CMSCY3001 Computer Networks

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Oct 23, 2013 (4 years and 17 days ago)

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Examiner: RA. Moderator: RP. Subject Leader CB.

CMSCY3001


23/10/13




Semester 1 Examinations 2000





CMSCY3001


Computer Networks






Duration: 2 Hours








Instructions to candidates


Do not open this question paper until you have been told to do so by the invigilator.

A figure in []

denotes the number of marks available for that question or part of
question.


There are 4 questions. Answer 3 questions.


If you present answers to more than 3 questions, the first 3 of these answers will be
assessed.


Questions carry 20 marks each.

The t
otal number of marks available is 60.


Calculators may be used.


A printed language dictionary may be used.

RA

Page
1

CMSCY3001

Semester 1


1.

(a)

The OSI 7
-
layer reference model describes a series abstractions for designing
computer networks beginning with the Physical Layer (layer 1) and

moving
up to the Application Layer (layer 7).


(i)

With the aid of a diagram describe how a message is passed
between two network applications using the OSI 7
-
layer model.


(ii)

Discuss the reasoning behind the layering approach to network
design.


(iii)

Discuss a criti
cism you have of the OSI model and suggest
possible improvements to address this criticism.

[12]



(b)

Each host on the Internet is currently assigned an IP address 32 bits long
(IPv4). IPv4 addresses are usually written as a series of four decimal numbers
bet
ween 0 and 255, e.g. 202.150.54.12. IPv4 addresses traditionally belong to
one of five classes of address, depending on the type of network.


(i)

Describe the structure of Class A, B and C addresses, and the types
of networks they are suited for.


(ii)

What are the

advantages of subnetting? Propose a scheme for
subnetting a Class B network. Identify the mask to be used and the
maximum number of hosts supported by each subnet in the scheme.

[8]




RA

Page
2

CMSCY3001

Semester 1

2.

(a)

The IEEE 802 series of standards describe both the Physical and Da
ta Link
layers of their respective technologies. Two important standards are 802.3 and
802.5, respectively known as CSMA/CD (Ethernet) and Token Ring.


(i)

Describe three major differences between Token Ring and
CSMA/CD technologies.


(ii)

Explain the mechanism for

collision detection and recovery in
CSMA/CD.

[7]



(b)

Fast Ethernet and FDDI are both used to improve on ‘slow’ LAN technologies
such as 802.3/802.5.


(i)

Discuss the different approaches of the two technologies, assessing
them in terms of their applicability.


(ii)

Explain with the aid of a diagram two methods to achieve fault
tolerance in FDDI, considering dual attach stations (DAS) and
single attach stations (SAS).

[7]



(c)

Consider a computer game server managing a networked game “ScaryBob”
with many players interact
ing via client machines. The need for players to
synchronise with each other requires the server to regularly update all clients’
clocks. Upon receipt of clock update the client acknowledges the update by
returning the time difference to the server. Outlin
e client and server functions,
based on the UDP protocol, to carry out this task. Explain how the functions
work and use pseudocode to show the processes required. (There is no need
for coding in any particular language, but include references to the socke
ts
API calls)

[6]

RA

Page
3

CMSCY3001

Semester 1

3.

A company wishes to network its offices together to enhance the overall business
process. The company has three offices, one in London and two in Liverpool (next
door to each other). The London office is on three floors whilst the Liv
erpool offices
are both small single floor offices. The company wishes to be able to handle a range
of data from email and documents to video conferencing. Finally the company wishes
to connect to the external Internet. Much data on the company’s computer
system is
highly sensitive and the company is concerned about the security issues of the new
network, both internally and externally.



(a)

Specify, with justification, a network that will meet these requirements. You
may ignore any application software that i
s required.

[10]



(b)

How might the company expand their network to cope with a rapid expansion
of their London office, both in terms of data throughput and number of nodes?

[6]



(c)

The Internet is based upon the TCP/IP protocol suite with further protocols
ope
rating either at the same layers as TCP/IP or above. Compare TCP and
UDP, giving two advantages of each and suggest an application using each.
Justify your answers.

[4]



RA

Page
4

CMSCY3001

Semester 1

4.

(a)

Data travelling on a network may either be Packet
-
Switched or Circuit
-
Switched. P
acket
-
Switched data may be transported using either Datagrams
(connectionless) or Virtual
-
Circuits (connection
-
oriented). Explain the
differences between the switching and service types, and the major application
of each.

[6]



(b)

Asynchronous Transfer Mode (
ATM) is a high
-
speed network technology.
ATM has a physical layer, an ATM layer and an ATM Adaptation Layer
(AAL). Four protocols at the AAL layer allow for the creation of a variety of
service classes.


(i)

What are the factors that determine the service clas
ses and what is
the purpose of such distinction?


(ii)

Discuss how AAL 5 is considered more appropriate for Internet
services, by comparing it to the other adaptation layer protocols
available.

[8]



(c)

A Video
-
On
-
Demand (VOD) server stores and distributes MPEG en
coded
films. Each film is 2 hours long and contains 3.5Gigabytes of data. The server
is connected to an ATM network. Calculate the required bandwidth (in
Megabits per second) of the ATM link required to support 50 different streams
simultaneously.

[6]




RA


CMSCY3001

Semester 1/2

EXAMINER'S STATEMENT

There are no corrections on this proof copy and the examination paper is now ready for printing. I am aware
that the printed papers will be produced directly from this proof.


Please print

100


copies.


Special printing instructions
for the examination paper


None





Signed .................................................................... Date ...............7
th

November 2000..........




STATIONERY AND OTHER REQUIREMENTS

SCHOOL

CMS


MODULE

CMSCY3001



ITEM

NUMBER REQUIRED
PER
STUDENT

TOTAL NUMBER
REQUIRED (ESTIMATE)

14 PAGE ANSWER BOOKS

1

100

6 PAGE ANSWER BOOKS

1

100




CONTINUATION BOOKS






DRAWING PAPER A1

0

0

DRAWING PAPER A2

0

0

DRAWING PAPER A3

0

0




GRAPH PAPER

0

0




ABSTRACT PAPER

0

0




DIMENSION PAPE
R

0

0




TRACING PAPER A1

0

0

TRACING PAPER A2

0

0

TRACING PAPER A3

0

0

DIAGRAMS

0

0

DRAWINGS

0

0

ANY OTHER REQUIREMENTS


0

0


Are there any special items which may be taken into the examination by candidates? If so, please list here



None





Si
gned .............................................................

Date .........7
th

November 2000..........