NATIONAL OPEN UNIVERSITY OF NIGERIA

UNIVERSITY VILLAGE, PLOT 91, CASDASTRAL ZONE, NNAMDI AZIKIWE EXPRESS WAY, JABI, ABUJA.
FACULTY OF SCIENCES
MARCH/APRIL 2016 EXAMINATION

Course Code:                     CIT 303

Time Allowed:                 3 hrs

Course Title:                      Principles of Communication

Instruction:  Attempt any five (5) questions. Each question carries 14 marks

Question 1

a) Outline the layers of Open System Interconnection Reference Model (OSI), in the right order.

b) Write briefly on the Physical Layer.

c) Cryptography algorithms (ciphers) can be divided into two groups, True or False? If True; Write briefly on any of the two groups

Question 2

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                                                                                                                                                                (7.5 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 3

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)