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# Mid term exam

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• 1. Name: Data Communication Date: Oct 26, 2012Student Id: Mid-Term ExamQ1. For Stop-and-Wait ARQ, for 10 data packets sent, _______ acknowledgments are needed. a. 1 b. 10 c. 2 d. None of theseQ2. Data link control deals with the design and procedures for ______ communication. a. Node to node b. End host to End host c. Process to process d. None of the aboveQ3. Error and Flow control algorithms are defined in the a. Data link layer b. Network layer c. Transport Layer d. Physical LayerQ4. You are running a PC at home, which is connected to the internet using a modem over a telephonecommunication link. The modem can transfer data at a maximum transmission rate of 28,8000 bits/sec. (a) How long will it take to download a 5000000 byte file from a server? (Assume propagation time is 0). (b) Now, assume that the server is located at a distance of 10,000 km from your PC. The data travel at a 8 speed of light, i.e., 3 * 10 meter/sec. How long will it take to download a 5000000 byte file from the server?Q5. Given a message M=1010001101, determine the CRC using the polynomial 5 4 2P=x + x + x + 1, i.e., the divisor is 110101.What is the final message?Q6. Consider the use of hamming code to send 11-bit message, 10011011011. You will need 15 bits.Show the hamming code.(hint) ********************Codeword | 1 0 0 1 1 0 1 1 0 1 1 --------|----------------------------------------------Position | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15******************************************Now invert the 13 bit and show that it is properly detected and corrected.
• 2. Q7. A data link layer uses the following character encoding:C: 01000011; E: 01000101; S: 01010011; FLAG: 01111110; ESC: 10011001For the eight character frame:C S ESC C E FLAG E EShow the big sequence actually transmitted in binary, with the flag bytes at each end (SFD, EFD) when bitstuffing is used.Q8. (Assume sliding window protocol is used)Each frame contains a sequence number S.The ACK number, K, acknowledges all frames up to K-1 and expects the next frame with sequence numberK.The sequence number is modulo W. i.e. S Modulo W returns the remainder when S is divided by W. Thisrestrains the value of S to stay between 0 and W-1. For ex: S=2 and W=8, S modulo W = 2. S=10 and W=8,S modulo W = 2. a) How many frames a sender can send without getting an acknowledgment, if the initial sequence number is S, and Window size is W. b) Assume windows size W= 8. The frames with sequence S = 5,6,7,0,1 are sent, but no ACK received yet. What frames is sender allowed to send? c) Assume window size, W=8. What is the value of Sf and Sn for (b) if the following ACK arrives a. Acknowledgment frame with value 2 arrives? b. Acknowledgment frame with value 6 arrives? For each ACK, also write the acknowledged frame and the draw the window showing the position of sf and sn. Q9. Fill in the numbers as explained below for the following illustrations.
• 3. Frames Accepted/ Rejeted Choose youranswer