CSC3420 – Mobile Internet Technology Semester 1 Page 1 of 5 NameMeaninglowfreqfreqwidthsnrnsymbolsdbdiffnchannelssymbolrateframelengthsubchannelfreqa frequency, in GHza spectral width, in MHza signal to noise ratio, in dBnumber of symbolsa difference, in dB, between the power of two signalsnumber of channelsa rate of sending of symbols, in symbols/sa duration, of a frame, in nsa frequency, in GHz Table 1: Table of exam parametersThis exam makes use of a number of parameters, each of which will be supplied to each student in an emailmessage prior to the exam. The parameters names, and meanings are listed in Table 1. To help make theseparameters easier to use, they are always written in a distinct font, of a certain colour, e.g. lowfreq.Question 1.(a) If a communication system uses the frequency range lowfreq GHz to lowfreq + freqwidth GHz, andhas a signal to noise ratio of snr dB, what is the upper bound on its capacity according to the formulaof Shannon? Provide a reasonable estimate of the capacity of such a system. 7 marks(b) Suppose there is difference in power of dbdiff dB between location A and location B of a communication system; What is the ratio of the power at A to the power at B, i.e. what is power of the signalat A divided by the power of the signal at B? 7 marks(c) Approximately, what frequency range of electromagnetic spectrum is typically used for the followingservices: 6 marks• Wifi,• Mobile phones,• optical communication through optical fibers?Total: 20 marksQuestion 2.(a) Suppose an OFDM system uses nchannels sub-channels, there are nsymbols possible symbols in usewhen transmission takes place in each frequency, and the bit-rate of the whole system is bitrate.What is the duration of each symbol? 7 marks(b) If the frame-length of an OFDM system is framelength ns, and one of the sub-channels has a frequencyof subchannelfreq, how many cycles of this frequency will occur in one frame? 7 marks(c) In a code-division multiple access system with codes of length 8, the codes 1111-1-1-1-111-1-111-1-1-1-11111-1-1 are orthogonal and sum to zero. Provide three more codes, also with zero sum, which are orthogonalto these codes and to each other? 6 marksTotal: 20 marksCSC3420 – Mobile Internet Technology Semester 1 Page 2 of 5Question 3.Suppose you are installing a communal wireless network in a block of 50 apartments. Review the options inregard to network security. Your task is to explain the alternative approaches to security to the committeeof owners, who will then decide whether to go ahead with the wireless system, and which option to choose.A list of feasible options for this situation are given below. In your answer to this question, please explainhow each option would be implemented and list the strengths and weakness of each approach. Include arecommendation which selects one of the options, and explains why it is the preferred one.Proposed solutions:1. Use the Wireless Protection Architecture I and II (WPA I and WPA II), which also has a single passwordwhich is passed on to all users of the LAN. 5 marks2. Use the Wireless Protection Architecture II (WPA II), which is linked to an authentication database,like Radius. 5 marks3. Use a Virtual Private Network access gateway. 5 marks4. Use routing through a mandatory proxy. 5 marks5. Use MAC address registration. 5 marksMarks will be awarded for the clarity and correctness of your explanations, rather than for having selectedthe right answer. If you feel that more than one of the proposed solutions is satisfactory, or that none ofthem are satisfactory, or that there is another solution which is preferable to all of the proposed solutions,explain that. Total: 20 marksCSC3420 – Mobile Internet Technology Semester 1 Page 3 of 5Question 4.A network as shown in Figure 1 was simulated and the results are shown in Figures 2a{2e. The capacity ofall the links in this network are 10 Mbps except the middle link, which has a capacity of 2 Mbps. Answerthe following question in connection with the results shown in these Figures:(a) Why are there sometimes long gaps between the sequence numbers for a traffic stream and thecorresponding acknowledgement numbers, in Figure 2d? 5 marks(b) Why does the buffer level, at the head of link 5, drop down dramatically sometimes? 4 marks(c) How much does the delay between sending a packet and its acknowledgement vary, for each of thetraffic streams whose sequence numbers and acknowledgement numbers are plotted in Figure 2d? 4 marks(d) During which periods of the simulation does congestion occur, in Link 5? 4 marks(e) Is the Link 5 ever idle after t = 1s? 4 marks(f) Why is the throughput of Stream 3 lower, on average, than that of Streams 1 and 7? 4 marksFigure 1: Simulated NetworkTotal: 25 marksCSC3420 – Mobile Internet Technology Semester 1 Page 4 of 5(a) Link throughputs in the simulated network for Question 1(b) Traffic throughput in the simulated network (c) Traffic transmitted in the simulated network(d) Sequence numbers in the simulated network (e) Congestion windows in the simulated networkCSC3420 – Mobile Internet Technology Semester 1 Page 5 of 5Question 5.Quality of service in the Internet varies randomly from time to time and from place to place depending onconditions. Explain why quality of service varies, how this affects the experience of users, and the strategieswhich are being used now, and are proposed to be used, to address quality of service. Make sure to addressthe following issues:(a) What are the main causes for quality of service to be worse than usual on some occasions or in someplaces? 5 marks(b) What the main strategies for ensuring that quality of service is satisfactory for a suffient proportion ofthe time? 5 marks(c) What strategies might be used in future to further improve quality of service? 5 marks(d) Are their devious strategies which could be used to enable some users to gain a service advantage overother users, and if so, please describe them. 5 marks(e) Are their approved strategies which could be used to enable some users to gain a service advantageover other users, and if so, please describe them. 5 marksTotal: 25 marks
