct and incorrect aspects;
▪ 1⁄4 of full mark – the answer is largely incorrect, however some minor parts of
the explanations/s are correct;
▪ 0 mark – no parts of the answer are correct and/or there has been no attempt to
address any of the requirements of the question.
▪ No plagiarism: an allocation of a zero mark is to be given.
Question-1 (Chapter 3): 10 marks
1) What is the bit rate? [2 mark]
2) If the number of signal level is 8 and the bandwidth is 3 MHz, what is the bit rate? [2
marks]
3) Suppose that the signal-to-noise ratio (SNR) is 6 dB. What is the capacity (in bits per
second) if the bandwidth is 3 MHz? [2 marks]
4) Discuss the difference between the data rate and the capacity (e.g., answer why they are
different?) [4 marks]

Question-2 (Chapters 6, 7, 8): 10 marks
1) Explain the purposes of multiplexing and demultiplexing? [4 mark]
2) Place one of “circuit-switching” and “packet-switching” into the followings sentences
[3 marks]
• “…” can further be divided into two subcategories – virtual-circuit approach and
datagram approach.
• “…” takes places at the physical layer.
• In “…,” the resources need to be reserved during the setup phases.
3) Suppose that a packet is transmitted from Host A to Host B. The VIC of the packet at Host A is “c”. What is VCI when the packet is arrived at Host B. [3 marks]
Incoming
Outgoing
Port
VCI
Port
VCI
0
a
0
d
1
d
3
c
2
c
1
b
3
b
2
e
0
e
1
h
1
f
3
g
2
g
2
e
3
h
0
f

Question-3 (Chapters 9, 10, 11):
1) Consider the parity-check code with 4-bit dataword [4 marks]
• What is the length of the codeword?
• How many codewords the code has?
• What is the minimum Hamming distance?
• How many errors can be corrected?
2) [6+3 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.
13 marks
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.
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 (Chapters 12 – 17):
1) Choose “Yes” or “No” for each item? [6 marks]
12 marks
Are there packet collisions?
Is carrier sensing required?
Is the number of the stations fixed?
TDMA
Polling
Slotted-ALOHA
CSMA/CA
* Each answer has 0.5 marks: For the final mark, the total sum is rounded. 2) Choose all wireless technologies in the followings. [2 marks]
• Bluetooth
• Gigabit ethernet
• 802.11n
• SONET
3) Choose “True” or “False” for each item? [4 marks]
True or False
In AMPS, CDMA has been used for channelization.
Handoff is used in a cellular system to support continuous transmission for a mobile station.
In CDMA-based cellular systems, the frequency-reuse factor can be 1.
CSMA/CD is used for wireless LAN as it can avoid the hidden node problem.

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