REAL-TIME COMMUNICATION Exam SEM2 PDF

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REAL-TIME COMMUNICATION Exam SEM2 Main Paper questions...

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Faculty of Creative Arts, Technologies and Science Department of Computer Science and Technology

CIS019-3 REAL-TIME COMMUNICATION

MAIN EXAMINATION PAPER

2015/16 SEM 2

DATE:

18 May 2016

TIME:

13:30hrs

DURATION:

3 Hours

UNIT COORDINATOR:

Dr Sijing Zhang, Dr Gregory Epiphaniou

INSTRUCTIONS TO CANDIDATES: •

Read the instructions in Parts 1 and 2

•

Basic, non-programmable calculators are allowed

DO NOT OPEN THIS PAPER UNTIL INSTRUCTED TO DO SO BY THE SENIOR INVIGILATOR CIS019-3 MAIN PAPER 2015-16 SEM 2

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PART 1 – Short Answer Questions (Compulsory) • •

Answer All Short Questions in Part 1 Each short question bears 4 marks

Question 1 Briefly state the difference between Network and Service convergence in the context of modern telecommunications. (4 marks) Question 2 Describe the problem of NAT and firewall Traversal in the context of SIPBased communications. Provide a discussion on the means used to circumvent this problem. (4 marks)

Question 3 List the four key limitations of Real-Time Protocol (RTP). (4 marks) Question 4 The packetisation delay results from the process of voice data encapsulation into packets prior to network transmission. Given that G.711 is used as the default codec with 160 bytes default payload, calculate the packetisation delay in milliseconds (ms). (4 marks) Question 5 List at least six best practices to impose security measures to converged networks. Briefly discuss considerations prior to their deployment. (4 marks) Question 6 Forward Error Correction (FEC) schemes, as effective fault-tolerance measures for restoring the lost packet at the destination node, have been used for some Internet applications. Describe one FEC scheme. (4 marks) CIS019-3 MAIN PAPER 2015-16 SEM 2

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Question 7 Why is a reduced protocol stack always preferred for real-time communication? (4 marks) Question 8 Although TCP is a connection-oriented transport protocol offering a reliable communication service, the protocol is generally considered unsuitable for transmitting time -constrained multimedia data. Explain why TCP is not considered as a practically suitable protocol for transmitting time-sensitive multimedia data. (4 marks) Question 9 Briefly discuss and justify the suitability of preemptive priority queuing and non-preemptive queuing for real-time communication over computer networks. (4 marks)

Question 10 Why is the Real-Time Transport Protocol (RTP) generally considered suitable for transferring real-time multimedia data over the Internet while it provides no guarantees about delivery and jitter? (4 marks)

CIS019-3 MAIN PAPER 2015-16 SEM 2

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PART 2 – Long Answer Questions • •

Answer any 3 questions only. Each question carries 20 marks.

Question 1 a) Discuss the main advantages and disadvantages of service convergence. (10 marks)

b) Critically discuss Quality of Experience (QoE) and its relationship to Quality of Service (QoS) as latter is manifested in the network impairments a transmission may suffer. Give an appropriate exemplar to distinguish the two different concepts. (10 marks) Question 2 a) Discuss the barriers unified Communication (UC) must overcome to fulfil its purpose. Use appropriate examples to justify your answers. (10 marks) b) Discuss the architectural elements of SIP. Draw a diagram that describes the SIP message exchange. (10 marks) Question 3 a) Discuss advantages and disadvantages of both the utilization-based approach and time-demand (e.g., response-time) approach for testing schedulability of real-time traffic in a hard real-time communication system. (10 marks) b) Peter’s boss gave him a detailed specification document about a set of periodic tasks which were derived based on one potential real-time application. The application was believed, once implemented, to be able to bring a much higher productivity of the company’s products. He asked Peter to quickly find out if the task set drafted during the initial design stage would be schedulable in practice as he knew Peter studied on a course called Real-Time Systems two years ago. Peter carefully checked the task set under the rate monotonic priority assignment scheme and found that the Rate Monotonic scheduling could not guarantee the task set (as he found the task set failed to pass both the utilization-based test and the response-time based analysis). Based on his knowledge on real-time computing, Peter drew the conclusion that the task set was unschedulable as he remembered that the rate-monotonic scheduling algorithm was one CIS019-3 MAIN PAPER 2015-16 SEM 2

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of a few optimal scheduling algorithms covered in his real-time systems course. Before reporting his finding to his boss, Peter phoned his old classmate Alice to double-check his conclusion. Alice replied to Peter over the phone as follows: “You are absolutely right !!! Because you already formally tested the task set using the Rate Monotonic Scheduling scheme which is in fact an optimal scheme, no other scheduling schemes including other optimal schemes like the Least Laxity First (LLF) can bring you a better solution. So just let your boss know that the task set is unschedulable.” Is Alice’s reply correct and why? (10 marks)

Question 4 Suppose that you are given a task of considering the possibility of using the VTCSMA-L algorithm to guarantee a given set of packets over an Ethernet LAN. Assume that the transmission time for each packet is TM = 15 and that the propagation time is τ = 1. Suppose that the packets arrive according to Table 1. Table 1: Packet Parameters Node 1 2 3 4

M 1 2 3 4

RC at arrival 0 10 20 24

DM 24 34 66 50

LM 8 18 50 34

a) Draw a VC-RC trajectory diagram to check if all considered packets can be successfully transmitted before their deadlines under η = 2 (i.e., the VC, Virtual Clock, runs twice as fast as the RC, Real Clock, when the channel is idle). You need to explain your diagram. (8 marks) b) Do the similar checking for the same set of packets under η = 4 . (Note: the same requirements as in a) apply, i.e., draw and explain the diagram.) (8 marks)

c) What conclusions can be drawn from the results in a) and b) above? (4 marks) CIS019-3 MAIN PAPER 2015-16 SEM 2

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