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Sample Question Paper for 9210-121
Graduate Diploma in Engineering
Computer networks
Duration: three hours
You should have the
following for this examination
• one answer book
• non-programmable calculator
• pen, pencil, drawing
instruments
General instructions
• This paper consists of nine questions.
• Answer any five questions.
• A non-programmable electronic calculator may be used but candidates must show
sufficient steps to justify their answers.
• Drawings should be clear, in good proportion and in pencil. Do not use red ink.
• All questions carry equal marks. The maximum marks for each section within a
question are shown.
1
a)
b)
2
a)
b)
c)
d)
3
a)
b)
The Open Systems Interconnection (OSI) model has seven layers. They can be
categorised as application centric layers and network centric layers.
i)
List the seven OSI layers appropriately under the above two categories.
Also state whether there are any layers that may not fit exclusively into
one category.
ii) Which OSI layers are communication-media dependants? Briefly explain
giving examples.
iii) In what way does TCP/IP relate to the OSI model? Explain using a diagram
of the protocol stacks.
Consider a communication that takes place between a client and a server in two
separate networks. These networks have routers that connect them to public
networks, and any remote client or server communication needs to happen
through these routers.
Show the interactivity between different OSI layers using a diagram. Your
illustration should indicate how the different layers of each of the devices
involved with the above communication interact.
List four advantages of IPv6 over IPv4.
Briefly describe the following.
i)
Link local address.
ii) Extension headers.
iii) IPv4 embedded IPv6 address.
What are private IPv4 addresses? Explain using examples.
IP is said to be an unreliable protocol.
How does Transmission Control Protocol (TCP) compensate for this unreliability?
i)
ii) If the transport layer uses User Datagram protocol (UDP), how can reliability
in communication be achieved?
Compare and contrast the client/server networking model and the peer-to-peer
networking model. In your explanation, identify the similarities, differences,
advantages and disadvantages of these two networking models.
Consider the following networking environment.
•
An international bank has many branches distributed across the world.
It has a Main Office for International Coordination (MOIC).
•
In each country where they have their presence, a Local Head Office (LHO)
is identified to locate a datacenter.
•
In each country, there may be many Local Branch Offices (LBO) depending
on the customer base.
•
All local network connections from LBOs are terminated at the LHO of
each country.
•
The bank uses centralised applications that can be accessed using a web
browser. However, for reliability and availability purposes, each LHO maintains
an instance of all centralised applications so that all local branches will be
able to gain access using the local connections, and less international traffic
is involved.
•
Each LHO synchronises data with the servers at the MOIC.
i)
Propose a high level network architecture for this bank. Use a diagram to
illustrate your answer, and use necessary labelling and descriptions in the
diagram itself.
ii) Identify three application protocols necessary for this bank to carry out its
business functions effectively and securely. Provide reasons why you think
that each protocol you identified is necessary.
2
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4
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7
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e)
Briefly explain what a socket in TCP is, and how TCP uses sockets in its connections.
Explain how a DNS request, by a client is served by a DNS server. Use diagrams to
illustrate your answer.
A client S has sent TCP segments to a recipient R with parameters as follows.
s1 seq=7000, ack=5000, data=2000, win=1000
s2 seq=9000, ack=6000, data=3000, win=1000
s3 seq=12000, ack=6000, data=1000, win=1000
Just before s1 is received by R, it had 1k of data to be sent and 6k of free buffer
space to receive data.
i)
When R received s1, what would have been the parameters sent by R to S?
ii) What can be the reasons for S to send s2 and s3 with ack=6000?
iii) If the segment s2 was lost, what would have been the parameters sent by R to S?
What factors are considered in determining the Round Trip Time (RTT) in TCP?
Briefly explain what Smoothed Round Trip Time (SRTT) is and why it is necessary.
Using diagrams if necessary, briefly explain the following TCP problems, and the
remedies proposed to solve these problems.
i)
Small packet problem.
ii) Silly window problem.
UDP is said to be unreliable. However, some applications are better implemented
using UDP than TCP.
What kinds of applications are better implemented using UDP? Give two examples.
i)
ii) What steps should be taken to compensate for the unreliability of UDP?
Consider the use of Cyclic Redundancy Code (CRC) for error detection. If the data
frame is 010011011011 and the generator code is 1011, calculate the polynomial
code checksum for detecting errors and produce the final transmitted frame.
Show the steps of your calculation.
Compare and contrast ‘Collision Detection’ and ‘Collision Avoidance’ concepts
used in IEEE LAN standards.
IEEE 802 series LAN standards use the Logical Link Control (LLC) layer as a common
layer for all of 802.3, 802.4 and 802.5. What advantages does this offer?
Assume that you need to design a LAN for a small organisation that has several
departments. The staff within these departments must have separate networks.
However, all these departments are situated in one rented floor of a building and
the owners of the building allow only one wall-mounted network equipment rack
in each floor. In this situation, explain how you would use VLANs in your LAN design.
Show how you will use VLAN tags in this design. Use diagrams if necessary.
Compare and contrast Wi-Fi, Bluetooth and Wimax wireless network architectures.
Note that it is not necessary to explain the protocol stack of each of these protocols,
and you can consider their features and attributes in your answer.
What advantages do wireless networks bring compared to wired networks?
List three advantages.
List three disadvantages of wireless networks.
What is unlicensed or Industrial, Scientific and Medical (ISM) band and what are
its purposes?
How does ATM achieve higher switching speeds than Frame Relay? Briefly explain.
3
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See next page
8
9
With the spread and popularity of the Internet, packet based protocols have become
popular for voice communication in place of the traditional circuit switching technology.
Similarly, video conferencing applications also have become popular today. Many TCP/IP
based voice and video applications have become common among the Internet community.
a) What advantages do today’s voice protocols provide compared to traditional circuit
switched voice communications?
b) List three voice applications that are popular among the Internet community, and
briefly describe them.
c) Explain the use of the H.323 protocol in VoIP and video conferencing applications.
d) Illustrate using examples the need for the Session Initiation Protocol (SIP) for
multimedia communication.
a)
b)
c)
d)
Briefly explain two differences between public key cryptography and private
key cryptography.
List two applications that use public key cryptography and state why private key
cryptography is not suitable for these applications.
List one application that uses private key cryptography and state why public key
cryptography is not suitable for this application.
Public key cryptography can be used to achieve both confidentiality and integrity.
i)
Show how confidentiality is achieved using public key cryptography.
ii) Show how integrity is achieved using public key cryptography.
iii) What is an X.509 certificate?
4
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