BSc (Hons) Computer Science with Network Security. Examinations for Semester 1

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BSc (Hons) Computer Science with Network Security Cohort: BCNS/15B/FT Examinations for 2015-2016 Semester 1 MODULE: DATA COMMUNICATIONS MODULE CODE: CAN1101C Duration: 2 Hours Instructions to

1 BSc (Hons) Computer Science with Network Security Cohort: BCNS/15B/FT Examinations for Semester 1 MODULE: DATA COMMUNICATIONS MODULE CODE: CAN1101C Duration: 2 Hours Instructions to Candidates: 1. Answer ALL questions. 2. Questions may be answered in any order but your answers must show the question number clearly. 3. Always start a new question on a fresh page. 4. Use of calculator is allowed. 5. Appendix is provided. 6. Total marks 100. This question paper contains 4 questions and 5 pages. Page 1 of 5

2 ANSWER ALL QUESTIONS QUESTION 1: (25 MARKS) (a) With the help of a diagram, name the five components of Data Communication. (3 marks) (b) Describe briefly the four fundamental characteristics of data communication. (c) Describe the features of any three physical topologies, showing the number of cable links requires for n devices in the network. (3 x 3 marks) (d) Describe briefly the five layers of the TCP/IP Protocol Suite. (e) Explain, with the help of examples, the four levels of addresses in TCP/IP protocols. QUESTION 2: (20 MARKS) (a) With the help of a diagram, show the frequency, amplitude and phase of a sine wave. (b) The electrical power we use at home has a Frequency of 60Hz. Calculate the period, T, of this sine wave. (c) A digitized voice channel is made by digitizing a 4-kHz bandwidth analog voice signal. We need to sample the signal at twice the highest frequency (two samples per hertz). We assume that each sample requires 8 bits. What is the required bit rate? Page 2 of 5

3 (d) Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Explain the three causes of impairment. (6 marks) (e) We have a channel with a 1-MHz bandwidth. The SNR for this channel is 63. What are the appropriate bit rate and signal level? (f) What are the propagation time and the transmission time for a 5-Mbyte message (an image) if the bandwidth of the network is 1 Mbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at m/s. QUESTION 3: (30 MARKS) (a) The conversion of the user data into a transmission signal is called Encoding. There is several line coding schemes. Given the bit pattern , explain each of the following scheme, and sketch the resulting waveforms: (i) (ii) (iii) (iv) (v) (vi) Unipolar Encoding NRZ-L Encoding NRZ-I Encoding RZ Encoding Manchester Encoding Differential Encoding Mention any assumption you make. (6 x 3 marks) (b) A constellation diagram can help us define the amplitude and phase of a signal element, particularly when we are using two carriers. The diagram is useful when we are dealing with multilevel ASK, PSK, or QAM. Draw the constellation diagram for the following: (i) ASK, with peak amplitude values of 1 and 3 (ii) BPSK, with a peak amplitude value of 2 (iii) QPSK, with a peak amplitude value of 3 Page 3 of 5

4 (iv) 8-QAM with two different peak amplitude values, 1 and 3, and four different phases. (4 x 3 marks) QUESTION 4: (25 MARKS) Networks must be able to transfer data from one device to another with acceptable accuracy. For most applications, a system must guarantee that the data received are identical to the data transmitted. Any time data are transmitted from one node to the next, they can become corrupted in passage. Many factors can alter one or more bits of a message. Some applications require a mechanism for detecting and correcting errors. (a) Suppose a sender wants to send the word world. Using the Even parity check error detection method, show the pattern of bits sent and how errors can be detected. (b) A frame transmitter is using the generator polynomial x 3 +x+1 and the header contains the bit pattern: Derive the transmitted CRC header checksum. (c) Hamming codes are a family of linear error-correcting codes that can correct one-bit errors. Given the bit pattern is sent, and the pattern is received, explain how Hamming codes can detect and correct this one-bit error. (7 marks) (d) One of the goals of multiplexing is efficiency by combining several channels into one. (i) Explain how Frequency Division Multiplexing (FDM) works. (ii) Give any two applications of FDM. (iii) Ten signals, each requiring 100KHz are to be multiplexed together using FDM. What is the minimum bandwidth of the link if there is a need for a guard band of 20KHz between the channels to prevent interference? *** END OF QUESTION PAPER *** ( marks) Page 4 of 5

5 APPENDIX Channel Capacity Shannon Capacity: C = B log 2 (1 + S/N) Nyquist Channel Capacity: C = 2B log 2 M where C = Channel Capacity; B = Bandwidth; M = Number of discrete signal levels S/N = signal to noise ratio ASCII numbers Character Decimal a 97 b 98 c 99 y 121 z 122 Page 5 of 5

BSc (Hons) Computer Science with Network Security BEng (Hons) Electronic Engineering

BSc (Hons) Computer Science with Network Security BEng (Hons) Electronic Engineering Cohort: BCNS/16B/FT Examinations for 2016-2017 / Semester 1 Resit Examinations for BEE/12/FT MODULE: DATA COMMUNICATIONS