nAmIB I A U n IVERS lTV OF SCIEnCE AnD TECHnOLOGY

FACUL TV OF COMPUTING AND INFORMATICS

DEPARTMENT OF COMPUTER SCIENCE

QUALIFICATION: Bachelor of Computer Science

QUALIFICATION CODE: 07BACS LEVEL: 7

COURSE: Internet and WAN Telecommunication COURSE CODE: IWT711S

DATE: June 2016 SESSION: 1

DURATION: 2 hours MARKS: 100

FIRST OPPORTUNITY EXAMINATION QUESTION PAPER

EXAMINER(S}

MODERATOR:

1. Answer all questions.

Mrs. Mercy Chitauro

Mr. Kudakwashe Madzima

THIS EXAMINATION PAPER CONSISTS OF 5 PAGES

(Excluding this front page)

INSTRUCTIONS

2. When writing take the following into account: The style should inform than impress, it should be formal, in third person, paragraphs set out according to ideas or issues and the paragraphs flowing in a logical order. Information provided should be brief and accurate.

3. Please, ensure that your writing is legible, neat and presentable. 4. When answering questions you should be led by the allocation of marks. Do not give too few or too

many facts in your answers. 5. Number your answers explicitly. 6. Clearly mark rough work as such or cross it out unambiguously in ink.

PERMISSIBLE MATERIALS

1. Calculators

1.

a. Compare and contrast NRZ-L and NRZI encoding schemes [4 marks]

b. State 3 ways/methods of evaluating or comparing various encoding schemes

[3 marks]

c. Use the methods you stated in question 'lb' to compare the two encoding schemes A and B

given in table 1 in such a way that would show the preferred encoding scheme in a WAN digital

link with low bandwidth. [6 marks]

A B

1000Hz 2000Hz

Alternating current Direct current

Embedded clocking External clock required

Immune to noise Immune to noise

Signalling rate < data rate Signalling rate > data rate

Error detection No error detection

Table 1

2. Given that a T1 TDM line is capable of 1.544 Mbps. How many of the following devices will be

accommodated by the Tlline if 2% of the Tlline capacity is reserved for synchronisation purposes.

3.

a.

i. 300-bps computer terminals

ii. 9600-bps computer ports

iii. 64-kbps PCM voice-frequency lines

b. Explain how the use of a statistical multiplexer would improve performance.

[5 marks]

[3 marks]

c. Explain the overhead incurred on frames when statistical multiplexing is used and why it is

necessary. [2 marks]

a. Highlight four characteristics defined for an interface by a communication standard. [4 marks]

b. State and explain two elements of an H.323 system. [4 marks]

c. Label the missing Pin specification from the diagram below. Choose the answers from the list

given below [ 4 marks]

• DCE ready

• DTE ready

• Received data

• Transmitted data

2

4.

5.

Shield Signal ground

Re.~ned

filr testin~ l.nasslgned Sew111d11 ry

clear tu send

A.

Tmns. signal elt:"mcnt timing

Rl'<IUl':'il to send/

Ready fm· rec.·l•i \'ing

•5

Rnr. signal cll'menl timing

B.

6 7

St"Cmadary requt"SI to

send

Secondar)' trunsmi lll•d

data

St'Cundary n·cci\·ed

data

I.OljtJ

loopbtll'k

Figure 1: V.24/EIA connector face

a. Explain cyclic redundancy check.

b. Explain the use of even parity error detection

Rec'd Hne sig. dt'tl'Clor

Rcsen·t•·d for le.sting

8

Rt•mutc loopb:tdil

Signal quruity detector

10

l>arra sig. nate st"lt'Cl

~c. reed. line sig.

detector/ .Datu si~:nal rnte sei~"Ctur

24 25

T~-st mode

Ring Tran-smit si~::nal indinttor dement timing

[3 marks]

[3 marks]

c. What will be the even parity bit set by the transmitter for the following bytes

i. 11110000

ii. 10110110

iii. 11001010

[3 marks]

d. If two communicating devices use single even parity check and the transmitter sends 10101010

and the receiver gets 11100010. Will the receiver detect the error? Why or why not?

[2 marks]

a. State two categories of WAN technologies. [1 mark]

b. Describe circuit switching in WANs. [2 marks]

c. Explain the difference between circuit switching and packet switching technologies. [2 marks]

d. Fill in the following table by stating whether the WAN technology listed is packet switched or

circuit switched. [5 marks]

3

WAN technology Packet /circuit switched ISDN T carriers Frame Relay Digital Subscriber Line Asynchronous Transfer Mode Table 2

6. Frame Relay is a WAN protocol that operates at the physical and data link layers of the OSI reference

model. A Tl/E1 leased line, connects the routers to the ISP Frame Relay switches.

a. What is the use of DLCI? [2 marks]

b. Suppose the configuration frame relay link was changed from DLCI 200 to DLCI 400. What will happen

after this configuration change?

c. Suppose the Committed Information Rate {CIR) is 2.5Mbps.

i. What is CIR?

ii. What happens to frames that are transmitted at 2.7Mbps?

[2 marks]

[1 mark]

[1 mark]

iii. What happens to frames that are transmitted at 2.7 Mbps if the network is congested? [1 mark]

7. Internet Protocol {IP) provides a means for data transfer over networks. 1Pv4 provides fewer addresses

than are actually needed to connect all networked devices. 1Pv6 provides more addresses and thus it is

being adopted for use in networks.

a. What is the 1Pv6 address space?

b. Describe 1Pv4 limitation besides the one of limited address space stated above.

c. Highlight 2 advantages 1Pv6 has over 1Pv4.

d. Fill in the table below by stating what type of 1Pv6 address is given.

1Pv6 address Type FE80::C001:1DFF:FEEO:O FF02::2 ::1 3001:0060:0000:0000:0000:0CB4: 1E24:981A/64 ::/128

[1 mark]

[4 marks]

[2 marks]

[3 marks]

4

Figure 2 illustrates Bakers Fresh network diagram. Use Figure 2 to answer question 8.

[>iQ> POOl tw-£5

'v\.AU 30: STAFF \\..AN-.o: SAlES

Figure 2: Bakers Fresh Network

S.O/OS'.o~~ 12.11.~ .. ·6.4-

s.o~~ ~ Gs.'ito~ ~- GS.0~/1

2811 Sc0/0/1 HQ 172. 16.50.5/JO

oca 200 SeO

192. 168. 100.38/24

2611 6RANCH1

loopbackO 172.16.60.25/24

172.16.70.S/2i

8. You are given that the ISP network is connected through Multi-Protocol Label Switching (MPLS). You are

also told that the ISP router is the first router in the MPLS network

a. What type of MPLS router is the ISP router? What is this type of MPLS router responsible for in

the MPLS network? [3 marks]

b. State and explain the use of any other two types of MPLS routers. [4 marks]

c. Describe two reasons why MPLS networks are popular. [2 marks]

d. If the ISP is implementing traffic engineering. Which Label distribution protocol is being used by

the ISP? [1 mark]

e. Name any other label distribution protocol you know

f. In which OSIIayer is MPLS?

g. Is MPLS circuit switched or packet switched?

[1 mark]

[1 mark]

[1 mark]

5

. ,. 'I

9.

IPSec tunnel between HQ and BRANCH2

B2·PC

Figure 3: ADSL

Use Figure 3 to answer question 9

a. State two valid ADSL modulation techniques that can be used between HQ and BRANCH1

[1 mark]

b. Assume DMT ADSL is used between HQ and BRANCH1. Two consecutive channels are operating using

the frequency 72.5 KHz and 77.5 KHz.

i. What is the size of each DMT channel?

ii. What are the frequencies of these two channels?

c. What is the actual size of each DMT channel used in real ADSL networks?

[1 mark]

[2 marks]

[1 mark]

d. DSL access multiplexer (DSLAM) is used to accept all of the ADSL connections and combines or

multiplexes them into one link. Assume DSLAM is getting 15 different signals? How many frequencies

does the DSLAM output to make one link? [2 marks]

e. Which multiplexing technique does the DSLAM use to achieve the result in '9d' above?

f . What are guard bands used for in DSL? [1 mark] [1 mark]

g. ADSL signals coexist with voice signals in the same physical medium. A DSL splitter is used to separate

voice and data signals. A signal of 6Hz will represent what type of ADSL signal? [1 mark]

10. State and explain two security concepts that are derived from maintaining integrity in a secure system.

[4 marks]

6