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 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) (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) For each of the situation described below, identify the layer of the TCP/IP model it refers to (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) Transmitting bits from a node to the next Adding a trailer to the end of a frame for error correction. Dividing a message into transmissible segments. Allow a user to access the network. Arbitrating between multiple nodes attached to a single medium Moving a packet from a network boundary to another network. Controlling flow so that sender does not overwhelm the receiver. Defining the pins in a RJ-45 cable. Decide between a connection-oriented or connectionless link. (9 marks) Page 2 of 6
QUESTION 2: (25 MARKS) (a) With reference to a HTTP packet, explain the use and give sample values for the four levels of addresses in TCP/IP protocols. (2 marks) (b) The electrical power we use at home has a Frequency of 50Hz. Calculate the period, T, of this sine wave. Draw the Time-Domain and Frequency-Domain of this sine wave. (1 + 2 + 2 marks) (c) 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) (d) 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? (e) Noisy channels use the Stop-and-Wait ARQ protocol to send frames with no loss. Show a diagram with several packets and losses over time to illustrate this protocol. Hence, using pseudocode, write the algorithm. (4 + 4 marks) Page 3 of 6
QUESTION 3: (25 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 0000 1111, describe the features of each of the following scheme, mentioning the characteristics (advantages and disadvantages) of each. (i) (ii) (iii) (iv) (v) (vi) Unipolar Encoding NRZ-L Encoding NRZ-I Encoding RZ Encoding Manchester Encoding Differential Encoding (6 x 2 marks) (b) Referring to the six encoding schemes in part (a), sketch the resulting waveforms. Mention any assumption you make. (6 x 2 marks) (1 mark for neatness of diagrams) 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. (a) Suppose a sender wants to send the word network. Using the Odd parity check error detection method, show the pattern of bits sent and how errors can be detected. (3 marks) (b) A frame transmitter is using the generator polynomial x 3 +x+1 and the header contains the bit pattern: 1010. Derive the transmitted CRC header checksum. Page 4 of 6
(c) Hamming codes are a family of linear error-correcting codes that can correct one-bit errors. Given the bit pattern 1011 1001 is sent, and the pattern 1011 1000 is received, explain how Hamming(7,4) encoding can detect and correct this one-bit error. (5 marks) (d) 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) QPSK, with a peak amplitude value of 3 (ii) 8-QAM with two different peak amplitude values, 1 and 3, and four different phases. (e) A cable company uses one of the cable TV channels (with a bandwidth of 6 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? (f) What is the one-way latency for a 3-Mbyte message (an image) if the bandwidth of the network is 1 Mbps? Assume that the distance between the sender and the receiver is 15,000 km and that light travels at 2.4 10 8 m/s. Assume a total delay of 8ms per 100 bytes of data. (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 ASCII numbers Character Decimal a 97 b 98 c 99 y 121 z 122 Page 6 of 6