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Autumn Examinations 2009-
Exam Code(s) 3BP Exam(s) Third Electronic and Computer Engineering Examination Module Code(s) EE Module(s) Communication Systems Engineering Paper No. 1 Repeat Paper No External Examiner(s) Prof. G. W. Irwin Internal Examiner(s) Prof. G. Ó Laighin Mr. L. Kilmartin
Instructions: Answer any five questions from seven.
All questions carry equal marks (20 marks each)
Duration 3 hrs
No. of Pages 6 Department(s) Electrical & Electronic Engineering Course Co-ordinator(s) Mr. L. Kilmartin Requirements : MCQ Handout Statistical Tables Graph Paper Log Graph Paper Other Material Standard mathematical tables
1. (a) Data is transmitted over a communication link in frames which have a fixed size of 500 bytes. The bit error rate on this link is 4x10-4. Using the expected number of bit errors in a frame as a guide, comment on the reliability of the data received from this link when the channel coding scheme in use on the link is: (i) An odd Parity based code (ii) A Hamming code based on even parity (iii) A Cyclical Redundancy Code with a generating polynomial of G(x)=x^4 +x^2 + [6 marks] (b) Figure 1 describes the structure of a convolutional encoder formed by a three-bit shift register which generates two output bits for every input bit. Firstly, draw state transition and trellis diagrams for this particular encoder structure. The receiver for this convolutional coder receives the following bit sequence: 10 01 00 01 11 Determine the most probable binary input sequence which was applied to the transmitter’s convolutional encoder. You may assume that this sequence was generated using the standard initial and final state constraints for a convolutional coder of this type. SSS 111 S 2 S 3 Xin V 1 V 2 Xout XOR Figure 1 - Structure of Convolutional Encoder [8 marks] (c) A source coding scheme must be designed to support the transmission of data which can have any one of six different values. The a priori probability of occurrence, P(Si), of these six values has been estimated and is given in Table 1. Si S 1 S 2 S 3 S 4 S 5 S 6 P(Si) 0.35 0.1 0.11 0.21 0.06 0. Table 1 – A priori probabilities of occurrence of the symbols Design and determine the efficiency of a Shannon-Fano source coding scheme for this system. [6 marks] 2. (a) Clearly distinguish between a synchronous and an asynchronous transmission system and hence explain how line coding/waveform formatting techniques are used to achieve bit or clock synchronisation. [4 marks] (b) Table 1 contains the sequence of waveforms which are received at a HDB3 receiver circuit. The symbols +, - and 0 represent a positive polarity pulse, negative polarity pulse and zero volts, respectively. Determine the binary values which these symbols represent and clearly indicate any violation/protection pulses present in the sequence. You may assume that the last pulse received prior to this sequence was of negative polarity.
(b) Explain what is meant by the term Maximum Transmission Unit (MTU) and outline how this value affects the process of IP packet fragmentation. Your answer should clearly identify and explain all fields in the standard IP header which are used during packet fragmentation. An internetwork is formed by two different networks as outlined below: Network A has a 4500 byte MTU Network B has a 2000 byte MTU A 5000 byte frame is transmitted by an IP host on Network A to another IP host on Network B. Determine how this PDU is fragmented on each network. Clearly indicate the values used in the relevant parameters in the IP header of each fragment. [5 marks] (c) An organisation requires in the region of 8000 IP nodes. Determine the address utilisation factor if the network is allocated a (i) single class B address and (ii) a suitable group of Class C addresses. Hence, explain how the concept of classless addressing could be used if the organisation was allocated specific class C addresses and give ONE advantage and ONE disadvantage of this approach compared to the traditional class based addressing schemes. [5 marks]
4. (a) In the context of a routing algorithm, what do you understand by the cost associated with the link in a network. Hence describe the impact which the following properties would have on the link costs of a network: Bite rate Latency Link utilization Your answer should also clearly indicate whether the impact of each of the quantities on the link costs would likely be static or a dynamic. [4 marks] (b) Clearly explain the difference between the aims and operations of inter-domain and intra-domain routing protocols in an IP network. Describe the basic operation of the distance vector routing algorithm. Give an example of how a count to infinity stability problem can arise when such a routing algorithm is implemented AND how it can be addressed in practice. [7 marks] (c) Figure 3 describes the structure of a simple data network. Express this structure in the form of two matrices namely the adjacency matrix A and the cost matrix C. The degree matrix D of a network is a diagonal matrix whose entries represent the number of links terminating at each node in the network. Determine the degree matrix for the network in figure 3 and state the relationship between the entries of A and those of D.
Figure 3 – Network Structure [4 marks] (d) Determine the routing table at node C in the network of Figure 3 using the forward search algorithm. Your answer MUST show the steps used in determining the final table. [5 marks]
5. (a) Comment on the support offered by (i) UDP and (ii) TCP for each of the following functionalities: (i) Transport Layer Addressing (ii) Reliability (iii) Flow Control (iv) Congestion Control By citing an example of the type of application that would utilise them, justify the need for both UDP and TCP transport layer protocols. [6 marks] (b) Explain why it is necessary to have an adaptive timeout duration for the TCP sliding window protocol and clearly identify the impact of utilising (i) too short of a timeout duration or (ii) too long of a timeout duration. A TCP implementation measures the round trip time (RTT) for a number of specific TCP segment as 120 ms, 130 ms and 110 ms, in that order. The estimated RTT prior to the first of these particular TCP segments being transmitted is 125 ms. Determine the TCP timeout duration at the end of this period of time using: (i) The original TCP timeout update algorithm (using =0.9). (ii) The Jacobson-Karels algorithm (with =1, =4, =0.1 and an initial deviation of 10 ms). If the real RTT process is actually normally distributed with a mean of 125 ms and a standard deviation of 20 ms, estimate the percentage of unnecessary re-transmissions which will take place based on the timeout value determined using (ii) above. [9 marks] (c) Clearly distinguish between the role and operation of the flow control and congestion control algorithms as implemented in standard TCP. [5 marks]
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(c) Discuss how digital speech interpolation technologies can be used to maximise revenues from voice traffic for operators of satellite communication links. [4 marks]
