NATIONAL OPEN UNIVERSITY OF NIGERIA

UNIVERSITY VILLAGE, PLOT 91, CASDASTRAL ZONE, NNAMDI AZIKIWE EXPRESS WAY, JABI, ABUJA.
FACULTY OF SCIENCES
MARCH/APRIL 2014 EXAMINATION

Course Code:                     CIT 303

Time Allowed:                   3hrs

Title:                                      Principles of Communication

Instruction: Attempt any five (5) questions

Question 1

a. Encrypt the word “IGNORANCE” with the following ciphers:  

i. Shift cipher with key=1                                                                                                                                              (1 mark)              

ii. Shift cipher with key= 26                                                                                                                                          (2 marks)            

iii. Shift cipher with key=0                                                                                                                                            (2 marks)            

iv. Transposition cipher where plain text: 1 2 3 4 5 6 7 8 9 and cipher text: 4 3 1 5 2 6 7 9 8 (with no bogus character)                                                                                                                                                                                            (1.5 marks)

b. Decrypt the word “ANCEIGNOR” with the following ciphers:

i. Shift cipher with key=1                                                                                                                                              (1 mark)              

ii. Shift cipher with key= 26                                                                                                                                          (2 marks)            

iii. Shift cipher with key=0                                                                                                                                            (2 marks)            

iv. Transposition cipher where plain text: 1 2 3 4 5 6 7 8 9 and cipher text: 4 3 1 5 2 6 7 9 8 (with no bogus character)                                                                                                                                                                                            (1.5 marks)

c. The amplification in a cable is usually defined in decibels per kilometer (dB/km). If the signal at the beginning of a cable with 2 dB/km has a power of 2 mW, what is the power of the signal at 5 km?                                                                                                                                                                                                                                                       (3 marks)

d. Briefly explain the term attenuation                                                                                                                  (1.5 marks)

Question 2

a. Define the term “network topology”.                                                                                                                                (1 mark)

b. With the aid of diagrams, briefly explain the following network topologies:

i. Ring                                                                                                                                                                                    (3 marks)

ii. Mesh                                                                                                                                                                                (3 marks)            

iii. Tree                                                                                                                                                                                                 (3 marks)

c. State and briefly explain 3 advantages and 3 disadvantages of using fiber optic cables in networking.                                                                                                                                                                                                                  (6 marks)

d. State 1 advantage and 2 disadvantages of STP cables in comparison to UTP cables.                      (1.5 marks)

Question 3

a. With the aid of a table ONLY, describe the five classes of IP addresses under the following columns:                  

i. IP Address Class           

ii. Format

iii. Purpose         

iv. Number of network/host bits

v. Max number of hosts                                                                                                                                                (10 marks)

b. Briefly explain each of the following terms:   

i. Linear filter                                                                                                                                                                     (2 marks)            

ii. Stable filter                                                                                                                                                                   (1 mark)              

iii. Unstable filter                                                                                                                                                              (1 mark)              

iv. Time-invariant filter                                                                                                                                                 (1 mark)              

v. Adaptive filter                                                                                                                                                              (1 mark)

vi. Linearity (in the context of linear-time invariant (LTI)                                                                                                 (1.5 marks)

Question 4         

i) Outline four (4) basic Network Topologies.      

ii) List three guided Media.                                                                                                                                          (1 mark)

iii) In one sentence, describe the OSI reference model                                                                                  (1 mark)

Question 5

a. Briefly explain 3 modes of communication (with regards to the direction of communication). In case, identify 2 examples of devices that utilize the mode of communication.                                                                         (9 marks)

i. 4 services/ functions provided by the OSI Physical Layer                                                                             (2 marks)            

ii. 4 services/ functions provided by the OSI Data Link Layer                                                                         (2 marks)            

iii. 4 services/ functions provided by the OSI Transport Layer                                                                       (2 marks)            

iv. 3 services/ functions provided by the OSI Session Layer                                                                           (1.5 marks)

Question 6

Contrast between:

i) Analog Signal Processing          

ii) Digital Signal Processing

Question 7

a) What is Data Communications?

b) Define Protocols.

c) The effectiveness of a data communications system depends on four fundamental characteristics.