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Final2015FallECE358(Solution) University of Waterloo ECE 358

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ECE 358 Final Exam., F’2015 Dept. of Electrical and Computer Engineering ECE 358 – Computer Networks Instructor: Sagar Naik Exam. Date and Time: Dec. 8, 2015, 12:30 PM—3:00 PM Room: PAC 9, 10 Instructions  This is a closed book, 2.5 hour exam.  Should there be a need, make reasonable assumptions and proceed.  All acronyms have their standard expansions as explained in class.  Unless otherwise stated, all network layer questions are about IPv4.  You may use a calculator. UW ID Your Name (Last, First) Signature Questions/ Marked by Marks Questions/ Marked by Marks Q1/ [SS] /10 Q2/ [SS] /7 Q3/ [SS] /6 Q4/ [AG] /11 Q5/ [AG] /8 Q6/ [AZ] /10 Q7/ [AZ] /8 Total = /60 Best wishes… Merry Christmas and Happy Holidays ECE 358 Final Exam/ Fall 2015 *** Sample Solution *** Page 2 of 15 You may continue writing on the reverse side of this page, except page 1. Q1. [10 marks] [6+2+2] [a] Clearly explain the non-persistent CSMA/CD protocol by means of a flow chart of the sender’s behavior. Show the receiver’s behavior as well. [b] Explain the importance of the random wait right after detecting the medium to be busy. [c] If you replace the binary exponential backoff (BEB) mechanism with a fixed backoff mechanism (that is, choose a random number from a fixed interval), how will the performance of the protocol be impacted? Answers: ------------ [a] There is a slide in the lecture notes showing the flow-chart for the sender. The receiver’s flow-chart can be drawn from the following text:  Receive a frame.  Check the CRC field for errors.  If there is CRC error, drop the frame, else pass it on to the upper layer.  Go back to the first step. [b] In the absence of the random wait right after sensing the medium to be busy, the following scenario can occur. Two nodes sense the medium to be busy and continue to sense the medium exactly at the same time. Eventually, they will sense the medium to be free and will start their respective transmissions, which will definitely collide. As a result, the performance of the protocol will degrade. [c] If the BEB is replaced with a fixed backoff mechanism, then the wait time will not be adaptive to network load, because every time the protocol enters the backoff phase, a random number will be chosen from the same interval. Consequently, the protocol’s throughput will be degraded.

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