CompNetExamFall09

CompNetExamFall09 - EPFL-logo Computer Networks Final Exam...

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Unformatted text preview: EPFL-logo Computer Networks - Final Exam Prof. J.-P. Hubaux and Dr. M. H. Manshaei January 18, 2010 Duration: 3:00 hours, closed book. Please write your answers on these sheets in a readable way. Poorly written answers will not be corrected. Use extra sheets if necessary (put your name on them). You may write your answers in English or in French. The total number of points is 60. This document contains 20 pages. Student First name: Last name: Division: 2 Communication Systems 2 Computer Science 2 Other (mention it): . . . . . . . . . Year: 2 Bachelor Year 2 2 Bachelor Year 3 2 Other (mention it): . . . . . . . . . (answers to the questions are shown in italic and blue) 1 Short questions (5 points) For each question, please circle a single best answer . 1. In the CSMA/CD protocol, what condition on the transmission delay T trans and the propagation delay T prop has to be satisfied to guarantee that a node always detects a collision? (a) T trans > T prop (b) T trans > 2 T prop CORRECT (c) T trans < T prop (d) 2 T trans < T prop 2. Alice and Bob communicate in an alphabet that has L letters. They use a monoalphabetic substitu- tion cipher with key K . Assume that Trudy obtains a plaintext m and the corresponding ciphertext c . Under what conditions can Trudy recover the key K completely? (a) m contains at least L- 1 unique letters. CORRECT (b) | m | ≥ L . (c) | m | ≥ | K | . (d) m is not empty. 3. The WiMaX protocol: (a) provides the maximum available data rate over a wireless channel. (b) uses unlicensed spectrum, such as WiFi. (c) provides a mechanism for scheduling. CORRECT (d) All of the above options are correct. 4. Two hosts simultaneously send data through a link of capacity 1Mbps. Host A generates data with a rate of 1Mbps and uses TCP. Host B uses UDP and transmits a 100bytes packet every 1ms. Which host will obtain higher throughput? (a) Host A . (b) Host B . CORRECT (c) They obtain roughly the same throughput. (d) They experience congestion collapse and negligible throughput. 5. What is the theoretical upper-bound on the number of simultaneous TCP/IPv4 connections that a host with a single IP address can handle? (Ignore memory limitations.) (a) 2 16 (b) 2 32 (c) 2 64 CORRECT (d) unlimited 2 6. Host A sends a TCP segment (Seq = 43, ACK = 103), to which host B replies with a TCP segment (Seq = 103, ACK = 57). The payload of the first TCP segment is (a) 14 bytes long. CORRECT (b) 43 bytes long. (c) 46 bytes long. (d) 57 bytes long. (e) 60 bytes long. 7. The Distance-Vector algorithm is not: (a) iterative. (b) asynchronous. (c) distributed. (d) used in RIP. (e) None of the above. CORRECT 8. Longest prefix matching is used: (a) in routers to know on which link interface to forward packets. CORRECT (b) in classless addressing to use the address space more efficiently than in classful addressing....
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CompNetExamFall09 - EPFL-logo Computer Networks Final Exam...

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