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EE444HW4.sln - EE 444 INTRODUCTION TO COMPUTER NETWORKS...

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EE 444 INTRODUCTION TO COMPUTER NETWORKS Spring 2009 HOMEWORK # 4 Deadline: April 24 2009 @ 23:59 (No late submission is allowed.) Posted on April 17 2009. Question # 1 Given the message 1010001101 and the CRC polynomial P(x) = x 5 +x 4 +x 2 +1. (a) Compute the final message to be sent including the CRC. (b) Show how the receiver performs the CRC check operation on the received message. Show all your work. Solution # 1 (a) P(x)= 110101 Sender side computes CRC 1010001101 00000 |110101 Quotient: 110101011 (not considered) 110101 _______ 0111011 110101 ________ 00111010 110101 ________ 00111110 110101 ________ 00101100 110101 ________ 0110010 110101 _________ 00 01110 Sent message: 1010001101 01110
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(b) Receiver checks CRC 1010001101 01110 |110101 Quotient: 1101010110 (not considered) _______ 0111011 110101 ________ 00111010 110101 ________ 00111110 110101 ________ 00101111 110101 ________ 0110101 110101 _________ 00000000 Remainder is 0, correct Question # 2 (a) A channel has a data rate of 4 kbps and a propagation delay of 20 ms. For what range of frame sizes does stop-and-wait give an efficiency of at least 50%? (b) Two nodes n 1 and n 2 communicate using Go-Back-N as the ARQ mechanism with a window size of 4 frames. Assume that n 1 is transmitting and n 2 is receiving. Show the window positions for the following sequence of events.
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