nAmIB I A UnIVERSITY OF SCIEnCE AnD TECH n 0 LOGY

Faculty of Computing and Informatics

Department of Computer Science

QUALIFICATION: BACHELOR OF COMPUTER SCIENCE HONOURS

QUALIFICATION CODE: : 08BCHC LEVEL: 8

COURSE: MOBILE NETWORKS AND ARCHITECTURES COURSE CODE: MNA810S

DATE: JULY 2016 SESSION: 2

DURATION: 2 HOURS MARKS: 100

EXAMINER(S)

MODERATOR:

SECOND OPPORTUNITY EXAMINATION QUESTION PAPER

PROF DHARM SINGH JAT

DR LINOH MAGAGULA

THIS QUESTION PAPER CONSISTS OF 3 PAGES

(EXCLUDING THIS FRONT PAGE)

INSTRUCTIONS

1. The question paper consists of 06 questions divided into two sections A and B.

2. Section A contains 2 questions. Answer all questions.

3. Section B contains 4 questions. Answer any three questions.

4. Write clearly and neatly.

5. Number the answers clearly.

PERMISSIBLE MATERIALS

1. Non-Programmable Calculator.

SECTION A [40Marks]

This section contains two questions. Attempt all questions.

Q1 a)

b)

Explain two functions of the Physical layer and two functions of the Data link layer [8] in a wireless and mobile environment. -2 marks for each correct statement for each layer

(i). Give 4 advantages of wireless LANs.

(ii). Give 4 disadvantages of wireless LANs.

- one mark for each advantage -one mark for each disadvantage

[4]

[4]

c) Give two differences between Ad-Hoc and infrastructure mode of wireless LAN. [4] Explain.

Q2 a)

- two marks for each correct statement.

Define the following digital modulation schemes

(i.) Amplitude Shift Keying (ASK) (ii.) Frequency Shift Keying (FSK) (iii.) Phase Shift Keying (PSK)

[2] [2] [2]

b) Two senders, A and 8, want to send data. COMA assigns the following unique and [10] orthogonal key sequences: key Ak = 010011 for sender A, key Bk = 110101 for sender B. Sender A wants to send the bit Ad = 1, sender 8 sends Bd = 0. Illustrate this example and explain how the receiver detects correct binary.

-Correct sent signa/for A -Correct sent signal forB -Correct received signals for A -Correct received signals forB

{3 Marks] {3 Marks] [2 Marks] [2 Marks]

c) Draw and explain the basic carrier sense multiple access with collision avoidance [4]

(CSMA/CA) mechanism, including "contention window and waiting time" in the diagram.

-one mark for each correct event in figure -one mark for each correct statement

1

SECTION 8 {60 Marks]

This section contains four questions Attempt any three questions.

Q3 a) If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses

a. One-cell reuse b. four-cell reuse c. seven-cell reuse

b) A total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex voice and control channels. If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of the three systems.

Q4 a)

b)

(i). One cell reuse (ii). four-cell reuse (iii). Seven-cell reuse

In an full -rate TDMA system used in United States Digital Cellular (USDC)

IS-54 standard are:

• Duration of a TDMA voice frame= 40ms • Number of time slots in a frame= 6 • Number of bits in a voice frame= 1944 • Number of bits in guard band= 6

Calculate (i). the duration of a time slot of a voice frame

(ii). the number of bits in a time slot of a voice frame

(iii). the duration of a bit in a voice frame

[iv ). the guard time

(i). Of the following, what values are possible for a cluster size in a cellular topology?

Assume a hexagonal geometry: 5, 8, 11, 13, 20, 21

(ii). Why?

-2marks: equal marks for each possible value - 6marks: equal marks for all correct events for each value

2

[3] [3] [3]

[3] [4] [4]

[3] [3] [3] [3]

[8]

Q5 a) As a Network Engineer you are given two scenarios to analyze and design a wireless network for Uranium Company in Namibia. Assume that the capital cost for hardware is approximately N$ 368 000 , if given the following prices for each item:

Scenario A Scenario B

Components Unit Price Components Unit Price

9 Access Point N$ 5000 5 Access Point (AP) N$ 5000 (AP) S-Layer 2 N$1000 4-Liayer unmanaged N$ 1000 unmanaged switches (Sport) switches (Sport) 5 -Layer 2 N$ 4000 11 -Layer 2 N$ 4000 managed switches managed switches (Sport) (Sport) 2-Layer 3 N$10000 3-Layer 3 N$10000 switches (24 port) switches (24 port) 50 Computers N$ 5000 50 Computers N$ 5000 S-IP Camera N$ 5000 3-IP Camera N$ 5000

The labour including installation and components are the same in both cases.

(i.) (ii.)

Which is the best scenario for wireless communication systems? Justify your answer in (i) above

b) Draw MSK signal for the data 10110 and frequencies as shown in following figure of Minimum Shift Keying (MSK).

Q6 a)

data

low frequen~·

high frequency

1 0 1 1 0

Figure

-0.5 marks for each correct event in the figure for even bit

-0.5 marks for each correct event in the figure for odd bit

-1 marks for each correct event in the figure for MSK signal

Discuss the two spread spectrum techniques with the help of diagrams.

b) Network and signalling are crucial aspects of any cellular mobile communication network.

(i.) Describe the roles of the VLR and HLR.

(ii.) Give two roles of the MSC in a GSM network.

GOOD LUCK

3

[3] [7]

[2.5] [2.5] [5]

[12]

[4]

[4]