Midterm Exam CSCE 232: Computer Networks Fall Instructions:

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1 Midterm Exam CSCE 232: Computer Networks Fall 2007 Last Name: First Name: Student ID: Instructions: 1. This is a close-book and close-notes exam. 2. There are seven questions in total. The number of points for each question is given in parentheses. There are 100 points total. You have two hours to answer the questions. 3. Please write down your name and student id on the top of this page first before you start answering any question. Answer all questions directly on the exam papers. If you need additional sheets, please let us know. 4. Partial credit is possible for an answer. Please try to be as concise and make your exam as neat as possible. 5. Good luck. Enjoy!

2 1. Quickies (20 points, 4 points each) Answer the following questions briefly. (a) Which one is preferred for data networks: circuit switching or packet switching? Why? (b) Which of the following encoding schemes is the best for clock recovery? Manchester or 4B/5B? Why? (c) The application wants to transmit the data pattern What bit sequence is actually transmitted on the wire in the body of the packet when special bit sequence is used for framing? 1

3 (d) Can we use a repeater to connect an Ethernet LAN and a Token Ring LAN? Why or why not? (e) Which is better under heavy load? Ethernet or Token Ring? Why? What about under light load? 2

4 2. HTTP Downloading (10 points.) How many round trips would it take to download a web page that contains N embedded onesegment objects? (a) nonpersistent HTTP with no parallel TCP connections (b) nonpersistent HTTP with N K parallel TCP connections (assume N K is an integer) (c) persistent HTTP without pipelining (d) persistent HTTP with pipelining 3

5 3. Delay in packet Switched Networks (10 points.) Suppose two hosts, A and B, are separated by kilometers and are connected by a direct link with bandwidth 1 Mbps. Suppose the propagation speed over the link is meters/sec. Consider sending a 1 MB file continuously as one big message from A to B. What is the transmission delay? What is the propagation delay? What is the maximum number of bits that will be in the link at any given time? 4

6 4. Error Detection (10 points.) Consider a 6-bit message This message is transmitted along with CRC computed using the generator polynomial x (a) What will be the resulting CRC? What will be the resulting bit sequence that is transmitted? (b) Suppose the second bit from the right in the transmitted sequence is in error. What will be the bit sequence received by the receiver? Can the receiver detect that there was an error? If so how? 5

7 5. Wireless LAN (15 points.) (a) Suppose an b station is configured to always reserve the channel with the RTS/CTS sequence. Suppose this station suddenly wants to transmit 1,000 bytes of data, and all other stations are idle at this time. As a function of SIFS and DIFS, and ignoring propagation delay and assuming no bit errors, calculate the time required to transmit the frame and receive the acknowledgment. Note that RTS, CTS and ACK have the same frame format as that of DATA, which contains 32 bytes of header and trailer, plus a payload field. The payload of RTS, CTS, and ACK contains 0 byte. The transmission speed of b network is 10Mbps. (b) What is the hidden terminal problem? What is the exposed terminal problem? If needed, you may use figures to explain your definition. (c) Give two reasons to explain why CSMA/CD could not be applied to wireless LAN. What is the basic media access protocol for ? 6

8 6. CSMA/CD (15 points.) Consider a CSMA/CD network running at 10 Mbps over 2-km cable with no repeaters. The signal speed in the cable is 200,000 km/sec. Suppose a station starts transmitting a frame of size 1K bytes at time T. (a) Can it happen that the station detects a collision at time T+7µs (micro seconds) if there was no collision detected between time T and T+7µs? Why? (b) Can it happen that the station detects a collision at time T+16µs, if there was no collision detected between time T and T+16µs? Why? (c) Can it happen that the station detects a collision at time T+29µs, if there was no collision detected between time T and T+29µs?? Why? 7

9 7. Bridging (20 points) Consider hosts A, B, C, D, E and learning bridges B1 to B7 with initially empty forwarding tables, as shown in the following figure. L1 C B2 D L2 L3 B3 L11 B1 L4 B6 L5 L6 B4 E L7 L8 B B5 A B7 L9 L10 (a) Indicate the ports/edges that are not selected by the spanning tree protocol. (b) Now after (a) suppose A sends to D. Which bridges learn where A is? Does B s network interface see this packet? 8

10 (c) Now after (b) suppose D sends to A. Which bridges learn where D is? Does E s network interface see this packet? (d) Now after (b) and (c) suppose C sends to D. Which bridges learn where C is? Does E s network interface see this packet? 9

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