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Spring Examinations 2007/ 2008
Exam Code(s) 3BN121, 3BP
Exam(s) Third Year Electronic Engineering
Third Year Electronic & Computer Engineering
Module Code(s) EE
Module(s) Communication Systems Engineering
Paper No. 1
Repeat Paper N
External Examiner(s) Professor P. Cheung
Internal Examiner(s) Professor G. Ó Laighin
Mr. L. Kilmartin
Instructions:
Answer any five questions
Duration
3 hours
No. of Pages 7
Department(s) Electronic Engineering
Course Co-
ordinator(s)
Requirements :
MCQ
Handout
Statistical Tables Y
Graph Paper Y
Log Graph Paper
Other Material
1. (a) Explain what you understand by the term source coding and identify one common example of source coding and clearly describe why it is an example of this technique. [4 marks]
(b) The following set of symbols for which the a prioi probabilities are known needs to be transmitted over a communication link:
Symbol S 1 S 2 S 3 S 4 S 5 S 6 S (^7) Number of Occurrences 0.2 0.15 0.025 0.1 0.225 0.05 0.
(i) State the number of bits per symbol required for this symbol set if NO source coding scheme is used. (ii) Determine the set of codewords used to represent each symbol if a Huffman code is developed. (iii) Determine the average number of bits per symbol for the code developed in (ii). (iv) Estimate the Entropy value for the above symbol set and the efficiency of the code developed in (ii). [7 marks]
(c) Explain what you understand by the terms BER and interleaving in the context of protecting information from errors induced in a communication channel. [3 marks]
(d) The following block of bits is received for processing by a channel decoding block.
1 1 0 1 0 0 1 0 0 0 1 0 1 1
Determine if there are bit errors present in this received block, and where supported clearly identify which bit is determined to be errored, where the channel coding scheme in use is:
(i) A CRC code with generating polynomial G(x) = x^4 +x+1. (ii) A Hamming code based on EVEN parity. [6 marks]
2. (a) What is the difference between asynchronous and synchronous transmission in the context of the physical layer of a data communication system and summarise the role of line coding in the process of achieving bit or clock synchonisation. [4 marks]
(b) The sequence of binary values contained in table 1 is to be transmitted by a HDB3 transmitter circuit. Determine the sequence of symbols transmitted to represent this sequence of bits. You may use the symbols +, - and 0 to represent a positive polarity pulse, negative polarity pulse and zero volts, respectively.
Your answer MUST clearly indicate the position of any protection or violation pulses which are present in this sequence.
(c) Determine the address class of each of the following IP addresses.
(i) 140.203.1. (ii) 22.121.1. (iii) 210.10.10.
Marks will only be awarded if some justification is provided relating to the address class that is indicated in your answer.
For the IP address (i), a subnet mask of 255.255.192.0 is in use on the actual physical network on which that node is located. Determine the subnet number for that physical network. [4 marks]
(d) An organisation expects to have in the region of 4000 IP nodes. Determine the address utilisation if the network is allocated a (i) single class B address and (ii) suitable group of Class C addresses.
Outline how the concept of classless addressing would be used if the organisation could only gain access to 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) Explain the difference between inter-domain and intra-domain routing in an IP network. [4 marks]
(b) Describe the basic operation of the distance vector routing algorithm and 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. [4 marks]
(c) Determine the routing table at node D in the network below using the forward search algorithm. Your answer MUST show the steps used to determine the final table.
D
C
B
A
[4 marks]
(d) Describe the function and operation of the Address Resolution Protocol in a broadcast based network technology such as Ethernet. [4 marks]
(e) Explain what is meant by the term DHCP including an example of a scenario where this protocol is commonly used. [4 marks]
5. (a) Explain why it is necessary to utilise a transport layer protocol in an IP network and describe in detail THREE relative advantages of the use of TCP compared with UDP as a transport layer protocol. [5 marks]
(b) Why is it necessary to have an adaptive timeout duration for the TCP sliding window protocol?.
A TCP implementation measures the round trip time (RTT) for a number of specific TCP segment as 120 ms, 130 ms and 100 ms, in that order. The estimated RTT prior to the first of these particular TCP segments being transmitted is 100 ms. Determine the TCP timeout duration at the end of this period of time using:
(i) The original TCP timeout update algorithm (using α=0.8). (ii) The Jacobson-Karels algorithm (with μ=1, φ=4, δ=0.2 and an initial deviation of 10 ms).
What would be the final timeout duration, using the above two algorithms, if the 100 ms sample RTT was measured as a result of a TCP timeout and segment retransmission? [8 marks]
(c) What is the difference between the advertised window size and the congestion window size in TCP and outline how the following procedures are used to update one of these parameters:
(i) Slow Start. (ii) Additive Increase\Multiplicative Decrease. (iii) Fast Retransmission. [7 marks]
6. (a) Describe, in detail, the operation of a persistent Carrier Sense Multiple Access protocol with Collision Detection supporting a truncated binary exponential back off algorithm. [4 marks]
(b) Clearly stating any assumptions made, derive an approximate formula for the efficiency of a CSMA bus network in terms of number of terminals on the network, the probability of a terminal attempting to transmit, system bandwidth, maximum bus length, frame length and speed of signal propagation on the bus. Hence, deduce the optimum efficiency for the network.
An 802.3 Ethernet network with a maximum propagation delay between any two stations of 51.2 μs and an average frame size of 10 kbyte is operating at an efficiency of 62.5%. If the probability of a terminal attempting to transmit on this network is 0.112 then determine the number of terminals on the network. [6 marks]
(c) Explain the meaning of the following terms in the context of an 802.5 Token Ring LAN:
(i) Minimum Ring Latency. (ii) Token Seizure Process. (iii) Frame Acknowledgement Process. (iv) Handling of Orphaned Frames and Frame Fragments. [6 marks]
(d) Explain what you understand by early token release in the context of a Token Ring Network and describe ONE advantage and ONE disadvantage of adopting this technique. [4 marks]
Ternary Output Signal Binary Running Digit Sum Value at the start of the four bit word Input -1 0 or +1 + 0000 +++ -+- -+- 0001 ++0 00- 00- 0010 +0+ 0-0 0- 0011 0++ -00 - 0100 +-+ +-+ --- 0101 0-+ 0-+ 0-+ 0110 -0+ -0+ -0+ 0111 00+ 00+ -- 1000 0+0 0+0 -0- 1001 +00 +00 0-- 1010 -+0 -+0 -+ 1011 +-0 +-0 +- 1100 +0- +0- +0- 1101 0+- 0+- 0+- 1110 -++ -++ --+ 1111 ++- +-- +--
Table 3 – 4B3T Encoding Table
