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MSc Degree Examinations 2020-1

Department: Electronic Engineering

Title of Exam: Internet & LAN Protocols

Time Allowed: 24 Hours (PLEASE NOTE: papers submitted more
than 30 minutes after the deadline will not be
marked)

Time Recommended: TWO hours

Allocation of Marks: All questions carry equal marks

Instructions for Candidates: You should attempt 2 of the 3 questions.

Write all of your answers by hand (unless you have a Student Support Plan
which allows use of a PC) and submit your handwritten answers in a single
pdf document.

Use only black or blue pen for any handwritten answers. Clearly write your
examination number on each page submitted. Begin each question on a
new page and clearly write the question number on each page.

More detailed instructions regarding submission of your work and conduct
during the open exam is given on the next page. Please read this carefully
before you begin, including the information regarding the conduct
expected of you during the exam.

Paper Queries:
Please see the detailed instructions on the next page regarding exam queries.
The exam setter for this exam is Dr Cuma Kanapathippillai who if necessary
can be contacted by email at kanapathippillai.cumanan@york.ac.uk.
Please copy any email that you send to the Chair of Board of Examiners
(eugene.avrutin@york.ac.uk) and to elec-genoffice@york.ac.uk.



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A note on Academic Integrity:
We are treating this online examination as a time-limited open assessment, and you are
therefore permitted to refer to written and online materials to aid you in your answers.
However, you must ensure that the work you submit is entirely your own, and for the whole
time the assessment is live you must not:
● communicate with departmental staff on the topic of the assessment, except for the
exam setter.
● communicate with other students on the topic of this assessment.
● seek assistance with the assignment from the academic and/or disability support
services, such as the Writing and Language Skills Centre, Maths Skills Centre and/or
Disability Services. (The only exception to this will be for those students who have
been recommended an exam support worker in a Student Support Plan. If this applies
to you, you are advised to contact Disability Services as soon as possible to discuss
the necessary arrangements.)
● seek advice or contribution from any third party, including proof-readers, friends, or
family members.
We expect, and trust, that all our students will seek to maintain the integrity of the assessment,
and of their award, through ensuring that these instructions are strictly followed. Failure to
adhere to these requirements will be considered a breach of the Academic Misconduct
regulations, where the offences of plagiarism, breach/cheating, collusion and commissioning
are relevant - see AM.1.2.1”

Please make sure you note the following detailed instructions:
• If you do not submit by the deadline you will score a mark of zero unless you submit in
the first 30 minutes after the deadline in which case your mark will be subject to a 5%
late penalty. The deadline is not flexible, it may take a while for your file to be
processed through the system. Make sure you submit in good time to allow for this.
• You must submit your answers as a single pdf file. Please check that all scanned
pages are clear and visible BEFORE you submit. The Adobe Scan app is a free app
that works on all phones and can be used to photograph and then connect together all
the scans into a single document - ensure all your pages are in the correct order.
• Please make sure that every page is clearly labelled with both your examination
number (NOT your name) and the question number. It would be useful if you
numbered the pages.
• The file size limit for anonymised work submitted online is 30MB per file. Please
see Compressing File Sizes (Student Guide) for details on reducing the file sizes. If
your submission remains too large or if you experience genuine technical difficulties
when trying to submit, then you should email your submission to elec-
genoffice@york.ac.uk. If the assessment is too large to send by email, you may use
the University of York Deposit Service. If using this service as the back-up please
specify elec-genoffice@york.ac.uk as the recipient of the drop-off. Please do not use
this backup option if the VLE submission has worked properly and you have received a
receipt.
• University policy is that corrections or clarifications to the questions in the exam paper
will only be issued within the first hour of the submission window. Therefore if outside
the first hour you should answer the question as best as you can and note the
assumptions that you have made to allow you to proceed.
• All submissions may be screened for collusion and plagiarism. By submitting your
assignment you agree to the University rules on Collusion and Plagiarism. The
Department will run submissions through Turnitin wherever possible. For more
information visit york.ac.uk/integrity
• You may make multiple submissions up to the assessment deadline but by default, we
will mark your last submission before the deadline.
IT IS YOUR RESPONSIBILITY TO MAKE SURE THAT YOUR SUBMISSION IS CORRECT.
Once you have submitted your assignment via the VLE, a 'receipt' screen appears that has a
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link to the submitted file, so you can click the link and check the file to make sure it is the
correct one. You can also click back into the submission point and see links to the file(s) you
have submitted.

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1
(a) [14 Marks in total]
(i) Discuss two advantages and two disadvantages of computer
networking. (4 marks)

(ii) Discuss two advantages and two disadvantages of OSI model
in computer networking as opposed to having no layers in the
protocols at all. (4 marks)

(iii) Describe the technologies that are used to solve the
incompatibility issues between IPv4 and IPv6 addresses. (6
marks)

(b) [5 Marks]
The distance between two stations M and N is L kilometers. All
frames are K bits long. The propagation delay per kilometer is
t seconds. Let R bits/second be the channel capacity.

Assuming that processing delay is negligible, derive the window
size for maximum utilization, when the sliding window protocol is
used. The answer should be in terms of L, K, t and R.
(c) [12 Marks in total]
A communication link runs from computer A to computer B
through two routers R1 and R2 as shown below. The bit rates and
lengths of individual links are given.






4 Mbit/s
200 m
2 Mbit/s
1500 km
12 Mbit/s
2500 m
A B
R1 R2
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This network uses a reliable transport layer protocol with stop-
and-wait error control. Assume that signal travels along the
cables at a speed of 2 × 108 ms-1 and state any assumptions you
make.
(i) Estimate the minimum time-out period of the retransmission
timer at the source node, if packets are 1500 bytes long.
Assume that there is no traffic on any of the links except that
between these two computers. (10 marks)
(ii) Determine the minimum delay through this network in the
case for packets that have to be re-transmitted once. (2
marks)
(d) [7 Marks in total]
To provide mobility, the link between computer A and router R1 in
(c) was replaced by 4 Mbit/s wireless link, and as result the
packet error rate on this link suddenly increased.
(i) To reduce the minimum delay through this network for
packets that have to be re-transmitted once, the wireless
data-link layer is made reliable. By how much does this
change the delay of packets that have to be re transmitted
due to loss in this first wireless link. Provide your answer in
microseconds (µs). (4 marks)
(ii) What are the disadvantages of making this change to the
data-link layer? (3 marks)
(e) [6 Marks]
If the stop-and-wait flow control in (c) was replaced by sliding
window flow control to increase the utilisation, what is the
minimum window size that could be used to keep this connection
running at its maximum capacity?
(f) [6 Marks in total]
The link from router R2 to computer B has bit error rate pbit = 10-6.
(i) Determine the packet error rate ppkt on the link. (3 marks)
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(ii) Determine the throughput of the link R2 to computer B with
a window size N when the errors are present if the link uses
go-back-N-ARQ with the packet error rate calculated in part
(f) (i). (3 marks)

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2
(a) [20 Marks in total]
(i) Subnetting is the procedure to divide the network into sub-
networks or small networks. Discuss the three advantages
and three disadvantages of subnetting. (6 marks)
(ii) Discuss how IPv6 addresses are assigned to devices and
hosts in the network. (5 marks)
(iii) Discuss the differences between IPv4 and IPv6 addressing.
(5 marks)
(iv) List three features of an IPv6 header that make IPv6 header
is simpler than an IPv4 header. (4 marks)
(b) [10 Marks in total]





Port B
Port A
Port C
Router R31 Gbit/s
Ethernet E3 (130 stations)
Ethernet E4 (61 stations)
To the
Internet
Port A Port C
Port B
Router R1
Ethernet E1 (58 stations)
100 Mbit/s
Router R2 Port B
Port A
Port C
Ethernet E2 (37 stations)
Port D
1 Mbit/s
Port A
Port B
Bridge B1
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The sub-network shown above is required to be connected to the
Internet. The IPv4 address range 144.32.64.0/23 has been
allocated. A web-server on Ethernet E1 should be assigned the IP
address 144.32.64.1.
(i) Suggest a suitable assignment of IP addresses to the four
Ethernet segments. (8 marks)
(ii) How would you assign addresses to the links between the
routers? (2 marks)
(c) [9 Marks]

Describe a possible routeing table for router R3, in the form:







giving the links appropriate cost functions. Justify the cost you
choose.
(d) [7 Marks in total]

(i) The network manager is considering an upgrade to IPv6.
What are the main advantages of using IPv6 over IPv4? (4
marks)
(ii) In particular, how would the process of assigning IP
addresses to computers change in IPv6? What are the
benefits of using this new approach? (3 marks)


Subnet mask Cost Port
X.X.X.X/Y Z P1
X.X.X.X/Y Z P1
… … …

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(e) [5 Marks in total]
Two subnetworks A and B are connected to a router R1 as shown
below. The IPv4 address range 152.72.45.0/24 has been
allocated.







(i) Describe a possible routeing table for router R1 in the form:


(3 marks)

(ii) Router R1 receives a packet with an IP address
152.72.45.12 from the Internet and the IP address matches
with two entries in the routeing table in part e (i). Explain
how the router determines which port to send this packet. (2
marks)




Router R1
Subnet A (28 stations) Subnet B (220 stations)
To the Internet
Port 1 Port 2
Port 3
IP Addresses Send Packet to: Send Packet from:


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3
(a) [18 Marks in total]
(i) Discuss five features of Enhanced Interior Gateway Routing
Protocol (EIGRP). (5 marks)
(ii) Why is IPv4 fragmentation required? How is fragmentation
done in IPv4? (4 marks)
(iii) List the three key differences between each of the following
pair of terms. (9 marks)

a. DNS and ARP
b. IPV4 address and MAC address
c. Internet checksum and Cyclic Redundancy Check (CRC)
(b) [11 Marks in total]
(i) When routers generate ICMP messages, to where do they
send them? Along with the ICMP header at the beginning,
what additional contextual information do routers include in
the messages? (4 marks)
(ii) Name two circumstances under which an end-host (not a
router) will send an ICMP message. (3 marks)
(iii) Briefly describe how the “traceroute” tool works (i.e., what
does it do in order to identify the routers that make up an
Ineternet path). (4 marks)
(c) [10 Marks in total]
(i) Discuss five differences between “Distance vector” routeing
protocol and “link-state” routeing protocol. (5 marks)
(ii) Name two problems that would arise if all routeing in the
Internet was done using link-state routeing protocol and
Dijkstra’s algorithm. Does Distance vector routing suffer
from these? How about path-vector? (5 marks)




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(d) [11 Marks in total]
(i) Briefly describe how Ethernet’s exponential backoff works.
What is one reason why Ethernet’s exponential backoff
might be better than randomizing retransmission attempts
over a fixed-length time interval? (4 marks)
(ii) A 2 km long broadcast LAN has a capacity of 107 bps and
uses CSMA/CD. The signal travels along the wire at
2×108 ms-1. What is the minimum packet size that can be
used on this network? (4 marks)
(iii) The message 11001001 is protected by a cyclic-reduncancy
check (CRC) with a polynomial x3 + 1. Determine the CRC
that need to be appended at the end of the message. (3
marks)


End of Paper