BSc (Hons) Computer Science with Network Security, BEng (Hons) Electronic Engineering Cohorts: BCNS/17A/FT & BEE/16B/FT Examinations for 2016-2017 Semester 2 & 2017 Semester 1 Resit Examinations for BEE/12/FT MODULE: DATA COMMUNICATIONS MODULE CODE: CAN1103C 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 6 pages. Page 1 of 6
ANSWER ALL QUESTIONS QUESTION 1: (25 MARKS) With the help of a diagram, name the five components of Data Communication. (3 marks) (b) Describe the three directions of Data Flow, giving an example of each. (6 marks) What are the criteria that a network must possess to be considered effective and efficient? (3 marks) (d) How many cable links are required in a mesh topology of 9 nodes? (3 marks) (e) Define the word protocol, in relation to networks. (f) For each of the situation described below, identify the layer of the TCP/IP model it refers to (i) (ii) (iii) (iv) (v) (vi) Representation of a stream of bits. Route the packet to the final destination. File transfer, access and management. A message is divided into transmittable segments, each segment containing a sequence number. Dividing the stream of bits received from the network layer manageable data units called frames. Moving a packet from a network boundary to another network. (6 marks) Page 2 of 6
QUESTION 2: (25 MARKS) Show, by means of a diagram, a sine wave of frequency 10Hz, amplitude 5V and phase 90 degrees. (b) The electrical power we use at home has a Frequency of 25Hz. Calculate the period, T, of this sine wave. Draw the Time-Domain and Frequency-Domain of this sine wave. (1 + 2 + 2 marks) A non-periodic composite signal has a bandwidth of 300 khz, with a middle frequency of 175 khz and peak amplitude of 10 V. The two extreme frequencies have an amplitude of 0. Draw the frequency domain of the signal. (d) The power of a signal is 9 mw and the power of the noise is 1 μw. What are the values of SNR and SNR db? (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 7-Mbyte message if the network is a 100BaseT network? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 10 8 m/s. Page 3 of 6
QUESTION 3: (25 MARKS) The user data can be in one of two formats: Analog: Human voice as converted by typical home telephones. Digital: Computer files. The transmitted signals, representing the data, can also be in one of two formats: Analog or Digital. Give the advantages and the disadvantages of digital transmission. (b) The conversion of the user data into a transmission signal is called Encoding. There is several line coding schemes. Given the bit pattern 1010 0101, list the features of each of the following scheme, mentioning the characteristics (advantages and disadvantages) of each, and sketch the resulting waveforms. (i) (ii) (iii) NRZ-L Encoding RZ Encoding Manchester Encoding (4 x 3 marks) The transmission of binary data across a link can be accomplished in either parallel or serial mode. Using diagrams, describe the parallel mode, the serial asynchronous and the serial synchronous modes of transmission. (3 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. Anytime 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. Suppose a sender wants to send the word data. Using the Odd parity check error detection method, show the pattern of bits sent and how errors can be detected. (2 marks) Page 4 of 6
(b) A frame transmitter is using the generator polynomial x 3 +x+1 and the header contains the bit pattern: 1000. Derive the transmitted CRC header checksum. Discuss the advantages and advantages of error detection against error correction. (d) Hamming codes are a family of linear error-correcting codes that can correct one-bit errors. Given that the pattern 111 0001 is received using Hamming (7,4) encoding, prove whether there was a one-bit error during transmission. Hence, give the correct data bit pattern sent. (5 marks) (e) A sets of constellation points is given below. The numbers in parentheses define the values of I and Q respectively. Four points at (1,1), (-1,1), (-1,-1) and (1,-1). Four points at (3,3), (-3,3), (-3,-3) and (3,-3). Draw the constellation diagram showing the peak amplitude value. Name the type of modulation. (5 marks) (f) A cable company uses one of the cable TV channels (with a bandwidth of 8 MHz) to provide digital communication for each resident. What is the available data rate for each resident if the company uses a 64-QAM technique? (5 marks) *** END OF QUESTION PAPER *** Page 5 of 6
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 Digital to Analog conversion The baud or signal rate can be expressed as: S = N / r bauds where N = data rate r = ratio between data element and signal element r = log2 L where L is number of signal elements ASCII numbers Character Decimal a 97 b 98 c 99 y 121 z 122 Page 6 of 6