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SUNYIT Computer Networks CSC-522 Midterm Exam Fall 2002 Answer only 5 questions. 1. Answer the following questions: a. b. c. d. e. Which layer of the OSI model is split into two and what are the names of the split layers? Will an intermediate node require all seven layers of the OSI model? Which layer(s) of the OSI model is/are the primary focus of Electrical Engineering? How far does a signal travel in one nanosecond? How long does it take to transmit a maximum size frame (1500 bytes) on a 10 Mbps Ethernet? 2. What is the open systems concept in the OSI model? Give one advantage and one disadvantage of an open system architecture. 3. Consider an asynchronous transmission system with 1 start bit, 3 data bits, 1 parity bit, and 2 stop bits. Assume p[bit error] = 0.05 and bit errors are independent. Show your work. (a) What is the maximum utilization possible on this link (assuming no errors)? (b) What is the probability that the received word has no errors? (c) What is the probability that a received word is detected as "good"? 4. What is circuit switching? Under what condition a circuit switching may be desirable than a store and forward packet switching on a distance spanned over h hops on links whose bandwidth is B bps? 5. Assume a 1 Mbps link of 1000-kilometers in length with 5 nanoseconds per meter propagation delay. Assume negligible time for ACK frames and processing delay. The sender always has data to send. For Stop-and-Wait (SAW), what is the minimum frame size required to achieve a utilization of no less than 75%? 6. A sender and receiver use CRC as the means of detecting errors. The generator polynomial, P, used by both the server and the receiver is, P = 101 (i.e in polynomial form x 2 1 ). Assume that the receiver receives a frame T = 111001. Is T without error(s)? Explain your answer carefully. You must show your work, on a separate sheet of paper, to get credit for this problem (a lucky guess won’t work). 7. Using a Hamming code a sender sends a 8 message bits and 4 parity bits (altogether 12 bits) in a codeword of length 12 bits. The receiver receives this as 1010 1011 0101. Comment on this. 8. The optimum capacity assignment formula on a link entertaining a traffic flow at an average rate 1 of i serving messages of average size of could be stated as i Ci i i k i i where k is a constant for the system. Assume that you have A B 16 25 C 36 three links as shown, and the traffic intensity i on the link A, B, and C are computed as 16, 25 i and 36 units, respectively. Obtain for these links the optimum capacity rates. 9. What is the purpose of a bridge B between two LAN segments as shown? L1 B L2 Suppose two enhance reliability we add one more bridge into this architecture. How will the System now work? Is a bridge same as a router? L1 B C L2