FACULTY OF SCIENCE AND AGRICULTURE
SPRING SESSION EXAMINATION 2003
ITC242 INTRODUCTION TO DATA COMMUNICATIONS
DAY & DATE:
WRITING TIME: Three (3) Hours
MATERIALS SUPPLIED BY UNIVERSITY:
MATERIALS PERMITTED IN EXAMINATION:
NUMBER OF QUESTIONS:
INSTRUCTIONS TO CANDIDATES:
TIME:
READING TIME: Ten minutes
1 x 24pp Answer Booklet
Pens and Pencils, Calculator
Part A: Forty (40) multiple choice questions
Part B: Ten (10) True/False questions
Part C: Twenty (20) fill in the spaces questions
Part D: Five (5) short answer questions
5.
6.
7.
8.
1.
2.
3.
4.
Write your answers to the written questions in the booklet provided.
Mark your answers to the multiple choice questions on the exam paper.
Mark your answers to the true/false questions on the exam paper.
Fill in your answers to the “fill in the spaces” questions on the exam paper.
Multiple choice questions are worth one mark each. (Total of 40 marks)
True/False questions are worth one mark each. (Total of 10 marks)
Fill in the spaces questions are worth one mark each. (Total of 20 marks)
Written questions are worth 6 marks each. Choose any 5 questions out of 8.(Total of
9.
30 marks)
A total mark possible is 100.
INSTRUCTIONS TO INVIGILATORS:
1.
QUESTION PAPER MUST BE COLLECTED
STUDENT NAME: STUDENT NO:
STUDENT SIGNATURE:

PART A: Multiple Choice Questions
1. In the OSI model, the ____________________ layer is concerned with finding the best path for the data from one point to the next within the network.
a.
Data Link b.
Network c.
Physical d.
Application
2. The ____________________ layer can perform ASCII to non-ASCII character conversions.
a.
Transport b.
c.
d.
Data Link
Presentation
Application
3. Large networks that encompass parts of states, multiple states, countries and the world are called: a.
MANs b.
c.
LANs
PANs d.
WANs
4. What is the baud rate of a digital signal that employs Differential Manchester encoding and has a data rate of 1000bps? a.
500 baud b.
1000 baud c.
d.
2000 baud
4000 baud
5. Which category of UTP cable is the most current and commonly implemented? a.
Category 4 b.
c.
Category 5
Category 5e

d.
Category 6
6. The ____________________ of a signal is the number of times a signal makes a complete cycle within a given time frame . a.
Bandwidth b.
Frequency c.
Amplitute d.
Spectrum
7. In the OSI model, which layer performs encryption or data compression?
a.
Application b.
c.
Network
Data Link d.
Presentation
8. In a sliding window Go-Back-N ARQ system, A sends packets 0,1,2,3,4,5 and 6.
Packet 3 arrives at B corrupted. What do A and B send to each other next? a.
B sends REJ-3 , A then sends packets 3,4,5,6,7,0 and 1 b.
c.
d.
B sends REJ-2, A then sends packets 3,4,5,6,7,0 and 1
B sends REJ-3, A then sends just packet 3
B sends REJ-2, A then sends just packet 3
9. The loss of signal strength is called: a.
Attenuation b.
Amplitute c.
d.
Noise
Crosstalk
10. The ____________________ class of digital encoding schemes solves the synchronization problem by ensuring that each bit has some type of signal change . a.
Manchester b.
NRZ c.
d.
4B/5B
Both a and b.

11. Ethernet is said to be non-deterministic because of which of the following? a.
It is not possible to determine how long it will take to get a frame from one device to another. b.
It is not possible to determine whether an error has occurred during the transmission of a frame. c.
It is not possible to determine if another device wishes to transmit. d.
It is not possible to determine the maximum time a device will have to wait to transmit.
12. ___________________ technology can be applied to wireless communications, making them virtually impervious to interception . a.
Pulse code modulation b.
c.
Frequency modulation
Spread Spectrum d.
None of the above
13. Electromagnetic interference is often generated by: a.
Motors b.
c.
Fluorescent lights
Power lines d.
All of the above
14. The ____________________ component of the interface standard describes the function of each pin or circuit that is used in a particular interface.
a.
Electrical b.
c.
Mechanical
Functional d.
b.
Procedural
15. ____________________ identifies terminating devices such as terminals and computers . a.
DTE
DCE

c.
d.
CSU
DSU
16. The multiplexer creates a frame that contains data only from those input sources that have something to send in ____________________ multiplexing . a.
Frequency Division b.
Statistical Time Division c.
Synchronous Time Division d.
Dense Wavelength
17. In frequency division multiplexing, the medium must be capable of carrying a range of frequencies that can support the ____________________ of all the individual frequency ranges a.
Difference b.
Sum c.
d.
Product
None of the above
18. ISDN basic rate interface (BRI) multiplexes only ____________________ separate channels.
a.
One b.
c.
d.
Two
Three
Four
19. Error detection is performed at the ____________________ layer of the OSI model ? a.
Data Link b.
c.
Transport
Network d.
Both a and b
20. ____________________ is a very powerful error detection technique and should be considered for all data transmission systems ?

a.
Vertical redundancy check b.
Cyclic redundancy checksum c.
Simple parity d.
Horizontal parity
21. In a ____________________ topology the workstations are connected by a linear coaxial cable.
a.
Bus b.
Star-wired bus c.
Star-wired ring d.
Ring
22. Which protocol requires that a host cease to transmit for a random interval once a collision is detected on the wire? a.
CSMA/CD b.
Round robin c.
Reservation d.
All of the above
23. Which layer addresses do routers use to determine a packet's path?
a.
Data Link b.
Network c.
Physical d.
Application
24. Which type of bridge observes network traffic flow and uses this information to make future decisions regarding frame forwarding ? a.
Remote b.
Source routing c.
Transparent d.
Spanning tree
25. Learning network addresses and converting frame formats are the function of which device ? a.
Switch

b.
Hub c.
MAU d.
Bridge
26. The device that can operate in place of a hub is a: a.
Switch b.
Bridge c.
Router d.
Gateway
27. The CSMA/CD network is described by which frame specification?
a.
IEEE 802.2 b.
IEEE 802.3 c.
IEEE 802.4 d.
IEEE 802.5
28. In a ____________________ subnet, no unique dedicated physical path is established to transmit the data packets across the subnet ? a.
Circuit-switched b.
Packet-switched c.
Large d.
Heavily loaded
29. A dial-up telephone system uses which type of subnet?
a.
Circuit-switched b.
Packet-switched c.
Broadcast d.
Logically switched
30. The postal system is a good example of which type of communication?
a.
Broadcast b.
Connection oriented c.
Connectionless d.
Both a and c

31. Which type of routing allows each node to maintain its own routing table ? a.
Distributive b.
Adaptive c.
Centralised d.
Static
32. The length of an IP address is: a.
8 bits b.
16 bits c.
32 bits d.
48 bits
33. ____________________ is a set of codes inserted into a document intended for display on a web browser a.
HTML b.
HTTP c.
FTP d.
Telnet
34. The protocol that allows dynamic assignment of IP Addresses to workstations is known as: a.
ICMP b.
DHCP c.
SMTP d.
SNMP
35. ____________________ is a packet-switched network designed for transmitting data over fixed lines . a.
ATM b.
ADSL c.
SONET d.
Frame Relay

36. An ISDN BRI consists of ____________________ channels.
a.
Two B b.
Two B and one D c.
Two D d.
Two D and one H
37. Which type of virus mutates with every infection, thus making it difficult to locate?
a.
Macro b.
Trojan horse c.
Polymorphic d.
Worm
38. A commonly employed encryption method used by businesses to send and receive secure transactions is: a.
Manchester encoding b.
Data Encryption Standard (DES) c.
Pulse Code Modulation d.
Kerberos
39. The equation to calculate availability is ? a.
(Total Time Available + Downtime) * Total Time Available b.
(Total Time Available - Downtime) c.
(Total Time Available + Downtime) d.
(Total Time Available - Downtime) / Total Time Available
40. The average time a device or system will operate before it will fail is called the: a.
MTBF b.
MTTR c.
Reliability d.
None of the above

(Circle the correct answer. Each question is worth 1 mark)
1. The main difference between infrared and Bluetooth transmission is that Bluetooth needs line of site whilst infrared does not. a.
true b.
false
2. 4B/5B encoding is more efficient than Manchester encoding. a.
true b.
false
3. Synchronous Time Division multiplexing will not allocate a time slot to a source that is idle. a.
true b.
false
4. Dumb terminals are used for simple data entry and data retrieval operations.
a.
true b.
false
5. The bottom layer in the OSI model, the data link layer, handles the transmission of bits over a communications channel.
a.
true b.
false
6. The physical organization of a star-wired ring is actually not circular.
a.
true b.
false
7. A source routing bridge keeps an internal table.
a.
true b.
false

8. In a virtual circuit packet-switched subnet, all packets that belong to a logical connection can follow the same path through the network.
a.
true b.
false
9. Every object on the Internet has a unique URL . a.
true b.
false
10. Ciphertext is data before any encryption has been performed . a.
true b.
false

(Each question is worth one mark)
1. Multiplexing occurs at the ______ physical ___________ layer of the OSI reference model.
2. The three basic components of all signals are _________ amplitude ____________ ,
_______ frequency ______________ and _________ phase ____________ .
3. ________ Impulse _________ noise is a non-constant noise that is one of the most difficult errors to detect because it can occur randomly.
4.
Transmitting multiple signals on one medium is called
_________ multiplexing ___________.
5. ________ Analogue ____________ data and signals are represented as continuous waveforms that can be at an infinite number of points between some given minimum and maximum.
6. The _______ Internet or TCP/IP _____ Model is a communications architectural model that incorporates TCP/IP and has 4 layers.
7. _______ Baseband _____________ coaxial technology uses digital signaling in which the cable carries only one channel of digital data
8. _______ Terrestrial _____________ microwave transmission systems transmit tightly focused beams of radio signals from one ground-based microwave transmission antenna to another .
9. In an _______ asynchronous _____________ connection, a single character, or byte of data, is the unit of transfer between sender and receiver .
10. The _____ local loop _______________ is the stretch of telephone wire that runs between a house and the telephone company's exchange .
11. ________ Code Division Multiplexing (CDM) ____________ multiplexing allows multiple users to share a common set of frequencies by assigning unique digital codes to each user.
12. A _______ collision _____________ results when two or more workstations listen to the medium at the same moment, hear nothing, and then transmit their data at the same moment.

13. Modern star-wired bus topology logically acts as a bus, but
___________ physically _________ looks like a star.
14. A _________ hub ___________ is a device that interconnects two or more workstations in a star-wired bus local area network and broadcasts incoming data onto all outgoing connections.
15. The process of adding transport header information on the front of the data packet is called ________ encapsulation _____________.
16. A ________ connection oriented ____________ network application provides some guarantee that information traveling through the network will not be lost and the information packets will be delivered to the intended receiver.
17. The ________ Internet ____________ is a massive collection of networks, routers, and communications lines.
18. The _________ Address Resolution Protocol (ARP) ___________ protocol takes an IP address in an IP datagram and translates it into the appropriate CSMA/CD address for delivery on a local area network.
19. A _______ Denial of service (DOS) _____________ attack is one where a computer site is bombarded with so many messages that the site cannot perform its normal duties.
20. ________ Benchmarking ___________ involves generating system statistics in a controlled environment and then comparing those statistics against known measurements.
(Each question is worth 6 marks each. Answer the questions in the Answer booklet provided).
Choose ONLY 5 Questions to answer
1. Name and briefly define the seven layers of the OSI model.
Answer
7. Application Layer – is where applications using network services reside. Example applications include
HTTP, FTP, Telnet.
6. Presentation Layer – presentation, encryption and compression of data between sender and receiver.
5. Session Layer – responsible for establishing, maintaining and terminating sessions between endpoints.
4. Transport Layer – reliable end to end transport.
Contains end to end error control and flow control.
3. Network Layer – Path determination, routing, the sending of packets through a network. IP addressing.

2. Data Link Layer – responsible for accessing the medium.
1. Physical Layer – transmission of bits over a communications channel.
Marking Guide
Deduct 1 mark for each layer not labeled correctly.
Deduct 1 mark for each layer not defined correctly.
2. Is stop-and-wait ARQ a half duplex protocol or a full duplex protocol? Explain your response.
Answer
Stop-and-wait ARQ is a half duplex protocol. That is because at any point in time only one endpoint may transmit to another. Endpoint A transmits one packet of data to endpoint B, then stops and waits for a reply from
B. If the packet arrives without error, B responds with a positive acknowledgement. If the data arrives with errors, B will respond with a negative acknowledgement.
Marking Guide
Deduct 3 marks for not stating that stop-and-wait ARQ is a half duplex protocol.
Deduct between 1 and 3 marks for not explaining why.
3. Describe the function of a switch. At what layer of the OSI model does it operate?
Answer
A switch is a combination of a hub and a bridge. It connects multiple workstations like a hub. It also filters frames like a bridge. Filtering occurs based on the layer 2 MAC Address within each frame. It performs filtering based on the MAC addresses of frames as they are transmitted through the switch. There are a number of different ways that a switch may perform this filtering.
The first is transparent bridging, where the switch will observe network traffic and make future decisions on frame forwarding based on learning out which switch port each MAC address resides. Source route bridging relies on information contained within each frame to make the filtering decision.
A switch operates at Layer 2, the Data Link Layer of the
OSI model.
Marking Guide
Deduct between 1 to 4 marks for not correctly describing the function of a switch.
Deduct 2 marks for not identifying the correct layer.
4. What are the advantages and disadvantages of fibre optic cable?
Answer

Advantages:
 Not affected by electromagnetic interference
 Low noise and error rates
 High speed transmission
 Difficult to wiretap
 Can be used over long distances
Disadvantages:
 Can send signals in one direction only, theirfore a pair of fibre is required between devices.
 Relatively higher cost of fibre.
 More expensive and difficult to install and terminate within a building.
Marking Guide
Allocate 1 mark for each correct advantage and disadvantage provided.
5. What is the difference between a deterministic and non-deterministic protocol?
Answer
In a deterministic protocol you can determine when a workstation will be allowed to transmit. The order of transmission by each workstation is known. Token Ring is an example of a deterministic protocol.
In a non-deterministic protocol, you cannot calculate or determine the time at which a workstation will transmit.
CSMA/CD is an example of a non-deterministic protocol, with Ethernet being an implementation.
Marking Guide
Deduct between 2 to 6 marks for not providing a correct difference.
Allocate 1 mark each for providing examples of each type of protocol.
6. What is the data transfer rate in bps of a signal that is encoded using phase modulation with eight different phase angles and a baud rate of 3000?
Answer
Eight different phase angles mean that at any point in time the signal may represent one of 8 states, or 3 binary numbers. (2^3 = 8)
Thus, data transfer rate = 3 x 3000
= 9000 bps.
Marking Guide
Full marks for obtaining the correct answer with working.
5 marks for obtaining the correct answer, with no working.
3 marks for showing that 8 phase angles allows for the transmission of 3 bits at a time.

7. Name four (4) types of multiplexing giving a brief explanation of each.
Answer
1.
Frequency Division Multiplexing (FDM): the assignment of non overlapping frequency ranges to each user of a medium. The oldest and one of the simplest forms of multiplexing. An example is the transmission of different television stations using different frequencies.
2.
Time Division Multiplexing (TDM): the division of available transmission time up into portions to be shared amongst each user. Different types of TDM are synchronous TDM and statistical TDM. ISDN is an example of TDM.
3.
Dense Wavelength Division Multiplexing (DWDM): uses different wavelength or colored light to transmit multiple signals down a fibre optic cable.
4.
Code Division Multiplexing (CDM): assigns unique codes to each user to allow them to share a common set of frequencies. CDMA is an example of CDM.
Marking Guide
Deduct 2 marks for each type of multiplexing not identified.
Deduct 1 mark for each occurrence of not providing an accurate brief explanation.
8. Define the main differences between the TCP and UDP protocols.
Answer
 Reliability: TCP is a reliable protocol, UDP is an unreliable protocol (best effort).
 Connection: TCP is a connection oriented protocol, UDP is a connectionless protocol.
 Efficiency: TCP has more overhead and is less efficient than UDP.
 Flow control: TCP provides flow control with windowing, UDP does not.
Marking Guide
Allocate 2 marks for each correct difference identified.

Section A
1 – b
2 – c
3 – d
4 – c
5 – c
6 – b
7 – d
8 – a
9 – a
10 – a
11 – d
12 – c
13 – d
14 – c
15 – a
16 – b
17 – b
18 – c
19 – d
20 – b
Section B
1.
false
2.
true
3.
false
4.
true
5.
false
6.
true
7.
false
8.
true
9.
true
10.
false
Section C
1.
physical
2.
amplitude, frequency, phase
3.
impulse
4.
multiplexing
5.
analogue
6.
Internet or TCP/IP
7.
baseband
8.
terrestrial
9.
asynchronous
10.
local loop
11.
code division multiplexing (CDM)
12.
collision
13.
physically
14.
hub
15.
encapsulation
16.
connection oriented
17.
internet
18.
address resolution protocol (ARP)
19.
denial of service (DOS)
20.
benchmarking
21 – a
22 – a
23 – b
24 – c
25 – d
26 – a
27 – b
28 – b
29 – a
30 – c
31 – a
32 – c
33 – a
34 – b
35 – d
36 – b
37 – c
38 – b
39 – d
40 – a